biological treatment of wwt

7/29/2019 Biological Treatment of WWT

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Wastewater Treatments 19

2.3 BIOLOGICAL TREATMENT

Biological treatment as indicated earlier, the biological wastes are rich in

decomposable organic matter and can be treated by biological processes

involving microbial decomposition of organic matter, aerobically or

anaerobically.

The main difference between aerobic and anaerobic processes is in thepresence or absence of oxygen.

The common methods of biological wastewater treatment are :

a) aerobic processes such as trickling filters, rotating biologicalcontactors, activated sludge process, oxidation ponds and lagoons,

oxidation ditches,

b) anaerobic processes such as anaerobic digestion, andc) anoxicprocesses such as denitrification .

The major biological wastewater treatment processes are shown in thenext Table 5 ./5/

2.3.1 AEROBIC PROCESSESThe basic principle of aerobic processes is the usage of aerobic

microorganisms which need oxygen for their metabolism in the process ofdecomposition of organic matter from the wastewater.

While the basic principles remain the same in all aerobic processes, the

techniques used in their application may vary widely, but may be broadlyclassified as either attached (film) growth or suspended growthprocesses .

A. ATTACHED GROWTH PROCESSES

Attached growth processes utilize a solid medium upon which bacterial

solids are accumulated in order to maintain a high population. Surface

growth processes include intermittent sand filters, trickling filters, rotatingbiological contactors, and a variety of similar devices.


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Table 5 Major biological treatment processes used for

Wastewater Treatment

Type Common name Use

AEROBIC PROCESSES

Suspended growth ACTIVATED SLUDGE PROCESS

- Conventional ( Plug flow ) Carbonaceous BOD

- Step aeration, Modified aeration - Contact Stabilization

- Extended Aeration, Oxidation Ditch

+ Nitrification

AERATED LAGOONS Carbonaceous BOD

AEROBIC DIGESTION HIGH

RATE ALGAL PONDS Carbonaceous BOD

Attached growth TRICKLING FILTERS

- Low rate Carbonaceous BOD

- High rate

ROTATING BIOLOGICAL

CONTACTORS

ANAEROBIC PROCESSES

Suspended growth ANAEROBIC DIGESTION

- Standard rate Stabilization

- High rate Single

Attached growth ANAEROBIC CONTACT PROC. Carbonaceous BOD

ANAEROBIC FILTER PROCESS

ANAEROBIC LAGOONS

ANOXIC PROCESSES - Suspended growth Denitrification- Fixed film

Source : NPC Report, /5/

2.3.1.1 Trickling filters


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This equipment consists of a bed of highly permeable media to whichmicro organisms are attached and through which wastewater is

percolated. The surface of the media that comes into contact with

wastewater develops a Zoogleal film rich in microorganisms. The

microorganisms in this film degrade the organic material present in thewastewater.

As the microorganisms grow, the slime layer increases. The adsorbedorganic matter is metabolised, before it can reach the microorganisms

near the media face. As a result of having no external organic source

available for cell carbon, the microorganisms near the media face enterinto an endogenous phase of growth and lose their ability to slime layer

stalks to grow. This phenomenon of losing the slime layer is called

sloughing and primarily a function of the organic and hydraulic loading

on the filter.

The trickling filter should also have an underdrain arrangement for

collecting the filtered effluent and a proper ventilation system tomaintain aerobic conditions.

A secondary settling tank should invariably follow the filter for removalof the sloughed off solids.

The principal flow scheme for an trickling filter treatment is shown inthe next Figure 24 ./11/

A typical trickling filter is shown in the Figure 25 ./12/

2.3.1.2 Rotating biological contactors

A rotating biological contactor consists of a series of closely spacedcircular disks of polystyrene or PVC, or other materials. The disks are

partially submerged in wastewater and rotated slowly through it.

In operation, biological growths become attached to the surfaces of the

disks and form a slime layer over the entire wetted surface area of the

disks. The rotation of the disks alternately contacts the biomass with theorganic material in the wastewater and then with the atmosphere for

adsorption of oxygen.

A principal scheme of an rotating biological disc plant is shown in thenext Figure 26 ./1/

B. Suspended growth


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Suspended growth processes maintain an adequate biological mass insuspension within the reactor by employing either natural or

mechanical mixing. In most processes the required volume is reduced

by returning bacteria from the secondary clarifier in order to maintain a

high solids concentration. Suspended growth processes includeactivated sludge and its various modifications, oxidation ponds, and

sludge digestion systems.

