BIOLOGICAL TREATMENT

Biological treatment is an important and integral part of any wastewater treatment plant that treats wastewater from either municipality or industry having soluble organic impurities or a mix of the two types of wastewater sources.

The obvious economic advantage, both in terms of capital investment and operating costs, of biological treatment over other treatment processes like chemical oxidation; thermal oxidation etc. has cemented its place in any integrated wastewater treatment plant.

AEROBIC it is important to briefly discuss the term aerobic .

Aerobic, as the title suggests, means in the presence of air (oxygen).

These term are directly related to the type of bacteria or microorganisms that are involved in the degradation of organic impurities in a given wastewater and the operating conditions of the bioreactor.

Therefore, aerobic treatment processes take place in the presence of air and utilize those microorganisms (also called aerobes), which use molecular/free oxygen to assimilate organic impurities i.e.

convert them in to carbon dioxide, water and biomass. The anaerobic treatment processes, on other hand take place in the absence of air (and thus molecular/free oxygen) by those microorganisms (also called anaerobes) which do not require air (molecular/free oxygen) to assimilate organic impurities.

Aerobic Treatment Principle

ACTIVATED SLUDGE PROCESS (ASP) SYSTEM

This is the most common and oldest biotreatment process used to treat municipal and industrial wastewater.

Typically wastewater after primary treatment i.e. suspended impurities removal is treated in an activated sludge process based biological treatment system comprising aeration tank followed by secondary clarifier.

The aeration tank is a completely mixed or a plug flow (in some cases) bioreactor where specific concentration of biomass (measured as mixed liquor suspended solids (MLSS) or mixed liquor volatile suspended solids (MLVSS)) is maintained along with sufficient dissolved oxygen (DO) concentration (typically 2 mg/l) to effect biodegradation of soluble organic impurities measured as biochemical oxygen demand (BOD5) or chemical oxygen demand (COD).

The aeration tank is provided with fine bubble diffused aeration pipework at the bottom to transfer required oxygen to the biomass and also ensure completely mixed reactor.

Roots type air blower is used to supply air to the diffuser pipework.

In several older installations, mechanical surface aerators have been used to meet the aeration requirement.

The aerated mixed liquor from the aeration tank overflows by gravity to the secondary clarifier unit to separate out the biomass and allow clarified, treated water to the downstream filtration system for finer removal of suspended solids.

The separated biomass is returned to the aeration tank by means of return activated sludge (RAS) pump. Excess biomass (produced during the biodegradation process) is wasted to the sludge handling and dewatering facility.

TRICKLING FILTERS

• Not a true filtering or sieving process• Material only provides surface on which bacteria to grow• Can use plastic media• lighter - can get deeper beds (up to 12 m)• reduced space requirement• larger surface area for growth• greater void ratios (better air flow)• less prone to plugging by accumulating slime

TRICKLING FILTERS

Types of Trickling Filters

• Super rate– synthetic plastic media units• modules or random packed• specific surface areas 2-5 times greater than rock• much lighter than rocks• can be stacked higher than rocks

– loading rates of 40-200 m3 wastewater/m2 filter cross-sectional area-day

– plastic media depths of 5-10 m

• High rate– single stage or two-stage rock media units– loading rates of 10-40 m3 wastewater/m2 filter cross-

sectional area-day– re-circulation ratio 1-3.

• Standard or low rate– single stage rock media units– loading rates of 1-4 m3 wastewater/m2 filter cross-sectional

area-day– large area required

TRICKLING FILTERS

Filter Material

Flow Diagram for Trickling Filters

Recycle

Primaryclarifier Trickling

filter

Finalclarifier

Wastesludge

FinaleffluentInfluent

Typical Trickling Filter

Trickling Filter

• Tank is filled with solid media – Rocks – Plastic

• Bacteria grow on surface of media • Wastewater is trickled over media, at top of tank • As water trickles through media, bacteria degrade BOD • Bacteria eventually die, fall off of media surface • Filter is open to atmosphere, air flows naturally through media • Treated water leaves bottom of tank, flows into secondary clarifier • Bacterial cells settle, removed from clarifier as sludge • Some water is recycled to the filter, to maintain moist conditions

