Low pH



Thiothrix-like, Beggiatoa, 021N

Low N (or P)

S. natans

with canning operations), since domestic sewage usually has a considerable excess of nitrogen and phosphorus.

Yet another type of bulking, which is rather less common than the filamentous version, is caused by a particular type of floc structure largely composed of the bacterium, Zoogloea ramigera, and for this reason this problem is referred to as zoogloeal bulking. These flocs are characterized by branched, fingerlike projections that can have an effect on settling similar to that of other filaments. Whereas zoogloeal flocs are seen in small numbers in perhaps 10% of suspended-growth systems, they are present occasionally in excessive amounts and cause bulking. They are favored by high organic loading rates (high F/M, low SRT). At one time, early investigators thought that these zoogleal species were the dominant, and perhaps only, floc-forming microorganisms in suspended-growth systems. This assumption is now known to be incorrect, but the (improper) use of the term zoogloea in reference to all floc forms (rather than the specific unusual type of floc) persists among some in the field.

Note that not every filamentous type has been linked directly with a specific cause of bulking. Also, there is still some debate about some of the relationships. For example, one filament, generically referred to as type 021N, in addition to being a sulfide oxidizer, has been associated by some with low dissolved oxygen (DO) and by others with low F/M. Part of this disagreement may stem from the likelihood that some of the filamentous types established using only the microscope may actually be composed of more than one species. Future efforts to apply molecular biology tools will undoubtedly help to clear up some of this confusion.

Reactor design may also influence bulking. For example, with completely mixed reactor designs, wastewater and recycled sludge are vigorously mixed and rapidly distributed throughout the tank, such that the incoming wastewater is rapidly diluted as it enters the reactor. This circumstance then leads to uniformly low substrate concentrations within the aeration tank, a condition that favors the low-F/M filamentous bacteria. Bacteria living within these flocs would be at a disadvantage to the extended filaments, which reach out into the bulk solution to an extent where they have far higher access to substrates. By comparison, an alternative plug-flow reactor configuration has no such initial mixing, with wastewater and return sludge being introduced at the head end of the linear reactor and then flowing together down the length of the tank, or through several tanks in series. In turn, there is far less initial dilution of the incoming substrates, and the elevated initial substrate concentration facilitates a higher rate of diffusion into the floc to an extent which then negates the physical advantage of extended filament growth. However, plug-flow systems are more apt to experience higher initial oxygen uptake rates at the head end of their operations, and unless care is taken to meet this higher demand, there may still be a contrary opportunity for filament growth due to low-DO bulking. Yet another innovative approach to the control of filament bulking is to use a special reactor configuration designed specifically to optimize and select for the growth of floc-forming bacteria at the expense of filaments. These selector systems with suspended-growth processes are typically quite small, usually less than 1% of the total aeration tank volume and placed just ahead of the main aeration tank and then mixed and/or aerated to promote aerobic or anoxic conditions favorable to floc formers.

Another approach for control of nuisance filaments would be to apply chlorine or other strong oxidant (e.g., hydrogen peroxide) to the return sludge on an intermittent basis. Because the filaments are more exposed to the solution than are the floc formers, being extended outward from the floc into the surrounding bulk solution, they are also more susceptible to toxins such as these oxidants. However, the recurring necessity for using this method can be costly, and in the case of high-level chlorination there are also secondary concerns stemming from the potential formation of toxic trihalomethane compounds within the mixed liquor.

Contrasted with the circumstance of excess filaments and bulking, though, there are also problems with settling stemming from other difficulties tied to unusual floc conformations or problems. One such condition, referred to as pin floc or pin-point floc, stems from the absence of any filaments whatsoever. These small-diameter floc forms are affected particularly by high-level aeration, which imposes excessive shear forces leading to floc disaggregation. Since there is no "net" of filaments that would otherwise strain out these particles physically, pin floc end up getting left behind during settling and thus contribute to a higher-than-desired effluent suspended solids concentrations.

