H3C - CH2 - CH2 - CH2 - SH
approach toward infrequent, intermittent loadings might impose a "feast-to-famine" regime that would then tend to disturb the desired metabolic harmony considered optimal for anaerobic digestion.
Taking into account the intermittent nature of these incremental inputs, the recommended loading rates [in terms of the daily applied volatile suspended solids (VSS) load per unit tank volume] range from 1.6 to 4.8 kg VSS/m3 • day. Based on the typical
characteristics of municipal wastewater sludge, these loading rates correspond directly to hydraulic retention times of 15 to 45 days, at which point these systems should be able to maintain volatile solids reductions on the order of 45 to 50% while producing 0.75 to 1.2 m3 of methane-rich gas per kilogram of volatile solids removed. Federal stipulations for the operation of these systems in a fashion to reduce their pathogen content significantly mandates minimal solids residence times of 15 days at a temperature of 35 to 55°C, and 60 days for sludge digested at 20°C.
Example 16.5: Preliminary Anaerobic Digester Design After completing the preceding series of four optional wastewater treatment designs, the Deer Creek, Illinois, consulting engineer is then asked for another set of preliminary design estimates relative to sludge processing. In this example, a preliminary design is developed for an anaerobic digester as might be used in conjunction with a conventional activated sludge system. Completion of this preliminary design involves four basic assumptions regarding sludge production and character. First, conventional wastewater treatment facilities typically generate about 0.25 kg total dry suspended solids per cubic meter of processed wastewater. Second, in its original wet state, raw primary plus secondary sludge generally contains a rather small solids fraction, approximately 1.2% by weight, or 12 kg/m3. Third, raw sludge is commonly prethickened prior to digestion, and for this example a reasonable value of ^6% (60 kg/m3) will be assumed. Fourth, blended primary plus secondary wastewater sludge typically has a volatile solids fraction of ^70%, the remaining ^30% being inert solids.
Preliminary Design Details
Sludge solids mass and volume, reactor sizing, and retention time
Design average wastewater flow: 605.7 m3/day Estimated daily total sludge solids mass: 151.43 kg TSS/day
(605.7 m3/day)(0.25 kg/m3) Estimated raw wet sludge volume: 2.5 m3/day
Estimated daily volatile sludge solids mass: 106 kg VSS/day
Design anaerobic digester solids loading rate: 1 kg VSS /m3 • day Design anaerobic digester volume: 106 m3
( 106 kg/day) / ( 1 kg/m3 • day) Design anaerobic digester HRT: 42 days
1. The estimated raw wet sludge volume represents more than a 200-fold concentration of contaminants from the original was-tewater flow into this residual sludge stream (i.e., 2.5 vs. 605.7 m3/day, or a ratio of 1:~242).
2. The design solids loading rate represents a typical norm for conventional low-rate anaerobic digesters.
3. Here again, this single reactor design does not afford the desired system redundancy associated with multiple units.
Digester off-gas generated by these conventional high-rate operations typically contains about two-thirds methane and one-third CO2, with a net heating value of approximately 22,400 kJ/m3. By comparison, natural gas (i.e., a mixture of butane, methane, and propane) has an energetic value roughly 50% higher. Moderate-to-large wastewater treatment operations will, therefore, typically collect and use this gas to run boilers and internal combustion engines that then supply heat and power, although many smaller plants just burn their gas in a flare as a waste product. Prior to using this gas within a boiler or diesel engine, though, low-level gas contaminants such as water vapor and H2S will routinely be removed to minimize their corrosive impact.
Although the vast majority of anaerobic systems currently in use for sludge digestion qualify as high-rate operations, experimental study and limited full-scale evaluation have demonstrated that further improvements in process efficiency and reliability can be obtained with appropriate revisions in the reactor staging, temperature and/or solids contact scheme. For example, even as early as the 1930s, it was noticed that thermophilic reactor temperatures from 50 to 55°C might provide distinctly higher metabolic rates for anaerobic fermentation, thereby resulting in significant reductions with necessary detention times. Unfortunately, though, these early thermophilic studies also revealed corresponding problems with process instabilities (e.g., higher levels of offgas odor, as well as elevated foaming tendencies) that would largely negate any metabolic gains.
Recognizing these relative trade-offs (i.e., benefits vs. shortcomings) with mesophilic and thermophilic systems, several new schemes have been developed in recent years to combine the advantages of both. Two such temperature-phased processing options are shown in Figures 16.45 and 16.46, with one using a thermophilic-to-mesophilic sequence and the other using a mesophilic-to-thermophilic arrangement. In either case, the operational premise is that the first phase serves as a solids preprocessor, which then expedites the efficiency of the trailing, second phase. The resulting improvement in processing efficiency achieved with these temperature-phased designs is reflected in their reduced HRT requirements, which on average (at approximately 15 days) are both comparable to the lowest permissible values with standard high-rate designs.
Yet another version of these phase-separated design schemes was developed not only to provide different sequential reactor temperatures but also to secure a beneficial separation of the acidogenic and methanogenic reactions. These types of acid-to-gas phased processing options (Figure 16.45), are designed with even smaller hydraulic retention times (e.g., to 3 days) for the initial, "acid" phase, such that the slower-growing methanogens simply cannot be retained within the first reactor. At that point, therefore, the first reactor's hydrolytic and acidogenic roles are uncoupled from that of the concluding, second-phase methanogenic transformation, thereby promoting optimal conditions for each of these subdivided reactions.
This type of mesophilic-to-thermophilic acid-gas scheme has been used with considerable success on a full-scale level (i.e., at the Woodridge facility in DuPage County, Illinois) with less than a two-week total retention time. Such acid-gas-phased anaerobic digestion processes typically carry a far higher level of volatile acids (i.e., commonly ranging from 6000 to 8000 mg/L, and sometimes as high as 18,000 mg/L) within the first tank, and as a result the average pH in this reactor (~5.6) is considerably below the desired, let alone tolerable level for the methanogenic conversion desired. Another telltale indicator to the success of this two-phase digestion scheme is that the vast majority (typically, ^95%) of the system's overall gas production takes place within the concluding thermophilic reactor. Conversely, gas production, particularly that of hydrogen, takes place at only a nominal level within the first reactor, such that the acidogens are not constrained by the thermodynamic difficulties (i.e., at higher hydrogen levels) that might otherwise be imposed.
Further efforts to secure better, or more stable, performance with anaerobic digestion systems have also been made using changes in the mode of initial contact (e.g., with upflow passage through a fluidized bed of anaerobic granules) and/or final separation of the solid-liquid matrix (e.g., using mechanically induced separation as opposed to natural settling and flotation). In whatever fashion these systems might be constructed, energy savings represent a significant benefit with anaerobic digestion, including the
Temperature-Phased^ "Meso^Thermo" Total HRT 12-17 days.
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