Mikkeli Wastewater Treatment Plant (WWTP) Case Study

Introduction to the problem

Wastewater treatment plant in Mikkeli (Eastern Finland) is located near

historical center and harbor area. Nowadays the city of Mikkeli faces an acute

question whether to replace WWTP with a new plant and new technologies or

not.

Currently activated sludge (AS) process is used in WWTP. Moving bed biofilm

reactor (MMBR) and membrane reactor (MBR) technologies are possible

option for replacement of AS process and are under consideration. The new

plant is planned to be situated inside a rock and process that will match the task

should be selected.

Main tasks of students are:

1. To choose the strategy of Mikkeli WWTP development. Will it be

oriented on AS or MMBR or MBR processes. Please note that

abovementioned technologies should not be considered as the only

possibility, other options can be selected as well. Please do not forget to

explain why you have selected this or that development strategy.

2. Once the strategy is chosen, please prepare detailed description of

suggested technological process (please remember to provide

characteristics of all equipment, estimate removal efficiency of

contaminants from water). Why chosen process is beneficial?

3. Estimate costs. It is possible to prepare brief business plan in order to

demonstrate if benefits from chosen process will cover costs.

Background information

The necessity of wastewater treatment became obvious when industrial

revolution hit Europe at the end of the 19th century and first wastewater

treatment plants WWTPs appeared in England and Germany. Both biofilms on

slate and activated sludge process were invented in between 1900-1910. Over

the years, activated sludge was adopted as the most versatile technology.

Please find more information about WWTP in Finalnd and which process is

mainly used.

With years more stringent requirements were adopted. At the moment, both

total nitrogen and total phosphorus are regulated.

Find information about water directives e.g. Wastewater directive 98/15/EC,

EU Council directive 91/676/EEC, Watershed directive 2000/60/EC.

What directives regulate wastewater discharges in Finland? Does it have more

stringent legislation?

The Mikkeli WWTP was built in the 1960s having activated sludge process as a

method of treatment. The reactor is organized as a recycling oxidation ditch,

which has quite efficient for both ammonia and carbon removal. See the water

treatment diagram below.

Activated sludge process

Basically activated sludge process contains three main components:

1. aerated reactor with microorganisms in form of suspension

2. sedimentation tank for solid-liquid separation

3. recycle system.

What is important in activated sludge system is formation of flocculent

settleable solids, which can be removed in sedimentation tank. Often AS

process is used together with physical and chemical treatment methods, which

are applied as preliminary step and disinfection, filtration as a post treatment.

In order to design AS process following parameters should be considered:

1. volume the aeration basin

2. the amount of sludge production

3. required amount of oxygen

4. wastewater characterization.

Benefits of AS process are:

• Relatively low installation cost

• Good quality of effluent

• Land requirements are not high

Disadvantages of AS process are:

1. Non flexibility of the method (In case of unexpected increase in the

volume of sewage, effluent of not high quality is obtained).

2. High operation cost

3. Sludge disposal

4. Sensitive to certain industrial wastes

More information concerning AS process can be found in “Wastewater

Engineering: Treatment and Reuse” George Tchobanoglous, Franklin Louis

Burton, H. David Stensel.

MBBR (moving bed biofilm reactor)

This is one of the methods which use surface biofilm for wastewater treatment.

It uses plastics pieces with high surface areas to grow biofilms. The concept of

using biofilms – so microbes that grow on a solid surface – originate back to the

begging of 20th century when slate slabs were used for biofilm growth. The

technology was displaced by activated sludge as a more convenient technology

but with the cheap production of plastic, it now become recognized as a helpful

method for some circumstances.

MBBR in its modern concept first originated from Norway in 1980’s, when

more stringent regulations for nitrogen loads became in place (Weiss et al.,

2005). In France this technology used only from 2006, 18 installations by 2011

by Vinci, 8 by Veolia. There are totally around 400 worldwide in 22 countries

(Rusten et al., 2006). With MBBR technology, there is better ammonium

removal than for ordinary AS (Di Trapani et al., 2010) and less solids

production (Weiss et al., 2005).

Although some concerns over nitrate removal (DI Trapani et al., 2010) MBBRs

are convenient way to remove nitrogen if incorporated into systems with

denitrification basins.

Other parameters of these systems are:

• No sludge recycling (Weiss et al., 2005).

• Better ammonium removal (Di Trapani et al., 2010)

• Compact (50% of space compared to activated sludge)

• Little temperature dependency

Note: IFAS (integrated fixed film activated sludge) is another name for moving

biofilm reactor when there is return activated sludge. Example is HYBAS by

AnoxKaldnes (www.anoxkaldnes.com).

There are certain disadvantages of his technology such as:

• increased chemical demands

• increased (doubled) power (aeration) demands (Rosso et al., 2011;Sen et

al., 2008)

• Sometimes similar efficiency for COD and ammonia in comparison to

conventional AS (Rosso et al., 2011)

Membrane reactors (MBR)

There are 800 operating plants (2009) out of which 566 are industrial (in Lin et

al., 2012) and they can provide better effluent quality, especially for suspended

solids. However, the use of membrane systems should be justified as in some

cases AS and MBRs have similar performance for COD ,P, ammonia. (Lerner et

al.,l 2007). Still, the market of MBR is growing steadily (Lesjean, Huisjes,

2008).