Trickling Filter System

Trickling Filter Process

BACTERIA REMOVAL

Design Criteria for Trickling FiltersT a b le 10 .5

T y p ic a l D e s ig n C r ite ria fo r T r ic k li ng F il te rs

Ite m L o w -ra te filte r H ig h-ra te f ilte r S up er- ra te f ilte r

H yd ra u l ic loa d in g (m 3/m 2-d ) 1 - 4 10 - 40 40 - 20 0

O rg an ic loa d in g (k g B O D 5/m 3-d) 0 .0 8 - 0 .32 0.3 2 - 1 .0 0.8 - 6.0

D e p th (m ) 1 .5 - 3 .0 1.0 - 2.0 4.5 - 12 .0

R e c irc u la tio n ra tio 0 1 - 3 1 - 4

F ilte r m e d ia R o c k , s la g, e tc . R o ck , s lag,syn th e tic s

F ilte r f lie s M a n y Fe w , la rva e a rew ash e d a w ay

Fe w o r no ne

S lo ug h in g I nte rm itte n t C o nt in uo u s C o nt in uo u s

D o s in g inte rv a ls < 5 m in < 1 5 s C o nt in uo u s

E f flue n t U sua l ly fu l lyn it rif ied

N itr ifie d a t lowlo ad in g s

N itr ifie d a t lowlo ad in g s

Advantages of Trickling filters• Simple and reliable process that is suitable in areas where large tracts of land

are not available for a WSP treatment system• Effective in treating high concentrations of organic material depending on

the type of media used;• Very efficient in removal of ammonia from wastewater;• Appropriate for small- to medium-sized communities• Ability to handle and recover from shock loads• Relatively low power requirements; They require power for pumping only

and do not need large power-hungry aeration blowers. From motor-driven rotary distributors are powered by fractional horsepower electric motors.

• They produce less sludge than suspended-growth systems. The sludge tends to settle well because it is compact and heavy.

• Level of skill and technical expertise needed to manage and operate the system is moderate

• The cost to operate a trickling filter is very low

Disadvantages of Trickling filters• Additional treatment may be needed for the effluent to meet strict

discharge standards.• It is a natural method for wastewater treatment that requires a

considerable amount of space.• Generates sludge that must be treated and disposed of;• Regular operators attention is needed;• Relatively high incidence of clogging;• Relatively low loadings required depending on the media;• Limited flexibility and control in comparison with activated sludge

processes. They require high maintenance costs of rotary distributor center mechanisms. Any maintenance service for the rotary distribution mechanism would require a crane and complete removal of the rotary distributor mechanism, guy rods, and arms.

• Potential for vector and odour problems

OXIDATION PONDS

• Oxidation Ponds are also known as stabilization ponds or lagoons. They are used for simple secondary treatment of sewage effluents.

• Within an oxidation pond heterotrophic bacteria degrade organic matter in the sewage which results in production of cellular material and minerals.

• The production of these supports the growth of algae in the oxidation pond.

• Growth of algal populations allows furthur decomposition of the organic matter by producing oxygen. The production of this oxygen replenishes the oxygen used by the heterotrophic bacteria.

• Typically oxidation ponds need to be less than 10 feet deep in order to support the algal growth. In addition, the use of oxidation ponds is largely restricted to warmer climate regions because they are strongly influenced by seasonal temperature changes. Oxidation ponds also tend to fill, due to the settling of the bacterial and algal cells formed during the decomposition of the sewage.

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• Lagoons or oxidation ponds– Anaerobic– Facultive– Aerobic

• Secondary treatment in rural areas• Polishing ponds • Serve 7% of population (1000’s)– >90% of ponds serve populations <10,000

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Anaerobic Lagoons• The primary treatment takes place in the anaerobic pond,

which is mainly designed for removing suspended solids, and some of the soluble element of organic matter (BOD).

• May be covered• High-strength WW – meat processing– dairy waste

• Temperatures must be high--- 75-82F• BOD loading 20 # per thousand ft3 per day• Gaseous end products of CO2 and CH4

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Facultative Lagoons

• During the secondary stage in the facultative pond most of the remaining BOD is removed through the coordinated activity of algae and heterotrophic bacteria.

• Anaerobic (bottom layer) and aerobic (upper)• Bacteria break down organics

– Nitrogen/phosphorous/CO2

• Algae and reaeration (wind) provides O2

• BOD <30 mg/l in warm weather• SS usually > 30 mg/l because of algae• Don’t operate well in cold weather• Can’t handle industrial ww’s

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Aerobic Lagoons

• The main function of the tertiary treatment in the maturation pond is the removal of pathogens and nutrients (especially nitrogen).

• Completely mixed.• First-stage treatment of municipal WW• Pre-treatment of industrial WW• Basins 10-12’ deep• Pier-mounted floating mechanical aerators• No algae• Odor-free if highly aerated

Advantages

• Low operational and maintenance cost.• lagoons provide effective treatment with minimal threat to the

environment.• Work well in clay soils where conventional subsurface on-site

absorption fields will not work.

Disadvantages

• Lagoons must be constructed in clay soil or be lined to prevent leakage.• May overflow occasionally during extended periods of heavy rainfall.• If there are extended periods of overcast windless days, a rare occasion• Offensive odours may occur for a brief time. Lagoons• Usually recover rapidly if this occurs.• Can not be installed on a small lot. Takes up a relatively large space for only one

use.• Lagoons are not aesthetically acceptable to some people. Some people consider

lagoons unsightly and unsafe.• As with any other open body of water, there is some potential danger.• Although lagoons are required to be fenced, this does not always prevent access

by people or pets.