Biotreatment of wastewater from an integrated oil refinery by activated sludge and by RBC was studied and reported by the authors in previous publications. The objective of this study is to develop a low cost process combination by utilizing facilities, which exist in most industries, as treatment means as well as recipient for the renovated effluent. The idea is to share the task of the bioprocess between: a) a partial primary stage biotreatment by two aeration ponds (AP); b) chemical clarification of the primary stage effluent for removal of MLSS and colloids synthesized in the first process, by utilizing the existing lime clarification-softening system; c) completion of the bioprocess by a recircuiated cooling system (RCS) which has a high bio-oxidation capacity and also reuses the clarified effluent as make-up. The paper analyzes experimental data obtained from a pilot plant including two aerated tanks in series with a total hydraulic residence time of 2 days, followed by a lime clarifier. The main characteristic of the AP process is that hydraulic and sludge residence times are identical; two days are considered as very short SRT, especially in petrochemical wastewater, while in activated sludge SRT is usually above 15 days. The AP removed 60% of the COD - 15% by stripping - and 80% of the BOD; the oil removal was up to 50%. The chemical lime clarification improved the AP effluent reduction; suspended solids from 160 to 8 mg/l; oil from 34 to 9 mg/l; total COD from 290 to 170 mg/l. The study includes data on volumes and characteristics of the chemical sludge produced. The organic matter residual from previous treatment stages, most of it in soluble form, will be biodegraded in the RCS. The characteristics of RCS working as biological reactor were studied by Rebhun et al. and reported elsewhere. This study emphasizes economy in capital investment and beneficial reuse of industrial wastewater. The combined system includes two biological barries, AP and RCS, combined with the chemical clarification barrier, providing a reliable and low cost process combination.

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