Membrane technology for water production in agriculture: Desalination and wastewater reuse

The problem of water shortage affects not only human and industrial activities but also an adequate and sustainable food production. Water quality in agriculture does not have the same requirements as that of drinking water and also properly treated wastewaters can be used for irrigation. The two possible alternative sources of water for agriculture are: desalinated water and wastewater. In this paper membrane operations usually utilized for water production in agriculture are discussed. Their main advantages are: (i) their mutual compatibility which offers the possibility of combining different membrane operations to achieve the desired water qualities and (ii) their flexibility and easy scale-up which allow passing from small to large scale, from centralized to decentralized systems. The membrane technology mostly used in desalination is reverse osmosis (RO) whereas membrane bioreactors (MBRs) deal with fresh water reclamation from wastewater streams. Novel membrane technologies are described, such as forward osmosis (FO) and membrane distillation (MD), a promising prospect for agricultural water production, and the possible recovery of nutrients from saline waters and wastewaters. Nevertheless, the development of some novel technologies needs to be accelerated to reduce the costs associated with treatment and to avoid further impacts of water scarcity on food production. © 2015 Elsevier B.V. All rights reserved.

Water and nutrients are the main constituents for plant growth. The transfer of elements in the water – soil – plant chain is part of the bio- chemical cycling of chemical elements. Several aspects control the nat- ural element fl ow from the non-living to the living species such as geochemical, climatic, biological and anthropogenic factors. Intelligent solutions can accelerate this process necessary to sustain the increasing food requirements due to the population growth. Among the available technologies, membrane operations, owing to their intrinsic character- istics such as good molecular separation and low energy consumptions, are emerging as best available technologies for wastewater treatment and reuse also in the agricultural sector. Moreover, they are already the most widespread processes used for fresh water production from seawater. Desalinated water, owing to its cost, is normally used for industrial and human needs. Early wastewater reuse was for irrigation of golf courses and lawns. Other uses include industrial reuse, irrigation of ed- ible and non-edible agricultural crops, and indirect potable reuse, such as groundwater recharge. Industrial reuse includes cooling towers, boiler feed, and the cooling cycles of power plants. However, water production for irrigation, either from desalination or from wastewater, will increase the expenses which most probably would return as a burden to farmers and then result in higher food prices. A study con- ducted in Greece found that 57.9% of the farmers are ready to pay for ir- rigation water a cost equal to the half of the fresh water. Moreover, it was proved that if water for irrigation is available, 33.6% do not want to use recycled wastewater and only 8.4% of the farmers are interested in using recycled wastewater at a cost lower than that of fresh water [112] . Different membrane operations c an be utilized for water treat- ment and reuse, such as MBR, FO and MD. MBRs are already applied for wastewater treatment at industrial level around the World. Fertilizer driven FO is an interesting combination of producing irri- gation water and fertilizer solutions. In this case the additional treatment of the draw solution can be neglected and the produced water/fertilizers can be applied directly on the fi eld. If other types of draw solutions should prove to be mo re effective for water production, FO can also be combined with MD for re-concentrating the draw solution and producing fresh water. Moreover, membrane operations such as FO and MD have fewer fouling problems compared to pressure driven membrane operations. Biofouling can be minimized by implementing quorum quenching. Hybrid RO – MD – MCr systems can increase the overall water production, decrease energy consumption and recover valuable salts from the brine [113] ( Fig. 10 ). Theoretical and experimental data of MD/MCr treating RO brine have shown that NaCl can be recovered from the solution at a concentration factor around 4.5 with respect to the RO brine [114] , therefore if the MCr process is stopped after NaCl recovery, the other components in the RO brine can be retained in the solution and eventually blended with fresh water to reach a concentration appro- priate for irrigation purposes. MCr c an also be utilized for the production of fertilizers as high quality crystals. In conclusion, membrane processes are promising technologies for approaching water solutions in the agricultural industry. The various techniques both as stand-alone units and in integration can address the different water qualities required by the agricultural industry ac- cording to crop selection, soil and existing water resources. Moreover, the possibility of having small and large scale plants together with cen- tralized and decentralized systems according to the speci fi c require- ments in the given area makes membranes an interesting answer for water production in agriculture. RO MD MCr Seawater Fresh water Fresh water Fresh water Irrigation water RO Brine MD Brine MCr Brine Salts e.g NaCl Fig. 10. Flow sheet of the hybrid RO – MD – MCr system for potable water, irrigation water and salt production. 30 C.A. Quist-Jensen et al. / Desalination 364 (2015) 17 – 32 Many studies highlight the effect of water quality with respect to crop yield, to the hazardous components that can accumulate in the soil and on fruit or vegetables or other crops consumed by humans due to their potential negative health effects. Wastewater treatment by membrane operations can also address the removal of substances harmful to human health (such as pathogens or other contaminants from, e.g., drug degradations). Improved membrane engineering, better membranes and enhanced integrated membrane systemsarenecessary toreduceproductioncosts, increase ef fi ciency and obtain water with quality adequate for the spe- ci fi c crops and with a better vision of the future in terms of sustainable food production.