Undesired floc breakdown and dispersed growth can also be encountered in a variety of instances. High-level influent concentrations of salt or surfactants have been found to destabilize floc structures, and short SRT systems with high-rate bacterial growth rates have shown a tendency toward dispersed rather than flocculated growth, due to a lack of exocellular polymer release. Toxic shock impacts have also been proposed as a factor behind dispersed growth, where a toxic shock initially kills off a portion of an existing biomass, and then the remaining organisms grow extremely fast, in an unflocculated fashion, because of the reduced competition for substrates.

Foaming is yet another problem that can occur in mixed liquors. Occasionally, this may be the result of a nonbiodegradable surfactant (a surface-active agent) that enters the system. Other times, degradable detergents and other foaming materials (including proteins and DNA) that are present in the wastewater may not be degraded quickly enough and may lead to surface tension reductions that trigger the release of foam. This typically occurs either during startup of a plant and with very short SRTs, or after washout of sludge during a storm. Both of these types of foam are usually white and can be controlled with defoaming compounds or water sprays. The most common and serious foaming problems, however, are the result of excessive growths of certain gram-positive filamentous bacteria. These seem all to be actinomycetes, and most are highly branched. They were originally called Nocardia, but it is now recognized that they also include several related genera, including Gordonia, Skermania, Rhodococcus, and Tsukamurella. Occasionally, the bulking organism Microthrix parvicella (which is not branched) also causes this type of foaming. These bacteria seem to have a waxy coating, which makes it hard to rewet them once they get lifted into the foam layer. They are slow-growing and therefore are favored by longer SRT conditions. However, once they form a foam, they may accumulate on the surface of the aeration and/or secondary settling tanks to an extent that control of waste to achieve a desired SRT is extremely difficult. In severe cases, half of the system's biomass may actually be in the foam, since it gets so thick. The foam may even overflow the tank, or get pushed out by the wind, creating a slippery safety hazard on walkways as well as an aesthetic problem. The foaming bacteria appear to be able to utilize hydrocarbons and other fats and oils that may not readily be available to many of the other organisms present. Defoaming compounds are not effective in their control, and in some cases may even be utilized by the organisms as a carbon and energy source. Instead, a combination of decreased sludge age and physical removal of the foam (by skimming or vacuum) is generally used. Improved oil and grease removal in primary treatment, and spray chlorination of the foam, may also be beneficial.

Occasionally, sludge will settle well in the clarifier and then suddenly bob back up to the top. This results from gas bubble formation in the sludge blanket while it is sitting at the bottom of the settling tank. This is called rising sludge if the bubbles are composed of nitrogen gas (N2), formed as the result of denitrification. Since denitrification can take place only if nitrate and/or nitrite are present while oxygen is absent, this means that sludge rising can occur only if nitrification has first occurred in the aeration tank and then the sludge has gone anoxic in the settling tank. In cases where no nitrate or nitrite are present, anaerobic sludge can float to the top as a result of the production of other gases, especially hydrogen (H2). These problems can be controlled by increasing the rate at which sludge is returned to the aeration tank, so that settled biomass within the clar-ifier's sludge blanket does not have a chance to go anoxic or anaerobic.

Another important aspect in the microbial makeup of a suspended-growth reactor's biomass involves the absence, or presence, of specialized bacteria uniquely acclimated for the metabolic uptake or conversion of specific contaminant species, including not only nutrients such as nitrogen and phosphorus but also a wide range of potentially inhibitory, and yet still biodegradable, compounds (e.g., phenolics, thiocyanates) (Bitton, 1999). The desired acclimation and retention of these specialized bacteria would be analogous to that of their development within fixed-film systems, and there is limited evidence to suggest that floc particles may also provide, at least partially, a comparable sequencing of cometabolic aerobes and anaerobes located within the exterior and interior regions respectively.

In the particular case of pursuing complementary oxidative and reductive nitrogen removal steps, design and control measures to secure the presence and vitality of nitrogen-oxidizing and nitrogen-reducing bacteria within biomass will determine whether nitrification and denitrification were able to occur. Given the low growth rates commonly maintained by nitrifying bacteria, the key to achieving nitrification within an activated sludge is that of retaining reactor biomass for periods (maintaining an SRT) in excess of 4 days (and perhaps as much as 8 to 10 days during cold-weather periods). At this point, a sufficient (albeit, even then rather small—at levels of a few percent or lower of the total viable cells) inventory of these nitrifying lithotrophs can be kept to effect the desired oxidation of ammonia to nitrate.

