Background: Treating latex rubber sheet wastewater often leads to the generation of a rotten-egg odor from toxic H₂S. To increase the treatment efficiency and eliminate H₂S, purple nonsulfur bacteria (PNSB), prepared by supplementing non sterile rubber sheet wastewater (RAW) with fermented pineapple extract (FPE), were used to treat this wastewater under microaerobic light conditions. The following 3 independent variables; chemical oxygen demand (COD), initial pH and FPE dose were investigated using the Box-Behnken design to find optimal conditions for stimulating the growth of indigenous PNSB (PNSBsi).

Results: The addition of 2.0% FPE into RAW, which had a COD of 2000 mg L^-1 and an initial pH of 7.0, significantly decreased oxidation reduction potential (ORP) value and stimulated PNSBsi to reach a maximum of 7.8 log cfu mL^-1 within 2 days. Consequently, these PNSBsi, used as inoculants were investigated for their ability to treat the wastewater under microaerobic light conditions. A central composite design was used to determine the optimal conditions for the wastewater treatment. These proved to be 7% PNSBsi, 0.8% FPE and 4 days retention time and this combination resulted in a reduction of 91% for COD, 75% for suspended solids, 61% for total sulfide while H₂S was not detected. Results of abiotic control and treatment sets indicated that H₂S was produced by heterotrophic bacteria and it was then effectively deactivated by PNSBsi.

Conclusions: The stimulation of PNSB growth by FPE under light condition was to lower ORP, and PNSBsi proved to be effective for treating the wastewater.