Transient Conditions And Stresses In Engineered Biological Treatment Systems

Various biological wastewater treatment processes operate with inherent dynamic conditions due to system configuration and operation; however, these processes tend to reach stable treatment performance and, therefore, are considered as controlled unsteady-state processes.13 Examples of these systems are sequencing batch reactors (SBRs); contact stabilization processes; activated sludge processes with selectors, subjecting biomass to different substrate concentrations along the treatment train; nutrient removal processes exposing biomass to alternating anaerobic, anoxic, or aerobic environments; and deep-shaft activated sludge processes. Most biotreatment processes strive to operate at steady state in order to provide constant effluent concentrations of pollutants within regulatory limits; however, common system variations may lead to uncontrolled unsteady-state operation that can violate effluent discharge limits.

The microbial response of a biological treatment process to variable operating conditions is of a transient nature, and has been related to poor treatment performance. Generally, these transient, dynamic conditions in biological wastewater treatment are caused by changes in substrate and nutrient characteristics and concentrations, and by changes in the environmental conditions to which biomass is exposed (e.g., dissolved oxygen (DO), pH, temperature).

Overall, the impact of transient conditions on the treatment performance of anaerobic reactors has been studied more extensively and under more controlled conditions than in aerobic reactors treating wastewater, as reviewed in the following sections.

17.2.1 Carbon Substrate Transients

Transient substrate conditions are related to variations in substrate concentration or type and have been more extensively studied than other transients, especially in activated sludge systems.14 Most of the work on substrate transients has been on input concentration transients (or loading rates) conducted in pure and mixed cultures after growth and acclimation in lab- or pilot-scale systems with defined substrates. A few respirometric batch experiments with spikes of a single readily degradable substrate have been conducted with activated sludge from full-scale plants.

Two main physiological responses occur under unbalanced microbial growth due to substrate concentration increases: internal storage of available substrate as poly-^-hydroxyalkanoates (PHAs) or glycogen14,15 and increased growth rate.15-17 Substrate storage is rapid, requiring little physiological adaptation because synthesis of storage polymers is simpler than that of other cell constituents, and polymers stored internally can be utilized for growth under substrate limited conditions. Substrate storage in mixed cultures has been studied under anaerobic, aerobic, anoxic, and mixed conditions.18 The increased growth rate response to substrate spikes is slower than polymer storage since it requires protein synthesis.14 Nevertheless, when microorganisms are still adapting to a specific environment and a substrate spike occurs, microorganisms are believed to almost immediately increase their specific growth rate.15

Additional mechanisms of response to high substrate concentrations include the release of soluble metabolites and of extracellular polymeric substances (EPS),14,19 and the accumulation20 and sorption of substrate. Accumulation refers to the transport of substrate into the cell where it is stored in an almost unchanged form or transformed into low molecular weight metabolic intermediates. Release of EPS may be relevant in biofilm systems where extreme extracellular polymeric release was observed to take place.21

In activated sludge microbial communities, feast and famine substrate dynamic conditions or substrate concentration gradients select for specific microbial populations with different physiological characteristics. This selection occurs due to the different maximum growth rates of microorganisms, for example, selection of floc-forming bacteria against filamentous bacteria in selectors or SBRs.13,14,22-24 It has been hypothesized that floc-forming bacteria have a greater ability for polymer storage under substrate transients than filamentous bacteria.24 For intermittently and continuously fed activated sludge, polymer storage is the main response to changes in substrate concentrations over time.

The effects of organic and hydraulic overloads on the treatment performance of anaerobic reactors have been broadly covered in stability studies, likely due to the greater sensitivity of anaerobic processes to disturbances compared to aerobic processes. Poor treatment performance, characterized by high concentrations of volatile fatty acids (VFAs), chemical oxygen demand (COD), and suspended solids in the effluent, and high sludge volume index (SVI), has been observed due to these types of transients, as reported elsewhere.25,26 These reports have been briefly reviewed by Nachaiyasit and Stuckey (1997) and Xing et al. (1997).27-29

Shifts in substrate types and increase in loading rates have been reported to disturb treatment performance and select for microorganisms with different phen-otypes within anaerobic granules. Under thermophilic conditions, a feed rich in acetate or volatile fatty acids was observed to select for acetoclastic methanogens over acidogenic and hydrogenotrophic methanogens, whereas a sugar-based feed selected for a symbiotic community of acidogenic and hydrogenotrophic methano-gens in an upflow anaerobic sludge blanket (UASB) reactor.30 The transitions between feeds were characterized by variations in methanogenic activity, COD removals, and changes in the granule properties. Compact and surface-homogeneous granules were formed with a sucrose feed, but not with an acetate feed, which promoted poor granulation.30 Increased loading rates of linoleic acid deterred methanogenic activity on different substrates, which was accompanied by granule disintegration and the release of free filamentous organisms in an expanded granular sludge bed (EGSB) reactor.31

