Soil fertility and global food security depend on a regular addition of plant-available nutrients, such as nitrogen (N) and phosphorus (P), either in the form of manufactured fertilisers or animal manure, to agricultural soils [1]. There is a thin line between the optimum amount and timing of N fertiliser and its over-supply. N over-supply can quickly lead to serious environmental problems, since excess N is typically lost from the soil system, contaminating bodies of water [2, 3]. In contrast to N, which is effectively unlimited in its atmospheric form, high-quality rock reserves of P are limited and expected to deplete within a few hundred years [1]. P does not leach through the soil but it is prone to excessive soil accumulation, and is subsequently exposed to the risk of erosion into water courses while being sorbed to soil particles [4].
Environmental problems associated with N and P use are particularly pressing in the Baltic Sea Region (BSR), since excessive inputs of nutrients coming from the surrounding land are among the primary causes of the Baltic Sea eutrophication [5]. N and P entering water bodies that originate from the application of synthetic fertilisers or farmyard manure are regarded as non-point source pollution. As of 2014, non-point sources in the BSR contributed 46.5% and 35.7% of total N and P riverine loads, respectively [6]. Point source pollutants are another significant N and P loads to BSR, and mostly originate from wastewater treatment plants (WWTPs). The contribution of point source pollutants to total riverine load entering into the Baltic Sea in 2014 was considerably smaller than that of non-point sources, i.e. 11.7% and 23.5% for total N and P, respectively [6].
In agriculture, nutrient recovery and reuse practices have a potential to address the most pressing problems related to nutrients use in the food chain, such as pollution, depletion of finite resources (such as P), and waste management. Agricultural waste consists of livestock manure, primary agricultural residuals (such as post-harvest crop residuals), and secondary agricultural residuals (from crop processing in agricultural industries). If not properly managed, this waste can be a significant environmental and economic burden [7].
Spreading manure on agricultural land constitutes approximately 53% of the P and 33% of the N applied annually to agricultural soils in the EU27 [8]. However, the spatial segregation of crop-intensive and livestock-intensive areas leads to uneven spatial distribution of manure, creating nutrient–deficient areas and nutrient hot-spots [9,10,11]. Finding cost-effective manure processing technologies to create safe and stable fertilisers from organic waste streams is thus a fundamental quest for sustainable agricultural production.
Domestic wastewater also represents an organic waste stream, from which nutrients can be recovered for agricultural use. The focus within the wastewater sector has, however, traditionally been on removal of organic matter, and P (including N to a certain degree) from the effluent by applying various treatment methods to protect receiving waters against eutrophication, rather than on nutrient recovery per se. Since recently, however, there is a shift in thinking towards circular economy that is defined as an economy where the value of products, materials and resources is maintained in the economy for as long as possible, and the generation of waste minimized [12]. This paradigm applied to the wastewater sector means a shift from the sole focus on waste treatment and nutrient removal to the recovery of energy and nutrients from waste and further reuse of these products [13, 14].
Some nutrient reuse from domestic wastewater (P especially) is being done via application of sludge to agricultural fields. The P content in sludge depends on whether P removal processes are applied at the WWTP where P removal from wastewater into the sludge can be achieved by different chemical precipitation or biological removal processes [15]. The suitability of sludge as a fertiliser in agriculture is, however, debated in many countries due to contaminants that can be found in it. In addition, WWTPs are typically not located close to the arable land where sludge from wastewater processing could be applied [11]. Furthermore, the recovery of N through sludge application is low when compared to P recovery rate, since most N is either removed by denitrification or remains in the treated wastewater at conventional WWTPs [13]. For example, Van der Hoek et al. [16] showed that for Amsterdam’s WWTP about 38% of the incoming N was captured in the sludge. Out of all N-carrying inflows to the WWTP, urine would be the most interesting source for N recovery if captured separately [16]. Thus, separate collection and treatment of blackwater (wastewater from the toilet only) captures this N-rich stream and is a technical solution on the rise in several countries [17].
