Removal of zinc by sulfide precipitation in completely-mixed anaerobic reactor

Abstract

Precipitation is a widely used method for treating heavy metals in wastewater. Sulfide precipitation is popularly employed due to its low hydraulic retention time and ability for metal recovery. Sulfide precipitation process can be employed using various sulfide sources. Generation of sulfide by sulfate reducing bacteria (SRB) is one method that can be applied for wastewater treatment plant. The treatment process is separated into two steps. In the first step, S2- was formed in the anaerobic water treatment system. The SRB used SO42- as an electron acceptor and the sulfate was transformed to S2-. In the second step, sulfide was precipitated with metals in metal sulfide form. The purpose of this research was to study the optimum condition for zinc precipitation by S2- in completely-mixed anaerobic reactor and the amount of zinc in the anaerobic reactor that can be efficiently removed. This experiment was separated into two steps. The first was starting up the reactor for eight days. The second was injecting zinc sulfate for 9 times (in 27 days). The concentration of zinc for the injection was as follows: 10 ppm for the 1st, 20 ppm for the 2nd to 5th, 50 ppm for the 6th, and the 7th, and 100 ppm for the 8th, and the 9th injection. From the experiment, the optimum for SRB in the anaerobic reactor was the pH between 7.0-7.2, ORP less than -l00mV, and temperature between 28-32 ℃. The injected zinc precipitated rapidly in the first 10 minutes at the concentration of 100 ppm of zinc. The concentration of 290 ppm of zinc accumulated in the reactor and made ORP to increase to higher than zero. The pH decreased to lower than 7.0 and strongly inhibited sulfate reducing process. The residual sulfide in the reactor decreased to zero and zinc precipitation did not occur and indicated the failure of the system.