Evaluation of alternative electron donors for denitrifying moving bed biofilm reactors (MBBRs).

The effectiveness of four different electron donors, specifically methanol, ethanol, glycerol, and sulfide (added as Na(2)S), were evaluated in post-denitrifying bench-scale moving bed biofilm reactors (MBBRs). With the requirement for more wastewater treatment plants to reach effluent total nitrogen levels approaching 3 mg/L, alternative electron donors could promote more rapid MBBR startup/acclimation times and increased cold weather denitrification rates compared to methanol, which has been most commonly used for post-denitrification processes due to low cost and effectiveness. While the application of alternative substrates in suspended growth processes has been studied extensively, fixed film post denitrification processes have been designed to use primarily low yield substrates like methanol. Bench-scale MBBRs were operated continuously at 12 degrees Celsius, and performance was monitored by weekly sampling and insitu batch profile testing. Ethanol and glycerol, though visually exhibited much higher biofilm carrier biomass content, performed better than methanol in terms of removal rate (0.9 and 1.0 versus 0.6 g N/m(2)/day, respectively.) Maximum denitrification rate measurements from profile testing suggested that ethanol and glycerol (2.2 and 1.9 g N/m(2)/day, respectively) exhibited rates that were four times that of methanol (0.49 g N/m(2)/day.) Sulfide also performed much better than either of the other three electron donors with maximum rates at 3.6 g N/m(2)/day and with yield (COD/NO(3)-N) that was similar to or slightly less than that of methanol.

Evaluating the role of microbial stress response mechanisms in causing biological treatment system upset.

It is known that microbial stress mechanisms play a significant role in short-term microbial adaptation to environmental perturbations, and activation of these mechanisms enhance a cell's chance for surviving the perturbation with minimal damage. Although the target of these mechanisms is protective at the cellular level, the effect may be disruptive at the macroscopic level in engineered bioreactor systems. In this paper, it is proposed that these mechanisms are activated in response to wastewater influent perturbations and may be a significant cause of activated sludge treatment process upset. Selected microbial stress responses are reviewed and hypotheses indicating their potential role in treatment process upset are proposed. A research approach that was previously used to identify the mechanistic cause of deflocculation during perturbation by electrophilic chemicals is summarized, and a protocol for future experiments geared toward establishing source-cause-effect relationships for a range of wastewater upset conditions is put forth. Identifying source-cause-effect relationships will provide a basis for development of new monitoring technologies and operational strategies for systems under the influence of influent chemical perturbations.

Stress protein expression in domestic activated sludge in response to xenobiotic shock loading.

Using the Western blot immunochemical analysis method, the heat shock protein, GroEL, was found to be either induced or repressed in activated sludge microorganisms exposed to a range of xenobiotics. At the EC25 concentration, pentachlorophenol (PCP), cadmium, nickel, 2,4-dichloroaniline, benzoquinone, 2,4-dinitrophenol, and 1,1,1-trichloroethane all rapidly induced measurable GroEL expression, even though the time-dependent response for each of these compounds was somewhat varied. Toluene and hydroquinone resulted in repression of GroEL expression to levels below that measured in the control mixed liquor. For PCP concentrations at or exceeding the EC25, there was a significant and consistent increase in effluent volatile suspended solids from activated sludge sequencing batch reactors relative to unstressed controls. These preliminary results indicate that stress proteins may serve as sensitive and rapid indicators of toxicity which can adversely impact treatment process performance in activated sludge systems.

The immunochemical detection of stress proteins in activated sludge exposed to toxic chemicals.

The heat shock protein, GroEL, was found to be induced in activated sludge cultures exposed to perturbations of chemicals (cadmium, pentachlorophenol, and acetone) or heat stress. In laboratory activated sludge reactors, GroEL was rapidly induced (within minutes) in the presence of 5 mg/l or greater total cadmium. At 5 mg/l cadmium, however, moderate to insignificant changes in activated sludge process performance indicators [effluent suspended solids concentration, chemical oxygen demand (COD) removal, and specific oxygen uptake rate] were observed. As total cadmium concentrations increased above 5 mg/l, there was a significant and consistent increase in effluent volatile suspended solids concentrations from activated sludge sequencing batch reactors relative to unstressed controls. These results indicate that stress proteins may serve as sensitive and rapid indicators of mixed liquor toxicity which can adversely impact treatment process performance, but that GroEL may not be a good candidate protein for this purpose.

Detection of GroEL in activated sludge: a model for detection of system stress.

GroEL is a ubiquitous constitutively synthesized protein that is also stress inducible. Activated sludge, which is a standard biological process used in wastewater treatment systems, is made up of a diverse microbial consortium. The synthesis of GroEL in activated sludge was significantly induced after heat (42 degrees C) shock. The increased level of GroEL expression was shown to be due to de novo protein synthesis. We have demonstrated a method which shows that stress proteins can be detected in activated sludge, and propose their use as specific indicators of system stress.