Aerobic digestion is an extension of the activated sludge aeration process whereby waste primary and secondary sludge are continually aerated for long periods of time. In aerobic digestion the microorganisms extend into the endogenous respiration phase. This is a phase where materials previously stored by the cell are oxidized, with a reduction in the biologically degradable organic matter. This organic matter, from the sludge cells is oxidized to carbon dioxide, water and ammonia. The ammonia is further converted to nitrates as the digestion process proceeds. Eventually, the oxygen uptake rate levels off and the sludge matter is reduced to inorganic matter and relatively stable volatile solids.
The primary advantage of aerobic digestion is that it produces a biologically stable end product suitable for subsequent treatment in a variety of processes. Volatile solids reductions similar to anaerobic digestion are possible. Some parameters affecting the aerobic digestion process are:
1. The rate of sludge oxidation,
2. sludge temperature,
3. system oxygen requirements,
4. sludge loading rate,
Chemical stabilization is a process whereby the sludge matrix is treated with chemicals in different ways to stabilize the sludge solids.
5. sludge age, and
Aerobic digestion has been applied mostly to various forms of activated sludge treatment, usually "total oxidation" or contact stabilization plants. However, aerobic digestion is suitable for many types of municipal and industrial wastewater sludge, including trickling filter humus as well as waste activated sludge. Any design for an aerobic digestion system should include: an estimate of the quantity of sludge to be produced, the oxygen requirements, the unit detention time, the efficiency desired, and the solids loading rate. Aerobic digestion tanks are normally not covered or heated, therefore, they are much cheaper to construct than covered, insulated, and heated anaerobic digestion tanks. In fact, an aerobic digestion tank can be considered to be a large open aeration tank. Similar to conventional aeration tanks, the aerobic digesters may be designed for spiral roll or cross roll aeration using diffused air equipment. The system should have sufficient flexibility to allow sludge thickening by providing supernatant decanting facilities. The advantages most often claimed for aerobic digestion are:
• A humus-like, biologically stable end product is produced.
• The stable end product has no odors, therefore, simple land disposal, such as lagoons, is feasible.
• Capital costs for an aerobic system are low, when compared with anaerobic digestion and other schemes.
• Aerobically digested sludge usually has good dewatering characteristics. When applied to sand drying beds, it drains well and redries quickly if rained upon.
• The volatile solids reduction can be equal to those achieved by anaerobic digestion.
Supernatant liquors from aerobic digestion have a lower BOD than those from anaerobic digestion. Most tests indicated that BOD would be less than 100 ppm. This advantage is important because the efficiency of many treatment plants is reduced as a result of recycling high BOD supernatant liquors. There are fewer operational problems with aerobic digestion than with the more complex anaerobic form because the system is more stable. As a result, less skillful and costly labor can be used to operate the facility. In comparison with anaerobic digestion, more of the sludge basic fertilizer values are recovered.
The major disadvantage associated with aerobic digestion is high power costs. This factor is responsible for the high operating costs in comparison with anaerobic digestion. At small waste treatment plants, the power costs may not be significant but they certainly would be at large plants. Aerobically digested sludge does not always settle well in subsequent thickening processes. This situation leads to a thickening tank decant having a high solids concentration. Some sludge do not dewater easily by vacuum filtration after being digested aerobically. Two other minor disadvantages are the lack of methane gas production and the variable solids reduction efficiency with varying temperature changes. In a typical plant operation the pollutants dissolved in the wastewater or that would not settle in the primary clarifiers flow on in the wastewater to the Secondary treatment process. Secondary treatment further reduces organic matter (BOD5) through the addition of oxygen to the wastewater which provides an aerobic environment for microorganisms to biologically break down this remaining organic matter. This process increases the percent removals of BOD and TSS to a minimum of 85 percent. A secondary treatment facility can be comprised of Oxygenation Tanks, Pure Oxygen Generating Plant, Liquid Oxygen Storage Tanks, Secondary Clarifiers, Return Sludge Pumping Station and Splitter Box, Sludge Thickeners and Pumping Station, Sludge Dewatering Building Addition and modifications to the existing Service Water Pumping Station. The Pure Oxygen Generation System often incorporates a pressure swing adsorption (PSA) system oxygen generating system A PSA system will provide a certain amount (as tons per day) of pure oxygen to the oxygenation system. As backup to the oxygen generating system, spare oxygen storage tanks containing liquid oxygen can be included in the design. Figure 5 illustrates what an aeration reactor looks like.
The oxygenation system is comprised of several covered oxygenation tanks, mechanical mixing system, and pressure-controlled oxygen feed and oxygen purity-controlled venting system. The primary effluent enters the head end of the tanks where it mixes with return activated sludge which consists of microorganisms
Air Requirement: 15 - 20 cfm per 1,000 cubic feet of digester capacity is adequate. The air supplied must keep the solids in suspension; this requirement may exceed the sludge oxidation requirement. A dissolved oxygen concentration of 1 to 2 ppm should be maintained in the aerobic digestion tanks.
Detention Time: Waste activated sludge only, after sludge thickening. 10 -15 days volumetric displacement time. If sludge temperatures are much less than 60°F, more capacity should be provided. Primary sludge mixed with waste activated or trickling filter humus. 20 days displacement time in moderate climates.
"activated" by the organic matter and oxygen. This combination of primary effluent and return sludge forms a mixture known as "Mixed Liquor". This mixed liquor is continuously and thoroughly mixed by the mechanical mixer in each tank. The oxygen gas produced in the PSA system is introduced into the first stage of each tank and then remains in contact with the mixed liquor throughout the oxygenation system. Secondary clarifiers like the one illustrated in Figure 6 are used in this process.
Once the mixed liquor goes through the complete oxygenation process, it flows to four secondary clarifiers where the biological solids produced during the oxygenation process are allowed to settle and be pumped back to the head of the system. These settled solids being pumped, called return activated sludge, mix with the primary effluent to become mixed liquor. Since the population of microorganisms is growing some microorganisms in the return activated sludge are removed from the system. This solids waste stream is called waste activated sludge ( WAS ) and flows to the secondary gravity thickener for solids processing . The cleaned wastewater flows over the weir of the secondary clarifier and on to the disinfection ( chlorination )-process. The activated sludge process describes is an aerobic, suspended growth, biological treatment method. It employs the metabolic reactions of microorganisms to produce a high quality effluent by oxidation and conversion of organics to carbon dioxide, water and biosolids (sludge). Basically the system speeds up nature and supplies oxygen so the aquatic environment will not have to. High concentrations of microorganisms ( compared to a natural aquatic environment ) in the activated sludge use the pollutants in the primary treated wastewater as food and remove the dissolved and non-settleable pollutants from the wastewater. These pollutants are incorporated into the microorganisms bodies and will then settle in the secondary clarifiers. Oxygen needs to be supplied for the microorganisms to survive and consume the pollutants.
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