This strategy for securing nitrifiers has subtle aspects that are inherently similar to that associated with fixed-film systems. A competitive advantage for nitrifiers in fixed-film systems is maintained by limiting the amount of incoming organics available to competing heterotrophs; on the other hand, suspended-growth nitrifying reactors maintained at high solids retention times are able to produce similarly low bulk solution levels of available organic carbon, such that the faster-growing heterotrophs are energetically constrained and unable to impose competitive stress. In fact, the process of nitrification has now become a rather commonplace requirement for a considerable number of waste-water treatment facilities given the inhibitory impact caused by low-level (sub-ppm) free ammonia concentrations on fish living within downstream receiving waters. Taken one step further, many newer activated sludge systems are also being designed to encompass full nitrogen removal, thereby matching ammonia oxidation (via aerobic nitrification) with nitrate reduction (via anoxic denitrification).

This combined nitrification-denitrification practice provides a number of potential benefits, including (1) the ability to achieve enhanced levels of overall nitrogen removal, thereby reducing the downstream impact of this prospective nutrient; (2) the ability to secure a recovery of the oxidizing "power" provided by nitrates, thereby conserving

Internal feedback recyle returning nitrates back to the denitrifying anoxic zone

Underflow recyle Waste returning settled biomass back to sludge the denitrifying anoxic zone

Figure 16.31 Representative two-tank plus two-recycle design scheme for biological nitrogen removal. (Note: This particular arrangement is referred to as the modified Ludzack-Ettinger system, based on its modified addition of an internal feedback recycle stream to the original Ludzack-Ettinger design scheme.)

Underflow recyle Waste returning settled biomass back to sludge the denitrifying anoxic zone

Figure 16.31 Representative two-tank plus two-recycle design scheme for biological nitrogen removal. (Note: This particular arrangement is referred to as the modified Ludzack-Ettinger system, based on its modified addition of an internal feedback recycle stream to the original Ludzack-Ettinger design scheme.)

the energetic investment previously made to create these oxidized products during aerobic nitrification; and (3) the ability to achieve a similar recovery of alkalinity during denitri-fication, thereby partially offsetting the alkalinity loss realized during prior nitrification.

Suspended-growth systems designed for combined nitrification and denitrification systems tend to fall into one of two alternative design categories, including zoned and temporally sequenced (i.e., flip-flopped aeration and mixing) arrangements to secure the necessary aerobic and anoxic phases. One such popular, zoned design strategy, known as the modified Ludzack-Ettinger (MLE) scheme is depicted in Figure 16.31. The first, anoxic stage in this MLE system mixes incoming raw wastewater with a nitrate-bearing flow brought back from the trailing aerobic nitrification reactor by way of an internal feedback recycle stream, thereby facilitating the conditions necessary for efficient reduction and removal of this recycled nitrogen. Within this initial anoxic zone, therefore, deni-trifiers employ incoming organics as their electron donor while reductively using nitrates as an electron acceptor, thereby producing nitrogen gas.

Example 16.2: Preliminary Activated Sludge Design After presenting their preliminary design estimate for a standard trickling filter with the Deer Creek, Illinois, community (see Example 16.1), the community's consulting engineer was asked to develop yet another preliminary design estimate for a standard secondary activated sludge system which might be used alternatively for this facility's secondary treatment process. As explained in Example 16.1, the expected average daily wastewater flow for this community will be 605.7 m3/day, and the carbonaceous organic loading (CBOD) entering this secondary activated sludge reactor system will be 72.7 kg BOD/day.

