A systematic review of food-waste based hydroponic fertilisers.

This review article seeks to evaluate existing research in hydroponic systems which utilise a fertiliser solution derived from food-waste, also known as "Food-waste based hydroponic fertilisers" (FWBHF). FWBHF research is rooted in addressing increasing concerns surrounding food security, addressing both non-productive waste streams and sustainable production of hydroponic fertilisers. In 2018, the world was predicted to have wasted 931 million tonnes of food, 17 % of the total food produced throughout the year (FAO, 2021). Meanwhile, existing hydroponic systems rely on synthetic fertilisers which are constituted from unsustainable processes, such as Haber-Bosch systems or mining for phosphate rocks. These practices contribute heavily to greenhouse gas emissions or rely on destructive exploitation of finite reserves, which researchers believe will increase in price as accessible reserves are exhausted (Liu et al., 2020 ; Cordell et al., 2011). With increasing population in urban areas, the demand of produce imported from regional areas grows alongside the density of waste generation. Thus, exploring methods to re-utilise urban food-waste in urban horticultural systems may help in improving food security, reducing waste, and providing a local source of fresh produce for consumers. The objectives of this review article are to : i) Utilise PRISMA protocol to collect and synthesize existing literature related to food-waste based hydroponic systems, ii) Identify major challenges found across literature which inhibit yield outcomes in food-waste based hydroponic systems, iii) Explore potential improvements using conventional or non-conventional methods, including chemical, physical, and biological modifications to existing systems, iv) Suggest a standardized reporting framework for future research in this area. Using the PRISMA protocol, 6840 papers were identified with key words: "Food-waste AND hydroponic AND fertiliser," "Organic AND hydroponic AND fertiliser," and "Organic AND Hydroponics." 308 papers were selected based on the relevance of their title and abstract. After considering quality, overlaps, and relevance, 37 papers were chosen to be part of this systematic review. Literature was chosen based on its contents utilising any form of processing to prepare waste generated from the food-waste industry for use in a hydroponic system. These papers utilised waste generated at i) Farm, ii) Industry, and iii) Consumer, levels as well as a range of novel methods such as fermentation, steaming, or composting. This review studies how both feedstock composition and processing methodologies play a role in determining the efficacy of a food-waste based hydroponic fertiliser. It was found that while feedstock plays a larger role in the final nutritional composition, categorisation by methodology offers greater clarity for future research. This is attributed to "methodology" being the primary factor researchers can influence, with feedstock being based heavily on regional, industrial factors.. Overall, no clear trends or processing methods were identified as being particularly effective, with the strongest relationship between a FWBHF and positive yield outcomes being its similarity to the corresponding synthetic control solution. Synthesis of existing literature identifies two key avenues for future research: i) nutrient/salinity imbalancesand ii) microbial activity. Nutrient deficiency and excess salinity are identified as the main limiting factors in FWBHF research and are closely related. N deficiency was the most recurring nutrient deficiency, although deficiencies in P were also common. Excessive salinity limits plant available macro-nutrients and was more frequent in research which utilised food-waste generated at "consumer" levels. Cultivating microbial agents may potentially improve overall plant yield by improving mineralisation of nutrients, assisting rhizosphere efficiency, or by antagonising pathogenic species. Finally this review suggests a standardized reporting system. The infancy of this research leads to irregular reporting standards. The suggested reporting procedure seeks to amend discrepancies by clearly establishing a list of 21 factors which have been reported across the available literature and attributes a priority ranking to the relevance of the paper based on its field of study. This review article seeks to develop a synthesis of existing research for a clearer direction of development for food-waste based hydroponic research. This research is important as it aims to accomplish three goals in urban environments: i) Provide economically beneficial, local, and sustainable use for food-waste, ii) Improve their own food security by increasing local food production, and iii) Develop sustainable urban horticulture practices. • There exists no significantly effective methodology for the consistent conversion of food-waste into an effective hydroponic nutrient solution. • The primary obstacle to developing effective food-waste based hydroponic fertilisers is nutrient density relative to salinity. • Utilising food-waste based nutrients challenges existing sterile cultures in hydroponic systems. • This review suggests a framework which seeks to amend discrepancies in reporting standards. [ABSTRACT FROM AUTHOR]