Valorisation of discharge water from substrate-grown crops in greenhouses

Due to mismatch of nutrients, high sodium concentration, growth inhibition or system failures, greenhouse growers with substrate-grown crops tend to discharge their drain water. To comply with Water Framework regulations, in future, they must re-use drain water streams as much as possible. With membrane distillation or reverse osmosis, about 80-90% of drain water may be recovered as irrigation water. The remaining concentrate must be valorised otherwise or disposed of at high cost. This paper explores possibilities for economic viable reuse of discharge streams. Fertilisers in drain water have an intrinsic value of about € 1.75/m3. Transport costs are high, about € 2.50/m3 per load at 25 km, so there is a preference for local or nearby reuse or to further reduce concentrate volume. Potential users are horticultural growers who irrigate additionally a salt-tolerant substrate, container or field-grown crop, or dairy farmers who may fertilise grassland with nutrient and sodium rich water. Fertiliser suppliers may process the concentrate after collection and sell it as a mixing fertiliser in agriculture. Algae cultivation for bio-fuel or fodder could eventually form a market. Each user has its own demands in terms of fertiliser composition and the presence of sodium, germs and/or pesticides. Therefore, processing prior to re-use is usually needed. More than 90% of the pesticides and 100% of the germs can be removed with advanced oxidation (H2O2-UV). Electro-chemical flocculation and O3 with activated carbon may eventually be used as well. Separation of monovalent and divalent ions by nano-filtration or electro dialysis can be used to meet nutrient composition requirements for using the concentrate as N-fertiliser. Valorisation of discharge streams is economically feasible, provided that there is a legal exemption for re-using waste water. However, this market must compete with other richer nutrient streams such as animal manure.