An Investigation Into Two-stage Anaerobic Process Treating Wastewater Under High Sulfate And Ammonia

Anaerobic process, especially upflow anaerobic sludge blanket (UASB) reactor, is widelyused in the treatment of high-organic-strength wastewater. However, the high-concentrationsulfate produces sulfide, as a result of sulfate reduction, which is reported to severely impairmethanogensis. Furthermore, the presence of high strength ammonia in wastewater can alsoaffect the anaerobic process.Tow-stage UASB process was adopted to treat high concentration wastewater under highsulfate and ammonia. The granulation of sludge could stimulate the anaerobic biodegradation,bear higher free sulfide and ammonia. And methane could bring out a lot of sulfide from thewastewater. In the first reactor, when CODcr, sulfate and ammonia of the influent were about 11000,2800 and 1000mg/L, respectively, the removal rates of CODcr and sulfate were over 87% and96%, respectively. The organic and SO42– loading rates were 15.3 and 4.3 kg/(m3·d), respectively. The value of COD/SO42– was an index for operating when treating wastewater under highsulfate. When COD/SO42– reduced from 5.0 to 1.5, the concentration of dissolved sulfide in theeffluent increased from 230mg/L to 470mg/L, and the free volatile acid increased from 250 to510mg/L. Meanwhile, the removal rates of CODcr and SO42– drecreased to 65% and 56%,respectively.The presence of ammonia was benefit to the anaerobic treatment.When the concentration of sulfate, ammonia, CODcr was 11500, 3800, 2000mg/Lrespectively, the removal rates of sulfate and CODcr were 92% and 84% respectively.It was practicable to treat wastewater under high sulfate and ammonia using a single UASBreactor, which could save investment and simplify the process. In the anaerobic biotreatmentprocess, there were complex interactions between H2S, VFA, CO2 and free ammonia, whichhelped the anaerobic biotreatment.The second UASB reactor was used to treat the effluent of the first reactor under low C/N(2.0-3.0) and alkaline condition at 20±1℃. After three-month continuous operating, the pH ofinfluent and effluent were 7.8-7.9 and 8.1-8.4, respectively. The removal rates of CODcr,ammonia, TN and TP were 60-70%, 20-25%, 10-12% and 30-35%, respectively. After two-stageanaerobic process, the total removal rates of CODcr and TP were over 95% and 50%,respectively.The second UASB reactor produced less gas than that of the first reactor. This was probablybecause the influent CODcr was low and there was little substrate could be utilized by bacteria.Furthermore, low C/N and alkaline condition wasn't fit for the growth of methanogensis.Granular sludge with diameter of 0.2-3.5 mm was formed in both UASB reactors. And thediameter was mainly distributed in 0.6-2.2mm.The SEM photographs showed that the mainmicroorganism in the granular sludge of the two reactors were different, which was probably dueto the difference of substrate and pH value between the two reactors. Through element analysis,it indicated that the metal elements in the granular sludge mainly consisted of Fe, Ca, Zn, Na andK, as well as small portion of Cu, Ni and Co.