Anaerobic degradation of sulfate laden organics involves sulfide generation and release of soluble microbial products (SMP). Sulfide causes direct toxicity to microorganisms and precipitation of key metals. On the other hand, SMP helps in keeping higher soluble concentration of essential metals through complexation and alleviating sulfide toxicity. The role of SMP is well recognized in biological treatment of wastes but many issues are yet to be fully elucidated. The present paper discusses the effect of sulfide concentrations on SMP production and effect of complexation on soluble Fe, Ni, Zn and Co concentrations in laboratory scale uplflow anaerobic sludge blanket (UASB) reactor, anaerobic baffled reactor (ABR) and hybrid anaerobic baffled reactor (HABR) degrading synthetic sulfate laden organics at 35°C for varying operating conditions. Results indicate increased SMP concentration with increasing sulfide concentration and significantly higher measured Fe, Ni, Zn and Co concentrations due to complexation in all three systems. Soluble Fe 0.43–0.66 mg/L; Ni 0.29–0.4 mg/L; Zn 0.15–0.19 mg/L; and Co 0.13–0.29 mg/L appears to be adequate for stable process operation and performance.

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