Desalination of a local oil refinery effluent to meet discharge limits

Abstract

The Sustainable Development Goal Six (SDG 6) – “ensure availability and sustainable management of water and sanitation for all” places huge responsibilities on stakeholders (industry, domestic and agricultural) to prioritize water saving, water reuse and proper wastewater treatment to make potable water accessible everywhere in the world. With the industrial sector consuming nearly 20% of the fresh water available, there is a corresponding generation of large volumes of effluents. This has been projected to increase, as population is skyrocketing and more economies are becoming more industrialized to accommodate the needs of the ever-increasing population. Over the years, stringent effluent discharge limits have been imposed on the industrial sector to minimize the pollution of the receiving environments, especially the water bodies. In addition, wastewater treatment for reuse is being encouraged, which will ease the stress on freshwater resources. The oil refinery industry is noted for the generation of large volumes of effluents. These effluents are heavy laden with toxic and refractory materials as well as high concentrations of salts which pose huge environmental risks and detrimental ripple effects on humans and animals if these effluents are not properly treated before discharge. Unfortunately, the use of conventional treatment methods to treat downstream oil refinery effluent (ORE) has been unsuccessful in the removal of these materials, especially the salts. This research therefore, aimed at desalinating the effluent from the effluent treatment plant (ETP) of a local South African waste oil refinery to meet discharge limits. The ETP, even though successful in the removal of organics (COD, turbidity and colour), consistently records high levels of sulphates, chlorides and carbonates as a result of the source of their raw material and other in-house processes that take place during the treatment process. The study assessed and compared the feasibility of applying three membrane processes, viz forward osmosis (FO), reverse osmosis (RO) and hybrid FO-RO systems in desalinating the ORE. The FO and RO were first run as standalone processes, where models were generated and used to optimize the important factors using the Box-Benhken design (BBD) of response surface methodology (RSM). Based on the optimized conditions, the hybrid FORO was investigated. The basis of comparison was their permeation fluxes, salt rejection and flux recoverability after membrane cleaning. A total of 45 experimental runs were conducted which catered for pure water flux tests of virgin membranes, optimization studies and confirmatory runs. The factors of interest for FO were feed solution flow rate (FS-FR) (7.5 – 9.4 L/h), draw solution flow rate (DS-FR) (7.5 – 9.4 L/h) and draw solution concentration (DS-C) (20, 35 and 50 g/L NaCl). With RO, focus was placed on operating pressure (14 – 18 bar), feed concentration and operating time (4-6 h). The results showed an average permeation flux of 3.64 ± 0.13 L/m2 h, Clenrichment (reverse solute diffusion (RSD)) of 35.5 ± 5.15%, SO4 2- rejection of 100%, CO3 2- rejection of 94.59 ± 0.32 and flux recovery of 86.01 ± 2.66% for FO. For RO, the average permeation flux achieved was 2.29 ± 0.24 L/m2 h, Clrejection efficiency was 90.54 ± 0.81%, SO4 2- rejection efficiency was 95.1%, CO3 2- rejection efficiency was 97.3 ± 0.4 and flux recovery after membrane cleaning was 62.52 ± 2.62%. The FO-RO hybrid process proved unsuccessful due to constraints from the filtration unit. As an intervention to make the hybrid process work, NF was used as the recovery process. However, results show a low permeation flux of 0.69 ± 0.10 L/m2h on average. From the results obtained, it was concluded that RO presents the best desalination option for treating the ORE using low pressure of between 14 – 18 bar. This will require no post treatment and there will be no contamination of feed due to RSD