Adapted Irom Jenkins, et al "
Adapted Irom Jenkins, et al "
sludge settleability tests. The diluted SVI (DSVI) or the stirred SVI at 3.5 g/L (SSVL^) produce the most reproducible results. '' The SVI is correlated with activated sludge settling characteristics and the measured SVI can be used to determine allowable secondary clarifier suspended solids loading rates."'
Flocculation Theory. Flocculation is the first step in building the compact, readily settleable floe necessary to optimize activated sludge settling characteristics. Its absence results in dispersed growth, as discussed above. The mechanisms of bioflocculation are poorly understood, in spite of numerous studies, attesting to the complexity of the phenomenon. One point upon which there is agreement, however, is that exocellular polymers (ECP) are central to the aggregation of individual bacteria into floe particles.' ' Several types of ECPs are involved in bioflocculation. Polysaccharides have received the most study and are generally thought to be of major importance.' 71 Nevertheless, it now appears that proteins also play an important role."' Possible sources of ECP are formation by microbial metabolism, release by cell lysis, and the wastewater itself. ' Evidence for the role of the wastewater itself comes from the observation that flocculation in activated sludge systems treating industrial wastewaters, which contain a limited number of organic compounds, is often more difficult than in systems treating domestic wastewaters, which contain a rich variety of large molecular weight organic materials. Nevertheless, the most important sources of ECP are metabolism and cell lysis. Exocellular polymers are produced by both the protozoa'* and the bacterial"7', found in activated sludge, although the relative importance of the two is unknown. Evidence for the role of protozoa comes from the observation that good bioflocculation and low effluent suspended solids concentrations are correlated with their presence. It is also known that floc-forming bacteria produce ECPs that are involved in aggregation,' although the factors that cause ECP formation are still a matter of speculation."1
The mere presence of ECPs is not sufficient to ensure bioflocculation. Rather, the milieu within which the activated sludge is growing also has an impact. For example, both ionic strength75 and divalent cations1" 5"' ' play important roles. Bacteria are negatively charged. Consequently, the ionic strength must be sufficiently large to allow individual cells to approach closely enough together for bridging by the ECPs to occur, but not so large as to cause deflocculation. Furthermore, ionic strength will influence the conformation of the ECP. One study suggested that an ionic strength on the order of 0.005 to 0.050 resulted in optimum floe stability.*6 Because both the cell surface and ECPs are negatively charged, divalent cations are thought to act as bridges between the two, allowing aggregation to occur. Consequently, the proper level of divalent cations is essential. One study found that the minimum concentration of calcium and magnesium required to obtain an activated sludge with good settling properties was in the range of 0.7 to 2.0 meq/L of each (14 to 40 mg/L of calcium and 8 to 24 mg/L of magnesium)."' However, the actual concentration required in a particular facility will depend on the ionic strength of the wastewater. Furthermore, the ratio of divalent to monovalent cations is also important because when that ratio is less than 0.5, deterioration of the settling characteristics results.1" This is thought to be due to the competition between divalent and monovalent cations for binding sites on the cell surfaces and the ECP.
Empirical observations suggest that the SRT must exceed a minimum value to achieve bioflocculation. This observation is consistent with both the role of protozoa and ECP production by bacteria. Protozoa generally have a lower maximum specific growth rate than bacteria, and the SRT below which bioflocculation does not occur could correspond to the minimum SRT required for protozoan growth. Alternately, the observation that the amount of ECP per unit of biomass increases and approaches a maximum as the SRT is increased14"1 suggests that the requirement for a minimum SRT could represent a balance between the rate of ECP production by the floc-forming bacteria and the rate of generation of new surface area by bacterial growth. Consequently, although ECP is produced on a continuous basis, at short SRTs the rate of generation of new bacteria exceeds the rate of ECP production and bioflocculation is incomplete. Only by reducing the rate of generation of bacterial surface area relative to the rate of ECP production by increasing the SRT can effective bioflocculation be achieved.
Figure 10.7 presents data illustrating the impact of SRT on bioflocculation in a pilot CMAS system receiving a synthetic wastewater consisting of glucose, yeast extract, and inorganic nutrients.K The proportion of activated sludge suspended solids that did not settle under quiescent conditions is plotted as a function of the SRT. A high proportion of the activated sludge solids (10% to 30%) would not settle when the process was operated at SRTs between 0.25 and 0.5 days, but that fraction was significantly reduced when the process was operated at an SRT of 1 day and it remained low as the SRT was increased up to 12 days. Settling velocity also increased with increasing SRT, as illustrated in Figure 10.8.h Microscopic analysis of the biomass produced at each operating SRT provided the results presented in Table 10.4."
Based on data such as these, an SRT of at least 3 days is generally recommended for achieving good bioflocculation in activated sludge systems. Nevertheless, many municipal wastewater treatment plants have been successfully designed and operated at SRTs as low as one day.:5'° Apparently, the presence of microorganisms in municipal wastewaters, which alters the relationship between SRT and specific growth rate as discussed in Section 5.2.3, and the presence of ECPs, as discussed
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