104471949 Energy Recovery Methods in Waste Water Treatment Systems

7/30/2019 104471949 Energy Recovery Methods in Waste Water Treatment Systems

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Table of ContentsChapter 1: Introduction ................................................................................................................ 5

Chapter 2: Overview of the processes of waste water treatment ................................................... 8

2.1- Introduction to waste water treatment .................................................................................. 8

2.2- Types of waste water .............................................................................................................. 8

2.3- Waste water treatment collection system ............................................................................. 9

2.4- Stages of waste water treatment ......................................................................................... 10

2.4.1- Preliminary treatment................................................................................................... 10

2.4.2- Primary treatment ........................................................................................................ 11

2.4.3- Secondary treatment .................................................................................................... 11

2.4.4- Tertiary treatment ........................................................................................................ 12

Chapter 3: Methods of energy recovery in waste water treatment .............................................. 13

3.1- Production of bio-gas ............................................................................................................ 13

3.1.1- Anaerobic digestion of sludge to produce methane (bio-gas) ..................................... 13

3.1.2- Sludge fermentation process ........................................................................................ 14

3.1.3- Exploiting the potential of waste water treatment sludge ........................................... 17

3.1.4- South Shore waste water treatment plant ................................................................... 18

3.2- Use of hydro-turbines to generate electrical power ............................................................ 21

3.2.1- The fundamentals of hydro-turbines ............................................................................ 21

3.2.2- Applying hydro-turbines to waste water treatment plants .......................................... 24

3.2.3- European waste water treatment plants that have adopted hydro technology .......... 24

3.3- Electricity generation using microbial fuel cells ................................................................... 28

3.3.1- How microbial fuel cells work ....................................................................................... 28

3.3.2- Microbial fuel cells for waste water treatment ............................................................ 30

3.3.3- Increasing electrical power generation using MFCs .................................................... 32

3.4- Power generation from plasma processing of sludge .......................................................... 33

3.4.1- How plasma processing works ...................................................................................... 33

3.4.2- The process of plasma gasification and the production of syn-gas .............................. 34

3.4.3- Plasma processing of sewage sludge ............................................................................ 34

3.4.4- Possible advantages and disadvantages ...................................................................... 36

Chapter 4: Case Studies .............................................................................................................. 39

4.1- Esholt Waste Water Treatment Plant, Yorkshire, UK ........................................................... 39


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4.1.1- Introduction .................................................................................................................. 39

4.1.2- Installation of new hydro-turbines ............................................................................... 39

4.2- Ringsend Waste Water Treatment Plant, Ringsend, Dublin ................................................. 42

4.2.1- Introduction .................................................................................................................. 42

4.2.2- Methane recovery from Ringsend waste water treatment plant ................................. 42

4.2.3- Potential power generation from hydro-turbines ........................................................ 44

Chapter 5: Conclusions ............................................................................................................... 48

5.1 Introduction .......................................................................................................................... 48

5.2- Overview ............................................................................................................................... 48

5.3- Final Comments & Recommendations ................................................................................. 49

5.4- References: ........................................................................................................................... 51

Books and Articles: ........................................................................................................................ 51

Websites: ...................................................................................................................................... 51


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I hereby certify that this material, which I now submit for assessment on the programme of

study leading to the award of Building Services Engineering Ordinary Degree is entirely my

own work and has not been taken from the work of others save and to the extent that such

work has been cited and acknowledged within the text of my work.

Signed:

_____________________

Student No:

Date:


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Acknowledgement

Firstly, I would like to extend many thanks to my dissertation supervisor ********, for all

her effort in helping and guiding me throughout the course of this dissertation. She has

been a constant and reliable means of gaining a good understanding of what this research

project is really about, and has helped enormously in seeing that the necessary tasks and

goals were achieved.

Secondly, I would like to thank Mr. ********** and all the staff of the building services

engineering department and the library who have been of constant support over the last 2

years in ensuring that my classmates and I were provided with all the necessary materials

we required to complete not just our research projects but our final year as a whole.

And finally, I would like to thank my parents, who have also been a constant source of

support not only in academic and financial terms but also in life and for that Im forever

grateful. To my mum who is severely ill at the moment, I would like to dedicate this to her.


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Chapter 1: Introduction

Due to the recent increase in energy prices over the past decade, the running costs of

operating waste water treatment plants has increased significantly. The function of waste

water treatment plants is to remove contaminants from the water in order to make it safe

to be used once again. In doing so, waste water treatment aims to prevent health hazards,

protect drinking water supplies, provide suitable water for recreational functions and

protect the ecology of our streams, lakes and other water bodies to which waste water is

discharged. For these reasons and the growing depletion of the Earths fossil fuel reserve,

the need for fuel conservation and energy management in waste water treatment is

important. However, if certain systems and processes are used, waste water treatment

plants have the potential to recover energy from various parts of the treatment process as

society demands increasingly scrupulous treatment to remove nutrients and chemicals from

waste water before it is discharged back into waterways.

However, energy consumption is coming under increasing questioning, with the financial

cost of energy and the cost of energy generation increasing new interest in the conversion

of waste water to energy. Incentives such as the emergence of earning carbon credits for

keeping greenhouse gas emissions low is also a driving force behind the need for carrying

out waste water treatment while also trying to conserve energy. Energy recovery can be

offered as a good alternative in the battle to lower fuel consumption. This problem is

putting pressure on local governments to seek more energy efficient ways in which to run

their waste water treatment plants or indeed build new ones as they consume vast amounts

of energy. I will be investigating the potential for energy recovery in waste water systems;

technology that is tried, tested and working at present and also technology that is still at the

experimental stages but has the potential for more advanced energy recovery in the future

as it becomes more refined. Some typical methods that are available at present for

recovering energy from waste water are; the collection of bio-gas (methane), generating

electricity using hydro turbines, fuel cell technology that generates power while it cleans

water and also power generated using plasma processing of sludge.


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Some of these methods are being used at present and are providing various waste water

plants around the world with considerable savings.

For example, the collection of methane gas from the waste water treatment process is not a

new technology and is also being used in many waste water treatment plants around

Ireland. Ringsend waste water treatment plant in Dublin is one of the biggest in Ireland and

is also one of the plants that use this type of energy recovery. Over fifty per cent of the

energy needed to operate the plant is generated from the sludge treatment. Sludge is a

product of waste water treatments primary and secondary stages which is high in energy

and can be used to harness methane gas. At the waste water treatment plant in Ringsend,

they use the processes of hydrolysis and digestion to treat the sludge. It is then before

thermally dried, in doing so, killing pathogens and producing an organic based fertiliser

called bio-fert. At Ringsend waste water treatment plant, they produce nearly twenty five

thousand tonnes of bio-fert each year. The sludge digestion process then produces methane

as by-product. This alone produces energy to cater for more than half of the heat and

electricity required at the waste water treatment plant.

This is the main theme of my dissertation. Energy recovery from waste water treatment

seems to be an area of energy recovery that doesnt seem to get a lot of attention and is

somewhat of an untapped source of energy generation in many ways. Waste water

treatment plants have the potential to one day generate their own electrical power and

become somewhat self sufficient and become less reliable on the national grid. Some of the

treatment plants that are mentioned in this research are producing up to half of their

operating needs, while others are selling some of the power generated back to the national

grid.

Bruce Logan, a professor of Environmental Engineering at Penn State University, USA said,

Were using something thought to be completelyuseless(Ehrenman, 2004)

The aim of this research is to provide an insight into some of the methods available for

energy recovery in waste water treatment plants that are functional at present and also

being developed. In present times, the economic landscape throughout the world demands

an energy efficient approach to all types of processes and for this reason, it is in the

interests of local governments to put methods like these into practice in order to save on


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energy costs and provide more sustainable and eco-friendly waste water treatment

systems.


