Liquid fertilizer products from anaerobic digestion of food waste: mass, nutrient and energy balance of four digestate liquid treatment systems
Graphical abstract
Introduction
Anaerobic digestion (AD) of food waste (FW) is increasingly used to produce renewable energy, in the form of heat and power or vehicle fuel, and nutrient-rich digestate for agriculture, to decrease the use of energy intensive mineral fertilizers (Laureni et al., 2013). However, the digestate has usually unbalanced nutrient ratios for plant growth (Camilleri-Rumbau et al., 2014). Large mass due to high water content increases the transportation need of the digestate, as the AD plants treating municipal FW are usually located far from agricultural lands (Babson et al., 2013). Digestate treatment by solid–liquid separation is an increasingly used treatment for the production of phosphorus containing solid digestate and liquid digestate containing water-soluble nitrogen and potassium. The solid–liquid separation of the digestate divides most of the mass into the liquid fraction decreasing its nutrient concentrations (Hjorth et al., 2010). Low nutrient concentrations and large mass complicate the use of the liquid digestate in agriculture and increase the transportation need (Chiumenti et al., 2013). To efficiently utilize the FW nutrients, the treatment of liquid digestate is needed to decrease its mass and increase nutrient concentrations.
The digestate liquid can be treated to remove water and simultaneously concentrate nutrients. This lowers the environmental impact (i.e. global warming potential and acidification) and reduces transportation costs to areas with nutrient deficits compared with digestate use as such (Rehl and Müller, 2011). In addition to the decreased transportation costs, the additional economic benefits of the digestate liquid treatment are related to the profit gained from the selling of the fertilizers (Fuchs and Drosg, 2013, Rehl and Müller, 2011). With the combination of solid–liquid separation and digestate liquid treatment, fertilizer products with optimal composition can be produced (Hjorth et al., 2010). Produced fertilizers can be designed to match the crop nutrient requirements and to achieve better control of the nutrient contents of the applied fertilizer to reduce the nutrient run-off and leaching. These products could be also used to supplement the raw digestate fertilization by replacing mineral fertilizers.
Technologies for digestate liquid treatment such as ammonia stripping, evaporation, struvite precipitation, membrane separation, as well as various combinations of these, have been previously studied considering nutrient recovery and production of nutrient-rich products with, e.g., digestate liquids, manure and urine (Antonini et al., 2011, Bonmatí et al., 2003, Bonmatí and Flotats, 2003a, Bonmatí and Flotats, 2003b, Chiumenti et al., 2013, Ek et al., 2006, Ledda et al., 2013). However, to ensure the usability and sustainability of different digestate liquid treatment techniques and to facilitate the agricultural utilization of the nutrients over longer transportation distances, the total digestate treatment chain and all produced mass flows should be taken into consideration as well as all the process inputs, e.g., chemicals and energy (Mehta et al., 2015). As life cycle assessment and energy efficiency studies have mainly concentrated on the use of raw digestate or separated solid digestate (e.g. Bacenetti et al., 2013, Berglund and Börjesson, 2006, Evangelisti et al., 2014, Pöschl et al., 2010, Smyth et al., 2009) only a few studies exist where the digestate liquid and its treatment has been taken into consideration (Rehl and Müller, 2011). In addition, these life cycle studies focus solely on environmental and ecological effects and do not evaluate the fertilizer products from the viewpoint of biogas plant efficiency or agriculture and plant nutrition. From these perspectives information about the mass, nutrient and energy balances of an AD plant with digestate liquid treatment is important, in addition to environmental aspects.
The aim of this study was to compare the potential of four digestate liquid treatment systems of a theoretical AD plant digesting municipal FW to produce fertilizer products with low water and concentrated nutrient contents. The studied treatment systems were different combinations of ammonia stripping, evaporation and membrane filtration, which have been applied in the full scale treatment of digestate or manure based liquids (see e.g. Boehler et al., 2015, Flotats et al., 2011, Fuchs and Drosg, 2013). For all four systems the mass, nutrient and energy balances were calculated and the nutrient recovery, mass reduction and energy efficiencies were compared based on typical literature values from laboratory, pilot and full scale studies. The performance of the treatment systems was also assessed in relation to the energy consumption of fertilizer product transportation to see the effect of digestate liquid treatment on the transportability of the products.
Section snippets
Overview of the theoretical AD plant
This study investigated a theoretical mesophilic AD plant which was assumed to digest source-segregated municipal FW (60 kt/y, kilotonnes per year). Fig. 1 presents the applied AD plant system boundaries which include pretreatment, a digester, digestate treatment and biogas upgrading. The FW was pretreated and hygienized (1 h at 70 °C) and subsequently diluted to a total solids (TS) content of 15% with processed water or water from the local water supply. The digestate treatment was assumed to
Mass and nutrient balance of AD and digestate treatment
The mass and nutrient flows of digestate and digestate liquid treatments formed the mass and nutrient balance of the systems showing the concentration of nutrients into fertilizer products. The calculations were based on the mass flow of the feedstock (FW + diluting water) and added chemicals as well as the characteristics of the digestate. The mass of the digestate accounted for 87% of the initial feedstock fed to the AD plant (60 kt of FW + 40 kt of dilution water), while 13% of the feedstock
Energy consumption of AD, digestate liquid treatment and transportation
The present results, based on typical literature values from laboratory, pilot and full scale studies, show that the processing of digestate through solid–liquid separation and digestate liquid treatments into concentrated fertilizer products consumed less than 10% of the produced energy in an AD plant treating 60 kt/y of FW. In total AD, solid–liquid separation and digestate liquid treatment accounted for 26% of the produced energy, of which around 19% was used in the AD and separation of the
Conclusions
This theoretical study showed the feasibility of FW nutrient recovery through AD and digestate liquid treatment and the production of transportable fertilizer products with the energy produced in AD. Despite the use of heat-demanding treatments, such as evaporation and stripping, the energy produced in AD was sufficient for digestate liquid treatment consuming fewer than 10% of the total energy produced in AD. The studied digestate liquid treatment systems were mostly considered as nitrogen
Acknowledgments
This work was funded by the Fortum Foundation (grant number 201400302). The authors are grateful to Karetta Timonen, Taija Sinkko, Sari Luostarinen, Juha Grönroos, Kaisa Manninen, Teija Paavola and Erkka Laine for the valuable comments and advice during a project that preceded this work.
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