Biological Oxidation of Dissolved Compounds in Oilfield-Produced Water by a Pilot Aerated Lagoon
Authors: Beyer A.H. , Palmer L.L. , Stock J. 
Published: December 2007

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Introduction

Despite the lack of documented adverse effects, environmental protection regulations severely restrict the offshore discharge of oilfield-produced waters. The California Ocean Plan (COP), adopted in 1972, regulates all domestic and industrial wastewaters discharged within California's 3-mile (4.8-km) zone. The 1972 COP includes strict limits on oil and grease, dissolved phenols and ammonium compounds, and toxicity to phenols and ammonium compounds, and toxicity to marine life. Federal guidelines proposed in 1975 for the offshore petroleum industry by the U.S. Environmental Protection Agency would require "zero pollutant Protection Agency would require "zero pollutant discharge" inside the 3-mile (4.8-km) zone by 1983.Besides small amounts of suspended oil, oilfield produced waters can contain dissolved organics (e.g., produced waters can contain dissolved organics (e.g., organic acids and phenols) and ammonium compounds that conventional oilfield gravity-separation and flotation processes are not designed to remove. Other processes are not designed to remove. Other water-treatment processes may be needed to remove such dissolved compounds and to allow ocean discharge of produced water to continue as an alternative to produced water to continue as an alternative to underground injection. Biological oxidation processes appear suitable for this application.The aerated lagoon is one of the most common biological oxidation processes. It is used widely for treating municipal wastewater and process water from oil refineries and chemical plants. The process is cost-competitive when sufficient land is available and is fairly resistant to sudden changes in feedwater characteristics. Water-retention times range from a few days to more than a month, and water depths range from 6 to 15 ft (2 to 5 m)Aerators are used to supply oxygen to the bacteria and may induce enough mixing in the lagoon to keep a significant portion of the bacteria in suspension. These bacteria then are carried out of the lagoon and contribute to suspended solids in the effluent. Therefore, clarification of lagoon effluent may be required to comply with suspended-solids limits.This paper describes a 19-month field pilot study conducted in southern California that demonstrated for the first time the feasibility of the aerated lagoon for biotreating oilfield-produced water.

Description of Pilot Plant

The two-stage pilot lagoon (Fig. 1) consisted of two 500-bbl (80-m3), plastic-lined steel tanks in series, each filled with 375 bbl (60 m3) fluid. Stage 1 was primarily for oxidizing suspended oil and dissolved organic compounds; Stage 2 was for oxidizing dissolved ammonium compounds. Each stage was aerated by a variable-speed, "egg-beater" type of mixer. Feedwater flowed continuously to the pilot plant, with each stage independently controlling the water-retention time.Feedwater to the lagoon was a mixture of produced waters from the Carpinteria and Summerland state leases after oil removal by induced-air flotation. The total dissolved solids (TDS) content of the mixture was 20,000 g/m3 . About 10 g of Na3PO4 (as P) was added per cubic meter of feedwater, to ensure adequate (though per cubic meter of feedwater, to ensure adequate (though not necessarily optimum) phosphorus for microbial growth.

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