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Chapter 2: Overview of the processes of

waste water treatment

2.1- Introduction to waste water treatment

Throughout our lives we use water for a whole different range of purposes and in turn it

becomes contaminated with many different substances. Water containing human waste will

contain disease-causing organisms, or pathogens. Such pathogens can cause diseases such

as hepatitis, polio, cholera and dysentery along with many others. The correct form of waste

water treatment can effectively help to control these and many other diseases being spread

throughout the water systems in which people come into contact with.

Oxygen is used in the process whereby organic wastes in water are broken down through

natural biological activity. As this occurs in nature such as a lake or a stream, if the content

of organic material is high it may actually consume enough of the oxygen in the water that

the fish and other aquatic wildlife may die. Water such as this contains a bad odour and is

deemed unfit for human consumption. It is also seen as hazardous for such waterrecreational activities such as swimming. Water that is polluted will be naturally purified by

streams and lakes, but this can take large amounts of time and space. In order to combat

this, waste water treatment uses biological, physical and chemical methods to minimise the

time and space needed for removing contaminants from water.

2.2- Types of waste water

Waste water can be described as domestic, industrial or storm water. Domestic sources of

waste water would include water used for activities carried out in homes, businesses and

other small to medium sized commercial buildings. However, classifying industrial waste

water is a little more difficult as it depends on the industrial application that the water is

being used for. A lot of industrial waste water can be treated the same way as domestic

waste water without much complexity but others can contain very dangerous toxic

substances or large amounts of organic materials or solids which may make treatment more

challenging. If this is the case, it is not uncommon for some industrial plants to pre-treat its


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waste water to remove these contaminants or lower them to more treatable levels before

they are released and received into a publicly owned treatment plant. Storm water though

usually very low in contaminants often goes to a treatment plant.

2.3- Waste water treatment collection system

Waste water is collected and transferred to the waste water treatment facility via a network

of pipes called a collection system. This network of pipes usually contains some of the

following components; building connections, laterals, mains pipes, interceptors, manholes

and lift stations. Building connections transport waste from the building to the collection

network. As the pipes return to the treatment facility, the diameters of the pipes increase in

size. These building connections feed into the smallest pipes of the system and these are

called laterals. The mains pipes are larger in diameter and receive flow from numerous

laterals. Interceptors however are major sewer pipes receiving flow from mains pipes and

transferring it to the treatment facility. Manholes are used for ease of access to the system

for tasks such as inspection or maintenance works. These components are installed where

problems in the system are most likely to occur such as changes in gradient, alignment, pipe

size or shape and also on long, straight runs of pipe work. Lift stations are pump installations

that are located at low points on the system where it is not possible to use gravity to move

the contents of the pipes through the line to the treatment facility. It is imperative that the

system is inspected and cleaned regularly in order to prevent the accumulation of grease,

sludge and sediment build-up in the pipe network and should be flushed at regular intervals

to avoid unnecessary blockages and pipe deterioration. Infiltration is water that seeps into

the collection network caused by broken pipes, root intrusion or joint failure. Additional

water like this may pose an excessive load on the treatment facility. Modern equipment is

available to combat this problem where a TV camera is inserted inside the sewer pipe where

it can correct infiltration by using chemical grout and other methods. In some areas roof

drains and catch basins are directly connected to the collection system and can make up a

high percentage of the water actually delivered to the waste water treatment facility. This

type of water collection is called inflow and this together with infiltration can affect the

overall efficiency of the treatment facility as there is no necessity to treat this type of water.


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2.4- Stages of waste water treatment

Waste water treatment facilities are designed to deal with their waste water in a number of

stages. The overall focus in waste water treatment is to remove contaminants from the

water by methods of getting them to either settle or to float and then removing this

material and allowing the left over water to move on to the next stage of treatment. Some

contaminants are uncomplicated to remove while others require more sophisticated

methods like converting the contaminants to a settle-able form in order for it to be

removed. Modern waste water treatment facilities may contain the following stages;

influent, preliminary treatment, primary treatment, secondary treatment, secondary

treatment, tertiary treatment, disinfection and effluent discharge, solids handling and side-

streams. Influent is the raw material that has been collected and transferred to the facility

for treatment. Influent is the waste water in its purest form and includes the water and

debris that entered the collection system. Awareness of the rate of flow of water into the

treatment facility is essential as it is the flow that determines how long the waste water will

be held in each of the different units in the facility. The rate of flow can vary significantly

with the flow usually being much larger during the day when human activities are at their

peak and much lower during the night when people are asleep. Efficient operation of the

treatment facility also depends on the operators knowledge of exactly what is entering the

plant. The types of treatment practiced on the waste water depend on the types of

contaminants in the water and as a result of this; the waste water must be tested

strenuously in a laboratory to establish what contaminants are present.

2.4.1- Preliminary treatment

Preliminary treatment is the first process of purification in which the influent will meetwhen it enters the treatment facility. The purpose of this process is to remove large objects

from the waste water. This is done in order to prevent the objects from clogging and causing

problems with pumps, pipes and other equipment later on in the treatment process. The

three main procedures of preliminary treatment are screening, grinding and grit removal.

Screening strains out large objects. These objects are either disposed of directly or sent to a

grinding station where the debris is ground and shredded and returned to the waste water

stream. Grit removal removes sand, gravel and other heavy debris by means of a grit


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chamber where the water flows slowly through the chamber and the grit settles to the

bottom. This is then sent to the landfill to be disposed of or it may however be treated

further before disposal.

2.4.2- Primary treatment

Primary treatment is the removal of the huge amounts of small solid particles that are still

suspended in the waste water such as silt, clay, algae, bacteria etc. The removal of these

small solids is carried out in large basins called primary clarifiers and are also known as

sedimentation basins or settling tanks. Inside the clarifiers, the waste water is stored for

many hours. This allows gravity to act on the particles by naturally separating the particles

that are heavier than water by sinking to the bottom. The build up of particles on the

bottom of the clarifier is called primary sludge. The particles that are lighter than water float

to the surface creating a scum. The primary sludge and the scum which is removed from the

surface of the water are then pumped to the solids-handling area of the plant. The material

that neither settles nor floats will overflow the clarifier and this is then transported to

secondary treatment. The overflow from the primary clarifier is called primary effluent.

Fig.2.4.1.1- Primary Clarifier

Source: http://www.ci.camarillo.ca.us

2.4.3- Secondary treatment

Secondary treatment is the part of the treatment process where the majority of the

unsettle-able material in the waste water is converted to a settle-able form. Activatedsludge, trickling filtration, activated bio-filtration and rotating biological contactors are all


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variations of secondary treatment. The common denominator between all these variations

is that they all utilise a mixture of micro-organisms which grow and form a stable biological

mass. The biological mass absorbs and breaks down the organic matter and captures the

suspended solids in the waste water. As the biological mass settles, it leaves a clarifiedeffluent. Another type of clarifier is used called a secondary clarifier to settle out the

biological or secondary sludge.

2.4.4- Tertiary treatment

The third stage of treatment is called tertiary treatment or advanced waste water

treatment. This type of treatment removes pollutants that were missed in the earlier stages

of treatment. Tertiary treatment focuses on removing nitrates, phosphates, sulphates, and

other inorganic compounds. It also employs the use of chemical and physical processes in

order to deal with highly complex organic compounds such as pesticides. Filters such as

sand filters may also be used in this stage of treatment to remove suspended solids.

