56. Water Management at the Dangote Refinery, Nigeria

This chapter describes the water treatment system at the Dangote refinery and petrochemical complex in the Lekki Free Zone near Lagos, Nigeria. The project is still under construction with mechanical completion and startup planned for the end of 2022. The raw water for diverse petrochemical processing steps will be drawn from the adjacent Lekki Lagoon and then treated, thus providing the entire complex with a high-quality supply. Treatment will include a sequential batch reactor and diverse filtration technologies, and seasonal fluctuations are to be taken into account. The effluent treatment plant will also deal with various refinery effluents. The system is to be based on effluent categorization and subsequent targeted handling, which will ensure efficient and cost-effective treatment. The wastewater will be divided into oily wastewater (oily used water), contaminated rainwater, sanitary wastewater (sanitary used water), cooling tower and boiler blowdown, each of which are to be decontaminated in individual treatment lines. Furthermore, complex effluents such as spent caustics and phenolic sour water will be handled separately. In the case of spent caustics, the core technology will consist of high-pressure, wet air oxidation, which offers a high level of sulfide and COD removal. By contrast, in order to remove excess cyanide, phenolic sour water will first be stripped and then subjected to an oxidation process. The effluents from the treatment plant are to be further decontaminated in a water reclamation plant comprised of an ultrafiltration treatment system, a two-stage, two-pass reverse osmosis unit, a downstream degasser tower, and final polishing in a mixed bed ion exchanger. The treated effluent will be of a high quality and thus suitable for reuse as boiler feed water. The water treatment system will include units for handling both sludge and, most importantly, rejects. As a result, the hazardousness of highly contaminated treatment by-products will be reduced and permit safe disposal.