Factors Affecting Performance

Many factors affect the performance of anaerobic treatment systems. These range from process loading factors such as the SRT, VOL, and hydraulic loading rate; to environmental factors such as temperature, pH, nutrient supply, and the presence of toxics; to operational factors such as mixing and the characteristics of the waste being treated. Historically, the stability and performance of anaerobic treatment systems have been considered to be poor in comparison to aerobic systems. However, with improved understanding of the factors that affect their performance, it has been possible to obtain stable and reliable performance. Consequently, a thorough understanding of these factors is critical to successful design and operation.

13.2.1 Solids Retention Time

The role of the SRT in controlling the performance of anaerobic processes is discussed briefly in Chapter 9 and has been referred to in previous sections of this chapter. Solids retention time controls the types of microorganisms that can grow in the process and the extent to which various reactions will occur. While SRT is the fundamental control parameter, it is difficult to routinely determine it in some anaerobic processes. Determination of the SRT is straightforward in flow-through systems such as anaerobic digesters, where it simply equals the HRT. Its calculation is also straightforward in anaerobic contact systems; the rate of solids wastage is controlled to achieve the desired SRT in the same fashion as in the activated sludge process. The SRT can also be measured and controlled in fluidized and expanded bed systems. The bioreactor bed is sampled to determine the VS inventory, and the biomass control device is adjusted to achieve the desired SRT. The SRT can also be measured and controlled in UASB and hybrid UASB/AF systems, but more often solids are simply wasted to maintain a set level for the granular and flocculent sludge layers. While it is possible to determine the solids inventory in low-rate anaerobic processes, solids wastage control is less certain. Likewise, while it is possible to determine the biomass concentration in pilot-scale AF and DSFF systems by removing sections of the media, this is not a practical approach for routine operation of full-scale systems. Consequently, it is not generally possible to control the operating SRT in low-rate, AF, and DSFF systems. Instead, process control is achieved by controlling the VOL, as discussed in the next section.

When the SRTs in pilot-scale anaerobic treatment systems are calculated, it is not unusual to find values of 30 to 40 days, with some systems ranging up to over 100 days.:,,:"s Such values are significantly higher than required for wastewater treatment, and represent the accumulation of excess biomass. Experience indicates that very stable performance can be obtained from some anaerobic treatment systems, particularly if long SRTs arc used. It also indicates that anaerobic systems can be shut down for extended periods of time (up to several months) and that good performance can be restored shortly after they are restarted."1 V In spite of these desirable features, it is possible that these long SRTs represent underloaded systems that could have been constructed more economically using shorter SRTs, while still achieving acceptable performance.

One benefit of increased SRTs is increased hydrolysis and stabilization of particulate organic matter. This can be particularly important for the stabilization of certain types of wastewater solids. More information will be provided on this topic in Section 13.2.9.

13.2.2 Volumetric Organic Loading Rate

Even though the VOL is not a fundamental parameter determining the performance of anaerobic treatment systems, it is related to the SRT through the active biomass concentration in the bioreactor. It is also a relatively easy parameter to calculate, and it has been used historically to characterize the loading on anaerobic treatment systems. Knowledge of the VOLs that can typically be achieved for a particular process quantifies how effectively the bioreactor volume is being utilized. Used in this fashion, the VOL provides useful information for the design and operation of anaerobic processes. The volumetric organic loading rate, rvs, can be calculated in units of kg COD/(nv • day) as:

where (Ssi, + Xso) is the influent wastewater strength in g COD/L (kg COD/m). F is the influent wastewater flow rate in mVday, and V is the bioreactor volume in m\ Substitution of Eq. 4.15 into Eq. 13.1 relates the VOL to the HRT:

This shows that the VOL is inversely proportional to the HRT, as illustrated in Figure 13.18. As discussed throughout Section 13.1, VOLs typically range from 1 to 2 kg COD/(nrday) for low-rate processes to between 5 and 40 kg COD/(m'• day) for high-rate processes.

The SRT is defined by Eq. 5.1, just as for all other biochemical operations. However, we saw in Section 9.4.1 that it is often convenient to use the net process yield to relate the biomass inventory to the mass input rate of substrate and the SRT. Rearranging that equation and expressing the biomass concentration and net yield on a VSS basis gives:

10 15 20 25 30 HRT, days

Figure 13.18 Effects of HRT and influent wastewater concentration on the volumetric organic loading of an anaerobic process.

10 15 20 25 30 HRT, days

Figure 13.18 Effects of HRT and influent wastewater concentration on the volumetric organic loading of an anaerobic process.

Combining Eqs. 13.2 and 13.3 gives:

Thus, it can be seen that the SRT and the VOL are inversely proportional to each other. Equation 13.4 also shows that for a fixed SRT, the VOL is increased as the biomass concentration is made larger, thereby allowing the bioreactor to be made smaller.

A similar approach is used for solids stabilization systems, such as anaerobic digesters, except that the VOL is expressed in terms of the mass of volatile solids applied, rather than COD, typically having units of kg VS/(m!-day). Figure 13.18 also presents the relationship between the anaerobic process HRT, the influent volatile solids concentration, and the resulting volatile solids VOL. For single-stage, anaerobic digestion processes, i.e., systems operated without solids recycle, the HRT and the SRT are identical. In these instances, the volatile solids VOL simply indicates how effectively the digester volume is being utilized. For two-stage digestion systems, which incorporate solids recycle, the SRT will be greater than the HRT and the volatile solids VOL indicates both the anaerobic digester volume utilization efficiency and the overall process loading. In both instances volatile solids VOLs typically range from 2 to 6 kg VSS/(m,-day).","x72 ?"