Although it is sometimes called the third stage of treatment, it doesnt necessarily have to

follow secondary treatment. Tertiary treatment can be integrated into any part of the

system depending on the type of waste water treatment and the problem that is trying to

be solved. (Delvecchio et al., 1981)


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Chapter 3: Methods of energy recovery in

waste water treatment

3.1- Production of bio-gas

3.1.1- Anaerobic digestion of sludge to produce methane (bio-gas)

Apart from water, sludge is the most valuable substance left over as a result of waste water

treatment. As sludge does not leave the waste water treatment plant directly, it can used as

a source of energy. It must contain a certain percentage concentration of organic matter in

order to be useful for sludge fermentation. The function of the primary clarifier is to collect

primary sludge with a large quantity of organic matter for sludge treatment. Sludge can also

be sourced from the biological treatment, although often times this is less rich in fuel. The

elimination of gravel and sand signals the commencement of primary sludge treatment.This

process is undertaken by hydro-cyclones (which are devices that separate particles in a

liquid suspension) and a combination of sand and gravel drops into the sand-washer.The

blend is then taken from the waste water treatment plant following washing. Once the

sludge enters over the hydro-cyclones, it then flows to the thickening vessels, which are

vessels that are round in shape and incorporate scrapers where the sludge is allowed to

revolve for approximately a week. Following this, the water is then separated from the

residue and the thickened sludge is then transported via pumps to the sludge fermentation

vessels.

A product which is created by the biological treatment of waste water is excess sludge.

However, if the excess sludge and the floating substances contain too much water content

to access the sludge fermentation vessels directly, they can then be thickened using

thickening centrifuges. This helps to literally squeeze the water out of the sludge. A polymer

can be added to the inlet of the centrifuge in order to gather a more even mix. The excess

thickened sludge is then prepared for the process of sludge fermentation. (Lockhart, 2002)


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Fig.3.1.1.1- Anaerobic digestion process

Source: http://www.sswm.info

3.1.2- Sludge fermentation process

The main purpose of the sludge fermentation process is to consume a portion of the raw

matter, which in turn, will reduce the total volume of sludge that will have need of

treatment.After this, bio-gas production is the next aim of the treatment process.65-70 per

cent of bio-gas is made up of methane and approximately 30-35 per cent consists of carbon

dioxide. The rest of the gas is made up of less significant quantities of other gases such as

ammonia. Sludge fermentation incorporates both biological and anaerobic processes, in

that they work with particular bacteria in an environment that has a temperature of 35-40

degrees Celsius. The sludge is stored in very large vessels for approximately two and a half

to three weeks where the process runs its course. The storage vessels are each installed

with plant such as a heat exchanger, heating system and a recirculation pump for the

sludge. The vessels are also installed with thermal insulation. The bio-generators produce

hot water which is used in the heating system and does not require supplementary energy.


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Fig.3.1.2.1- Sludge fermentation vessel

Source: http://www.r-e-a.net

In order to operate the process as efficient as possible, the contents of the storage vessels

are continually rotated to guarantee a sufficient contact between the bacteria and the

sludge.Due to the diameter of the sludge fermentation vessel, a decent blend is onlycertain

with bio-gas circulation. This is because the bio-gas is drawn in over and above the sludge

level, flows through the compressors and is then transported into the vessels which

incorporate sprayers. An overflow pipe and an isolation valve are fitted to each

fermentation vessel. A large drop pipe is used to transport the sludge to the buffer vessel

which deals with fermented sludge and is then passed through to the dewatering

centrifuges. The goal of this process is to include a polymer which will aid in the drying out

of the sludge which can then be used for incineration following its removal from the waste

water treatment plant. Bio-gas is continuously produced in the process where the organic

matter is endlessly consumed. Within the sludge fermentation vessels, the bio-gas is

harnessed at a level that is higher than that of the sludge and is then moved through a

piping network where it is stored in gas membrane vessels.They act like buffer tanks in that

they keep the system somewhat pressurised. The internal membranes of the vessels are

enclosed by thick layer of strong steel that provides the protection that the vessels require.

The gas is transported using miniature compressors from the storage vessels to supply the

bio-gas generators where it is burnt to generate electricity which is used for operations


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within the waste water treatment plant. This type of energy recovery reduces the running

costs and allows the waste water treatment plant a degree of energy independence. Heat

that is created from the operation of the thermal motors can be reused and can assist in the

process of heating up the sludge fermentation vessels to the correct temperature in orderto produce a suitable environment for the growth of methanogenic bacteria . (Anthony,

2007) ((Crowe et al., 2009, Zitomer and Adhikari, 2005)

Using the equation below, the volume of methane gas can be calculated throughout the

digestion process and can be worked out using:

Where;

= volume of methane produced at standard conditions

= flow rate, m3 /day = ultimate biochemical oxygen demand (BODL) in influent, mg/L = ultimate biochemical oxygen demand (BODL) in effluent, mg/L = theoretical conversion factor for the amount of methane produced from the

complete conversion of 1kg BODL to methane and carbon dioxide, m3

CH4/kg BODL

oxidized

= net mass of cell tissue produced per day, kg/dayThe production of methane gas is typically estimated from the total percentage of volatile

solids reduction. Values of methane gas produced can range from 0.75 to 1.12 m 3/kg of

volatile solids destroyed. The ultimate capture of methane depends on the composition and

biodegradability of the organic feedstock, however, the creation rate will depend on the


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number of microorganisms present, their growth environment and their fermentation

temperature.

3.1.3- Exploiting the potential of waste water treatment sludge

The production of gas can vary over a wide scale depending on the content of the sludge

feed and the biological activity in the digester. A per capita basis can be used to roughly

approximate the amount of gas which is produced. Normally, the approximate amount

captured per person/day is 15 to 22 litres in primary plants which treat domestic waste

water. On the other hand, the approximate amount of gas produced per person/day in

secondary treatment plants is about 28 litres. (Eddy, 1991)

Table 3.1.3.1 1

Methane production rates

Types of waste water Rate of methane (m3CH4/kg COD removed)

Synthetic 0.29

Domestic 0.48

Leachate 0.227

Olive Mill 0.41

Poultry breeding 0.31

Cheese whey 0.34

(Berktay and Nas, 2008)

The methane gas must be cleaned once it has been collected from the reactor from other

bio-gas constituents e.g. carbon dioxide, hydrogen sulphide and excess moisture. The

removal of carbon dioxide is a costly operation and is only financially viable if the gas is to

be sold commercially. The use of a scrubber, which is the absorption through a chemical

solution, is the most common method of removing carbon dioxide. Carbon dioxide isremoved to in order to decrease the volume and increase the gas value. Following the


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scrubbing and cleaning of the gas, it can then be stored and accessed for later usage or it

can be consumed straight away. The methane can be burned by direct firing or within a gas

engine. It can be used as a fuel to power hot water boilers, water pump motors, blowers

and electrical generators. That being said, a very common applications for methane gas thatis produced is to fire incinerators or burned to heat up the influent sludge during pre-

treatment. The most beneficial part of the process is when the fuel is consumed by the plant

reducing energy costs. However, gas that is excess to operations in the waste water

treatment plant can be sold commercially.

Methane gas that is produced during the anaerobic digestion process in waste water

treatment plants is somewhat comparable to natural gas. In spite of this, the constituent

makeup of natural gas is different than methane as it contains a number of other

hydrocarbons such ethane, propane and butane. Therefore, the calorific value of natural gas

will always be higher than pure methane. The methane content of bio-gas is generally

expected to be in the range of between 55-80%. The energy content which is to be expected

of pure methane is 9.25-11KW/m3. The energy content for natural gas will be approximately

10% more due to the fact of added gases such as butane being present. At present methane

is regularly used in combustion engines to generate electricity. (Berktay and Nas, 2008)

It is well known that the recovery of methane gas from waste water treatment plants is not

a new technology; however, advancements have been made around the world to speed up

the process of methane production and also create more purified methane gas.

