Assessing the Biotreatability of Produced Water from a Qatari Gas Field

Biological treatment is generally regarded as the most cost-effective method for the removal of organics from wastewater. Biotreatment of produced water (PW) presents many challenges when compared with municipal wastewaters or industrial waste streams. These challenges are typically linked to its high level of organics (including refractory organics) and salinity which can dramatically reduce biological floc formation and biomass settleability in conventional clarifiers. In a membrane bioreactor (MBR), an ultrafiltration membrane separates the treated water from the biomass and settleability is not a factor. This paper summarizes the results of tests conducted in Qatar with three parallel bench-scale membrane bioreactors to assess the biotreatability of PW from a local gas field. The testing was partially funded by Qatar National Research Fund which is testament to the importance of the research and the value it brings to Qatar and the world. The test program followed a Box-Behnken experimental design requiring 13 separate tests with input parameters of hydraulic retention time, solids retention time and temperature and response parameters of % COD removal and oxygen uptake rate (OUR). The results indicated ≈60% of the COD is removed through treatment in a membrane bioreactor. Statistically, only SRT was shown to be a factor in the percentage COD removal. At an SRT of 60 days, the average percentage COD removal was 62%, 4% higher the 58% average COD removal obtained at an SRT of 120 days. The OUR ranged from 0.10 to 0.19 mgO2/L-min and was shown statistically to be only dependent upon HRT with the highest OUR obtained at the shortest HRT of 16 hours. It is concluded that if biotreatability is shown to be cost-effective, it can contribute as part of an overall system to treat PW prior to recycle or reuse. This can reduce the facility's demands for fresh water and can thereby make existing potable water supplies available for other important uses.