Preliminary Design Details

Hydraulic loading, system sizing, and retention times

Design hydraulic retention time (HRT): 4h (see Table 16.4) Design solids retention time (SRT): 8 days (see Table 16.4) Design reactor depth: 5 m

Projected reactor volume: (605.7 m3/day) (4 h) = 100.9 m3 Projected reactor surface area: 100.9 m3/5 m = 20.2 m2

Biomass Estimated theoretical biomass yield (YT): 0.5

content Estimated biomass endogenous decay and death rate: 0.05 day-1

Estimated active reactor biomass concentration:

(SRT/HRT)(Yt/(1 + kd • SRT)(CBODinfluent - CBODeffiuent) = 1920 g/m3

Estimated active biomass mass: (1920 kg/m3) (100.9 m3) = 193.8 kg Estimated total suspended solids mass: (active biomass/0.75) =

258.4 kg Projected organic loading:

Projected F/M (food/microorganism) ratio:

Related notes 1. As with Example 16.1, this preliminary design was based on a single reactor receiving an average daily flow, as compared to multiple reactors and peak etc. design flow conditions.

2. The total suspended solids vs. active biomass differences was based on a typically observed biomass fraction of ^75% vs. an additional inert solids content of ^25%.

3. The biomass concentration derivation given above takes into account composite biomass growth for CBOD depletion, plus decay and death, with an assumed effluent BOD of 10 mg/L); further details regarding this analysis can be found in Wastewater Engineering (Metcalf and Eddy, 2003).

4. In comparison to the prior trickling filter design estimate, there are several important design differences, including:

a. Considerably different contact times (^hours at activated sludge vs. ^seconds or minutes for the trickling filter).

b. Considerably different total reactor volumes (the activated sludge system is ^2+ times smaller).

c. Roughly comparable active biomass levels but sizably different total biomass levels (with a total of ^4+ times higher in the trickling filter, due to attached underlying anaerobic and inert biofilm).

Going beyond nitrogen removal, biological phosphorus removal can also be achieved in full biological nutrient removal (BNR) systems designed specifically to provide sequenced anaerobic and aerobic zones which would encourage the growth of a highly unique bacterial group largely believed to fall within the Acinetobacter genus. These unusual cells consume low-molecular-weight volatile fatty acids (VFAs) during an initial anaerobic zone while releasing internal reserves of polymerized phosphate (i.e., a polyphosphate known as volutin). Moving from the anaerobic to aerobic zones, though, these same cells reverse the process, balancing their accelerated uptake of soluble phosphorus against the oxidation of previously stored VFAs. The end result, therefore, is that of a biochemically mediated phase transfer of phosphate, shifting incoming soluble

Organic loading

1st internal feedback recyle returning biomass back to the initial anaerobic zone

2 nd internal feedback recyle returning nitrates back to the denitrifying anoxic zone

Raw wastewater [ influent

Organic carbon fermentation,I VFA formatinn and upd ate, Acinetobacter promotion [

Organic carbon oxidation a nd nitrate reduction (Denitrification)

Organic carbon oxidation nnd ammonia oxidatian (Nitrification)

I_I Clarifier II

J ^\|/freated effluent

Underflow recyle returning settled biomass back to the denitrifying anoxic zone

Waste sludge

Figure 16.32 Representative three-tank plus three-recycle design scheme for combined biological nutrient removalm including nitrogen and phosphorus removal. (Note: This particular arrangement is often referred to as UCT or VIP systems, based on their respective geographic development at the University of Cape Town and Virginia; there are numerous alternatives using similar sets of anaerobic, anoxic, and aerobic reactors.)

phosphorus contained within the raw wastewater stream into phosphate-rich solid-phase intracellular storage granules contained within cells eventually wasted from the system.

Further design refinements to secure an overall goal of complete biological nutrient removal, encompassing both nitrogen and phosphorus, typically involve the addition of yet another separate, anaerobic zone or phase at the head end of the reactor train, as shown schematically in Figure 16.32. The second and third reactors in this sequence, offering anoxic and aerobic environments, respectively, will again play much the same roles, including oxidation of organic carbon in both stages as well as nitrification in the aerobic stage and denitrification in the anoxic stage. By adding an additional internal feedback recycle loop, though, biomass moved from the anoxic stage to the new headend anaerobic stage is able to shift rapidly into an anaerobic condition (since it carries little, if any, nitrate or dissolved oxygen). In turn, fermentative transformation of incoming organics into volatile fatty acids sets up the initial conditions necessary for Acineto-bacter's phosphorus-related metabolism.

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