3.1.4- South Shore waste water treatment plant

In South Shore waste water treatment plant in the south of Milwaukee, USA, a research

study was carried out by Marquette University to investigate how to produce extra methane

from municipal anaerobic digesters. Feedstock from four different companies from around

the region were used by the research team at Marquette University including waste from

the Miller beer company, Lesaffre yeast fermentation, South-eastern Wisconsin Products

fermentation and Pandls food restaurant. Bio-solids were mixed with these potent waste

products at the South Shore waste water treatment plant. The plant treats approximately

340 million litres of waste water each day. The peak capacity that the plant can treat on any

given day is in the region 1.1 billion litres of municipal waste water. A three phase study was


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carried out where the wastes initially examined for biochemical methane potential and

were also analysed using anaerobic toxicity assays. Secondly, digesters of a laboratory scale

were activated in the laboratory and thirdly, a full scale demonstration was carried out in

that the wastes from the four companies mentioned previously were fed to the plantsanaerobic digesters. The values for average maximum BMP (ml CH4, per gram COD) are

shown below. COD (chemical oxygen demand) is how the organic matter content or

concentration is measured in a liquid.

Table 3.1.4.1 1

Average maximum BMP values (ml CH4/g of COD)

Miller Brewery 413

Lessafre Yeast 2274

South-eastern Wisconsin Products 943

Pandls Restaurant 488

(Zitomer and Adhikari, 2005)

As seen from the table above, Lessafre Yeast and South-eastern Wisconsin Products have

the highest figures by far. This signifies that they can initiate methane production from

background COD existing present in the biomass, which was digested sludge from the South

Shore treatment plant. The COD present in the waste from the Miller Brewery and Pandls

restaurant was in essence entirely convertible to methane gas. Throughout the research,

laboratory style anaerobic digesters were applied. The digesters had a capacity of 2 litres

and each one used a different mixture of one high strength waste and also municipal waste

water solids. After this, each digester was fully mixed and ran on a 2 week solids

preservation time at a temperature of 37 degrees Celsius, plus or minus one degree. The

findings from the testing were that Lassafre Yeast and South-eastern Wisconsin Products

waste water can be effectively co-digested for all mixture proportions examined (20-80%

waste water/municipal waste water sludge). Likewise, the waste from Pandls restaurant

can also be effectively co-digested for all mixture proportions analysed (3-11% food type

waste/municipal waste water sludge). Although the Miller Brewery waste water did not


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effectively digest by itself in the laboratory digester which was used in the test, it did

however eventually digest when it was mixed with municipal waste water solids from the

treatment plant. Another positive finding of the research was that it found that all of the

wastes used in the testing had metal concentrations which were within the regulations setout by the Wisconsin Department of Natural resources restrictions for the application of

bio-solids on land.

By the end of the research, it was concluded that a seventy per cent increase in methane

creation was recorded as a result of South-eastern Wisconsin Products waste water being

co-digested. It was also found that the increase in methane production was not as a result

of the extra COD; however it may have potentially been due to a synergistic effect caused by

the existence of bio-available nutrients such as iron, which is necessary for microbial

growth. Due to this, the extra production of methane gas can be used to produce

approximately 16,300kWhr/day of electrical power and this could save the treatment plant

$200,000 per year incorporating an active methane powered electrical generator at the

treatment plant. The research team from Marquette University have provided

recommendations that the treatment plant management should consider the co-digestion

of South-eastern Wisconsin Products waste water and municipal waste water solids. Theybelieve that this will result in a continual increase in methane gas production. They have

also provided recommendations to other anaerobic digestion plants to consider using

South-eastern Wisconsin products waste water and other related yeast or fermentation

wastes as an addition to boost methane production. (Zitomer and Adhikari, 2005)


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3.2- Use of hydro-turbines to generate electrical power

3.2.1- The fundamentals of hydro-turbines

Fig.3.2.1.1- Components of a Kaplan turbine and electrical generator

Hydro-turbines are a type of hydro-power which create electrical power from the flow and

head of water. The fundamentals of hydro-turbines are that they are positioned in

waterways that include an intake and a discharge. Hydro-turbines are usually made up of

the following components; a stator, rotor, shaft, wicket gate, and the turbine blades to

harness the kinetic energy that exists in the flowing water. In the renewable energy sector,

hydro-power outweighs any of the other types of renewable energy for electricity

generation. The following are some advantages of using hydro-turbines as a source of

energy:

They are environmentally friendly as they produce no emissions.

There is relatively no cost to operate, apart from maintenance and initial purchase.

Operation of hydro-turbines will not be effected by rising fuel costs.

They can be retro-fitted relatively easy to cater for rising electrical demand and

energy needs.


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The potential power generation from a stream of water can be calculated using:

Where:

P = power (w)

= turbine efficiency

= density of water (kg/m)

g = acceleration of gravity (9.81 m/s)

h = head (m). For still water, this is the difference in height between the inlet and outlet

surfaces. Moving water has an additional component added to account for the

kinetic energy of the flow. The total head equals thepressure headplus velocity

head.

= flow rate (m/s)

Hydro-turbines can be used in new and existing waste water treatment plants by using the

influent and effluent water at strategic locations to rotate turbines inside of a generator.

Artificial structures such as declines and steep drops are also being used to create the

necessary head and volume needed to rotate the turbines and create electricity. In 2010,

Sonal Patel wrote in an article on powermag.com that;

An Australian sewage plant this April began using treated wastewater falling down a 60-

meter (m) shaft to produce its own power. The unique 4.5-MW hydroelectric plant, installed

as part of a A$150 million (US$124 million) upgrade to North Head sewage treatment plant,

is one of three new units the New South Wales government is installing in Sydney Waters

water and sewerage networks.The new plant was built by Worley Parsons and Energetics

and is supported by the New South Wales governments climate change fund. Along with a

methane gas cogeneration unit that was also recently installed, the North Head plant now

generates nearly 40% of its own power.


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(Sonal Patel, 2010 source:http://www.powermag.com)

Fig.3.2.1.2- Inside a Pelton turbine

Fig.3.2.1.3- Kaplan turbine

Fig.3.2.1.4- Francis Turbine with spiral casing
http://www.powermag.com/http://www.powermag.com/http://www.powermag.com/


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Fig.3.2.1.5- Reverse Pump set being used for treated waste water

3.2.2- Applying hydro-turbines to waste water treatment plants

As mentioned earlier, the possibilities for generating electricity using hydro-turbines in

waste water treatment plants are using the influent water (before the waste water

treatment plant) and the effluent (after the waste water treatment plant). If the hydro-

turbine is located before the treatment plant, the wastewater infrastructure will lead to a

fore-bay incorporating a thin trash rack that uses a rack cleaner. The waste water is then

transported via a penstock to the waste water treatment plant. The waste water treatment

plant is at a lower elevation, where the water passes through the turbine prior to the

normal treatment processes that that the plant performs. The hydro-turbine must be

located as near as can be to the elevation of the water treatment basin in order to take full

advantage of the head that is available. The second option available is to locate the hydro-

turbine after the waste water treatment plant. In this situation, treated waste water leaves

the treatment plant and is guided downwards through a penstock to the hydro-turbine prior

to be released in to a waterway of some kind. In this case the waste water treatment plant

is usually located at a higher elevation than to that of the waterway in which it is discharging

effluent into. Hydro-turbines can also be located at the two locations of a waste water

treatment plant and there is an example of this in Jordan called As Samra project, where

electricity is produced using hydro-turbines on the inflow and outflow sides of the waste

water treatment plant.

