Aerobic digestion of excess activated sludge may be considered to be a continuation of the activated sludge process. Figure 4.1 illustrates this process graphically. When the soluble substrate (food) is completely oxidized by the microbial population, the microorganisms begin to consume their own protoplasm to obtain the energy for cell maintenance. This phenomenon of
obtaining energy from cell tissue, known as endogenous respiration, is the major reaction in aerobic digestion.
The cell tissue is oxidized aerobically to carbon dioxide, water, and ammonia. As the digestion process proceeds, the ammonia is subsequently oxidized to nitrate. In actuality, only about 65 to 80% of the cell tissue can be oxidized. The remaining 20 to 35% is composed of inert components and organic compounds that are not biodegradable. The material that remains after completion of the digestion process is at such a low-energy state that it is essentially biologically stable. The biooxidation of biomass results in the reduction of the volume of residual solids requiring disposal. However, this objective of volume reduction has not been fully realized in many facilities because of the problems with effective dewatering of the digested biosolids.
The first step of the aerobic digestion process, direct oxidation of biodegradable matter, can be illustrated by the equation organic matter + O2 bacteria > cellular material + CO2 + H2O (4.1)
The formula for the cell mass of microorganisms is C5H7O2N. The second step, endogenous respiration, can be illustrated by the following equations (Enviroquip, 1997):
Destruction of biomass:
Nitrification of released ammonia nitrogen:
NH+ + 2O2 ^ NO3 + 2H + + H2O (4.3) Overall equation with complete nitrification:
C5H7O2N + 7O2 ^ 5CO2 + 3H2O + HNO3 (4.4) Using nitrogen as an electron acceptor (denitrification) :
C5H7O2N + 4NO- + H2O ^ NH+ + 5HCO- + 2NO2 (4.5) With complete nitrification/denitrification:
2C5H7O2N + 11.5O2 ^ 10CO2 + 7H2O + N2 (4.6) With partial nitrification:
In the destruction of biomass [equation (4.2)], oxygen is used to oxidize cell mass to carbon dioxide and water. This reaction also produces ammonium bicarbonate. The important thing here is that it is NH3 that is released, which then combines with some of the CO2 that is produced to form ammonium bicarbonate.
In the second reaction [equation (4.3)], nitrification of released ammonia creates nitrate and 2 mol of acidity, which is shown as 2 mol of hydrogen ion (H+). Therefore, the destruction of biomass produces 1 mol of alkalinity as ammonium bicarbonate, then nitrification destroys 2 mol of alkalinity. Complete oxidation and nitrification take 7 mol of oxygen, as shown in equation (4.4).
If the oxygen in the nitrate is used as the oxygen source just as is done in liquid stream processes, denitrification is possible. This is shown in equation (4.5): biomass plus nitrate producing ammonia, nitrogen gas, and alkalinity in the form of bicarbonate. So, taken together, biomass destruction and deni-trification produce alkalinity, and nitrification consumes alkalinity.
Theoretically, approximately 50% of the alkalinity consumed by nitrification can be recovered by denitrification. If the dissolved oxygen is kept very low (less than 1 mg/L), nitrification will not occur. Therefore, cycling of the aerobic digester between aeration and mixing by mechanical means can be effective in maximizing denitrification while maintaining pH control. The complete nitrification-denitrification equation (4.6) shows a balanced stoi-chiometric equation. If all the ammonia released is nitrified and denitrified, there is balance. Biomass plus oxygen is converted to carbon dioxide, nitrogen gas, and water, with no net consumption of alkalinity.
What is often seen in aerobic digestion is in fact partial nitrification; that is, a portion of the nitrogen is being left as ammonia. The system will nitrify until the pH drops enough that it begins to inhibit the nitrifying bacteria. This is illustrated by equation (4.7). This is what is normally seen in many digesters, a mixture of both ammonia and nitrogen. This would be the case where there is only an inconsequential amount of alkalinity in the sludge that is fed to the digester. In situations where the buffering capacity is insufficient, resulting in pH depression below 5.5, it may be necessary to feed chemicals such as lime to maintain the desired pH.
When WAS is aerobically digested, the predominant phase that is maintained is endogenous respiration. However, if primary sludge is included in the process, the overall reaction can shift to a lengthy phase of direct oxidation of biodegradable matter. Most of the organic matter in primary sludge becomes the external food source for the active biomass in the biological sludge. Therefore, longer detention times are required to accommodate the metabolism and cellular growth that must occur before endogenous respiration conditions are achieved.
Theoretically, about 1.5 kg of oxygen is required per kilogram of active cell mass (1.5 lb/lb) in nonnitrifying systems, whereas about 2 kg of oxygen per kilogram of active cell mass (2 lb/lb) is required in nitrifying systems. In a system with complete nitrification-denitrification, it provides the opportunity to (1) reduce the oxygen requirements (a 17% reduction), (2) avoid alkalinity depletion because the alkalinity produced in denitrification is used to offset the existing alkalinity that is required for nitrification, and (3) and nitrogen is removed. The third benefit may or may not be of consequence. However, the first two result in savings in operating costs.
The oxygen requirements for mixed primary and activated sludge digestion are substantially greater than what is required simply for activated sludge digestion because of the longer time required to oxidize the organic matter in primary sludge. The oxygen uptake rates in the digestion process vary depending on the characteristics of the feed sludge. Thickened sludge has a very high oxygen uptake rate at the beginning of the digestion process. An adequate amount of air should be supplied to the digester to keep the solids in suspension and to avoid their settling to the bottom of the tank. An air supply rate of 1 m3/m3-h (17 cfm per 1000 ft3) is adequate to keep the solids in suspension, although some state standards might dictate much higher air requirements for mixing (Turovskiy, 2001).
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