Attached versus Suspended Growth Anaerobic Reactors: Response to Toxic Substances

Abstract

Anaerobic methane fermentation has significant potential for treatment of industrial wastewaters. Application of the process has been limited, due in part to the belief that the process cannot tolerate chronic and transient toxicity inherent in many industrial wastewaters. Experiments with cyanide, chloroform, formaldehyde, ammonium, nickel and sulfide have shown that the methanogens have the ability to recover from and acclimate to relatively high concentrations of toxicants. The key is proper attention to solids retention time. Of the two general reactor types used in biological waste treatment, attached growth systems offer two major potential advantages over suspended growth systems for treating wastewaters containing toxicants: (1) higher inherent solids retention times at relatively low hydraulic retention times and (2) the potential for a quasi plug-flow hydraulic regime that allows rapid elution of toxicants. For transient toxicity, the ability of a plug-flow, attached growth system to rapidly elute toxicants resulted in dramatically reduced down times when compared to a complete-mix, suspended growth system. For chronic toxicity, the solids retention capability of attached growth systems allows for acclimation to occur prior to washout of active biomass. Effluent recycle to plug-flow, attached growth systems should allow for more gradual exposure to chronic toxicity, thus maximizing acclimation potential.