Simultaneous removal of sulfate and heavy metals from wastewater by mixed microbial cultures under anaerobic conditions

Abstract

This study focused on determining the optimum ratio of sulfate, COD and heavy metals that facilitate, in anaerobic bioreactors, simultaneous lowering of COD and sulfate with precipitation of metal sulfides, with methane production using high performance mixed-microbial assemblages. Three phases of the research were performed. Phase 1, mixed-bacterial from five different sources. i,e., septic tank, a coastal area, a brewery wastewater treatment plant, acidic sulfate soil, and leachate from landfill were compared. Glucose at 20,000 mg COD/l was used as synthetic waste. The initial loading factor of each reactor was controlled to by equal. Five reactors of 6-liter capacity, which were equipped with a gas collection system, were operated in a batch mode at room temperature of 31.5°C and pH of 7. Another set of ten reactors were set to study sulfate reduction and heavy metal removal. Sulfate at 2,100 mg/l was added to all of these reactors and copper at 10 mg/l was added into the five reactors. Mixed-bacterial culture from a brewery wastewater treatment plant showed the optimum ability to promote methane production and sulfate reduction. The methane production yield coefficient was 324 ml at STP per gram of COD removal and the specific methane production rate was 0.0072 ml at STP/(mg MLVSS/l-hour) while the endogenous decay coefficient was not found during the experimental period. Copper showed a positive effect on sulfate reduction due to sulfide precipitation. The specific sulfate reduction rate was 9x10-7(l/mg MLVSS-hour). This mixed-culture showed the highest performance in recovery of the system to higher levels of COD removal as a result of metal sulfide precipitation thereby avoiding system inhibition. This mixed-bacterial culture was selected to study in Phase 2 and 3. Phase 2, the experiments to find optimum reactor conditions for simultaneous control of sulfate and heavy metals in complex wastewater streams were carried out in reaction bottles with 100 ml working volume by a batch test at a pH of 7 and at 35±l° C. Glucose at 3,200 mg COD/l was used as synthetic waste. Sulfate was added in the range 0-2,600 mg/l giving rise to a COD:S ratio in the range of 13.7-3.7. At a COD:S ratio of 9, the COD reduction reaction and methanogenesis were highly promoted. The highest specific methanogenic activity of 0.5815 (gCH4 gas COD)/(g COD removal-g MLVSS) was shown at this condition. The methane produced was 196 ml ( at STP) and the sulfate reduced was 0.356 gram, expressed per gram of COD removal. Phase 3, the toxicity of cadmium, copper, and zinc were studied in both single and combined metal forms. Experiments were conducted similarly to those of Phase 2. The optimum COD:S ratio of 9 was provided in all reactors. Heavy metals could negatively impact the activities of anaerobic bacteria. The relative toxicity of heavy metals in organic degradation was Cu > Cd > Zn. Combined heavy metals caused synergistic inhibition on the bacterial activity but the inhibition could be antagonized by Cu, when the concentration was not more than l mg/l. The antagonism was not observed when CD combined with Zn. The criterion model for the prevention of the synergistic inhibition was proposed. The metal loading, K, of ( Zn/32.7+Cd/56.2+Cu/31.8)/W has to be less than 15 meq/kg MLVSS. At this metal loading, the relative methane production activity was not lower than 80%. Where K is the ratio of the sum of Zn, Cd, and Cu (in meq/l) divided by the MLVSS, W (in kg/l). Zn, Cd, and Cu are the concentration of the metal in solution (in mg/l). The final pH was elevated but did not exceed 8. These experimental results elucidated the performance of the reactor involved in sulfide detoxification and heavy metal remediation processes.