"Operating SRTs necessary for nitrification vary with temperature: >4 days in summer, >8 days in winter. Source: Adapted from Metcalf and Eddy (2003).

"Operating SRTs necessary for nitrification vary with temperature: >4 days in summer, >8 days in winter. Source: Adapted from Metcalf and Eddy (2003).

The control strategies used with suspended-growth systems are more sophisticated than those used in attached-growth wastewater processes (Bailey and Ollis, 1977), including that of controlling the aeration plus clarifier recycle and wastage flow rates. The major operational objectives for suspended-growth systems, though, are to maximize effluent quality while minimizing costs for disposal of waste biomass or sludge (the production of which decrease with SRT) and energy used with mixing and aeration (which increase with SRT), and in this context, controlled biomass wastage represents the key control factor. In turn, there is a delicate balance with controlling SRT to maximize effluent quality while minimizing operating costs, to an extent that considerable experience is necessary for optimal process control given the dynamic impacts of seasonal temperatures, changes in wastewater character, and so on.

In general, control regimes designed to sustain higher rather than lower SRT conditions are often preferable, in that their correspondingly reduced (i.e., starvation) growth rates would correspond with low-level effluent substrate concentrations. As was the case with similarly starved biomass growth within adhesively bound biofilms, suspended-growth biomass maintained at these elevated SRT levels will also tend to produce beneficial, floc-forming exocellular polysaccharides.

In turn, the development of improved floc-forming biomass would be similarly beneficial in terms of desired settling performance. It is imperative that the suspended biomass be able to settle and compact effectively, forming a sludge blanket, so that these solids can be recycled back to the aeration tank rather than being lost in the settling tank's effluent. Settling behavior is, consequently, tracked in most systems. The oldest and still likely to be the most common method of monitoring settling is that of the sludge volume index (SVI). The SVI is defined as the volume in milliliters occupied by 1 g of sludge after 30 minutes of settling. A settling test is completed by adding mixed liquor to a settleo-meter (a vertical glass or plastic cylinder) or standard laboratory graduate cylinder (Figure 16.21). Although there are a number of variations, the basic test consists of allowing the sludge to settle for 30 minutes, then noting the volume of settled sludge [settled volume (SV)]. This value is then divided by the MLSS concentration:

An acceptable SVI value (i.e., producing good settling) is considered to be in the neighborhood of 100 mL/g. An SVI greater than about 200 indicates bulking conditions, where settling performance is less than satisfactory. Values in between must be judged depending on the design and operating conditions of a particular plant. The settled 1-L sample shown on the right-hand side of Figure 16.21, for instance, has a 30-minute settled volume of 250 mL, and with a solids concentration of 2.5 g/L, the corresponding SVI would be 100 mL/g. Although rather crude, this simple test provides an approximate feel for the settleability of these solids, and when tracked on a recurring basis, the operating staff can predictably qualify "good" vs. "bad" behavior.

Beyond the SVI test, though, suspended-growth reactors are monitored in a fashion that pays a higher level of attention to the biological aspects of the involved biomass than is the case with the various attached-growth processes, taking into account such issues as expected rates of cell growth, decay, and respiration, as well as the overall morphology and consortia maintained within the involved biomass. For example, dissolved oxygen levels in the aeration reactors are monitored routinely, both to ensure appropriate

Figure 16.21 Sludge volume index testing for suspended-growth biomass.

oxygen transfer and to track biomass activity qualitatively. More precise measurements of mixed-liquor oxygen utilization rate (OUR) levels are also completed at many facilities on a frequent basis, using respirometers to evaluate biomass oxygen uptake such that they might quickly pinpoint potential problems with incoming wastewater toxins or other problematic metabolic conditions.

In yet another attempt to upgrade their monitoring efforts, many suspended-growth operations have also adopted routine measures to examine their floc with a light microscope on a recurring basis. This effort can be used to track chronological changes in its overall appearance, conformation, and approximate average diameter of a system's floc particles. At the same time, density counts taken with other higher life-forms, including those for protozoans and rotifers, may be recorded to evaluate their relative presence, or absence, as this factor may also signal symptomatic changes.

Floc particles may, in fact, have less microbial and metabolic diversity than biofilms (which often contain both aerobic and anaerobic zones), but they do enjoy several apparent advantages in terms of potentially securing maximal degradation rates. First and foremost, the combined effects of submillimeter floc diameters and their inherently open structure as a whole means that their cells will have far closer proximity, and easier access, to substrates and oxygen coming from the surrounding bulk solution. In turn, it

Figure 16.22 Light microscope image of suspended-growth floc structure with Vorticella protozoans.

is highly likely that a far larger fraction of the biomass will be able to sustain oxidative, aerobic activity compared to that found in biofilms. Furthermore, this enhanced transport of substrates and nutrients into the floc would be mirrored by a comparable improvement in the release of unwanted metabolic products. Finally, the overall surface area of these floc particles, on a cumulative basis that factors in both outer surface and internal subsurface exposure, will be considerably higher than that of a comparably sized (similarly loaded) biofilm system. Here again, the higher surface area not only provides for better kinetics but also contributes to improvements in a range of supplemental impacts, including sorption, filtration, and chelation.

Most floc structures will also include any number of higher life-forms, including attached and free-swimming protozoans (Figures 16.22 and 16.23) and rotifers (Figure 16.24). These organisms are highly beneficial and symptomatic of desirable floc conditions in that their presence sets up a tiered food chain which at the bottom end helps to maintain a clean effluent by scavenging finer, unattached solids and free-swimming bacteria that might otherwise linger during clarification as a fine, turbid haze. The stalked protozoan species, Vorticella, seen in Figures 16.22 and 16.23 represents one such desirable scavenger, whose life-style and mode of operation is similar to that of a free-wheeling vacuum cleaner. This organism's contracted cilia, seen in top-down view in Figure 16.23, will open and extend outward into the bulk solution and then whirl rhythmically to create rotational fluid currents that pull unattached particulates in the bulk solution toward, and then into, this organism's mouthlike opening, where they are then ingested.

Problems with floc conformation and behavior may, however, be encountered, including various difficulties with settling or clarification. Table 16.5 provides a synopsis of these conditions. In fact, a considerable number of suspended-growth process problems are related to poor settling and clarification. Although settling is a purely physical step, it is largely floc microbiology and conformation that determines the settling characteristics of the sludge.

Figure 16.24 Light microscope image of suspended-growth floc structure with rotifer.
TABLE 16.5 Suspended-Growth Floc Problems and Possible Contributing Factors

Type of Floc Problem

Operational Difficulties

Possible Contributing Factors

Bulking floc

Poor thickening, resulting in

Low dissolved oxygen concentration

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