Research On Coagulation-A/O Process Of Treating Tannery Wastewater

Tannery wastewater is complex and harmful to water environment, containinghigh concentrations of toxic and hazardous substances, such as sulfide, heavymetals, tanning agents, surfactants, fats and proteins. Therefore, it is difficult totreat tannery wastewater.In this paper, the application of FeSO47H2O on tannery wastewater treatmentwas optimized. Microbial degradation characteristics of several typical pollutants intannery wastewater were determined. The effect of hydraulic retention time （HRT）,temperature and the composition of tannery wastewater, as well as adding fillers onthe performance of A/O reactor treating tannery wastewater was investigated. Thetreatment of tannery wastewater using a pilot-scale coagulation-A/O process wasstudied.The results of single factor test showed that the optimum operationalparameters for removal of COD、total chromium and sulfide by FeSO47H2Ocoagulation was that, FeSO47H2O dosage was1600-2000mg/L, cationicpolyacrylamide dosage （C-PAM） of5-10mg/L and initial pH value of9-11.According to the results of orthogonal experiments, the optimum conditions were asfollowing: FeSO47H2O dosage was1800mg/L, C-PAM dosage was7.5mg/L andinitial pH value was10.The COD removal rate of the A/O can reach70%-80%with HRT30h. TheCOD removal was not significantly influenced by prolonging HRT or decreasingtemperature（not less than7C）. However, the ammonia removal was clearlyaffected by the influent concentration and temperature. When the influentconcentration was too high, higher than120-150mg/L, the effluent ammoniaconcentration increased with increasing influent concentration, and the effluent pHgot acidic. Nitrification function almost stopped when temperature was lower than12°C. The trend of total nitrogen removal was similar to ammonia. After adding thepacking to the last four compartments in the aerobic section, COD removalefficiency increased from72.45%to78.56%. However, removal efficiency ofammonia and total nitrogen was not clearly changed.The operation parameters of the pilot-scale mechanical clarifiers-A/O processtreating tannery wastewater in this study were as following: FeSO₄·7H₂O dosagewas1800mg/L-2000mg/L, coagulant aid dosage was7.5mg/L and initial pHvalue was9.0-10.0. The process achieved a removal rate of COD and totalchromium was7.09%-24.55%and86.48%-98.24%, respectively. When HRT was30h, R（nitrification liquid reflux ratio） was200%and （rSludge return ratio） was 75%, the removal efficiency of COD, ammonia and total nitrogen was77.06%,10.90%and16.64%. The removal of ammonia and total nitrogen in this study wassignificantly affected by low temperature.