2.3.1.3 Activated sludge processIt is a treatment process in which biologically active mass, called

activated sludge is continuously mixed with the sewage flow in an

aeration basin in the presence of abundant oxygen. The liquid in the

aeration basin is called mixed liquor. The oxygen is supplied to the

mixed liquor either by diffused compressed air or by mechanicalagitation. The activated sludge is subsequently separated from the mixed

liquor by sedimentation process in a clarifier and a part of this sludge isre-circulated to the aeration basin. The rest of the sludge, which is the

excess production of biological cell material, is disposed-off.

A principal scheme of an activated sludge process is given in the next

Figure 27 ./5/

Many modifications in basin configuration and aeration techniques have

been made in past. The more important types of activated sludge processare described below :

- theconventional process (known also as the plug flow type )

consists of a aeration basin, a clarifier and a solid-return linefrom the clarifier bottom. The return solids are mixed with the

incoming wastes and pass through the reactor in a plug flow

fashion.Air is provided uniformly along the aeration basin.

Conventional activated sludge process is shown in the nextFigure 28 ./13/

- tapered aeration processes attempt to match the oxygen supplyto demand by introducing more air at the head end. The process

is otherwise the same as described for conventional aeration

process. The main advantage of tapered aeration is the optimum

use of air.

- step aeration processes distribute the incoming flow to a number


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of points along the aeration basin, thus avoiding the locally high

oxygen demand encountered in conventional and taperedaeration process.

A principal scheme for an step aeration is shown in the next

Figure 29 ./13/

- contact stabilization orbiosorption takes advantage of the

observed adsorptive properties of activated sludge.The sewage is mixed and aerated with activated sludge for a

short period ( - 1 hours) and the mixed liquor then passes into

a clarifier, where final effluent and sludge are separated bygravity. The settled sludge in the clarifier is re-aerated in a

separate sludge re-aeration ( stabilization ) tank for a period of

some 3 6 hours. After the aerated sludge is fed into contact

tank for intimate mixing with the inflow. This process is suitable

for high organic loadings.

A principal scheme for an contact stabilization is shown in thenext Figure 30 ./13/

- completely mixedor ahigh rate aeration processes disperse theincoming waste and return sludge uniformly throughout the

aeration basin. The main objective of high rate aeration is to

reduce the cost of construction.

A principal scheme of an completely mixed high rate aeration isshown in the next Figure 31 ./13/

- extended aeration is a completely mixed process operated at a

long hydraulic retention time and a high sludge age. The processis limited in application to small plants where its inefficiency is

outweighed by its stability and simplicity of operation. The

advantage of this process is that usually there is no provision forexcess activated sludge necessary.

A principal scheme of an extended aeration process is shown inthe next Figure 32 ./12/

- short-term aeration orhigh-rate activated sludge is apretreatment process where retention times and sludge age are

low, which leads to a poor effluent and relatively high solids

production. A possible application of this process is as the first

stage of a two-stage nitrification process.

- High purity oxygen activated sludge systems have been


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developed in an attempt to permit easier matching of oxygen

supply to oxygen demand and higher-rate processes throughmaintenance of higher concentrations of biological solids. / 7 /

A principal scheme for an oxygen activated sludge system is

shown in the next Figure 33 ./1/

2.3.1.4 Waste Stabilization PondsWaste stabilization ponds are low cost wastewater treatment units whichusually depend on natural processes. These systems are sometimes also

called oxidation ponds or lagoons. In these ponds the organic matter

from the wastewater is degraded by natural oxidation processes by the

action of microorganisms, both algae and bacteria.

If these processes take place in a stream water then it will be known as aself-purification process.

Waste stabilization ponds or lagoons are classified according to the

biological process taking place as aerobic, anaerobic, facultative andmaturation ponds . /3/

The general reactions which occur in aerobic and facultative ponds are

illustrated schematically in the next Figure 34 ./14/

Aerobic ponds are shallow ponds, less than 1 m in depth, wheredissolved oxygen is maintained throughout the entire depth, mainly by

the action of photosynthesis.

Facultative ponds are ponds between 1 to 2.5 m depth, which have ananaerobic lower zone, a facultative middle zone, and an aerobic upper

zone maintained by photosynthesis and surface reaeration.

A schematic presentation ( diagram ) of a facultative pond operation is

given in the next Figure 35 ./14/

Anaerobic ponds are deep ponds that receive high organic loadings

such that anaerobic conditions prevail throughout the entire pond depth.

