Use of reclaimed wastewater as fertilizer in hydroponic greenhouse systems

Maximum nutrient utilisation from wastewater reuse is one of the main issues in circular economy and it affects the use of freshwater. In December 2019, the European Union launched a directive on use of wastewater for irrigation purposes. This project investigates the possibility to reuse both blackwater and kitchen wastewater as fertilizers in closed hydroponic systems, in order to close the loop of nutrients. Blackwater and liquid kitchen wastewater were collected from a water reclamation plant (RecoLab, Helsingborg, Sweden) before and after anaerobic digestion. Microbiological and chemical analysis were performed to see process stability and product consistency at four and three incidents, respectively. The microbiological analysis and characterization were based on determinants specified in the EU directive. The microbial analysis showed that the anaerobic digestion process itself is affecting only some of the microbial target and index organisms in a significant way. In the culture dependent analysis significant reductions have been found for all the target organisms (enterococci, total coliforms, E. coli and Legionella pneumophila) in blackwater samples after anaerobic digestion; in the culture independent analysis significant reductions have been obtained only for E. coli and Legionella in blackwater. The liquid kitchen wastewater didn’t show significant reduction in most of the target microorganism after the AD process. Both culture-dependent and culture-independent methods showed that the results are affected by broad ranges and standard deviations, thus the anaerobic process lacks in directionality. Similar results have been obtained with the chemical analysis of both influents to and effluents from the anaerobic digestion process, that displayed no accumulation or depletion of nutrients after the anaerobic digestion process. Trials for wastewater efficiency as fertilizer were performed on two crops, in two types of cropping systems: hydroponic subirrigation systems and soilless systems using a peat-coir based growing medium. Both types of digested wastewater were trialled in a greenhouse hydroponic system on a vegetative phase crop, komatsuna (Brassica rapa var. perviridis) whereas only digested liquid kitchen wastewater was used in the soilless system, employing tomato as a model crop. Pure or 50% diluted liquid kitchen wastewater was not suitable to fertilize hydroponically grown komatsuna. When fertilized with adjusted to maximum tolerable sodium concentrations with and without compensation with inorganic nutrients, the development and biomass production of hydroponically grown komatsuna was significantly lower than the one of komatsuna grown in full-strength nutrient solution based on synthetic fertilizers. When used as a fertilizer in a soilless grown tomato crop, tomato fertilized with 50% or full strength of AD treated liquid kitchen wastewater showed similar plant performance as tomato fertilized with 50% or full strength of nutrient solution prepared with synthetic fertilizers. However, there was a trend to impaired tomato fruit quality due to increased incidence of blossom end rot when tomato plants were irrigated with 100% of liquid kitchen wastewater. From the findings in the present study it can be concluded, that (i) the microbial quality and the chemical consistency of digested blackwater and liquid kitchen wastewater are very variable, that (ii) the load of microbes and chemicals post process cannot be predict by the pre-process properties, that (iii) raw digested liquid kitchen wastewater and blackwater need to be optimized to be used as sole nutrient sources in hydroponic culture of komatsuna and soilless culture of tomato.