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The corrosion of sewers and the control of odor are the major operational and maintenance problems in wastewater collection systems. The generation of hydrogen sulfide and subsequent sulfuric acid results from microbially mediated reactions, by sulfate-reducing bacteria (SBR) and sulfide-oxidizing bacteria. This review covers pertinent information about sulfate reduction-induced problems in general and SBR in particular. Metabolism with respect to carbon, energy, and sulfur sources, ecology, growth factors (dissolved oxygen, temperature, pH, and sulfide), and the competitive effects of methane-producing bacteria on SBR are discussed. Because metals react with sulfide to form metal sulfide precipitates with extremely low solubilities, metal interactions in sulfate reduction environments are discussed.

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AbstractÐThe role of humic substances in general and hydrophobicity of humic substances in particular on membrane permeate¯ux is unclear. The main goal of the present study is to evaluate the eect of fractionated humic substances on ultra®ltration (UF) performance. A commercial humic solution was subjected to a hydrophobic resin for fractionation of the hydrophobic and hydrophilic fraction. These fractions were further fractionated into dierent molecular wight groups, using gel ®ltration chromatography. The hydrophobic fraction accounts for 85% organic carbon recovered and has a signi®cantly high THM (trihalomethane) formation potential, or 190 mg/mg C. The hydrophilic fraction exhibits the worst¯ux decline despite little solute rejection. The results of molecular weight fractionation of both hydrophobic and hydrophilic fractions further indicate that those molecules with the largest molecular weight (6.5±22.6 kDa) exhibit the worst¯ux decline. The UF system evaluated is unable to remove a signi®cant portion of THM precursors, resulting in potentially high THMs in the permeate. The use of powdered activated carbon (PAC) for the pretreatment of hydrophobic/ hydrophilic humic substances or in an integrated PAC-UF system exhibits an enhanced membrane fouling. #