Please find major advantages of using membranes for WWT.

An example of membrane reactor is given below.

Fig.1. Membrane reactor (Picture taken from lenntech.com)

Please check the Fig.1 and comment how the reactor works.

The table below compares the parameters of some technologies and this will

help you in your final decision making.

BOD5/COD removal rate HRT (hours) N removal rate sludge production water quality, footprint, other factors

Conventional AS 3 kg BOD5/m3*d

4-8 up to 12 for nitrifying AS

20 – 80 g TN /m3*d (total N, including 50% denitrification)

yes

if higher water quality is required, larger foot print and O&M costs (e.g. so-called 4 stage Bardenpho technology)

depending on configuration

Membrane reactors same or large than for AS

shorter HRT than for conventional AS same as for AS

probably less sludge (e.g. Judd, 2008)

largely disinfected permeate/small foot print. e.g. 0,08 TP and 0,4 NH3-N 20 mg/l TSS (Oleszkiewicz and Barnard, 2006)

larger aeration demand due to higher concentration of suspended sludge in the aeration tank, foaming larger, less readily dewaterable sludge, greater sensitivity to shock loadings (Judd, 2008).

MBBR

Per reactor volume: 0,8-2,2 kg BOD5/m3*d (Cemagref data) Per area: 1,3-3,6 g BOD5/m2*d (Cemagref data) 1,8-20,5 g COD/m2*d (Orantes and Gonzalez-Martinez, 2003)

0,3-0,5h for COD, no nitrification (Leikes, Ødegaard, 2001) 0,8-7,6 h (Orantes and Martinez, ) 1,3 – 2,5 h for BOD5; 2,5-11 h for NH3-N Cemagref data)

Per reactor volume: 0,15 kg N/m3*d (Weiss et al., 2005) 0,1-0,3 kg N/m3*d (Rusten et al., 2006) 0,23 kg NH3-N/m3*d (Suhr, Pedersen, 2010) 0,13 kg NH3-N/m3*d (cemagref data) Per area: g NH3 -N/m2*d 0,1 -1,6 (McQuarrie,

1,45 TSS/kg BOD5 removed (more than of activated sludge)

foot print 50% of activated sludge

chemicals required at primary stage, sedimentation through coagulation and flocculation required or other particles removal technology, high 4 -6 mg/l of oxygen for nitrification, increased power demands Better nitrification at low temperatures

6-15 BOD5/m2*d (Cemagref)

0,5-1 h (Sen et al., 2

Boltz, 2011) 0,27; 0,73 (Suhr, Pedersen, 2010) 0,2 g NH3-N/m2*d (Cemagref data) (54;65%) 54-65% at HRT 2,25-5 h 94-96% at HRT 10-11 h temp 8-12oC 0,3 kgN/m3*d (Weiss et al., 2005)

RBC (rotating biological contractor)

10-30 g COD/m2*d 2 g BOD/m2*d 1-30 g/m2*d

15-24 12 but usually HRT is shorter 0,7-8 h

1-3 g NH3-N/m2 *d After Cortez et al. (2008) 0,7-2 g/m2*d

low sludge small footprint

BAF

Per volume: Biostryr® 0,3-0,7 kg NH3 - N/m3*d 0,8 – 2,0 kg NO3 - N/m3*d (Holloway et al., 2008) 0,1 kg NH3 - N/m3*d (Suhr, Pedersen, 2010)

Table 2. Economics of WWT by different methods

Type of process Capital costs Running costs (Operation and Maintenance costs, O&M)

labor, maintenance energy chemicals

Conventional AS yes, coagulants,

flocculants

Membrane reactors

high,

membranes account

for 25-50% of total

capital costs

high,

membrane replacement 25-33% of total

O&M costs

Higher than AS

0,5-1,8 kWh/m3 (Verrecht et al., 2010)

membrane aeration 74%, biology aeration 11%

0,55 -1,7 kWh/m3 (1)

yes

MBBR

not clear,

installation of grids,

and carriers but

50% of space

regular

20 -30 % higher than conventional (Faletti and Lino,

2007)

8-9 kWh/kg BOD5 (in theory possible to 3,5) (Cemagref)

0,15-0,18 kW/m3 for mixing

3 -3,2 kW/m3 total

(Cemagref)

yes

Table 3. Characteristics of wastewater of MWWTP

year amount of

wastewater, m3/d

max. amount of wastewater,

m3/d BOD, kg/d total P, kg/d total N, kg/d COD, kg/d Suspended solids, kg/d

2008 13678 29972 3106 132 748 6696 3995 2009 11673 18328 3150 129 743 6751 4039 2010 11682 22864 3196 130 758 7085 4180 2011 12639 26325 3319 126 783 7184 4401 2012 13501 23379 3360 118 756 7184 4325

Table 4. Degree of wastewater purification at MWWTP

year BOD, % total P, % total N, % COD, % Suspended solids, %

2008 98 97 44 93 98 2009 98 98 44 94 98 2010 97 95 44 93 96 2011 98 97 43 94 98 2012 98 96 37 94 97

average production of dry sludge at MWWTP is 3615 tonn/d

References

1. http://bvwater.files.wordpress.com/2009/05/abstract_siw09_wallis-

lage.pdf

Cortez, S., Teixeira, P., Oliveira, R., Mota, M., 2008. Rotating biological

contractors: a review on main factors affecting performance. Rev. Environ. Sci.

