Microalgal bioremediation of petroleum-derived low salinity and low pH produced water

Abstract

Due to the presence of toxic organic compounds and heavy metals, it is essential to treat the produced water before reuse or discharge to the environment. In this study, produced water sample was collected from a local petroleum company. The sample had the following characteristics: pH: 4.17, total organic carbon (TOC): 720 ppm, total nitrogen (TN): 52.5 ppm, total phosphorus (TP): 0.21 ppm and salinity of 4.3 ppt. As a pretreatment, NaOH was added to raise the pH to 7.1 which removed 40% TOC, 38.3% TN, and 19% of TP. The growth viability of six microalgae strains (three freshwater and three marine) in the pretreated produced water (PPW) were studied in two conditions: (1) without additional nutrients and (2) with N and P supplementation. Out of these strains, only Chlorella sp. and Scenedesmus sp. were able to grow in both experiments. Chlorella sp. had the maximum biomass density on both occasions; it produced 1.2-g L−1 biomass density in the nutrient-supplemented PPW. Next, Chlorella sp. was grown in 1 L PBR to study the contaminant removal efficiencies. After 15 days of growth Chlorella sp. could remove 92% of the TN and 73% of the TOC from the PPW. Fish bio-assay was conducted using raw, pretreated, and biotreated produced water. Results indicate that the bioremediation process greatly reduced the toxicity of the produced water. Furthermore, the bioremediation process simultaneously generated 1.72 g L−1 biomass. Therefore, microalgal bioremediation of produced water provides an opportunity to produce a large quantity of biomass that can be used as feedstock for many products.