Abstract
BackgroundA regular supply of nutrients such as nitrogen and phosphorus to agriculture is needed for global food security, and increased recycling of nutrients back to agriculture from organic waste streams is necessary for increased rural–urban sustainability. Anaerobic digestion of sewage sludge and agricultural wastes is widely applied to stabilize the substrate and capture some of its energetic value via biogas production. Anaerobic digestate is a concentrated source of nutrients to which nutrient recovery technologies can be applied. By combining anaerobic digestion and nutrient recovery technologies on the digestate, both energy and nutrient recovery can be achieved. Two promising technologies that could increase nutrient recycling from different types of wastewater are struvite precipitation and ammonia stripping. This review examined the effectiveness of these ecotechnologies for the recovery of nitrogen and phosphorus from anaerobic digestate with the aim of reducing the impact of waste on the environment.MethodsWe searched for academic and grey literature published after 2013. Searches were performed in 5 bibliographic databases in English, in the search engine Google Scholar in English, Swedish, Finnish and Polish, and across a range of organisational websites in English, Swedish, Finnish and Polish. Eligibility screening was conducted at two levels: ‘title and abstract’ and ‘full text’. Included eligible studies were subject to a critical appraisal that assessed external and internal study validity. We extracted information on study characteristics, intervention, comparators, effect modifiers, and measured outcomes. Data synthesis included narrative synthesis of each study of sufficient validity. We performed quantitative synthesis on a subset of studies.Review findingsThe review included 30 studies on struvite precipitation and 8 studies on ammonia stripping. Both pH and Mg:PO4 ratio were found to have a clear influence on the effectiveness of struvite precipitation process (and thus nutrient removal rates). The response to pH was found to be non-linear, resembling a bell curve with a maximum around pH 9.5. Mg:PO4 ratio was found to have a positive effect on removal up to a ratio as high as 4:1. However, it should be noted that high removal efficiencies were sometimes achieved at a ratio as low as 1:1 as well. Although the effects of pH and Mg:PO4 ratio were clear, the model developed could not accurately predict removal based on these two parameters alone. Studies on ammonia stripping were relatively heterogeneous. Due to the small size of the evidence base, and the heterogeneity between studies, no conclusions are presented regarding the influence of different process parameters on the outcome of ammonia stripping.ConclusionsIn conclusion, when performed under the right conditions (i.e. pH around 9.5 and Mg:PO4 ratio of at least 1:1), available evidence suggests that struvite precipitation is an effective technology for the recovery of nutrients from the liquid phase of anaerobic digestate. The evidence base is limited for ammonia stripping. We provided suggestions of which data to report in future studies.
Highlights
A regular supply of nutrients such as nitrogen and phosphorus to agriculture is needed for global food security, and increased recycling of nutrients back to agriculture from organic waste streams is necessary for increased rural–urban sustainability
In conclusion, when performed under the right conditions, available evidence suggests that struvite precipitation is an effective technology for the recovery of nutrients from the liquid phase of anaerobic digestate
The evidence base is limited for ammonia stripping
Summary
The review followed the Collaboration for Environmental Evidence Guidelines and Standards for Evidence Synthesis in Environmental Management [25] and conformed to ROSES reporting standards [26] (see Additional file 1). I.e. pH, Mg source, removal efficiency and molar ratios of P, N and Mg was extracted from the struvite precipitation studies that were included in the quantitative synthesis. Narrative synthesis for struvite precipitation evidence base Twenty-five studies were excluded during critical appraisal due to: presence of competing or additional interventions that were not controlled for (14), design flaws (3), reporting bias (2) and clarity issues (5). 2 studies were excluded since they lacked data on process parameters, and 5 experiments were removed as outliers Results of meta‐analyses of the struvite precipitation dataset The descriptive analyses of the struvite precipitation dataset showed that the recovery of struvite was substantial regardless of the type of substrate and other conditions in the performed experiments This was confirmed by the meta-analysis, in which a variance component model was fitted to the dataset already subjected to descriptive analyses (See Table 1 in Additional File 9). A more detailed version of the quantitative synthesis, including sub-group analyses with different substrates as well as a summary of different models fits, is available in Additional file 8
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