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Digested sludge

Thermophilic 'Gas PPass'

(Methanogenesis)

Thermophilic 'Gas PPass'

(Methanogenesis)

Acid-Gas Phased

Digested sludge

Figure 16.46 Acid-gas phased anaerobic sludge digestion process schematics.

being well stabilized, but produces much less of an odor problem than that of anaerobi-cally digested sludge. The main disadvantages of aerobic digestion is the significant energy cost of operation; the loss, compared to anaearobic digestion, of the energy value of the waste; and the production of high levels of oxidized nitrate-nitrogen, which may complicate subsequent land application options.

Compared to the complicated process of anaerobic digestion, aerobic sludge digestion represents a far more simplistic and in large measure a single-step process. However, there are still several important biological issues with aerobic digester operations that must be considered and accommodated. The level of oxygen uptake maintained within these reactors provides a useful indication of the level of organic solids stability and digestion efficacy. This parameter is quantified as a specific oxygen utilization rate (SOUR) by which the measured oxygen uptake rate is divided by the existing volatile suspended solids concentration to derive the specific (i.e., per mass of solids) value. The generally accepted benchmark for suitably stabilized biosolids is a SOUR value of no more than 1.5 mg of oxygen consumed per gram of total solids per hour.

The pH levels with standard, mesophilic aerobic sludge digestion operations can typically be expected to drop with time given the fact that the organic nitrogen released from

Figure 16.47 Aerobic sludge digestion system.

the degrading cells will hydrolyze to ammonia and be nitrified. This oxidative transformation releases 2 mol of hydrogen ions for every mole of oxidized nitrogen, which corresponds to an alkalinity consumption rate of 7.14 mg alkalinity (as CaCO3) per milligram of fully oxidized ammonia-nitrogen. With incoming solids levels measured in percentile figures, and with commensurate ammonia-nitrogen releases of 1000+ mg N/L, it conceivable that this sort of pH drop could well shift the reactor to a sufficiently low level (i.e., <pH 5.5) that this nitrification process would actually be discontinued.

Without this sort of pH disruption, though, the effluent nitrate-nitrogen levels commonly observed in aerobically digested biosolids residuals would be quite high (measured in 100s if not even the 1000+ mg NO3-N/L range). The problem posed by these high residual nitrate levels is that unlike ammonium-nitrogen, NO3 -N has no cationic affinity for soils. As a result, subsequent land application of these aerobically digested solids will require close attention to the potential migration of these nitrates to the groundwater (for which the U.S. EPA's Safe Drinking Water Standard is 10 mg NO3-N/L).

The regulatory stipulations on aerobic digester holding times (i.e., hydraulic retention times) relative to temperature stem from the necessity to maintain suitable reductions in pathogens. However, at the mesophilic conditions under which most aerobic digesters are maintained, retention times on the order of multiweek to multimonth periods are not uncommonly necessary to achieve volatile solids destruction levels up to or beyond the 38% range [i.e., the benchmark national standard for vector attraction reduction (VAR) with stable biosolids established by 503 Rule federal regulations (i.e., Code of Federal Regulations, 1993)]. Furthermore, during cold-weather periods, and with a corresponding drop in aerobic digestion efficacy, even longer periods will often be necessary.

As a proactive approach to escalating the standard aerobic digestion process, therefore, this technology has followed a trend analogous to that of the newer anaerobic schemes whereby reactor temperatures are increased to secure higher metabolic rates. This new design strategy, commonly known as autothermal thermophilic aerobic digestion (ATAD) (Figure 16.48) involves reactor temperatures starting in the mid-50°C range and in most cases reaching the 60+ °C range, at which point these processes experience far faster rates of lysis and oxidation. In addition, these ATAD operations also realize a sizable acceleration in their rates of overall disinfection.

Relatively little is known as yet about the microbial character and makeup of these types of systems, but it does appear that they may offer several new avenues for degrading a number of organic, possibly even hazardous compounds in addition to that of conventional wastewater sludge. In both cases, the levels of oxygen tension at which these high-temperature systems appear to operate tends to be much lower than what is usually seen with standard aerobic treatment reactors (e.g., ^0.5 mg of dissolved oxygen per liter with ATAD vs. 2+ mg/L in standard mesophilic aerobic digesters). In fact, at these DO levels, it is rather likely that the microbial consortia involved includes both aerobic and quasi-anaerobic microbes which live and work in metabolic harmony. Rather interestingly, the issue of nitrate buildup and release is also a moot concern at these thermophilic levels. Not only do the autotrophic bacteria responsible for nitrification effectively stop working at temperatures of about 40°C, but there is also a pronounced tendency to volatilize free

Off-gas

Off-gas

Figure 16.48 Autothermal thermophilic aerobic sludge digestion process schematic.

Figure 16.48 Autothermal thermophilic aerobic sludge digestion process schematic.

ammonia (NH3) at this thermophilic level. Off-gas treatment (e.g., biofiltration) may, however, be required to deal with this released ammonia, let alone the release of other odorous compounds (e.g., reduced sulfur gases such as mercaptans and hydrogen sulfide).

Aerobic digestion is widely used as a sludge stabilization process at smaller wastewater plants (i.e., at flow rates below about 20,000 m3/day). Batchwise operating regimes are common, but it is also possible to use intermittent batch (with cyclic settle, decant, and refill steps) and continuous-flow formats as well. Compared to anaerobic digestion, this aerobic option tends to be easier to maintain, and in the case of mesophilic systems has considerably less potential to release troublesome odors. These aerobically digested solids are also biochemically stable (i.e., resistant to further decay) and low in residual ammonia-nitrogen.

The key metabolic factor with aerobic digestion is that of the endogenous decay rate of the solids involved, which for most cells is in the neighborhood of 0.05 day-1 (i.e., 5% solids decay/day) at a temperature of 20°C. Of course, this rate of decay varies according to temperature and the degradable characteristics of the processed solids.

The design criteria typically used for sizing these units is therefore that of the retention time for these solids, for which values of 20+ days would accordingly provide volatile solids reductions of approximately 63% (37% remaining) when completed under batch conditions:

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