Several studies report that anaerobic processes, although sensitive to substrate transients, can develop robustness against perturbations in substrate concentrations. High biomass concentrations in anaerobic baffled reactors have helped in the fast recovery of system performance from short-term substrate concentration and hydraulic transients.27 Nachaiyasit and Stuckey27,28 (1997) have argued that a high biomass concentration in anaerobic reactors can be used to increase reactor stability, since a significant amount of biomass remains after washout from imposing low hydraulic retention times (HRT). Xing et al.29 (1997) observed the adaptation of an anaerobic microbial community and a new steady-state performance in a completely stirred tank reactor (CSTR) under long-term (>400 days) periodic glucose concentration changes. Increasing sulfate concentrations in an UASB reactor also showed gradual assimilation of the shock loads with overall good treatment performance over a period of 266 days.32 Some studies even report no impact on treatment performance of increasing COD loading rates in a pilot-scale anaerobic hybrid reactor 33 and full-scale UASB reactor followed by trickling filters.34

Previous studies on cell decay and death, defined as the inability of cells to reproduce and respire further,35 have shown that cell death due to lack of substrate hardly ever occurs.36 Sporulating bacteria form spores when exposed to starvation whereas non-sporulating bacteria become dormant, and can be reactivated in the presence of specific extracellular compounds. Internal decay leading to a loss in microbial weight and activity, but not necessarily in the number of microorganisms, is active in all microbial systems exposed to starvation.35

17.2.2 Periodic Substrate Oscillations

The studies on substrate oscillations in biological treatment of wastewater indicate that microbial acclimation leading to stable treatment is achievable. The stable operation of a chemostat resulting from periodic substrate variations due to the establishment of a mixed culture of competing microbial populations or in pure cultures is well reported in the literature.37-39

Long-term periodic changes in substrate concentration have also shown to induce stable treatment in anaerobic processes. Xing et al.29 (1997) observed the adaptation of an anaerobic microbial community and CSTR steady-state performance under long-term (400 days) periodic substrate perturbations. An anaerobic micro-bial consortium is sensitive to short-term perturbations due to complex symbiotic microbial interactions and slow growth rates of acetoclastic methanogens. However, the diversity in populations within the consortium can also give an advantage to anaerobic communities under long-term perturbations since they can adapt selectively, as studies on changes on community structure suggest.40,41 For example, hydrogen-utilizing methanogens can quickly adapt to changes in environmental conditions.29

17.2.3 Environmental Transients

The most common environmental transients in biological treatment systems are with respect to DO concentrations, toxic compounds, pH, and temperature. Although transient conditions have been studied in anaerobic systems for more than two decades, the impact of these transients in aerobic treatment has received more attention only relatively recently.

17.2.3.1 Dissolved Oxygen (DO) Transients

The literature on DO transients in biological wastewater treatment systems is limited. Most of the studies have addressed the impact of low DO levels or anaerobic conditions as stresses on activated sludge. This literature is reviewed in Section 17.3 as stress conditions leading to deflocculation.

The few studies on DO transients tend to agree in that variable or low DO levels select for microorganisms that thrive under these conditions. However, a thorough characterization of the physiology and metabolism of the microorganisms selected by these transients has not been conducted. Sharp decreases in DO levels associated with sudden substrate overloads in activated sludge have been shown to favor the growth of filamentous bacteria.42 Pernelle et al.42 (2001) showed experimentally that the continuous proliferation of filamentous bacteria (Type 021N, Thiothrix, Nostocoida limilicola II, and especially Haliscomenobacter hydrossis) in an activated sludge pilot plant leading to sludge bulking was triggered by the combined transient of a substrate overload (meat and vegetable extracts and sugar) with an imposed DO deficit. Separately, the DO deficiency and the increase in substrate caused transitory, small-scale proliferation of some of the filaments. Similarly, but on a positive side, alternating aerobic and anaerobic conditions in an activated sludge plant promoted a microbial community shift toward facultative organisms without affecting COD removals or settling, as SVI.43

The effects of periodic DO oscillations resulting in the selection of a mixed community of nitrifiers, aerobic heterotrophs, and denitrifiers in activated sludge have only been simulated using the AQUASIM model,44 but experimental validation is required.

