Effects of operating parameters on biological siudges decantation of an oil refinery waste water plant
The wastewater treatment systems built in the seventies for treating the effluents from oil refineries often fail to comply with new environmental standards. The present study focuses mainly on the problem of sludge settling in the clarifiers. The purpose of this work was to determine the effect of various parameters on the settling characteristics of the activated sludge, to evaluate the rote of alun as a flocculating agent and to characterize the operation of the clarifiers primarily as it relates to the settling problem. The influence of the quantity of the alun added and of the stirring speed on the settling characteristics of the sludge were determined in jar tests. The effects, on the performance of the clarifiers, of the level of dissolved oxygen in the bioreactor, of the pH and of the concentration in total suspended solids (TSS) maintained in the mixed liquor, were also investigated. The studies on the sludge settleability led to the following conclusions : - the concentration in dissolved oxygen and the concentration in TSS in the aeration tank are two parameters which have a critical effect on the effluent quality. The optimal operating intervals for these parameters are from 0.9 to 1.3 ppm for dissolved oxygen and from 4 300 to 4 800 mg/L for the TSS; - the pH of the mixed liquor is not a main factor and does not seem to have any effect on the sludge settleability; - the addition of alum in excess of 50 mg/L has an adverse effect on the sludge settling characteristics. Addition of alum below this level, however, improves the clarity of the residual liquor. Nevertheless, the addition of this flocculating agent is not recommended; - the high flowrates and violent mixing in the piping system used to feed the mixed liquor from the aeration tank to the clarifiers adversely affects the solid-liquid separation in the clarifiers. By simply stopping the addition of the flocculating agent, maintaining the dissolved oxygen concentration between 1 to 2 ppm and the TSS between 4 300 to 4 800 mg/L in the wastewater treatment unit, it was possible to improve substantially the quality of the effluent and thus meet the environmental standards.