Biotechnol. 7, 155-172

Di Trapani, D., Mannina, G., Torregrossa, M., and Viviani, G., 2010.

Comparison between hybrid moving bed biofilm reactor and activated sludge

system: a pilot plant experiment. Water Sci. Technol. 61.4, 891-902

Faletti, L., and Conte, L., 2007. Upgrading of activated sludge wastewater

treatment plant with hybrid moving bed biofilm reactors. Ind. Eng. Chem. Res.

46, 6656-6660

Holloway, R., Zhao, H., Rinne, T., Thesing, G., Parker, J., Beals, M., 2008. The

impact of temperature and loading ion meeting stringent nitrogen requirements

in a two-stage BAF – a comparison of pilot and full scale performance.

WEFTEC

Judd, S., 2008. The status of membrane bioreactor technology. Trends

Biotechnol. 26(2), 109-116

Leiknes, T., Ødegaard, H., Moving Bed Biofilm Membrane Reactor (MBB-M-

R): Characteristics and Potentials of a Hybrid Process Design for Compact

Wastewater Treatment Plants, Proceedings, Engineering with Membranes,

Granada, Spain, June 3–6, 2001

Lesjean, B., Ferre, V., Vonghia, E., and Moeslang, H. 2009. Market and design

considerations of the 37 larger MBR plants in Europe. Desalin Water Treat. 6,

227-233

Lesjean, B., Huisjes, E.H., 2008. Survey of the European MBR market: trends

and perspectives. Desalin. 231, 71-81

Lerner, M., Stahl, N., and Galil, N.J., 2007. Comparative study of MBR and

activated sludge in the treatment of paper mill wastewater. Water. Sci. Technol.

55(6), 23-29

Lin, H., Gao, W., Meng, F., Liao, B.-Q., Leung, K.-T., Zhao., L., Chen, J., and

Hong, H., 2012. Membrane bioreactors for industrial wastewater treatment: a

critical review. Crit. Rev. Environ. Sci. Technol. 42, 677-740

McQuarrie, J.P., Boltz, J.P. 2011. Moving bed biofilm reactor technology:

process applications, design, and performance. Water Environ. Res. 83(6), 560

– 575

Oleszkiewicz, J.A., and Barnard, J.L., 2006. Nutrient removal technology in

North America and the European Union: a review. Water Qual. Res. J. Canada.

41(4), 449-462

Orantez, J.C., and Gonzalez-Martinez, S., (2003) A new low-cost biofilm

carrier for the treatment of municipal wastewater in a moving bed reactor.

Water. Sci. Technol. 48(11-12), 243-250

Rosso, D., Lothman, S.E., Jeung, M.K., Pitt, P., Gellner, W.J., Stone, A.,

Howard, D. 2011. Oxygen transfer and uptake, nutrient removal, and energy

footprint of parallel full-scale IFAS and activated sludge processes. Water. Res.

45, 5987-5996

Rusten, B., Eikebrokk, B., Ulgenes, Y., Lygren, E., 2006. Design and operation

of the Kaldnes moving bed biofilm reactors. Aquacult. Eng. 34, 322-331

Sen, S.P.E., Occiano, V., Wong, P.E.P., and Landworthy, A. 2008. Comparing

implementation of MBBR versus BAF on a space constrained site. Proceedings

of the Water Environment Federation, WEFTEC 2008: Session 81 through

Session 90 , pp. 6442-6455(14)

Verrecht, B., Maere, T., Nopens, I., Brepols, C., Judd, S., 2010. The cost of a

large-scale hollow fibre MBR. Water Res. 44, 5274-5283

Weiss J.S., Alvarez, M., Chi-Chung, T., Horvath, R.W., and Stahl, J.F., 2005.

Evaluation of moving bed biofilm reactor technology for enhancing nitrogen

removal in a stabilization pond treatment plant. WEFTEC, 2085 – 2102

Internet:

http://www.ohiowea.org/docs/Wed0800Res_Sludge_Truths.pdf

http://www.watermaxim.co.uk/submerged-aerated-filters.php

http://www.aaees.org/images/e3competition-winners-2011honor-research03.jpg

http://www.madep-sa.com/english/wwtp.html

http://www.eu-etv-

strategy.eu/pdfs/08_Nutrient_removal_biofilm_reactors_Rusten.pdf

Look here also, these are quite interesting:

http://typo3.kuster-hager.ch/fileadmin/dokumente/EnhanceingPerformance.pdf

http://www.beknowledge.com/wp-content/uploads/2010/09/1091.pdf

Difference between AS and membranes http://www.forskningsplatformen-

vand.dk/Documents/Annual%20meeting%202012/Presentations/Kragelund_D

WRP12.pdf