Stepwise Growth Process Of Filamentous Organism

fioc particles (deflocculated)

Source: From Ref. 8.

Source: From Ref. 8.

above, result in a reduction in the SRT required to achieve good bioflocculation. This reduction is reflected in the typical SRT operating ranges presented in Figure 9.3.

Bioflocculation forms the microstructure of activated sludge floe.'1" The resulting floes are relatively weak, however, and can readily be broken into smaller particles by turbulence. Consequently, if bioflocculation is the only mechanism of floe formation, a variety of particle sizes will be present, ranging from large floes that settle rapidly to small particles that will not settle. A sludge of this type will compact well, but will leave a poor quality, turbid supernatant. A filament backbone is necessary to provide strength to the floe, resulting in a rapidly settling floe that is also strong enough to produce a clear supernatant.

Role of Filamentous Bacteria. The relative proportion of floc-forming and filamentous bacteria in floe determines its macrostructure,"1 as illustrated in Figure 10.9.^ In an ideal activated sludge floe (Figure 10.9a), the filaments provide a strong backbone around which the well flocculated bacteria grow. This results in a large, dense, compact floe that settles rapidly and compacts well in the clarifier. A clear supernatant is also produced since few small, slowly settleable particles are present. An activated sludge composed of such floes will have a low SVI, typically less than 100 mL/g. Figure 10.9b illustrates pin-point floe as described above, consisting primarily of individual floe particles with little or no filamentous bacteria present to provide floe strength. Figure 10.9c illustrates a filamentously bulking sludge. Excessive numbers of filamentous bacteria are present, causing them to extend beyond the activated sludge floes. The resulting floes are strong and produce a clear supernatant when they settle. However, because they are large and because the filamentous bacteria extend beyond them, the floe particles settle slowly and compact poorly. The slow settling rate negatively impacts the capacity of the clarifier, and the poor compaction results in a dilute settled sludge for recycle to the bioreactor.

Figure 10.10 illustrates that the extension of filaments beyond the floe particles results in poor sludge settling characteristics as measured by the SVI test/" It also illustrates why an SVI of 150 mL/g is used as an indicator of the onset of filamentous bulking. Beyond that point small increases in extended filament length lead to large increases in SVI, indicating significant deterioration of sludge settling and compaction characteristics. Although not shown by the figure, both impacts of filament

Activated Sludge Formation

Figure 10.9 Effect of filamentous growth on activated sludge structure: (A) ideal, non-bulking activated sludge floe; (B) pinpoint floe; (C) filamentous bulking activated sludge. (From D. Jenkins, M. G. Richards, and G. T. Daigger, The Causes and Cures of Activated Sludge Bulking and Foaming, 2nd ed.. Lewis Publishers, Ann Arbor. Michigan, 1943. Copyright ■£> Lewis Publishers. Reprinted with permission.)

Figure 10.9 Effect of filamentous growth on activated sludge structure: (A) ideal, non-bulking activated sludge floe; (B) pinpoint floe; (C) filamentous bulking activated sludge. (From D. Jenkins, M. G. Richards, and G. T. Daigger, The Causes and Cures of Activated Sludge Bulking and Foaming, 2nd ed.. Lewis Publishers, Ann Arbor. Michigan, 1943. Copyright ■£> Lewis Publishers. Reprinted with permission.)

growth, expansion of the activated sludge floe and filament extension beyond the Hoc particle, negatively affect sludge settling and compaction.

The conceptual model presented in Figure 10.9 allows a more complete analysis of the results presented in Figures 10.7 and 10.8, and in Table 10.4. When the CMAS systems were operated at SRTs between 0.25 and 2 days, they produced a large proportion of dispersed growth, corresponding to failure to attain adequate biofloc-culation. At SRTs between 2 and 9 days, well formed, average size floes of low to medium density were produced, reflecting a balance between floc-forming and filamentous bacteria. Operation at SRTs between 9 and 12 days produced irregularly shaped pin point floe, which typically result from an inadequate proportion of filamentous bacteria. Thus, it seems likely that the continuous increase in settling ve-

Extended Filament Length, p m/mL

Figure 10.10 Effect of extended filament length on SVI. (From J. C. Palm. D. Jenkins, and D. S. Parker, Relationship between organic loading, dissolved oxygen concentration and sludge settleability in the completely-mixed activated sludge process. Journal, Water Pollution Control Federation 52:2484-2506, 1980. Copyright © Water Environment Federation. Reprinted with permission.)

