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1、3 Biological Wastewater TreatmentApplication of microbial degradation and removal of undesirable constituents in industrial and municipal wastes is not a new concept. It is commonly used process for general wastewater treatment activities and has been for many years. As the awareness of chemical con
2、tamination of the environment, much research on biological degradation of toxic chemicals has occurred. Among the range of treatment technologies, biological degradation ranks among the most effective. Its management and application is enhanced by the potential to apply biological treatment in seque
3、nce with other chemical and thermal processes.Activated sludgeThe activated sludge process is s typical type of suspended growth biological treatment system and probably the most widely used biological process for the treatment of organic and industrial wastewaters. However, it can only treat aqueou
4、s organic wastestreams having less than 1% suspended solid content, and can not tolerate shock loading of concentrated organics. Therefore, the wastestream entering this process will usually have passed through a pretreatment process which includes a clarifier (primary clarifier) and an equalization
5、 basin. The primary clarifier is used for removal of grit, oily and fatty material and gross solid material, while the equalization basin is used to dampen wastewater flow variations and to provide more uniform organic loading to the activated sludge system.Activated sludge process are used to treat
6、 municipal and industrial wastes since they are versatile, flexible, and can be used to produce an effluent of desired quality by varying process parameters. The process was so-named because it produces an active mass of microorganisms capable of aerobically stabilizing a waste. Many versions of the
7、 basic process exist but all are fundamentally similar.The term activated sludge is applied to both the process and to the biological solids in the treatment unit. The mixed liquor suspended solids or activated sludge contains a variety of heterotrophic microorganisms such as bacteria, protozoa, fun
8、gi, and larger microorganisms. The predominance of a particular microbial species depends upon the waste that is treated and the way in which the process is operated.The activated sludge process is currently the most widely used biological treatment process. This is partly the result of the fact tha
9、t recirculation of the biomass, which is an integral part of the process, allows microorganisms to adapt to changes in wastewater composition with a relatively short acclimation time and also allows a greater degree of control over the acclimated bacterial population.An activated sludge system consi
10、sts of an equalization basin, a settling tank, an aeration basin, a clarifier, and a sludge recycle line. Wastewater is homogenized in an equalized basin to reduce variations in the feed, which may cause process upsets of the microorganisms and diminish treatment efficiency. Settleable solids are th
11、en removed in a settling tank.Next, wastewater enters an aeration basin, where an aerobic bacterial population is maintained in suspension and oxygen, as well as nutrients, are provide. The contents of the basin are referred to as the mixed liquor. Oxygen is supplied to the aeration basin by mechani
12、cal or diffused aeration, which also aids in keeping the microbial population in suspension. The mixed liquor is continuously discharged from the aeration basin into a clarifier, where the biomass is separated from the treated wastewater. A portion of the biomass is recycled to the aeration basin to
13、 maintain an optimum concentration of acclimated microorganisms in the aeration basin. The remainder of the separated biomass is discharged or “wasted”, The biomass may be further dewatered on sludge drying beds or by sludge filtration to disposal. The clarified effluent is discharged.The recycled b
14、iomass is referred to as activated sludge. The term “activated” is used because the biomass contains living and acclimated microorganisms that metabolize and assimilate organic material at a higher rate when returned to the aeration basin. This occurs because of the low food-to-microorganism ratio i
15、n the sludge from the clarifier.For the treatment of industrial wastewater, supplemental nutrient sources are often needed to provide sufficient nitrogen and phosphorus. In most cases, nitrogen is added as ammonia and phosphorus as phosphoric acid. A proper pH range (6 to 8) and s sufficient dissolv
16、ed oxygen concentration (a minimum of I to 2mg/L) must also be maintained in the aeration basin to support a healthy and active system.The aeration basin hydraulic retention time (HRT) and sludge residence time (SRT) are important operational factors. HRT is defined as the ratio of the volume of aeration tank to the influent liquid flow rate, and SRT is the total amount of sludge in the system divided by the rate of sludge leaving the system as waste. Sufficient time mus