3.2.3- European waste water treatment plants that have adopted hydro technology

Some waste water treatment plants have adapted multi-purpose schemes that have

incorporated hydro-turbines across Europe and a few are listed below;


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The Plobb Seefeld waste treatment plant, Seefeld Zirl, Austria commissioned in 2005

Nyon wastewater treatment plant, Nyon, Switzerland commissioned in 1993

The La Louvre plant,Lausanne, Switzerland commissioned in 2006

The primary task of the waste water treatment plant in Seefeld, Austria is to discharge

treated wastewater (effluent) to a river so that dilution criteria can be carried out, while the

other is to produce electricity from this treated wastewater.In order for effluent to reach its

destination of the Inn River, it has to be pumped to travel over an obstructing hill and then

passes through the turbines in the hydro-power plant. Following passing through the

turbines, the water is then dispatched through a de-foaming machine before being

discharged into the river. A permanently accessible by-pass is installed to allow these types

of discharges. The hydro-turbine combined with its by-pass is incorporated in a key process

control system for automatic functioning. The investment of 2.2million and the feasibility

of the project are warranted for a number of reasons. The first being that hill that lies

between the waste water treatment plant and the Inn river is quite small and is in favour of

the project feasibility as regards how much pumping will have to be carried out by the

treatment plant to get the water to the turbine station. The head for the pumps is 94m and

the head for the turbine is 625m. This means that the treatment plant can produce electrical

power that surpasses the 1,500,000kWh/yr of energy consumed by the pump and also the

500,000kWh/yr of energy consumed by the waste water treatment plant itself. Excess

energy that is surplus to demands by the waste water treatment plant can be sold to the

grid network of electrical distribution. The plant has an electrical output of 1,192kW and

produces 5,500,000kWh/yr or the electricity consumption of approximately 1,220 European

households.


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Fig.3.2.3.1- Treated waste water turbine

In the waste water treatment plant in Nyon, Switzerland, its secondary function after

treating waste water is to produce electricity by means of hydro-turbines. About two

decades ago, a waste water treatment plant was built in Nyon City about 110m higher than

its discharge point, Lake Geneva, due to the lack of construction space that was available

close to the lake. Ever since, untreated waste water is gathered in a basin not far from the

lake and it is then transported to the waste water treatment plant via a pumping station.

After that, the water is then treated and subsequently passed through a hydro-turbine prior

to being emitted into the lake. The gross head for hydro-turbine is 94m.The electrical

generation of the waste water treatments hydro-turbine represents half of the pumping

stations energy and one third of the energy consumption of the waste water treatment

plant. The type of turbine use at the treatment plant is a reverse pump type which was

designed specifically for its application. The operators of this waste water treatment plant

conducted an investigation into the replacement of the existing turbine with a Pelton

turbine so that they could gain flexibility, increase overall production of electricity and

reduce the noise pollution from the plant. The treatment plant has an electrical output of

220kW and produces 700,000 kWh/yr which is the same as the electricity consumption of

approximately 160 European households.


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Fig.3.2.3.2- Pumping Station, Nyon, Switzerland

Fig.3.2.3.3- Reverse pump (generates electricity)

The function of the plant at La Louvre in Lausanne, Switzerland is to split the discharges of

nearby river from the main sewer in the area and also to recover electrical energy in the

process. Originally the waste water from the area of Lausanne was released into a stream

that then made its way into Lake Geneva. The stream became buried in a tunnel at the end

of the nineteenth century and was no longer active. In 1964 a new waste water treatment

plant was built not far from the lake. Due to the growth of the population in the area over

the years in Lausanne, it raised concerns over the capacity of the plant especially when

storms hit the area seeing as the water from the stream had no reason to be treated. In light

if this, it was decided to separate the stream water from the waste water to reduce the load

on the waste water treatment plant. In order to reduce the effects of this problem, an

intake was constructed and it collected the discharges of the river in La Louvre by means of


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a penstock pipe which was put in place in the tunnel. The penstock pipe came to an end in a

power station near the lake in which the hydro-turbine, generator and control panel were

located. The treatment plant has a gross head of 180m. It has an electrical output of 170kW

at maximum conditions and generates 460,000kWh/yr or the equivalent amount ofelectricity it takes to power of 100 European households. (San Bruno et al., 2010, Griffin,

2000, Saket, 2008)

Fig.3.2.3.4- Pelton turbine, La Louvre, Switzerland

3.3- Electricity generation using microbial fuel cells

3.3.1- How microbial fuel cells work

Microbial fuel cell technology is not a relatively new type of technology but only until

recently has it been advanced into a new approach for generating electricity in waste water.

The first type of advancement of electrical current produced by bacteria is by and large

credited to M.C Potter in 1911. Following this, hardly any advancement had been made in

this area of electrical production really until the 90s, where fuel cells became of more

interest to researchers and experimentation with MFCs began to become more popular.

The technology for MFCs works by where microorganisms breakdown organic matter and

by doing so, create electrons that pass through a chain of respiratory enzymes in the cell

and produce energy for the cell in the form of ATP (adenosine tri-phosphate). A (TEA)

terminal electron acceptor will accept the electrons that have been released and then

become reduced. A number ofTEASs such as oxygen, nitrate and sulphate willingly diffuse

into the cell where they receive electrons creating products that can diffuse out of the cell .


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However, it is understood that a number of bacteria can transmit electrons exo-genously to

a TEA, for example, a metal oxide such as iron oxide. Bacteria such these can exo-genously

transmit electrons that can be used to produce electrical power by means of a MFC. This

type of electron generating process is known as electro genesis, with the bacteria being exo-electrogens and the reactor type being an MFC. (Logan, 2007)

Inside the anode camber of the MFC, oxygen will prevent the generation of electricity. In

order to combat this, the system must be designed so that the bacteria will be separated

from any oxygen. Separation of this kind can be achieved by inserting a membrane that will

stimulate the transfer of electrical charge between the two electrodes and in doing so,

creating two separate chambers. The first chamber is the anode chamber, where the

bacteria are grown and the second chamber is the cathode chamber where the electrons

react with the catholyte. Air is provided to the cathode so that oxygen can dissolve for the

reaction to start. A wire acting as a load connects the two electrodes together, however, in

a laboratory environment; a resistor is used to replicate the load.

Fig. 3.3.1.1- Microbial Fuel Cell Schematic

Source: http://www.sciencebuddies.org

It is important that the (PEM) membrane is permeable in order to allow the protons that are

produced in the anode chamber to travel to the cathode where they can then combine with

electrons that are transferred by means of the wire and oxygen and in doing so, producing


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water. The electrical current that is produced by an MFC is usually determined in a

laboratory situation by observing the drop in voltage across the resistor using either a

voltmeter or a multi-meter which can be connected to a computer for accurate continuous

data readings. (Logan, 2007)

3.3.2- Microbial fuel cells for waste water treatment

The idea of marrying the technology of microbial fuel cells and its potential to create

electricity from waste water treatment can be largely credited to Bruce Logan, a professor

of environmental engineering at Pennsylvania State University and a team of researchers

from the USA. Logan and his team were funded $87,000 through a project grant from the

National Science foundation Small grants for Explanatory Research program. As microbial

fuel cells use the anaerobic oxidation of organic matter in a material to produce electricity,

waste water was the idea behind the whole project.

The microbial fuel cell in which Logan and his team developed was a single chambered

Plexiglas device which measured approximately 6 inches long by 2.5 inches in diameter. The

interior of the fuel cell consisted of eight graphite anodes which surrounded the cathode

which consisted of carbon/platinum catalyst and a proton exchange membrane layer which

fused to a support tube made of plastic. The graphite rods were constructed so that they

(Ehrenman, 2004) Fig. 3.3.2.1-Single chamber microbial fuel cell


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could be roughened and the reason for this is to make it simpler for the bacteria to make

contact with them. A copper wire is used to make the connection in the circuit and this

provides the pathway for the electrodes to travel. Waste water is pumped into the chamber

that feeds the bacteria and the waste waters organic matter is broken down releasingelectrons into the electrical circuit hydrogen ions that are positively charged into the waste

water mixture. The positively charged hydrogen ions manage to reduce the mixtures oxygen

demand. These hydrogen ions also travel though the proton-exchange membrane layer to

arrive at the cathode, which is made exposed to air. It is at the position of the cathode that

oxygen which is present in the air, hydrogen ions which pass through the membrane layer

and electrons travelling down the circuit meet to produce clean water.