Extended Filament Length, p m/mL

Figure 10.10 Effect of extended filament length on SVI. (From J. C. Palm. D. Jenkins, and D. S. Parker, Relationship between organic loading, dissolved oxygen concentration and sludge settleability in the completely-mixed activated sludge process. Journal, Water Pollution Control Federation 52:2484-2506, 1980. Copyright © Water Environment Federation. Reprinted with permission.)

locity with SRT in Figure 10.8 was a result of a continuous decrease in the proportion of filamentous bacteria. Although observations such as these have led to the recommendation that SRTs be maintained in a moderate range, say from 3 to 15 days, operation outside of that range is entirely feasible, provided an appropriate balance is maintained between the floc-forming and filamentous bacteria.

Types of Filamentous Bacteria and Their Control. The preceding discussion emphasizes the need to control the relative populations of floc-forming and filamentous bacteria in activated sludge systems. As seen in Section 2.3.1, many types of filamentous microorganisms can exist in activated sludge systems, and the most common are listed in Table 2.1. Consequently, efforts to control their growth require knowledge of the particular type of filamentous bacteria that could potentially be present. Fortunately, the conditions that favor the growth of many of them are known, as shown in Table 2.2. This link between a specific environmental condition and a particular type of filamentous organism can be used to identify and correct activated sludge settling problems, as suggested by Table 10.5.

Individual types of filamentous bacteria have high affinities for different limiting nutrients, allowing them to out-compete floc-forming bacteria for them. Some filamentous bacteria have a high affinity for dissolved oxygen, some have a high affinity for readily biodegradable organic matter, and others have a high affinity for nitrogen and phosphorus. Furthermore, as indicated in Table 2.2, the filamentous bacteria Thiothrix, Beggiatoa, and 02IN can also obtain energy from the oxidation of hydrogen sulfide, which provides a further advantage for them when it is present. Low pH will encourage the growth of filamentous fungi. Consequently, the key to

Table 10.5 Proposed Filamentous Organism Groups

Group I—Low DO Aerobic Zone Growers

Features • readily metabolizable substrates

Organisms S. nutans, Type 1701, H. hydrossis Control • aerobic, anoxic, or anaerobic selectors

• increase aeration basin DO concentration

Group II — Mixotrophic Aerobic Zone Growers

Features • readily metabolizable substrates, especially low molecular weight organic acids

• moderate to high SRT

• sulfide oxidized to stored sulfur granules

• rapid nutrient uptake rates under nutrient deficiency Organisms Type 02IN, Thiothrix spp.

Control • aerobic, anoxic, or anaerobic selectors

• nutrient addition

• eliminate sulfide and/or high organic acid concentrations (eliminate septicity)

Group III—Other Aerobic Zone Growers

Features • readily metabolizable substrates

• moderate to high SRT Organisms Type 1851, N. iimicola spp.

Control • aerobic, anoxic or anaerobic selectors

Group IV—Aerobic, Anoxic, Anaerobic Zone Growers

Features • grow in aerobic, anoxic and anaerobic systems

• possible growth on hydrolysis products of particulates Organisms Type 0041, Type 0675, Type 0092, M. parvicella Control Largely unknown but:

• maintain uniformly adequate DO in aerobic zone and stage the aerobic zone

Adapted from Jenkins, et al."

controlling the growth of filamentous organisms is to control the concentration of the growth limiting nutrient. It is generally desirable to use permanent filament control methods such as those listed in Table 10.5. However, in certain instances it may be more economical to use nonspecific toxicants such as chlorine or hydrogen peroxide to control filament growth. The use of such techniques is discussed in Section 10.4.3.

The general objective of the activated sludge process is to remove biodegradable organic matter. This is achieved by creating conditions in which it is the limiting substance. Consequently, the presence of filamentous bacteria with a high affinity for nitrogen, phosphorus, or dissolved oxygen (Groups I and II) indicates that these nutrients may be limiting bacterial growth. The solution to problems caused by excessive growths of these filamentous bacteria is addition of the limiting nutrient. For nitrogen and phosphorus, residual concentrations of approximately 1 mg/L are desired. For DO, the required residual concentration is a function of the process loading factor, as illustrated in Figure 10.11/" This relationship exists because DO concentrations are measured in the bulk solution while bacterial growth occurs within the floe particle. As the process loading factor is increased, the biomass uses oxygen at a faster rate and a higher bulk DO concentration is required to ensure the penetration of DO throughout the floe particle.

Control of some filamentous bacteria results from an understanding of the relative growth kinetics of filamentous and floc-forming bacteria. Figure 10.12 illustrates the typical relationship. In general, for a particular substrate, floc-forming bacteria have higher (1 and Ks values than filamentous bacteria. In other words, the floe formers can grow faster when the substrate concentration is high, but the filamentous bacteria have a higher affinity for the substrate and can grow faster when its concentration is low. For example, if the substrate concentration is S, in Figure 10.12, the specific growth rate of the floc-forming bacteria is higher than that of the fila-

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