Nutrient recovery technologies can be applied to different waste streams, where a higher starting nutrient concentration will make the waste stream in question obviously more valuable. Anaerobic digestion of sludge, agricultural waste and blackwater is widely applied. Anaerobic digestion produces biogas which can be used as renewable energy. Dewatering of the digestate, often applied to reduce its weight, results in a liquid and a solid digestate phases. The liquid phase of anaerobic digestate is a concentrated source of nutrients, such as N and P, to which then nutrient recovery technologies can be applied. By combining anaerobic digestion and nutrient recovery technologies, a treatment process can be achieved that provides both renewable energy and nutrients for plants. Van der Hoek et al. [16] showed that the digestate has a potential of recovering 27% of the incoming N to the WWTP. Nutrient recovery from wastewater and agricultural wastes could decrease the need for mineral N and P fertilisers, reducing the pressure on respective biogeochemical cycles [18, 19] and further is an important and integral contribution of the wastewater sector to a circular economy.
Potential solutions
Two promising technologies for N and P recovery identified in systematic maps of ecotechnologies for recovering nutrients and carbon from domestic wastewater [20] and agricultural waste streams [21] are struvite precipitation and ammonia stripping.
Struvite precipitation and recovery is an ecotechnology that can be used mainly for P recovery and was one of the most represented ecotechnologies identified in both systematic maps (unpublished data). Struvite is a crystalline mineral composed of equimolar concentrations of magnesium, ammonium and phosphate with the chemical formula MgNH4PO4*6H2O. Struvite is formed under alkaline conditions and the process depends on specific and controlled molar ratios in the liquid, pH, aeration, reaction time and temperature. Usually, magnesium must be added in order to achieve sufficient struvite precipitation when the technique is applied to domestic or manure wastewater. Under optimal conditions, up to 94% of the P can be recovered as struvite and approximately half as much N, because struvite contains approximately 0.5 kg of N per kg of P [22]. Struvite is an effective, slow-release fertiliser with a relatively low content of contaminants, which can replace fertilisers produced from phosphate rock [22]. The value of struvite as fertiliser has only been recently understood and it is now the focus of increasing research attention [23].
Ammonia stripping is applied to liquids containing high concentrations of ammonia [24, 25], and using this method up to 98% of the ammonia in the liquid can be removed in a given flow stream [25]. High temperature and pH increase efficiency of ammonia stripping since this leads to a larger fraction of N as gaseous ammonia. The stripped ammonia gas is then recovered by absorption to an acid, commonly sulphuric acid. The resulting product is a low pH ammonium sulphate, used as a fertiliser recommended for use on soils with alkaline or neutral reaction [24].
Both struvite precipitation and ammonia stripping could potentially be incorporated into existing WWTPs and manure management processes to enhance nutrient recovery, improve WWTP function, and contribute to an increased P recovery. The liquid phase of anaerobic digestate is a concentrated stream of nutrients, commonly produced in the current management process of both manure and wastewater. Therefore, the focus of this review is on the liquid phase of anaerobic digestate as a source of nutrients for recovery. We have chosen struvite recovery and ammonia stripping for this review in order to focus on both P and N recovery, since the N content of struvite is too low to be considered as a N fertiliser.
Although there are some relevant reviews on the topic [9, 26, 27], to our knowledge, no systematic reviews of effectiveness of modern ecotechnologies for reuse of N and P from anaerobic digestate have been conducted. Here, we define ecotechnologies as “human interventions in social-ecological systems in the form of practices and/or biological, physical, and chemical processes designed to minimise harm to the environment and provide services of value to society” [28]. This definition was produced by a thematic synthesis of definitions in the literature, encompassing both hard (e.g. mechanical or chemical) and soft (e.g. behaviours and practices) technologies and has been used in two other, preceding systematic maps [20, 21].
Stakeholder engagement
The topic for this review was initially proposed by the research funder BONUS (https://www.bonusportal.org/). The scope of the project was then refined through expert discussions as part of the process of drafting an application in response to the call by the research funder. The scope and the search strategy were further refined by a stakeholder group consisting of the broader BONUS RETURN project consortium members (see https://www.bonusreturn.com/), local stakeholders from the three BONUS RETURN case study areas in Finland, Poland and Sweden, as well as external experts from these countries, which explains the Baltic Sea basin focus.
