Secondary Waste Treatment

7/28/2019 Secondary Waste Treatment

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Sewage treatment or domestic waste water treatment – physical , chemical, biological processes to remove contaminants from water

to produce environmentally safe fluid waste and solid for disposal

Treatment – pretreatment, primary, secondary , tertiary waste treatment

Primary treatment-holding the sewage in a basin where heavy solids settles to

the bottom , fats ,oils and lighter solids floats on surface.

Secondary treatment-removes dissolved and suspended biological matter

using water born micro-organisms.

Tertiary treatment-treated water is disinfected physically or chemically before

discharging to river , pond , lagoon and wetland or can be used for irrigation.



The objective of secondary treatment is to remove the residual organics and

suspended solids which can not be removed by primary treatment.

Secondary treatment usually follows the primary treatment and involves the

removal of biodegradable dissolved and colloidal organic matter by

microorganisms.

Microbes metabolize organic matter in waste water , there by producing more

micro organisms and inorganic end products principally CO2,NO3,SO4 and

PO4.

Microorganisms must be separated from the treated waste water by

sedimentation to produce clarified secondary effluent.



The sedimentation tanks used in secondary treatments are usually called ,

secondary clarifier.

The biological solids removed during sedimentation , called secondary or

biological sludge are normally combined with primary sludge for sludge

processing.

The principle group of organic substances found in waste water are protien ,

carbohydrate , fats and oils . some synthetic organic compounds are

surfactants , phenols and pesticides



BOD – amount of oxygen required by the micro organisms to decompose the

organic matter.

COD – amount of oxygen required to decompose the organic matter by a

strong chemical oxidizing agent in an acidic medium.



Secondary treatment can be aerobic or anaerobic . In aerobic process bacteria andother microbes consumes organic matter as food . Anaerobic treatment isemployed for the digestion of sludge

Degrade the biological content of the sewage which are derived from human waste

, food waste , soaps and detergents using aerobic biological processes.

Bacteria and protozoa consumes biodegradable soluble organic contaminants (likesugar ,fat , short chain organic carbon molecules )

Reduce the BOD level to more than that attained by sedimentation process

Dissolved and colloidal organic compounds and colours present in waste water isremoved or reduced and organic matters get stabilized.



Secondary treatment

- fixed film process, where the biomass grow on a mediaand sewage passes over its surface. (trickling filter , rotating biological

contactor, MBBR,AFBBR,packed bed reactor)

- suspended growth where biomass is mixed with the

sewage. (activated sludge , oxidation pond/ditch , aerated lagoon)



Activated sludge

Aerated lagoon

Trickling filter

Rotating biological contactor Packed bed reactor

AFBBR

Membrane bio reactor

Oxidation pond



Most versatile biological oxidation method for treatment colloids , dissolved

solids and organic matter.

Waste water aerated in a reaction tank in which microbial floc is suspended .

Bacteria in activated sludge process are gram negative and include members

of the genera Psuedomonas , Zoogloea,Achromobacter,Flavobacterium and

the two nitrifying bacteria, Nitrosomonas and Nitrobacter

Biological degradation of waste by aerobic bacteria to carbon dioxide and

water.

Bacteria flora grows and remain suspended in the form of a floc , which is

called “activated sludge”





Effluent from the reaction tank is separated from the sludge by settling and

discharged

A part of the sludge is recycled to the same tank to provide microbial

population for fresh treatment cycle.

Efficient aeration of 5 to 24 hours required for industrial sludge.

BOD removal of 90-95% achieved by this process



Consist of circular or rectangular beds, 1m to 3m deep broken stone ,pvc ,coal, synthetic resin , gravel etc of size 40mm to 150mm size

Waste water sprinkled uniformly over the entire bed with the help of a slowlyrotating distributor such a s rotatory sprinkler equipped with orifice or nozzles

Waste water trickles through the media . Air enter at the bottom counter current to the effluent flow

A gelatinous film of bacteria and aerobic microorganism is formed on thesurface of the filter media , which thrive on the nutrient supplied by wastewater

The organic impurities in the waste water are adsorbed on the gelatinous filmduring its passage are oxidized by bacteria and microbes.



as the micro organisms grow, the thickness of the slime layer increases, and

the diffused oxygen is consumed before it can penetrate the full depth of the

slime layer .Thus an anaerobic environment is established near the surface of

the media.

