Inhibition of methane production from whey by heavy metals - protective effect of sulfide

A whey solution was used as a substrate for methane production in an anaero bic fixed-bed reactor. At a hydraulic retention time of 10 days, equivalent to a space loading of 3.3 kg (m(3) day)(-1), 90% of the chemical oxygen de mand was converted to biogas. Only a little propionate remained in the effl uent. Toxicity tests with either copper chloride, zinc chloride or nickel c hloride were performed on effluent from the reactor. Fifty per cent inhibit ion of methanogenesis was observed in the presence of greater than or equal to 10 mg CuCl2 l(-1) greater than or equal to 40 mg ZnCl2 l(-1) and greate r than or equal to 60 mg NiCl2 l(-1), respectively. After exposure to CU2Zn2+ or Ni2+ ions for 12 days, complete recovery ol. methanogenesis by equi molar sulfide addition was possible upon prolonged incubation. Recovery fai led, however, for copper chloride concentrations greater than or equal to 4 0 mg l(-1) If the sulfide was added simultaneously with the three heavy met al salts, methanogenesis was only slightly retarded and the same amount of methane as in non-inhibited controls was reached either 1 day (40 mg ZnCl2 l(-1)) or 2 days later (10 mg CuCl2 l(-1)). Up to 60 mg NiCl2 l(-1) had no effect if sulfide was present. Sulfide presumably precipitated the heavy me tals as metal sulfides and by this means prevented heavy metal toxicity.