What evidence exists on ecotechnologies for recycling carbon and nutrients from domestic wastewater? A systematic map
Abstract Background Eutrophication of the Baltic Sea, and many other water bodies, is partly the result of point-source emissions of nutrients and carbon from wastewater. At the same time, nitrogen and phosphorus planetary boundaries have been breached. There is a need for more efficient resource management, including the recovery and reuse of nutrients and carbon in waste. The aim of this paper is to collate evidence on ecotechnologies intended for use in the wastewater sector globally to facilitate the recovery or reuse of carbon and/or nutrients. Methods Searches were performed on literature published between 2013 and 2017 and in 5 bibliographic databases, 1 search engine, and 38 specialist websites. Database searches were performed in English. Searches in specialist websites were also performed in Finnish, Polish and Swedish. There was no geographical limitation. Screening was conducted at title and abstract level, and on full texts. Apart from bibliographical information, we extracted information on ecotechnology type, intervention, details of the recovery or reuse, the type of wastewater stream to which the ecotechnology is applied, the study location, type and design. Prior to screening and coding, we conducted consistency checks amongst reviewers. We generated a searchable database of coded studies. Findings were synthesised narratively and visualised in a geographical information system (i.e. an evidence atlas). We identified a series of knowledge gaps and clusters that warrant further research. Results The search resulted in 4024 records, out of which 413 articles were retained after the screening process. In addition, 35 pre-screened studies from the specialist website searches were added. Together, these 448 articles contained 474 individual studies of 28 types of ecotechnologies. A combination of ecotechnologies (16.7%), followed by microalgae cultivation (14.1%) were the most frequent ecotechnologies in the evidence base. Ecotechnologies for recovery composed 72.6% of the evidence base. The most common wastewater streams for recovery were mixed wastewater and sludge (73.8%). There was a relative lack of studies on recovery from source-separated wastewater. The most common type of recovery was energy (27.3%), followed by simultaneous recovery of nitrogen and phosphorus (22.1%). Reuse of recovered substances was described in 22.8% of the studies. The most common type of reuse was of nitrogen and phosphorus (57.4%), followed by joint reuse of organic carbon, nitrogen and phosphorus (35.2%). Reuse ecotechnologies were mostly focused on the use of wastewater for irrigation or reuse of biosolids, and not on the nutrients that had been extracted through e.g. precipitation of struvite. In 22 studies both recovery and reuse were described. In total, 60 different study countries were reported in the evidence base, and the most common study location was China. Conclusions We found substantial evidence for the recovery and reuse of nutrients and carbon from wastewater sources. The relative abundance of studies where substances are recovered compared to studies where they are reused, suggests a knowledge gap on reuse of recovered nutrients and carbon. The majority of studies on reuse were on irrigation with treated wastewater or reuse of biosolids, and not on reuse of extracted nutrients such as struvite.