The reason for Logan and his team not adopting the two chamber design is that you need to

push air into the water to provide oxygen in the dissolved form to the cathode. The single

chamber microbial fuel cell used by Logan uses the system of passive direct airflow rather

than forced airflow which is used in the two chamber design, which reduces the expensive

aeration cost. This is a method still used in traditional waste water treatment plants. The

waste water used in his experiment was effluent from the primary clarifier from

Pennsylvania States waste water treatment plant. The pH of the waste water used rangedfrom 7.3-7.6 and had a COD of 210-220mg/l. The variation in the waste water flow did not

influence the performance of the MFC too much, however, the higher the organic matter

that existed in the water, the more electrical power it could potentially produce. It was also

found that waste water containing content from food processing plants etc. would provide

the perfect type of fuel for the MFC. The MFC can generate approximately 26 milli-watts of

electrical power per sq. metre of electrode surface. For example, a Christmas decoration

style light would require roughly 38 sq. metres of surface area in order to illuminate it. In

light of this, the process that takes place can still remove up to 78% of the organic matter

from the waste water, as measured by BOD demand, and approximately 50 -70% of the COD

demand. Logan says the waste water produced by 100,000 people has the potential to

generate 2.3MW of electricity, if you could recover all of the energy, it would be enough to

power 1,500 homes (Ehrenman, 2004)

The proof of this statement can be backed up by an example calculation that Bruce Logan

does in his book to find what the potential energy benefit of maximum energy recovery


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using domestic waste water to a town of 100,000 people. The maximum energy production

is calculated assuming 500L/d per capita, 300mg/L COD and 14.7 kJ/g COD. The potential

electricity produced is also calculated assuming electricity is worth $0.44/kWhr. The amount

of homes that can be powered from is then calculated assuming each home uses 1.5kW.

**It must also be noted that these calculations are assuming 100% energy recovery, which is

not a realistic figure for energy recovery. It is envisaged to recover 25-50% of the energy

that is available which means that the above calculation should be reduced by half to obtain

more practical results for this calculation. (Logan, 2007)

3.3.3- Increasing electrical power generation using MFCs

The initial design of the MFC has proved that the potential is there to generate power by

means of electricity and clean water at the same time using waste water as the medium for

both. The focus now is to work on ways to further advance the power production of the

MFC, reduce the cost of production and makes the transition of a laboratory type device to

a mass produced product that can be used for energy recovery in the waste water

treatment industry. Bruce Logan and his team at Penn State are looking into trying toreduce the cost of the materials, especially the PEM. Other areas that are matters of

discussion are the catalyst, configuration and the design of the device. Although the power

output of the initial device that Logan and his team designed and operated was quite low, a

new model that they have been working on has capacity to power a small electric fan.

Although this is still way off the mark as regards applying this technology to waste water

treatment plant, the teams goal is to produce a stable level of 500kW of electric power. If

this were to be achieved using MFC technology, it would have the potential to power


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approximately 300 homes. Figures produced by the NSF state that approximately 33 billion

gallons of domestic waste water are treated by waste water treatment plants in the US

every year. This results in a cost of $25 billion. The majority of this cost goes towards the

payment for the energy required to operate the processing systems. In spite of this largecost, if the MFC technology can be further advanced and made more efficient, it has the

potential to significantly reduce the cost of energy in the waste water treatment industry.

(Ehrenman, 2004)

3.4- Power generation from plasma processing of sludge

Fig. 3.4- Plasma power plant system using waste water sludge

3.4.1- How plasma processing works

Plasma processing is carried out by the use of plasma torches in order to offer efficient

measures of melting solids or waste materials into a lava or magma type form after a short

time period between the plasma, which can reach up to and over 5000K, and the solids. The

plasma interacts with the waste, and the plasma density decreases with distance. Plasma

processing can treat a number of different kinds of waste including municipal waste water

dry solids. High temperature plasmas can be used for treatment of solids, liquids and gasses.


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They can be used for the melting of waste and the formation of non-leachable products.

Also, these plasmas can provide thermal decomposition of toxic molecules into simpler

molecules that are less violent. (Leal-Quiros and Villafafie, 2007)

3.4.2- The process of plasma gasification and the production of syn-gas

Plasma processing at very high temperatures of more than 5000K has the potential to

convert inorganic materials into syn-gas, which is made up of CO and H2. The creation of CO2

is impossible. The reason for this is because; this molecule separates itself at less than

1500K. This process is hugely important and is one of the main advantages of plasma

processing. The fact that it is physically impossible to produce C02, which is a well known

unwanted green house gas, means that it is ozone friendly. The gasification process allows

waste water sludge to be transformed into syn-gas with a left over product of vitreous slag.

The syn-gas which is produced consists of contaminants such as particulate matter, HCI,

dioxins, furans, sulphur oxides and others. Syn-gas can act as raw materials for the making

of methanol, ethanol or other industrial chemicals. The plasma gasification process needs a

source of heat in order for the reaction to take place. One way in which this heat energy can

be provided is plasma torches. Plasma exists as an ionised gas which is capable of

conducting electric current. Plasma is produced when a gas is exposed to a high energy field

that occurs between the two electrodes. Plasma which is produced is slowly moved into the

reactor where it will heat up the wastewater sludge and drive the gasification reaction. The

syn-gas produced is recovered through a vent at the top of the reactor and the vitreous slag,

which is in liquid form, exits at the bottom of the reactor at approximately 1900K. After the

syn-gas has been produced from the waste water sludge, it can be pumped to number of

different units of operation depending upon the process that is taking place. The syn-gas can

be piped to a boiler in order to generate steam and electricity for the plant.(Dollard, 2010)

3.4.3- Plasma processing of sewage sludge

The energy content in which the sewage sludge contains is also known as the heating value.

The value for the energy content depends on a number of factors:

The waste water source, whether it is industrial, domestic or of some other source.


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Types of waste water treatment processes in which the waste water has received

e.g. primary, secondary, tertiary. It is also worth noting that the waste water may

not have received any treatment.

Table 3.4.3.1- LHV values of various combustibles

Combustible LHV in KJ/kg

Light Fuel 42000

Granulated tire rubber (crumb rubber) 37000

Coal 29000 - 32000

Municipal WWTP sludge 20% DS 920

Municipal WWTP sludge 33% DS 3200

Municipal WWTP sludge 60% DS 8000 - 12000

Municipal WWTP sludge 90% DS 13000 - 18000

Table 3.4.3 above shows the typical low heating values of a number of different

combustibles including municipal waste water treatment sludge at various different dry

solid percentages. As the dry solid content of the waste water treatment sludge increases,

the LHV increases also.

Puerto Rico is one of the main producers of sludge and they are concentrated primarily on

secondary treatment plants. The secondary treatment process removes approximately 80-

90% of the total suspended solids which can accumulate approximate total solids sewage of

45,395 tons/ year. From Table 3.4.3, it can be determined that the total potential energy of

the dry sewage sludge that is generated per year equals 9.48 x 1014 J/year, which has a total

power output of 30MW. The approximate energy potential which can be obtained is shown

below for their forecasted dry sewage sludge that results from converting all primary waste

water treatment plants to secondary treatment plants:


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The figure of 30MW is sourced from the US environmental protection agency data. From

their data, they estimate that approximately 19kg of organic dry solids of waste water

sludge are produced by a single person per year. Therefore, if 1kg of organic dry substance

has for instance a calorific value of approximately 25000kJ 7kWh, the potential energyvalue can be worked out to be in the region of 19kg x 7kWh = 133kWh per person/year or

47500kJ. Of a population of roughly 4 million people, around half of the population of

Puerto Rico have working sewage systems. From this information, it can be estimated that

the potential energy generation per year is equal to:

It can be seen from these calculations that the use of plasma processing of primary sludgefrom waste water treatment plants, it is possible to generate electrical power in the region

of 30MW. This figure relates to the energy consumed, on average, for approximately 22500

homes in the United States.(Leal-Quiros and Villafafie, 2007)

3.4.4- Possible advantages and disadvantages

The advantages of plasma gasification of municipal waste water solids are valuable products

(energy, industrial chemicals and construction material), a solution for waste water

treatment sludge and potential environmental impact and risk reduction when compared to

other management methods. Although the use of plasma torches is nothing new, the

process however is and performance data is limited. However, two plasma gasification

plants are operating in Japan, Utashinai and Mihama-Mikata. Engineers have the luxury of

using these two plants as reference plants for operational, economic and environmental

data. Although this is not an ideal situation as data is only used from two sources, it is


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however better than estimates calculated by other engineers based purely on operational

assumptions.

