Reuse of significant quantities of produced water (PW) extracted during gasfield operations requires treatment to remove both organic and inorganic materials. Biological treatment is generally regarded as the most cost-effective method for organics removal. For industrial waste waters, biotreatment faces distinct challenges because the PW composition can dramatically affect sludge settleability, a critical parameter in the operation of conventional biotreatment systems. Membrane bioreactors (MBRs) have an inherent advantage and have proved to be successful in the treatment of industrial waste waters because a membrane filter is used to separate the treated water from the sludge rather than separation being contingent on biomass settleability.
The outcomes of a bench-scale experimental study on the application of an MBR to the biotreatment of PW from Qatari gas fields are presented for three operating parameters: hydraulic-retention time of 16 to 32 hours, solids-residence time of 60 to 120 days, and temperature of 22 to 38°C. The impact on chemical-oxygen-demand (COD) removal was evaluated through experimental testing by use of three parallel bench-scale MBRs. Low sludge concentrations (0.3–1.5 g/L of volatile suspended solids) were attained throughout, with instantaneous-flux values ranging from 3 to 15 L/(m2·h).
Results indicated that the COD removal averaged 60% (54–63%), approximately one-third of this value being attributed to physical removal, with the operating parameter values shown to have no statistically significant effect on removal. Although trends were consistent with some previously reported studies performed on refinery waste water, overall removals were lower than expected. The pH of the bioreactor sludge ranged from 4.9 to 6.0, averaging 5.2, compared with a feedwater pH of 4.3, possibly contributing to the low carbon removal recorded. Adjustment of the feed pH to more than 6.5 caused a precipitate to form that contributed to membrane fouling. However, all feedwater acetate and more than 90% of the oil and grease were removed by the MBR treatment.
Treatment appeared to be carbon-limited, accounting both for the absence of nitrification (with all removed organic nitrogen apparently being assimilated into the sludge) and for the low sludge-solids concentrations attained. Evidence suggests the feedwater contains a significant fraction (approximately 40%) of highly recalcitrant organic compounds presumed to be nitrogen-containing field chemicals (e.g., scale inhibitors and corrosion inhibitors).