Aerobic digestion is the most widely used sludge stabilization process in wastewater treatment plants that treat less than 20,000 m3/d (5 mgd). It is a process of oxidation and decomposition of the organic part of the sludge by microorganisms in special open or enclosed tanks with the presence of oxygen. It is a suspended growth biological process that is similar to the extended aeration-type activated sludge process. The process produces a stable product, reduces mass and volume, and reduces pathogenic organisms. Advantages of aerobic digestion compared to anaerobic digestion include:

• Lower capital cost

• Odorless end product

• Volatile solids reduction only slightly less than that obtained in anaerobic digestion process

• Easier to operate

• Operational control of volatile acids-alkalinity relationship not necessary

• Lower BOD, TSS, and ammonia nitrogen in supernatant liquor

• Suitable for digesting nutrient-rich sludge solids while recovering more of the basic fertilizer values

• Safer to operate with no potential for gas explosion and less potential for odor problems

Major disadvantages of aerobic digestion compared with anaerobic digestion include:

• The operating cost is higher, in the form of the power cost for supplying the oxygen required.

• Methane gas, a useful by-product, is not produced.

• The process is dependent on temperature.

• The efficiency of the process is reduced during cold weather.

• The performance is affected by the concentration of solids, type of sludge, location, and type of mixing-aeration system

As discussed in Chapter 1, aerobic digestion is one of the processes defined in Part 503 regulations to meet the PSRP requirements. According to the regulations, the solids retention time in aerobic digestion must be at least 40 days at 20°C and 60 days at 15°C to meet the class B requirements. Most older aerobic digestion facilities have less than 40 days of SRTs. Therefore, if these facilities need to meet the class B requirements for pathogen reduction, additional capacities have to be added to the digesters, or thickeners have to be built to thicken the feed sludge. However, those facilities that do not meet these SRTs can meet the class B requirements by monitoring the process and demonstrating that the pathogen reduction criterion has been met. Monitoring is also required to demonstrate that the volatile solids reduction requirements to comply with the vector attraction reduction criterion have been met.

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