In petroleum production operations a large amount of waste water needs to be treated before disposal to meet environmental regulations. Fundamental mechanisms involved in the treatment of produced water are described for some of the important processes available including carbon adsorption, polymer flotation, wet oxidation and air stripping. These processes are compared in terms of their effectiveness as well as economics. It is shown that traditional waste water treatment processes such as granular activated carbon adsorption, even though effective in reducing the TOC level, is more expensive to operate than the newer processes described in this paper.
Water produced with oil and gas contains various organic components of the crude oil such as paraffins, naphthenes and aromatics. In addition, organics added to protect equipment such as antifreeze, corrosion inhibitors, etc. as well as those added to enhance oil recovery such as polymers and surfactants are also found in produced water. All these organic compounds contribute to an increase in the dissolved total organic carbon (TOC) in produced water. The TOC content of produced water may be as high as 2000 ppm while the maximum TOC level permitted may be as low as 50 ppm. For example, the produced water from Shell's Peace River in-situ project contains 350 ppm of TOC (1)while produced water from Gulf of Mexico offshore platforms contains about 1,244 ppm of TOC(2). In addition to TOC, total dissolved solids (TDS) also needs to be removed. In many cases a high concentration of TDS may be found. Shell's Peace River in-situ project water contains about 1770 ppm of TDS along with dissolved calcium and magnesium salts(1).
The quantities of water produced varies from field to field and depends on many factors including the type of process. For example different Oil-Shale retorting processes may produce between 0.1 to 22 barrels of contaminated retort water per barrel of oil produced(3).
Many of the TOC causing organic compounds originate from the crude oil itself. Many low molecular weight volatile organic compounds of crude oil including paraffins such as n-butane, n-pentane, n-hexane etc. and aromatics such as benzene and toluene are moderately soluble in water. Some other organic compounds are added for operational purposes. For example formic and acetic acids may be added as biocides, methanol and ethanol may be added as antifreeze and surfactants or polymeric compounds may be added as mobility control agents. The low molecular weight volatile components are also known as volatile organic carbons (VOC). The VOC's are easier to remove from the waste water than the heavy molecular weight, non volatile compounds.
There are many waste water treatment processes available. Some involve removal of TOC causing organics by one means or another while others involve conversion of organics into CO2 by oxidation. The type of process to be selected depends on the type and concentration of contaminants and the degree of purity required in the treated water.