Suspended-growth processes used for wastewater treatment, such as that shown in Figure 16.18, have largely surpassed the attached-growth options over the past halfcentury in terms of general application, to the point where they have become the accepted state-of-the-art, best available practice for wastewater processing. There are several reasons for this preference, covering both performance and economic issues. These systems do have a higher energy demand to aerate and mix their suspended biomass, but they have earned a higher degree of confidence in terms of expected performance levels. These suspended-growth processes, commonly referred to as activated sludge systems, also tend to have better economy of scale in construction costs, whereas attached-growth is often used in smaller installations, due to its ease of operation and simpler mechanical design.
The key biological structure within suspended-growth processes is that of an amorphous, clustered structure (see Figure 16.19) known as floc, whose constituent microbial population sustains a desired biochemical oxidation of wastewater contaminants. Like its biofilm counterpart, floc is composed of a complex aggregate of various bacterial and higher life biota, plus an added quantity of nonliving organic and inorganics material. Suspended-growth floc particles exist as free-floating structures whose physical, chemical, and biological exchange with the surrounding bulk solution functions in three dimensions, as compared to a biofilm sheet which is confined against the inert surface to which it is attached. Floc suspensions are referred to as mixed liquor, and the suspended solids concentration of this is denoted mixed liquor suspended solids (MLSS). These floc concentrations are usually maintained at total volatile suspended solids concentrations ranging from 1 to several grams per liter.
As was the case with biofilm development, the circumstance of having bacterial cells stick together in floc particles depends on starvation conditions which trigger an exocel-lular release of adhesive exocellular polysaccharides. In turn, individual cells are able to self-adhere or flocculate into larger, enmeshed floc clusters. Floc dimensions are considerably smaller than multimillimeter biofilms, though, with typical diameters of 250 to 500 mm, and this limit is probably controlled by shear forces encountered during routine mixing, aeration, and pumping. Given that floc biomass is composed largely of bacterial cells, the specific gravity of these particles also tends to be only slightly higher than water (about 1.03 to 1.05), such that these relatively light floc particles can be kept in suspension with only moderate agitation and aeration.
Flocculation behavior also plays a role in terms of bacterial survival as a whole. When an organism metabolically releases any substance outside its boundaries, there is a cost that must be accompanied by a return. Only under conditions of scarcity is this investment worthwhile for the microbe, and in the case of floc physiology the benefit is that of enhancing the entrapment of particulate substrates. The secretion of exocellular digestive
Recycled Solids Figure 16.20 Suspended-growth design schematic.
enzymes by a single cell could result in most of that enzyme uselessly diffusing away from the cell, such that a large fraction of any dissolved substrate formed by digestion of a particle would also diffuse away. While clustered within an aggregated floc community, though, bacteria are cooperating by releasing enzymes that benefit them collectively.
The physical structures used in suspended-growth or activated sludge systems include both a reactor basin and a settling tank (Figure 16.20). Within the reactor basin, biodegradable substrates are then absorbed by the floc particles, along with oxygen and other key metabolic elements (e.g., nutrients, trace metals), and used to sustain a complex set of catabolic and anabolic reactions by which the wastewater is then cleansed biochemically. In turn, a subsequent settling tank is used to settle suspended floc biomass in preparation for recycle or wastage.
To maintain desired aerobic conditions inside the reactor, oxygen must be added routinely either by introducing compressed air or oxygen via dispersed air bubbles or through various mechanical aeration mechanisms. Extending beyond these reactor and clarifier components and their affiliated aeration equipment, suspended-growth systems will also require a complementary set of valves, pipes, and pumps used to control the reactor feed, as well as additional hardware for clarifier underflow recycling and wastage plus effluent discharge.
The settling tank, also called the secondary clarifier, has one basic function, to separate the floc biomass from the effluent. This operation can be divided into two complementary functions: clarification and thickening. Clarification is intended to produce a clear effluent that is nearly free of suspended solids. Thickening is the production of a concentrated underflow sludge which is then pumped from the bottom of the clarifier. Most of the thickened sludge is returned to the aeration tank [as a recycle or return activated sludge (RAS) stream], but a portion of the sludge will also be wasted and removed from the process for further sludge processing. In turn, this wastage turns out to be the key control parameter for a suspended-growth process, which by varying the age of this biomass (its SRT) controls net growth rate, will then regulate overall operational efficiency.
In this context of qualifying discrete reactor and settling operations, though, it should be noted that some suspended-growth systems do not follow this convention. For example, sequencing batch reactor (SBR) systems use individual tanks as both reactors and clari-fiers, where aeration and settling phases are then sequenced during batchwise operating cycles. Another suspended-growth system, which is just beginning to see wider utilization in full-scale systems, is that of the membrane bioreactor (MBR), where semipermeable synthetic membranes are used for biomass separation in lieu of a settling tank. In these units, mixed liquor is passed across and through the membranes to obtain an effluent that is essentially free of all suspended solids, thereby directly retaining floc biomass within the aeration tank.
Numerous design options exist with respect to the manner in which the waste is introduced to these suspended-growth reactors, how the reactor system might be arranged in physically or temporally separated stages, the condition (i.e., aerobic, anoxic, anaerobic) of these stages, and the rates employed for hydraulic throughput and solids discharge. The vast majority of systems are maintained in a fully aerobic state for oxidative biodegradation of incoming organics, and in recent years an escalating number of these aerobic systems have also been used to maintain an additional oxidation of reduced nitrogen (ammonia and organic nitrogen) to nitrate by way of nitrification. In addition, there are also a variety of suspended-growth design options by which full nitrogen removal, including denitrification, as well as biological phosphorus removal can be achieved by, respectively, using an additional complement of suitably designed and controlled anoxic and anaerobic phases.
Six design variables are considered in the development of suspended-growth systems, including hydraulic residence time, solids residence time, specific substrate loading (mass substrate applied per mass cells per time), volumetric loading, reactor configuration, and recirculation rate (recycle flow relative to raw influent flow) ratio. Table 16.4 provides a general synopsis of the typical design criteria relative to four of the basic options for configuring these reactors, including conventional, high-rate, extended aeration, and nitrification systems. In addition, further engineering consideration must be given to the anticipated reactor solids levels, excess sludge production rates, solids-liquid separation methodologies, and overall environmental conditions (e.g., operating temperature).
The HRT and SRT were described in Section 16.1.1. The third variable, specific substrate loading, is often referred to in terms of a food-to-microorganism (F/M) ratio. Of course, the form of the food term will vary according to the purpose of the reactor and whether its intended goal was that of carbonaceous BOD, COD, ammonia-nitrogen, and so on, removal. Similarly, there are several techniques for quantifying "mass," but the most common procedure is to use mixed liquor volatile suspended solids (MLVSS). At steady state, the F/M ratio is inversely related to SRT. Specifying one is equivalent to specifying the other, and either one can be used for design purposes.
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