Transients Or Stresses In Biological Treatment Systems

17.3.1 Transients Leading to Biomass Deflocculation

Perturbations or stress conditions are known causes of biomass deflocculation, such as sudden variations in toxic compound concentrations, DO levels, pH, ionic strength, and temperature. Increases in the effluent turbidity have been related to defloc-culation and weak flocculation due to several stress conditions: oxygen-supply disruption79-81 or anaerobic storage82; 1000-mg/l phenol spikes 45,83; the addition of metabolic inhibitors and decrease of temperature from 20°C to 4°C84; decreased19 and increased85 substrate loadings; low86 or high87 ionic strength; glucose spikes45; Hg (30 to 1000 mg/l), Cd (30 to 1000 mg/l), and Zn (100 to 1000 mg/l) shock doses88; temperature increases71, and Cr(VI) shock doses (0.5 to 5 mg/l).89 Activated sludge deflocculation has also been reported due to the chemical reduction of Fe(III) to FeS by the presence of sulfide,90 and the activity of the external addition of polymer-degrading enzymes, such as deoxyribonuclease71 and pronase.91

Toxic shocks have been reported to cause activated sludge deflocculation, poor treatment performance, and increased effluent turbidity. Batch shocks of aluminum sulfate and paper optical brightener concentrations were reported to decrease activated sludge settleability, and increase discharges of effluent suspended solids.50 Similarly, continuous exposure to Orange 3 dye in a pilot activated sludge plant resulted in decreased SOURs, increased effluent solids losses, and dispersed, pin-point growth.49 Sludge deflocculation has been reported due to a pH shock (up to 9 for 9 h) caused by a spill of 50% sodium hydroxide solution into an activated sludge plant treating pulp and paper mill effluent,50 due to sudden increases in hydrogen peroxide concentrations,47 and increases in potassium concentrations (0.5 to 1 g/l influent).92 Increased sludge supernatant turbidity due to cell detachment from flocs was also observed due to increasing surfactant concentrations (i.e., decreasing water surface tension) in batch experiments.93

Sludge deflocculation has been characterized by turbidity, polymeric, and suspended solids measurements in the supernatant of settled activated sludge mixed liquor, and by microscopic quantification and observation of pin-point flocs.2,19,94 Turbidity has been correlated to the number of cells in the supernatant, and the concentrations of proteins, carbohydrates, and humic substances.81,84 Total polysaccharides or carbohydrates have also been measured in the supernatant of deflocculated sludge.45,83,86

Few researchers have isolated bacteria from activated sludge and studied their flocculating characteristics via deflocculation experiments. One example is the work of Sakka et al.71 (1981), in which deflocculation of aggregates of Pseudomonas sp. was caused by washings with distilled water, guanidine-HCl, and NaCl, and temperatures above 40°C. Reflocculation occurred when solutions of KCl and MgCl2 were added, and the temperature was decreased to approximately 4°C.71

Changes in substrate types and loading rates have been observed to change the diversity of bacterial populations in anaerobic granules and render them more susceptible to disintegration. Acetoclastic methanogens have been selected over hydrogenotrophic methanogens and acetogenic bacteria due to the shift from a sugar-rich feed (alcohol distillery wastewater) to an acetate feed in a thermophilic UASB reactor.30 Anaerobic granules with a predominant population of acetate-utilizing methanogens (Methanosaeta sp.) developed a porous surface and were prone to disintegration, whereas anaerobic granules with a predominant symbiotic community of acetogens and hydrogenotrophic methanogens developed smoother surfaces and compact granules.30 Increased loading rates of oleic acid, one of the toxic long-chain fatty acids (LCFAs) to acidogens and methanogens, in an expanded granular sludge bed (EGSB) reactor have been associated with granule disintegration.31 Anaerobic granule disintegration was related to the surfactant effect of LCFA lowering water surface tension and hindering aggregation, and to the washout of acetogenic bacteria in the granules.31 However, the specific mechanisms of deflocculation were not identified.

17.3.2 Mechanisms of Biomass Deflocculation

Overall, the mechanisms whereby deflocculation under transients or stresses occurs are not well understood; however, these mechanisms can be divided into abiotic or biotic. Deflocculation via abiotic mechanisms proceeds via physical interactions or chemical reactions. Conversely, microbial metabolism or physiology mediates deflocculation via biotic mechanisms.

The abiotic mechanisms of sludge deflocculation are better understood than the microbially mediated ones. Deflocculation via physical means has been shown to occur due to anaerobic reduction of Fe(III) by the presence of Fe(III)-reducing bacteria and sulfide-producing bacteria84,86,95; low86 and high87 ionic strength; the imbalance between divalent (Ca2+,Mg2+) and monovalent (Na+) cations,91 and the removal of Ca2+ from sludge flocs86 or ionic exchange of Ca2+ by H+,Na+, K+, or Mg2+.96 Sludge deflocculation has been observed to increase with increasing solids concentration and turbulent shear rates.2 These effects have been described by an adhesion-erosion equilibrium model based on thermodynamic considerations.2,3 The process of sludge floc deflocculation and reflocculation or reaggregation appears to be somewhat reversible97 involving floc constituent exchange, at least under variable shear stress.5

Microbially mediated sludge deflocculation has been explained hypothetically under different conditions: anaerobic conditions,81,98 phenol spikes,83 cadmium spikes,99 decreased metabolic activity,84 decreased substrate loadings,19 and due to adverse environmental conditions, such as drastic shifts in pH, temperature, and concentration of toxic compounds.35 Deflocculation of activated sludge has been shown to occur due to the activity of exocellular proteases (pronase)91; however, it remains to be demonstrated that this occurs under physiological conditions.

Sludge deflocculation has been related to a reduction in microbial metabolic activity.84 Wilen et al.84 (2000) showed that improved sludge bioflocculation (stronger flocs) or less deflocculation (measured as less supernatant turbidity) was attained only if the microorganisms were metabolically active under aerobic conditions or in the presence of an electron acceptor, such as oxygen or nitrate under anoxic conditions. The lack of aerobic metabolic activity by the addition of azide and chloramphenicol, by anaerobic conditions, and by the reduction in temperature from 20°C to 4°C caused deflocculation. Similarly, phenol shocks were found to lower sludge ATP levels and reduce the oxygen uptake rates and cause deflocculation simultaneously.45,83 These results suggest that microbial floc stability is directly maintained by microbial metabolism.

Transient conditions in biological treatment processes represent stresses or perturbations that seem to have similar impacts as in suspended-growth systems, that is, deflocculation and decreased activity. The mechanisms leading to these effects, however, appear to be different depending on the stress. Although the same response mechanisms may result from different transients, such as microbial cell lysis, the causes leading to these mechanisms could be different (e.g., membrane damage under phenol spikes vs death of strictly aerobic bacteria under anaerobic conditions), and are not well identified.

The poor understanding of the mechanisms of sludge deflocculation is reflected in the many hypotheses explaining its causes. Deflocculation has been explained in association with EPS, microbial community shifts, bacterial physiological responses, microbial metabolism, and abiotic processes, as outlined in Table 17.2.

Little work has been done in linking microbial physiological stress responses with the effects of stress or transient conditions on biological treatment performance.

0 0

Post a comment