Atlantic Canada Offshore R&D: Treatment of Oilfield Produced Water by Chemical Coagulation and Electrocoagulation
Authors: Younker J. , Lee S.Y. , Gagnon Graham , Walsh M.E. 
Published: May 2011

Produced water is a wastewater generated during oil and gas production; it is typically discharged from offshore platforms into the open ocean following treatment. The current regulations and treatment technologies for effluent quality focus on the total oil and grease content of the wastewater (30 mg/L per day in Atlantic Canada), despite concerns about the potential toxicity of dissolved species. Coagulation with metal salts has long been used in the drinking water industry and has been shown to remove dissolved natural organic matter. However, this requires constant chemical input. This research assesses an alternative treatment technology, electrocoagulation (EC), for the treatment of produced water, using chemical coagulation (CC) as a benchmark. A synthetic wastewater containing a crude oil emulsion in a brine solution was treated at bench scale using EC and CC in a jar tester with dissolved air flotation (DAF). Tests were conducted at pH 5 and 8, with ferric chloride (FeCl3) coagulant used in the CC tests and a pure iron anode in the EC tests. The highest chemical oxygen demand (COD) reduction (62%) was obtained during CC testing at pH 8 at a FeCl3 dose of 80 mg/L. The best COD removal with EC (56%) occurred at a dose of 20.6 mg/L of iron at pH 5. CC and EC had comparable performances with respect to COD removal; however, future work focusing on dissolved hydrocarbon removal is necessary in order to evaluate EC for the treatment of produced water.

Introduction

Produced water is the largest wastewater stream in the oil and gas industry, and is generally disposed of from offshore oil and gas platforms by discharge into the open ocean (Gomes et al. 2009). Globally, it is estimated that 250 million barrels of produced water are generated per day, with a corresponding oil production of 80 million barrels per day (Ahmadun et al. 2009). In other words, about three barrels of produced water are generated for every single barrel of crude oil (Gomes et al. 2009).

Produced water commonly contains compounds such as dissolved and dispersed oil, formation minerals, and production chemicals (Ahmadun et al. 2009). In Atlantic Canada it is required that "the 30-day weighted average of oil in discharged produced water does not exceed 30 mg/L and that the 24-hour arithmetic average of oil in produced water does not exceed 60 mg/L" (OWTG, 2002). While current treatment standards focus on the removal of total oil and grease, dissolved hydrocarbons are known to be major contributors to produced water toxicity but are not readily removed by current treatment technologies (Veil et al, 2004). Coagulation has been used to successfully remove dissolved natural organic matter from surface waters, and thus is a promising technique for removing dissolved oil from produced water (Gregor et al, 1997).

However, conventional coagulation requires constant chemical input and acidifies the water (Droste, 1997). The purpose of this project is to develop a new approach to produced water treatment using electrocoagulation, which uses electrochemically generated metal hydroxides to target the removal of dissolved hydrocarbons.

This research is investigating electrocoagulation (EC) with dissolved air flotation (DAF) as a treatment technology for the treatment of offshore oilfield produced water, using conventional (chemical) coagulation (CC) as a benchmark.