Dimambro, M.E., Cambridge Eco Ltd(free)
A comparative study of the properties (nutrients, pH, electrical conductivity) of ten whole and liquor digestates produced from a range of feedstocks including food waste, maize and slurry was carried out. The aim was to ascertain whether these digestates could be used in protected horticulture (e.g. glasshouses or polytunnels) as a liquid fertiliser, for soilless crop production or as a growing media ingredient. The digestate pH was the least variable characteristic, ranging from 8.2 to 8.8. Total nitrogen in the fresh digestates ranged from 2.4 to 6.2 g/kg, with the majority of mineral nitrogen being NH4-N. The digestates produced from food wastes generally had higher NH4-N than digestates produced from maize and slurry, in addition to higher electrical conductivity, due to higher sodium concentrations. These results highlight the variability between digestates from different sources. A review of published trials indicates that suitably diluted digestate can generally achieve similar or better yields compared to standard growing practices, provided the recommended nutrient (including NH4-N) concentrations, pH and electrical conductivity levels are adjusted for each crop. Adoption of digestates within the protected horticulture sector could result in a reduction in the use of inorganic fertilisers.
Keywords Digestate, biogas slurry, liquor, horticulture, nutrients, horticulture, glasshouse
In Europe approximately 80 million tonnes of digestate are produced in around 13,000 anaerobic digestion (AD) facilities (Anonymous, 2013). In the UK in 2013 over 2 million tonnes of digestate were produced from 117 AD plants processing food waste, farm waste, manures and crops, with 98% used on agricultural land (Scholes and Areikin, 2014), as compared to 105,000 tonnes of digestate in 2008 (Anonymous, 2010). With over 450 AD plants still in the planning stage in the UK alone, (Anonymous, 2015), this volume of digestate is set to increase further.
The benefits associated with the use of digestate in agriculture are well known (Warnars and Oppenoorth, 2014; Heslop and McCabe, 2012; WRAP, 2013), although the reliance on a single agricultural market is risky, and AD industry resilience would be improved if a wider range of markets could be developed. Indeed, the UK Defra/DECC Anaerobic Digestion Strategy and Action Plan identified the need to find appropriate markets for quality digestates (Defra and DECC, 2011).
The digestate produced from AD is either used whole or can be separated using a variety of techniques to produce a liquid fraction, often termed liquor, in addition to a solid fibre faction. These are either utilised directly or the fibre may undergo further processing such as drying or composting (Möller and Müller, 2012).
The concept of using digestates, either whole or separated, as a nutrient source for hydroponic systems, as a fertiliser, and in growing media has been suggested in a number of reports and reviews (Hogg et al., 2007; Rigby and Smith, 2011; Arvanitoyannis, 2008). Moreover, many authors have highlighted the need to consider the properties of each digestate before use (Formowitz and Fritz, 2010; Lichti et al., 2012). The aim of this study was to compare the properties of whole and separated liquor digestates representing a range of common UK feedstocks, and to consider their potential for use in protected horticulture.
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