Achromobacter , flavobacterium , psuedomonas and alcaligenes are among

the bacterial species commonly associated with the trickling filter



As the micro organisms utilize the

organic matter , thickness of the

slime film increases to a point

where it can no longer be

supported on the solid media and

gets detached from the surface.

This process is known as

“sloughing". A settling tank

following the trickling filter

removes the detached bacterial

film and some suspended matter.



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Bacterial growth is fixed on the media.

All solids from the settler are wasted.

Less sensitive to shock loading Low operating cost

Produce insects and odours

Less effective in removing disease causing organisms.

Bacterial growth is suspended as a dispersed floc.

Solids from the settler are partially recycled.

More sensitive to shock loadings , require closer process

control

High operating cost

More effective in removing pathogens



Large holding tanks or ponds having a depth of 3-5m and lined with cement ,

polythene or rubber

Effluent from primary treatment are collected in this tank and are aerated with

mechanical devices such as floating aerators , for 2 to 6 days

During this time a healthy flocculent sludge is formed which bring about

oxidation of dissolved organic matter

BOD removal of about 99% could be achieved by this process.

Major disadvantage is large space requirement. The fundamental difference

between aerated lagoon and activated sludge , sludge is not recycled in aerated

lagoon



Large shallow pond where stabilization of organic matter in the waste is brought about by bacteria and to some extent by protozoa.

Oxygen requirement is provided by surface aeration and by photosynthesis of

algae present in the pond.

The oxygen released by the algae through the process of photosynthesis , isused by the bacteria in the aerobic degradation of organic matter.

The nutrients and carbon dioxide released in the degradation are in turn used by the algae.

Symbiotic relationship between algae and bacteria.BOD removal of 95%achieved.Also called waste stabilization ponds.







http://en.wikipedia.org/wiki/File:Rotating_Biological_Contactor.png



Combination of activated sludgetreatment with a membrane liquid-solidseparation process.

The membrane component uses low pressure ultra filtration or microfiltrationmembrane and eliminates the need for clarification and tertiary filtration

The key benefit of an MBR system is thatit effectively overcomes the limitationsassociated with poor settling of sludge inthe conventional activated sludge process.

Two configurations-internal/submerged ,where membranes are immersed ,external where membrane are separateunit process requiring an intermediate pumping step.





Waste water is fed to a bed of 0.4 to 0.5 mm sand or activated carbon.beddepth are in the range of 3 to 4 m. Up flow velocities are 30 to 36m/hr.

As the bio film increases in size, the packing become lighter and accumulates

at the top of the bed where it can be removed.

Effluent recirculation is necessary to provide the fluid velocity within thenecessary treatment detention times.

For aerobic process recirculated effluent is passed through an oxygen aerationtank to predissolve oxygen.

AFBBR are frequently used for treatment of groundwater contaminated withhazardous substances



1.storage tank

2.centrifugal pump

3.valves

4.rotameter

5.air sparger

6.supporting mesh

7.fluidizing section

8.discharge valve



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Non biodegradable toxic compounds can be adsorbed onto theactivated carbon.

High quality effluent is produced low in TSS and COD

concentration.

The system operation is simple and reliable.

The oxygenation method prevents stripping and emission of toxicorganic compounds to the atmosphere

As the micro organisms in the bio film multiply there is an

increase in the bio film thickness .This limits the diffusion of

oxygen or organic substrate to the deeper layer of the bio film .

Starvation of microorganisms at the base of the bio film causes

pieces of bio film to detach and leads to in effective bioreactor operation.







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Secondary Waste Treatment