MIC 203: Environmental Microbiology Sewage Treatment

Stages in typical sewage treatment

Wastewater treatment processes

Primary Sewage treatment Large floating materials (cans, sticks, plastic packets etc.) are screened out by bar screen. Grit chamber removes sand, grit, stones, broken glass etc. Floating debris is shredded & ground

Primary Sewage treatment After that sewage pass through sedimentation tank where colloidal and dissolved organic matters are removed. Skimmers remove floating oil & grease Sewage solid collecting on the bottom is called primary sludge. 40-60% suspended solid are removed Primary treatment may remove 70-80% of BOD

Primary Sewage treatment (contd.) The sludge are drawn off from bottom & is subjected to anaerobic digestion before disposal in landfills or agriculture fields Biological activity is not important. Some digestion of sludge & dissolved organic matter occur during holding time The effluent then undergoes secondary treatment

Primary Sewage treatment (contd.)

Secondary Sewage treatment Predominantly biological Removes most organic matter & reduces BOD Sewage undergoes strong aeration to encourage growth of aerobic bacteria & other microbes that oxidize dissolved organic matter to CO2 & H2O (mineralization).

Secondary Sewage treatment (contd.) Two method are commonly used: Activated sludge system Trickling filters Combination of primary & secondary treatments reduce original BOD of sewage by 80-90%

An activated sludge system of secondary sewage treatment

An activated sludge system Popular & common method Effluent from primary treatment is aerated in a large aeration tank. Slime bacteria e.g. Zoogloea ramigera grow & form flocs. Protozoa, small animals, filamentous bacteria and fungi are attached to the flocs.

An activated sludge system (contd.)

An activated sludge system (contd.) Holding time is generally 5-10 hrs. During this time, a part of of organic matter is mineralized and another part is converted into microbial biomass. The content of the tank is transferred to a settling tank where the flocs settle.

An activated sludge system (contd.) Some floc materials are used as inoculum (hence the name of the process) and the rest is sent to the sludge digester. The clear effluent is disinfected and discharged.

An activated sludge system (contd.) Advantages: Effective & flexible Relatively small space is needed Widely used for treatmnet of domestic & industrial sewages Reduces the numbers of intestinal pathogens, such as Salmonella, Shigella, E. coli, enteroviruses etc.

Trickling filter

Trickling filter

Trickling filter (condt.) common method Wastewater is sprayed over a bed of crusted stone or molded plastic with drains at the bottom. The bed is designed such a way that air can circulate throughout the bed. The spray also saturates the sewage with O2

Trickling Filter

Trickling filter (condt.) A biofilm consists of aerobic microbes grows on the stone or plastic surface and completely mineralizes organic matter to CO2, NH3, NO3-, SO42- and PO43-. As a result BOD reduces. Insect mainly fly larvae consume the extra microbial biomass.

Trickling filter (condt.) Advantages: Simple & inexpensive Removes 80-85% BOD Can be used for several times

Trickling filter (condt.) Disadvantages: Less effective than activated sludge system Nutrient overload impairs the operation of the filter Cold weather reduces effectiveness Make nuisance to nearby residence Produces aerosols

Oxidation pond

Oxidation pond Shallow pond (2-4 ft in depth) to allow algal growth on waste water efluent. Oxygen is supplied from the air but release of O2 during photosyntensis by alga Chorella provides additional source of O2.

Oxidation pond (condt.) Advantages: Inexpensive to build & operate Have a large holding capacity

Oxidation pond (condt.) Disadvantages: Requires large areas of land & long retention time. Create odor problem. Largely restricted to warmer climate.

Rotating biological contractor (RBC)

Rotating biological contractor (RBC) Aerobic secondary sewage treatment process. Also called biodisc system and is an advanced biofilm design.

Rotating biological contractor (condt.) Several closely palced disc are continuously rorated in a trough containing liquid sewage. Discs are partially submerged in the liquid sewage. Biofilm develops on the discs. Continuous rotation keeps biofilm well aerated and in contact with sewage.

Rotating biological contractor (condt.) Advantages: Require less space Efficient & stable in operation. Produce no aerosol. Disadvantages: Require higher initial investment

Rotating biological contractor (condt.) Micobial community: Complex & diversed Diverse bacterial, protozoan, metazoan populations. Filamentous microbes: Sphaerotitus, Beggiatoa, Nocardia & Oscillatirio

Rotating biological contractor (condt.) RBC typically has two layers: An outer layer: whitish layer containing Beggiatoa filaments & An inner layer: balck layer containing Desulfovibrio. Black color is due to the precipitation of FeS from reaction between Fe and H2S produced by sulfate-reducing Desulfovibrio.

Anaerobic sludge digester An anaerobic sludge digester is designed to encourage the growth of anaerobic bacteria (especially methane-producing bacteria) that degrade these organic solids to soluble substances and gases mostly CH4 (60-70%) and CO2

Anaerobic sludge digester

Major microbial processes occurring during anaerobic sludge digestion

Anaerobic sludge digester The digester contains high amounts of suspended organic matter. A considerable part of this suspended material is bacterial biomass, viable count can be as high as 109-1010 bacteria/ml. Anaerobic counts are typically 2-3 orders of magnitude higher than aerobic counts.

Anaerobic sludge digester The operation of anaerobic digester requires close control of several parameters, e.g. : Retention time Temperature pH C:N & C:P Reactions are usually heated 35-37 oC for optimal performance. Control is necessary for pH to stay within 6.0-8.0 with pH 7.0 being optimal.

Anaerobic sludge digester Extremes of pH, influx of heavy metals and toxic materials upset the operation. The reactor must be cleaned and charged with large volume anaerobic sludge or appropriate methagenic bacteria from an operational unit. Heavy metals-especially cadmium, which can accumulate in plants and copper, which is toxic to plants- are a potential problem with land disposal.

Anaerobic sludge digester Advantages: CH4 is used as a fuel for heating the digester Sludge contains soil-conditioning materials e.g. humus and mulch and can be used a fertilizer.

Anaerobic sludge digester Disadvantages: Expensive to construct. Require expert maintenance

Disinfection & release

Septic tank Short note

Septic tank

Tertiary sewage treatment Its importance

Tertiary sewage treatment Process

Tertiary sewage treatment Advantages

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