Interestingly, experience indicates that a maximum COD stabilization activity of 1 kg COD/(kg VSS-day) is achieved in a wide variety of anaerobic treatment processes.2' Although higher values have been reported, especially in conjunction with the treatment of wastewaters rich in acetate, this value can be used to develop an initial estimate of the capability of a particular anaerobic process to stabilize organic matter.

13.2.3 Total Hydraulic Loading

In contrast to the suspended growth systems considered in Part II and the rest of Part III, some of the high-rate anaerobic processes are influenced by the total hydraulic loading (THL) applied to them. This is characteristic of the attached growth processes considered in Parts IV and V, and a detailed discussion of the effects of THL on them is presented in Chapters 16, 18, 19, and 21. This section presents the most important impacts of THL on UASB, AF, hybrid UASB/AF, DSFF, and FB/ EB processes. With the exception of UASB systems, all of these anaerobic processes contain attached growth biomass. However, because of the physical similarity between UASB granules and the bioparticles in FB/EB systems, UASB systems also behave like attached growth systems.

The THL is simply the total flow applied to the bioreactor (including recirculation) divided by the bioreactor cross-sectional area perpendicular to the flow. It is calculated as:

where FK is the recirculation flow rate and A,. is the cross-sectional area. The THL is a superficial velocity, i.e., a theoretical velocity based on the empty bed cross-sectional area.

The THL affects process performance in several ways. For upflow processes with sludge blankets, such as UASB, hybrid UASB/AF, and FB/EB systems, maximum allowable values of the THL correspond to the settling velocity of the particles to be retained in the bioreactor. If the THL exceeds these values, the particles will be washed out of the bioreactor. As a result, the desired biomass inventory and associated SRT cannot be maintained, and the process will fail. Procedures for calculating maximum THL values for FB/EB processes are presented in Section 18.2.2 and typical values are presented in Section 21.2.3.

For UASB and hybrid UASB/AF processes, the maximum allowable THL depends on the nature of the solids developing in the bioreactor/2'4'' For granular solids, the daily average THL should not exceed 72 m/day when treating fully soluble wastewater, and 24 to 30 m/day when treating partially soluble wastewater. The THL can be temporarily increased to 144 m/day for fully soluble wastewaters and 48 m/ day for partially soluble wastewaters. For flocculent solids, the daily average THL should not exceed 12 m/day and the maximum THL should not exceed 48 m/day. The factors that lead to development of granular versus flocculent solids are discussed in Section 13.2.9. Knowledge concerning appropriate values of the THL for these bioreactor types continues to evolve, and the reader is urged to consult the literature for the most recent information.

For some bioreactors, a minimum THL must be maintained for various reasons. For AF and DSFF processes a minimum THL is needed to achieve uniform distribution of flow across the bioreactor cross section to minimize short circuiting. For AF processes, values in the range of 10 to 20 m/day appear to be appropriate.7" For FB/EB processes, a minimum THL must be maintained to fluidize or expand the bed, as discussed in Chapters 18 and 21. As with the UASB and hybrid UASB/AF processes, THL criteria for these processes continue to evolve and the reader should consult the literature for further information.

THL constraints can affect the configuration of the anaerobic bioreactor The bioreactor cross-sectional area must be adjusted to produce THL values within the necessary range. In some instances, recirculation must be initiated to maintain minimum required THLs. The impacts of the THL constraints on the design of anaerobic processes is discussed briefly in Section 13.3.2 and in substantially more detail in Chapters 19 and 21 for other attached growth systems.

13.2.4 Temperature

As with all biological processes, the performance of anaerobic processes is significantly affected by operating temperature. Best performance is typically obtained by operation in the optimal region of one of the two higher temperature ranges, i.e, 30°C to 40°C for mesophilic or 50°C to 60°C for thermophilic, and most anaerobic processes are designed to do so. These two regions generally represent the optima for growth of the methanogens. Nevertheless, it is possible to grow methanogens at lower temperatures, provided that longer SRTs are used to compensate for the lower maximum specific growth rates. Although anaerobic activity can be sustained at temperatures approaching 10°C, operating temperatures in the 20°C to 25°C" range appear to be the lower limit from a practical perspective.17

Although the preceding paragraph focussed on methanogens, operating temperature affects hydrolytic and acidogenic reactions as well. For wastewaters consisting largely of simple, readily biodegradable organic matter, the effect of temperature on methanogenesis is the primary concern. However, for wastewaters consisting largely of complex organic compounds or particulate materials, the effects of temperature on hydrolysis and acidogenesis will be the primary concern. Table 13.4 presents |i and K, values for biodégradation of VFAs at temperatures of 25°C, 30°C, and 35°C. These data may be used to characterize the impact of temperature on the anaerobic biodégradation of simple organic compounds.

Figure 13.19 shows the combined effects of SRT and temperature on the anaerobic digestion of municipal primary solids. Essentially complete stabilization of biodegradable volatile solids is achieved at an SRT of 10 days when operating at a temperature of 35°C. A moderate increase in SRT to about 15 days is required when operating at a temperature of 25°C, but the stabilization is not complete as indicated by a residual VS concentration at SRT values as long as 60 days. The required SRT

Table 13.4 Average Values of Kinetic Parameters for Anaerobic Enrichment Cultures Grown on Various Volatile Fatty Acids'

Volatile fatty acid

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