17.2.3.2 Toxicity and pH Transients

Phenol and hydrogen peroxide (H2O2) have been used to study toxic transient effects on microbial metabolism. Sudden increases in these toxic compounds have been shown to cause deleterious effects on biological wastewater treatment. Phenol spikes and shock loadings have been shown to cause activated sludge deflocculation and the immediate reduction in sludge oxygen uptake rates (OURs),45 and to decrease the sludge substrate removal capacity in respirometric assays.46 Phenol is known to inhibit growth rate kinetics in activated sludge systems, but it is degraded after sludge acclimation.45 Similarly, Larisch and Duff47 (1997) observed that sudden H2O2 increases in the influent (5 to 50 mg/l influent) to lab-scale activated sludge reactors led to decreased mixed liquor volatile suspended solids (MLVSS) levels, weakly bonded flocs, and high effluent suspended solids (ESS).

If microbial metabolism is not completely eliminated by high toxic concentrations, microbial communities are known to undergo adaptive changes in structure and function capability in order to tolerate and degrade xenobiotic compounds.48 Preacclimation of activated sludge to toxic compounds tends to buffer the impacts of toxic spikes, as shown for increased phenol shock loading spikes45 and increased H2O2 concentrations47 in activated sludge.

Some studies have reported decreased OUR and settleability in activated sludge respirometric assays due to the toxicity of chemicals used in the manufacturing of paper and pulp.49-51 These studies, however, have not been able to provide conclusive mechanistic explanations of the observed effects. Chemicals that have inhibited microbial activity, assessed as reduced OURs,49,50 include a cleaning solvent, turpentine, microbiocides containing 2,2-dibromo-3-nitrilopropionamide (20%), methylenebis-tiocyanate (9%), and carbamate, several paper dyes (e.g., Orange 3, Violet 5, Red B), monochloro acetic acid (80% in water) and soft soap. In batch experiments,50 the same biocides decreased the OURs and the dissolved organic carbon removal efficiencies; acrylic latex increased sludge settleability probably by adsorbing onto flocs, while aluminum sulfate and an optical brightener decreased sludge settleability probably due to floc charge instability. A simulated black-liquor spill into a lab-scale activated sludge decreased the biological oxygen demand (BOD) and COD removals, the sludge-specific adenosine triphosphate (ATP) concentration, and increased ESS and effluent toxicity.51

The impacts of chemicals on activated sludge are dose- and type-dependent.52 Some chemicals have no effect on the OUR or on settleability49,50 and others even increase the OUR.49,50,53 The chelating nature of chemicals, such as that of diethylene triamine pentaacetic acid (DTPA), has decreased the OURs in a dose-dependent fashion in batch experiments and lab-scale continuous activated sludge reactors treating bleached kraft mill effluent.47 DTPA has also decreased the BOD removal (39%) and caused sludge floc disintegration.47

Depending on the nature of the compound, chemical transients can be associated with pH stresses. Overall, bacteria seem to be more sensitive to low pH than to high pH values.36,54,55 Bacteria in activated sludge flocs appear more resistant to chemical and pH stresses than microorganisms in more dispersed biomass (i.e., from lagoons) due to the high-biomass-density structure of activated sludge flocs.55 Both acid and basic shocks decrease activated sludge metabolism,36,55 and decrease specific ATP concentrations,36 which is a measurement of decreased microbial metabolic activity.

Toxicity in anaerobic systems is generally related to the inhibition of methanogens by the increase in concentration of VFA, particularly, butyrate and acetate, but it can also be associated with sulfide and heavy metals, as reviewed in refs 56 and 57. The inhibitory effect of VFA is related to the lowering of pH. Since substrate concentration and temperature transients can lead to the accumulation of VFA, the effect of these transients can be compounded by a pH inhibitory effect on methanogens.

17.2.3.3 Temperature Transients

Temperature transients in biological wastewater treatment can result from seasonal variations and from the operation of batch units and shutdowns/start-ups in upstream industrial processes. Industrial treatment systems may be subjected to frequent and drastic temperature transients that affect treatment performance. On the contrary, sewage treatment systems may experience mainly seasonal transients of which winter may represent the most challenging due to reduced microbial activity.

Temperature transients have been identified in activated sludge plants treating pulp and paper mill effluents during the summer.58 Deterioration of activated sludge metabolic activity was detected by respirometry in several pulp and paper mills at temperatures exceeding 38°C, and effluent cooling towers were installed to counteract these effects.58,59 Cocci and McCarthy60 (1998) also reported the installation of wastewater cooling towers prior to SBRs treating pulp and paper mill effluents in Canada. The cooling towers in these systems generally reduced the raw wastewater temperature by approximately 10°C. The reported mean winter and summer operating temperatures of the SBR systems were 28°C and 37°C, respectively. Several mills, probably those without or with undersized cooling systems, were reported to operate with temperatures over 40°C, but with concerns about system performance.60

Similarly, temperature increases in full-scale anaerobic reactors treating industrial wastewater have been reported.61,62 An accidental increase in temperature from 36°C to 50°C in a full-scale anaerobic filter reduced the COD removal efficiency from 85% to 45% over a seven-hour exposure at 50°C.61