Fig.3.4.4.1- Plasma gasification

Source: http: //www.netl.doe.gov

The plant in Utashinai commenced operations in 2003 and was originally designed to treat180 metric tonnes/day of a 50/50 blend of municipal waste water sludge and auto shredder

residue. There are two process lines in total, one of which to two updraft reactors to

produce syn-gas and electricity for the operation of the plant and sale to the grid. The dry

solids and the auto shredder residue need to be shredded before being fed to hopper

blending operations and further feed to gasification reactors. Each of the reactors is coated

with refractory and heated by means of four plasma torches rated at 80 300kW each.

From feedback from the plant managers, the performance of the reactors and plasma

torches has been of a high standard. The way in which the plant was designed means that

each reactor operates with three torches in order for maintenance to be carried out on the

fourth torch. The time period between torch maintenances is 500 hours of operation.

Challenges that face the plant are the problems that shredding before gasification present.

Due to the changing makeup of municipal sludge composition, jamming can occur in the

shredder if any small metal objects come through. This can result in the halting of

processing and also very costly repairs to the shredder. Also, as the auto-shredder residue is


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gasified, it produces a build-up of sticky slag. The slag then forms stalactites as it builds up.

When they finally decide to break off, they fall and can damage the refractory at the bottom

of the combustion chamber.

The Mihama-Mikata plant also commenced operation in 2003 but was designed to only

process 22 metric tonnes per day of solids waste. From this figure, 4.8 tonnes per day is

sewage sludge. There is no electricity produced at the plant. The plants operating cycle is 2

and a half months of operations followed by a two week shutdown in order to carry out

necessary maintenance, which means the plant operates 10 months out of 12. The plant

shares a lot of the same operational benefits as the one in Utashinai but it has not had the

same amount of problems especially relating to the challenges with auto-shredder residue

in the feed to the reactors. The reason for this is because the slag in Mihama-Mikata has a

different composition. The plant has the added benefit of producing and selling the vitreous

slag as a construction aggregate. Utashinai was unable to produce aggregate due to the

processing of auto-shredder residue with the municipal solids. The plant at Mihama-Mikata

produces 1.5 tonnes of slag per day. Another advantage is the ability to produce industrial

chemicals from the syn-gas which is produced. A major drawback of this type of energy

recovery is the high initial capital cost for plant construction. For example, one of the largestprojects which are still under consideration is a 3000 ton per day processing facility in St.

Lucie, Fla., with an estimated cost of $425 million. (Dollard, 2010)


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Chapter 4: Case Studies

4.1- Esholt Waste Water Treatment Plant, Yorkshire, UK

4.1.1- Introduction

The Esholt waste water treatment plant which is located in Yorkshire treats wastewater

from the regions of Bradford and the north of Leeds, which have a population of

approximately 730,000 people. The incoming flow of influent into the plant is in the region

of 13.5m3/s and peak flow to treatment of 3.2m

3/s. Esholt waste water treatment plant has

the capacity to treat roughly 12% of Yorkshires waste water capacity. The site on which the

plant is built dates back to the early 1900s. In light of this, there was a need to restore a

number of the old structures and equipment with new state of the art processing units. The

new waste water treatment plant project can produce up to 45% of the power demand

from the on-site sources of the new combined heat and power (CHP) engines and hydro-

turbines. The new project at the plant was completed in 2009 and also within the

authorised budget. The plant is now delivering effluent qualities which are significantly

better than in the past and also the plants operation costs are below forecasted levels.

4.1.2- Installation of new hydro-turbines

The new hydro-turbines commenced operation in 2009. There are two Archimedean type

screw turbines installed in series at the plant, each with a capacity of 90kW.Screw turbines

can be used in any application where water flows from a higher level to a lower one, such as

rivers, industrial effluent plants and within or at the outlet from water treatment plants.

Costs from the use of this type of technology can be kept at a minimum as there is no needfor fine screens and the equipment used is suitable for many low head applications.

Following the initial period of commissioning, the turbines are now running 24 hours a day

and providing approximately 180kW of energy, hence reducing the plants operating costs.


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Table 4.1.2.1

Esholt WWTP Technical Data

Flow up to 3200l/s

Head 8.2m

Installed Power 2 x 90kW

Diameter 1.8m

Screw Length 14m each

Total Weight 64tn

Medium Screened Waste Water

Output Max 180kW

Source: //www.spaansbabcock.com

The electricity produced from the hydro-turbines is used to offset the imported electrical

power demand of the waste water treatment plant.This leads to a saving of nearly 410 per

day and 150,000 per year in electricity costs. The hydro-turbines are located between the

inlet works grit collectors and the primary settlement tanks, where a flow of roughly 3200l/s

is pushed through a 1.8m diameter pipe to the screw turbines. Esholt waste water

treatment plant is the first in the UK to use untreated sewage for hydro power generation.

Each hydro-turbine is controlled using a regenerative drive unit to control speed and output

to the power grid. The hydro-turbines installed at the plant were provided by a company

called Spaans-Babcock and have provided Yorkshire water with various equipment including

screw pumps for over forty years. The installation of theses hydro-turbines allows Esholt

waste water treatment plant and Yorkshire water towards its objective reducing its Carbon

Footprint through the use and installation of renewable energy sources.The design of the

turbines are also fish friendly and help in meeting regulations set out by the environmental

agency.


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Fig.4.1.2.1- Screw turbine at Esholt waste water treatment plant

Source: http: //www.spaansbabcock.com

Fig.4.1.2.2- Section of Screw turbines in seriesSource: http://www.spaansbabcock.com


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4.2- Ringsend Waste Water Treatment Plant, Ringsend, Dublin

4.2.1- Introduction

Ringsend waste water treatment plant was constructed in order to provide the processes ofpreliminary, primary, secondary and tertiary treatment to a population of approximately 1.7

million people. The construction of the plant began in 1999 while operation processes

commenced in 2003. The waste water treatment facilities in which were originally on the

site remained operational during the course of the new treatment plant. The site on which

the new treatment plant is builtwas originally an old landfill and required large scale piling

in order to support the weight of the new structures of the new plant. Ringsend is also the

proud owner of the largest double-decker sequencing batch reactor in the world.

Fig.4.2.1- Ringsend waste water treatment plant

Source:http: //conorcreighton.wordpress.com

4.2.2- Methane recovery from Ringsend waste water treatment plant

The treatment plant produces between 80-100 dry matter tonnes of dry sludge each day a

by-product of the waste water treatment process. The sludge is a mixture of primary sludge

and secondary sludge, which is sourced from the settlement tanks. This is then process in

the processed in the sludge treatment section of the plant. A typical mass balance would be

as follows, 80 tonnes of sludge sent to the 4 anaerobic digesters. This is fed at a

concentration of 10% Dry Solids. The plant can run high a solid as the sludge is hydrolysed in

a THP plant firstly, decreasing its viscosity. The solid fraction of sludge is approximately 85%

Volatile Solids (which can be digested and converted to Biogas). Approximately 40 tonnes of


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the original 100 tonnes of sludge are digested in the anaerobic digesters and converted to

methane rich biogas. The biogas is formed by the anaerobic digestion process. The digesters

are mesosphilic (operated at 40deg C) and completely air-tight. Following the digestion

process, the remaining sludge is dried out and spread on the land. The biogas isapproximately 60% methane. The waste water treatment plant produces 35,000 to 40,000

m3

of biogas each day. The plant utilise 99% of all biogas produced. The remaining 1% is

Fig.4.2.1.1- Settling tank

Source: http: //www.caw.ie

flared to the atmosphere. The biogas is used to power 4 CHP steam engines. The plant

recovers extra energy on their engines by use of steam generators on the exhausts. Thissteam is then used in a hydrolysis process for sludge treatment. These can produce up to

75% of the treatment plants daily energy needs. The engines are rated 995 at kW/hr. The

treatment plant produces upwards of 92 MW/hrs each day from these engines. This type of

power generation greatly offsets the plants need to import electricity from the national grid.

