Effectiveness of struvite precipitation and ammonia stripping for recovery of phosphorus and nitrogen from anaerobic digestate: a systematic review

Abstract

Background 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. 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. Methods We 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 findings The 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. Conclusions In 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.

Potential production of struvite from the anaerobic digestion of food waste: analysis in one-phase and two-phase configurations

“…Regarding other environmental benefits, PSP obtained through the two-phase configuration is a potential alternative for the traditional ammonia production methods, such as ammonia stripping (Sheets et al 2015) and the use of magnesium hydroxides (Castro et al 2017), and presents two important aspects associated with energy and economic consumption due to the need for external processes and inputs (Lorick et al 2020). In addition, it can also generate an annual revenue of ∼US$ 26,505 for a population of ,15,000 people, which is in line with the concepts of circular economy and biorefineries associated with comprehensive and sustainable FW management (Margallo et al 2019).…”

“…Given that food security at a global level depends on a regular supply of soil nutrients, such as nitrogen (N) and phosphorus (P), the demand for chemical fertilizers has been steadily increasing for the last 100 years. For these purposes, N is largely synthesized through the energy-demanding Haber-Bosch process, while much of the P is extracted from finite mineral reserves (phosphate rock) (Lorick et al 2020). The excessive demand for these elements has not only led to water resource eutrophication but also raised concerns regarding the future supply of fertilizers and food production (Cordell & White 2014).…”

“…One of the alternatives to chemical or inorganic fertilization is using biofertilizers, such as struvite, which naturally precipitate as crystals whenever the Mg:NH 4 :PO 4 concentration ratios exceed 1:1:1 (Castro et al 2017). Unlike other fertilizers, struvite is characterized by low water solubility, which increases its performance by reducing uncontrolled nutrient insertion in the soil and contributes significantly to the protection of the environment (Lorick et al 2020).…”

“…Vinardell et al (2021) theoretically analyzed the production of struvite from the AD of FW (AD-FW) in one-phase configuration. Finally, Lorick et al (2020) performed a systematic review of struvite production from organic waste, mainly sludge and animal manure; similarly, the present study focused on the production of this biofertilizer in one-phase configuration. For this reason, many authors, such as Ma et al (2018), assert the need to explore alternatives that can be used to recover valuable nutrients from AD-FW as these nutrients represent an important biofertilizer source.…”

Technologies for recovery and reuse of plant nutrients from human excreta and domestic wastewater: a protocol for a systematic map and living evidence platform

“…There are numerous reviews that summarise a subset of the literature on recycling nutrients from human excreta and domestic wastewater to agriculture. Typically, these reviews have focused on a specific: nutrient, e.g., phosphorus [6]; recovery technology, e.g., struvite precipitation [20]; input stream, e.g., digestate [25]; or combination thereof, e.g., struvite precipitation from digestate [13,14].…”