Beneficial Reuse of Production Water for Irrigation
Authors: Burgos M. , Lebas G. 
Published: December 2015

An increasingly challenging consequence of hydrocarbon exploitation is the associated production of important volumes of water (PW). The water that is not reinjected after treatment is –if allowed- discharged into the environment, but regulations are tight: severe specifications must be met to limit the impact on ecosystems, with the associated treatment cost.

This paper aims to demonstrate the feasibility of PW reuse for non-food crop irrigation, which offers both an innovative treatment alternative (especially in areas with strong water stress) and potential production of added-value materials like biomass for biofuel. Food crops (human/animal consumption) were ultimately not retained because of higher prohibitive costs associated with exhaustive water pre-treatment for required safety level, along with the risk of reduced acceptability.

Considering the geographical zones of interest and PW characteristics, candidate plant screening and preliminary germination tests have resulted in four energy crops: Reed Canary Grass (Phalaris Arundinacea), Switchgrass (Panicum virgatum), and 2 Sorghum (Keller and a new seed). A set of laboratory tests has started to study the bio-and geo-chemical processes in the soil-water-crop system, to identify the resulting impacts of reusing PW for these crops. Synthetic PW with different concentrations (considering targeted electro conductivity values and several dilution rates) has been used: solution "A" contains salt, metals and organic substances while solution "B" doesn't include organics, to allow further result comparison. While not determinant at laboratory scale, the effect of the two selected soil types (sand and silt clay) on the plant absorption process is deduced; additional tests -such as effects on soil structure/permeability and clay dispersion- are proposed.

Two types of tests have been performed at different growth stages (up until 6 weeks): hydroponics (nutrient solution + PW) and tests on a soil matrix. Phytotoxicity, growth rate and biomass quality have been evaluated (analysis of total elements, metals, etc.), and tolerance levels were determined. While an identified high concentration of salt has proven lethal for all individuals (ref. Food and Agriculture Organization, 1985), an encouraging growth rate and production quality have been observed at several concentrations. Organic substances have a direct toxicity effect on the plants; soil seems to positively buffer the negative effect of these organic substances. Volatilization phenomena is also observed yet hardly measurable (absorption/adsorption rates require precise quantification with greater plant production). Several elements (Ni, Zn, K) are found in plants in larger quantities after tests.

Larger-scale pilots are recommended to validate these results and determine additional impacts of irrigation using PW (e.g. accumulation of biofilms in pipe systems, corrosion or clogging), prior to full-scale implementation. The use of innovative crops associated with an optimized water treatment can offer an alternative to current exhaustive and expensive treatments.