KEY DATA:

Population Equivalent: 1.7 million

Dry Weather Flow: 4.65 m3/s

Flow to Full Treatment: 11.1 m3/s

Maximum flow to storm: 11.5 m3/s

Maximum flow to works: 22.6 m3/s

BOD: 98,400 kg/d


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TSS: 101,000 kg/d

Fig.4.2.2.2- Plant data

Source: http: //www.caw.ie

4.2.3- Potential power generation from hydro-turbines

Fig.4.2.3.1

As part of the research regarding Ringsend waste water treatment plant, an interesting

study carried out was to find out the power output it could potentially generate by use of

hydro-turbines. The reason for this is that it is not an energy recovery method which is usedat the plant at present but could perhaps be one for the future. The effluent flow rate from

the waste water treatment plant on average is 4m3/s and can rise to levels of 10m

3/s at

periods of high demand. The formula which has been used to do the calculations is shown

earlier in section 3.2.1 to calculate the potential power output of a turbine for Ringsend. The

turbine chosen for the sake of the calculations has an efficiency of 95%. This formula has

also been used to calculate various power outputs in kW with the head (m) and the flow

rate (m3/s) varying. The graph shown above Fig- 4.2.3.1 shows the results of these

0

100

200

300

400

500

600

0 1 2 3 4 5 6

Power output

potential (kW)

Water head (m)

Ringsend WWTP hydro-turbine power output potential in

relation to head and effluent flow rate

flow rate: 4m3/s

flow rate: 5m3/s

flow rate: 6m3/s

flow rate: 7m3/s

flow rate: 8m3/s

flow rate: 9m3/s

flow rate: 10m3/s


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calculations which were carried out in Microsoft excel and then plotted on a scatter graph. It

can be seen from the graph that at a flow rate of 4m 3/s (which is the average flow rate per

day of the plant) and head of 1m that the turbine has the potential to generate nearly 50kW

of electrical energy. As the flow rate and head height increase, so too does the poweroutput. It is clear to see from this graph that there is huge potential for energy generation

from this type of technology in Ringsend waste water treatment plant. Along with the

production of bio-gas, Ringsend waste water treatment plant would have the potential to

generate enough power to substantially lower its reliability on the national grid and even

perhaps generate funds in the sale of electricity to the grid. It is also a possibility to

construct some sort of an artificial head height where the plant could feed effluent water

into to increase the power output of the turbine. An initiative such as this would have large

initial costs but the energy produced by the turbine would offset the payback period of the

project in a small amount of time.

The potential power generation from a stream of water can be calculated using: (formula used for

calculations)

Where:

P = power (w)

= turbine efficiency

= density of water (kg/m)

g = acceleration of gravity (9.81 m/s)

h = head (m). For still water, this is the difference in height between the inlet and outlet

surfaces. Moving water has an additional component added to account for the

kinetic energy of the flow. The total head equals thepressure headplus velocity

head.

= flow rate (m/s)


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Table- 4.2.3: Data table of varying flow rates and head heights (matches graph shown above)

Flow rate: 4m3/s Flow rate: 5m

3/s

Water Head(m)

Generated power Output

(kW) Water Head(m)


(kW)

1 44.73 1 55.92

1.5 67.1 1.5 83.88

2 89.47 2 111.84

2.5 111.84 2.5 139.8

3 134.21 3 167.76

3.5 156.58 3.5 195.72

4 178.95 4 223.68

4.5 201.32 4.5 251.64

5 223.69 5 279.6


3/s

Water Head(m)


(kW) Water Head(m)


(kW)

1 67.1 1 78.28

1.5 100.65 1.5 117.43

2 134.2 2 156.58

2.5 167.75 2.5 195.73

3 201.3 3 234.88

3.5 234.85 3.5 274.03

4 268.4 4 313.18

4.5 301.95 4.5 352.33

5 335.5 5 391.48


3/s

Water Head(m)


(kW) Water Head(m)


(kW)

1 89.47 1 100.65

1.5 134.2 1.5 150.98


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2 178.93 2 201.31

2.5 223.66 2.5 251.64

3 268.39 3 301.97

3.5 313.12 3.5 352.3

4 357.85 4 402.63

4.5 402.58 4.5 452.96

5 447.31 5 503.29

Flow rate: 10m3/s

Water Head(m)


(kW)

1 111.83

1.5 167.75

2 223.67

2.5 279.59

3 335.51

3.5 391.43

4 447.35

4.5 503.27

5 559.19


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Chapter 5: Conclusions

5.1 Introduction

Over the last decade or so, the engineering world has taken a dramatic change in the way in

which processes such as waste water treatment are carried out and managed. New energy

buzz words such as efficiency, green, sustainable and recovery are all taking centre stage in

the engineering landscape. The reason for this is that the Earths fossil fuel reserve is

depleting, this in turn drives up the price of fuel costs and therefore makes it more

expensive on the process operator. In light of this, it is in the interest of the operators

(governments) of waste water treatment plants to use methods of energy recovery in their

facilities in order to reduce fuel costs and become less reliable on the grid. In the past,

waste from treatment plants was not utilised efficiently. Today however, the technology is

there to deal with the various types of waste from these plants and convert it to energy that

can be used to generate electricity within the plant and the sale of any further excess.

As part of this research, four main methods of energy recovery in waste water treatment

systems were examined; the harnessing of methane gas, the use of microbial fuel cells,

hydro-turbines and plasma processing.

5.2- Overview

It is relatively clear the most widely used method of energy recovery in this field is by

means of harnessing methane gas from the waste water sludge. This gas is then stored and

burned in a generator in order to produce electricity to meet part or all of the plants

electricity load. Methane gas is harnessed at Ringsend waste water treatment plant and

used in conjunction with heat recovery to provide up to 75% of the treatment plants daily

energy needs. This type of energy recovery is used in a lot of different waste water

treatment plants in developed countries around the world.

Hydro-turbines as a method for energy recovery in waste water treatment is still at a

relatively early phase of development, with only a handful of plants around the world

adopting this type of method. The Esholt waste water treatment plant in Yorkshire, UK is a

prime example of the savings that can be made from this type of technology. The plantmakes a saving of nearly 410 per day and 150,000 per year in electricity costs due to the


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installation of its two turbines in 2009. As this technology is being developed in the UK,

Australia and the US, there is no reason that this style of energy recovery could not be

incorporated into Irish waste water treatment plants.

Microbial fuel cells are a type of energy recovery method that is still really at the stage of

laboratory testing. However, they have huge potential and could be a major future source of

energy recovery in the next few years. Bruce Logan of Penn State University in the USA has

carried out various types of research on these types of cells and has at present been able to

successfully generate approximately 26 milli-watts of electrical power per sq. metre of

electrode surface. For example, a Christmas decoration style light would require roughly 38

sq. metres of surface area in order to illuminate it. This may seem quite minuscule but the

process that takes place can still remove up to 7

104471949 Energy Recovery Methods in Waste Water Treatment Systems

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