Evaluation of Optimal Chemical Aids and Process Parameters in Oil Refinery Wastewater Flocculation and Flotation

Abstract

Wastewater treatment of oil refinery effluents is a pre-requisite for refinery operations in a continuously stricter environmental regulatory framework. One effective method of treating typical refinery wastewaters with oil in water emulsion is chemical destabilisation by metal salts, followed by separation of oil by flotation. The aim of this work was to improve the performance of the Naantali Refinery wastewater treatment plant with respect to oil, chemical oxygen demand, suspended solids, phosphorus, and turbidity removal. This was done experimentally, by assessment of chemicals, pH, and dissolved air flotation (DAF) process parameters. The performance of aluminium sulphate, polyaluminium chloride, ferric sulphate and ferric chloride and six different polyacrylamide flocculants were evaluated by jar testing. Ferric sulphate and low charge density cationic and anionic polyacrylamides were then evaluated in industrial scale experiments. The novel chemicals and DAF parameters recommended were found on average to reduce all the flotation overflow impurities by more than 50 % compared to the conventional process. The optimal operating parameters in the industrial scale were found to be 1-2 mg/l of flocculant, an average dose of 35 mg/l ferric sulphate and pH 7-8. Various process disturbances can require up to 180 mg/l of ferric sulphate, which increases purification in a dose-dependent manner. The DAF system airflow was raised from 20 Nm3/h to 30 Nm3/h, which reduced suspended solids by 34 % compared to the reference. For future research it is recommended to assess the performance of combinations of Al and Fe based coagulants for disturbances caused by high levels of hydrogen sulphide.