Practical applications in environmental biotechnology

IntroductionWastewater treatment processes (WWTP) can be considered the largest industry in terms of volumes of raw material treatedIndustrial expansion and urban population growth have increased the amount and diversity of wastewater generateThe WWTP has become part of a production process, e.g. for fresh water reuse purpose

Wastewater treatment

Activated sludge process Most widely used aerobic suspension wastewater treatment system 1 treated effluent introduced into an aeration tank Rapid development of microorganisms is stimulated by reintroduction of settled sludge (i.e. activated sludge) The microbial suspension decomposes and utilizes the waste organic matters

Activated sludge process (cont)process dealing with the treatment of sewage and industrial wastewaters.Sludge process is strictly aerobicAerobic source : Atmospheric air or pure oxygenAeration tank suspended-growth reactor containing microbial aggregates (flocs) of microorganisms termed the activated sludge

Two Types

1) Attached growth or fixed system- Organisms attached to some inert media like rocks or plastic. 2) Suspended growth- Organisms are suspended in the treatment basin fluid. This fluid is commonly called the mixed liquor.

Attached Growth or Fixed Film Reactors

Trickling Filters

Consists of a reactor called the - aeration tank - a settling tank - solid recycle : from the settler to the aeration tank - sludge wasting line

Suspended Growth Process

Characteristics of Activated SludgeMicrobial ecology2 crucial characteristics i) wide variety of microorganisms - e.g. prokaryotes (bacteria), eukaryotes (protozoa, crustacean, nematodes and rotifer), bacteriophage and fungi. ii) -most are held in flocs by naturally produced organic polymers and electrostatic forces

Primary consumers of organic wastes heterotrophic bacteria (dominant community)Majority of the bacterial are Gram-negative e.g. genera Pseudomonas, Arthrobacter, Comamonas, Lophomonas, Zoogloea etc.Other bacteria: Micrococcus, Arthrobacter, Coryneform bacteria, Mycobacteria and Sphaerotilus spp.

Protozoa serve as an indicator of process performance.Rotifers, nematodes and other multicellular forms roles in process not obviousBacterial virus or phage role not well documentedSome bacteria e.g. Zooglea ramigera produce slimes that held the microbial biomass together, forming flocsThese flocs are removed from suspension by settling A portion of the settled sludge is recirculated back to the aeration tank

However, the effectiveness of sludge settling depends on the microbial composition which is affected by Food to Microorganisms Ratio and toxicity of the wastewater

Plug floworiginal or conventional activated sludge systems in which the wastewater enters at one end and exits through the otherWater/sludge mixture moving constantly through an aeration tankAlong the flow is a gradient in substrate concentration and oxygen demand

Step-aerationAlso called step feeding (a more accurate descriptor), developed to circumvent some of the problems of plug-flow approachImproves the concerning O2 demandKeep O2 demand uniform by adding the waste water step wiseProduces well settling sludge

Complete-mixCSTR with settling and recycle, evolved in the 1950sSimplest lay out for activated sludge processultimate approach for spreading the wastewater throughout the treatment systemThe disadvantage removal efficiency for an individual organic compound is not as high as in a well operating plug-flow system in which high concentrations are not a problem

Contact stabiliztionpermits high efficiency treatment to occur in a significantly reduced total reactor volumeWaste water is mixed in a contact reactor short detention time (15-60 min) for oxidation Enters a stabilization tank after the settler tank (for volume saving)Disadvantage requires substantially more operational skill and attention

Activated sludge systemHas a selector before the aeration tankTo overcome the sludge bulking or sludge that does not compact well in the settling tankSludge bulking poor settling characteristics in sedimentation tanks caused by the growth filamentous bacteria and algae.Selector change or select the ecology of the activated sludge system towards organisms with good settling characteristic

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