In anaerobic treatment processes, a better understanding has been achieved of the effects of temperature shifts on reactor performance, granule characteristics, and microbial community structure from conducting systematic studies.25,62-65

In general, poor treatment performance has been reported due to temperature shifts in anaerobic reactors. Short-term (8-9 and 5-24 h) temperature shifts from mesophilic (30°C and 37°C to 39°C) to thermophilic (46°C, 55°C, 61°C, and 65°C) conditions have decreased the removal efficiencies of volatile fatty acids and biomass methanogenic activity in UASB reactors treating a sulfate-containing wastewater,62 VFA, and vinasse.64,66,67 Thermophilic temperature shifts from 55°C to 65°C have increased the concentration of total organic carbon in the effluent in association with granule disintegration in an UASB reactor,63 and have increased the effluent VFA concentrations and decreased the methanogenic activity in an anaerobic digester.65 Short-term (6 and 12 h) temperature downshifts from 35°C to 25°C and 15°C increased the concentrations of COD, VFA, and ESS in an anaerobic fluid-ized bed reactor treating ice-cream wastewater.68 Interestingly, temperature shifts from mesophilic temperatures (30°C and 37°C to 39°C) to 45°C62,64 or from 35°C to 42°C67 have had no significant effect on the treatment performance of UASB reactors.

The literature on temperature shifts in anaerobic treatment indicates that the recovery of anaerobic reactors from short-term mesophilic-thermophilic temperature shifts is possible and is dependent on the growth rate of microorganisms. For instance, methanogens are more sensitive than sulfate-reducing bacteria to shifts from 30°C to 55°C and 65°C due to relatively slower growth rates.62 Exposure of mesophilic anaerobic granules to temperatures above 60°C may completely eliminate methano-genic and acetogenic activity.64 However, even after a drastic temperature increase from 38°C to 75°C that almost eliminated anaerobic digestion in an UASB reactor, the system began to recover after 12 days.67 Anaerobic digestion has also recovered from a thermophilic temperature shift from 55°C to 65°C, in which the activity of methanogens and acidogens was reduced, but not that of hydrolytic and fermentative populations.65

There is agreement in the literature that temperature upshifts from mesophilic to thermophilic or within thermophilic conditions cause granule disintegration in anaerobic treatment systems. Disintegration of anaerobic granules has been reported under a six-h thermophilic shift from 55°C to 65°C in an UASB reactor treating a sucrose-containing wastewater,63 and under temperature shifts from mesophilic to thermophilic conditions during UASB startups.66,67 Anaerobic granular disintegration under the thermophilic temperature upshift seems to be gradual, and takes place long after the temperature has been increased (80 days). A gradual increase in effluent VSS and decrease in the reactor's VSS was observed over a period of 80 days at 65°C in an UASB,63 and complete granule disintegration was observed 2 to 3 months after subjecting mesophilic granules to temperatures above 45°C during the startup of thermophilic UASB reactors.66 It is probable that some level of biomass detachment occurs from biomass carriers in an anaerobic fluidized bed reactor due to temperature downshifts from 35°C to 25°C and 15°C, as indicated by an increase in effluent suspended solids.68

Although observed in practice, the effects of temperature transitions on activated sludge metabolism, microbial community structure, settling characteristics, and bioflocculation are not well understood. Table 17.1 summarizes different types of studies where temperature transients have been reported in aerobic treatment of wastewater. The transitional conditions from an initial to a final stage of a temperature shift have received little attention in aerobic treatment of wastewater. Until very recently, little work had been conducted on the effects of temperature disturbances on the response and overall performance of wastewater aerobic biological treatment processes. Most of the studies reporting temperature transients in aerobic wastewater treatment are not systematic and tend to be anecdotal as part of experiments at different temperatures, but under steady-state conditions (e.g., see ref. [69,75,77] as per Table 17.1). Most of these experiments have not addressed temperature transients in long-term experiments or have not been able to show reproducibility of the effects (e.g., see ref. [36,51,70,72,74] as per Table 17.1).

In general, microbial growth rate and cellular metabolism increase with increasing temperature within a survival temperature range (e.g., from 10°C to 40°C for mesophiles) up to a maximum value.54,78 Beyond this maximum value, growth rate and metabolism decrease. Therefore, a temperature shift over the optimal temperature limits, for example, from a mesophilic to a thermophilic range, is likely to lead to decreased microbial activity and represents a disturbance leading to poor treatment performance. The effects of temperature shift in the upper limit of mesophilic treatment or between mesophilic and thermophilic treatment remain to be fully characterized and fundamentally understood. Recent work in our group has focused on understanding how these transients affect aerobic treatment.

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