Maturation or tertiary ponds are ponds used for polishing effluents

from other biological processes. Dissolved oxygen is furnished through

photosynthesis and surface reaeration. This type of pond is also known

as a polishing pond.


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Aerated lagoons are these ponds which are oxygenated through the

additional action of surface or diffused air aeration.

A typical aeration system for the aerated lagoons is a static tube aerator

as shown in the next Figure 36./7/

Oxidation ditches are simple artificial ditches, dug in the ground for the

purification of wastewater using a very simplified technique. The basic

form of the plant is a ring shaped circuit or ditch. Oxygen as well ascirculation is provided by mechanical rotors . In fact, oxidation ditch is

a modified form of extended aeration of activated sludge process.

Some typical oxidation ditches are shown in the Figure 37 ./12/

2.3.1.5. Natural systems

Natural treatment systems are designed to take advantage of physical,chemical and biological processes which occur in the nature when water,

soil, plants, microorganisms and the atmosphere interact.

The natural treatment systems are :

- the soil-based or land-treatment systems, (infiltration, irrigation, etc.),

- the aquatic-based systems (constructed and natural wetlands andaquatic plant treatment systems.

Aquatic plant systems are schematically presented in the Figure 38 ./11/

2.3.2 ANAEROBIC PROCESSESThe anaerobic waste treatment is an effective method for the treatmentof highly concentrated organic wastes. In the absence of oxygen,

anaerobic bacteria convert organic material into gaseous end-products

such as CO2 , CH4 and H2S.

This biological process is called asanaerobic digestion and is one of the

oldest processes used for the stabilization of sludges.

Anaerobic digestion is a fermentation process which involves the

decomposition of organic and inorganic matter in the absence of

molecular oxygen. The process stabilizes the organic matter byconverting it as completely as possible into methane and carbon-dioxide

gas through the two stages:


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- acid fermentation or liquefaction,- methane fermentation or gasification.

In the first stage, so called liquefaction or acid fermentation, acid

forming bacterial group consisting of acid-producing bacteria isresponsible for converting complex organic compounds ( like fats,

proteins, carbohydrates ) into more simple organic compounds ( like

acetic, propionic and butyric acids ) which become a source of food for asecond group, the methane-forming bacteria.

In the second stage, so calledgasification or methane fermentation, the

organic acids will be converted into gases, mainly methane ( CH4 ) andcarbon dioxide ( CO2 ).

The stages of oxidation of organic matter of sludge is given in the next

Figure 39 . /14/

Two types of digesters are at present in use, standard-rate and high-

rate.In the standard-rate digestion process the content of the digester are

usually unheated and unmixed. Detention times for this process vary

from 30 to 60 days.In a high-rate digestion process the contents of the digester are heated

and completely mixed. The required detention time is 15 days or less.

A combination of these two basic processes is known as the two-stage

process digestion.

Some types of anaerobic sludge digesters are shown in Figure 40. /14/

2.3.3 ANOXIC PROCESSESUnder anoxic conditions, a group of facultative anaerobes calleddenitrifiers utilizes nitrites ( NO2-) and nitrates ( NO3-) as the terminal

electron acceptor.

The removal of nitrogen in the form of nitrate from the wastewater byconversion to nitrogen gas in the absence of oxygen is a biological

process calledanoxicdenitrification.

Questions to the Chapter . :

1. Name the biological wastewater treatments !2. Describe briefly all above mentioned biological treatments!


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3. The purification of sewage in oxidation ponds is carried out by :

a)algae; b) bacteria; c) both ; d) non of these .

4. What is zoogleal film ?5.

In the trickling filter process : a) much skilled labour is required;b) operation cost is higher than in ASP unit; c) has low adaptability

with varying load; d) power requirement is low as compare to ASP;

6. Sketch a principal schematic of a trickling filter !

7. Distinguish between suspended- and attached- growth processes !

8. Describe short RBC systems !

9. Sketch a principal scheme of a RBC plant !

10. Describe short an ASP !

11. Sketch a principal scheme of an ASP !

12. Which types of ASP do you know ?

13. Describe short a process of an extended aeration !

14. Distinguish between the diffused aeration and surface aeration !

15. Explain short the terms step aeration and tapered aeration !

16. When is usage of pure oxygen in ASP required ?

17. What are stabilization ponds ?18. How are the stabilization ponds classified ?19. Sketch different types of stabilization ponds !20. Sketch an oxidation ditch !21. What is anaerobic digestion ?22. Distinguish between anaerobic and anoxic processes !23. Explain the denitrification process !

biological treatment of wwt

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