oa Biotreatment Of Produced Water Containing Hydrate Inhibitors

With proper treatment to remove organics and inorganics, the produced water (PW) generated during oil and gas extraction can be reused as process water. Biotreatment is generally regarded as the most cost-effective method for organics removal and although widely used in industrial wastewater treatment, PW biotreatment installations are limited. The research described in this paper focused on the aerobic biotreatment of PW from the Qatari "North Shore" gas field supplemented with either 1.5% kinetic hydrate inhibitor (KHI) or 1.5% thermodynamic hydrate inhibitor (monoethylene glycol, MEG). KHI and MEG are "field chemicals" added off-shore during the winter months. This research was part of a larger project assessing the biotreatability of produced water from both summer and winter seasons. Although the feed pH was 4.5 and the biomass used as seed was cultured at pH 5.5, the bioreactor pH stabilized at 2.6 when KHI or MEG were added. Active biological oxidation was demonstrated in our tests for a period of 7 months through COD tests and in-situ dissolved oxygen (DO) and oxygen uptake rate (OUR) measurements. When 1.5% KHI in PW was added to the bioreactor, the DO decreased sharply and the OUR increased quickly from 0.2 to 1.9 mg O2/L.min. The COD results indicated that 43% and 81% of the organics present were removed through biotreatment of PW dosed with 1.5% KHI or MEG respectively. The concentration of 2-butoxyethanol (one of the two main components in KHI) was reduced from >5,000 mg/l to <10 mg/L indicating effective biodegradation of these chemicals even under the acidic conditions of our reactor. Removal of KHI and MEG by simple stripping was also investigated and results indicated that stripping was not responsible for significant COD removal. Literature references on aerobic biological activity at pH 2.6 are scarce. We postulate that the biological activity in our reactor is producing acids that resulted in the pH depression. One possible mechanism by which the acid production could have occurred through the bio-oxidation of either KHI and MEG has been developed and will be included in this presentation. A discussion of the specific fungus believed to be responsible for this unexpected biological activity is also included. The results are aligned with the Qatar National Vision (QNV) 2030 and fall directly within "Energy and Environment" Research Theme of the QNRS in two areas: i) water security, and ii) energy security. With the recent announcement that Qatargas is building a water reuse system incorporating membrane bioreactor and reverse osmosis technologies, the research results are particularly relevant.