Wetland Treatment of Oil and Gas Well Waste Waters

Constructed wetlands are small, on-site systems that possess three of the most desirable components of an industrial waste water treatment scheme: low cost, low maintenance and upset resistance. The main objective of the present study is to extend the knowledge base of wetland treatment systems to include processes and substances of particular importance to small, on-site systems receiving oil and gas well waste waters. A list of the most relevant and comprehensive publications on the design of wetlands for water quality improvement was compiled and critically reviewed. Based on our literature search and conversations with researchers in the private sector, toxic organics such as phenolics and b-naphthoic acid, (NA), and metals such as Cu(II) and Cr(Vl) were selected as target adsorbents. A total of 90 lysimeters equivalent to a laboratory-scale wetland were designed and built to monitor the uptake and transformation of toxic organics and the immobilization of metal ions. Smectitic clays (hectorite and montmorillonite) modified by alkyl diamine-type and quaternary ammonium-type surfactants were shown to adsorb strongly metal ions (Cu(II) and Cr(Vl) ) and toxic organics such as b-naphthoic acid (NA). The surfactant-modified clays are expected to act as inexpensive additives to a wetland to enhance its sorption potential for a number of pollutants, organic and inorganic. Similarly, metal uptake studies with unicellular green alga, Chlorella vulgaris, were undertaken to evaluate the ability of algae to enhance the metal uptake potential of wetlands. It was shown that Chlorella vulgaris was a potent adsorbent for Cd(II) ions from aqueous media. Studies on the uptake of toxic organics such as phenol and b-naphthoic acid (NA) and heavy metals such as Cu(II) and Cr(VI), the latter two singly or as nonstoichiometric mixtures by laboratory-type wetlands (LWs) were conducted. These LWs were designed and built during the first year of this study. The uptake of phenol by the wetlands was found to be quite rapid, and nearly complete in 50 hours, but it was also observed that there was a small, but measurable evaporative loss of phenol from the supernatant water during the same time period, especially during Summer months. A lower water depth in LWs resulted in a slightly higher evaporative loss of phenol, but the major removal mechanism of phenol was shown to be sorption to various components of a LW and degradation by its microbial constituents. Phenol sorption by LWs was enhanced by the addition of peat. A mass balance model was developed to quantify the fate of phenol in LWs. It was assumed that the fate of phenol in LWs is determined by evaporation of the solute and the solvent, adsorption of phenol onto various components of LW and its biodegradation, both in solution and at solid-liquid interface. Both zero order and first order kinetics for the overall disappearance of phenol fitted the experimental data. Evaporative loss of water appeared to be more important than the loss of phenol through evaporation. 13-naphthoic acid (NA) sorbed quite slowly and there was no indication of evaporative losses in the case on NA. The uptake of Cu(II) followed a tri-phasic behavior, attributed to partial hydrolysis and precipitation , sorption onto wetland components and a slow dispersion into underlying pore water of the LW. The addition of peat was observed to have only a minimal effect on Cu(II) immobilization by a LW. The uptake of Cr(VI) by laboratory-type wetland systems (LWs) appeared to be quite effective, and the addition of peat to the wetland produced only minor enhancement in Cr (VI) uptake. The simultaneous uptake of Cu(II) and Cr(V1) by LWs from a binary mixture of the two ions was also studied. Non-stoichiometric mixtures of metal ions were used to minimize precipitation. The uptake of Cr(VI) in presence of Cu(II) by laboratory-type wetland systems (LWs) followed first order kinetics with an average half life of 25 hours irrespective of the initial CuiCr ratio. On the other hand, Cu(II) removal was more complex with an average half life of only 3 hours. A road map and guidelines for a field-scale implementation of a wetland system for the treatment of oil and gas wastewaters have been suggested. Two types of wetlands, surface flow (SF) and sub surface flow (SSF), have been considered, and the relative merits of each configuration have been reviewed.

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