The solid residues from anaerobic digestion of organic waste is referred to as either digestate or biosolids. Biosolids is the residue from anaerobic treatment of municipal sewage waste, while digestate is the residue from anaerobic digestion of other organic waste. Both materials can be beneficially used as a soil fertiliser and conditioner, although stricter regulatory limits apply to the disposal of biosolids to land in order to minimise potential health risk for humans and the environment. Digestate which is suitable for use as a fertiliser is called biofertiliser.
Processing of residual organic waste in an anaerobic digester produces biogas and a digestate which when ready for market is called biofertiliser. The residual organic wastes are the remaining material after the quantities of waste have been reduced by minimisation, recycling or reuse.
Efficient and trouble free anaerobic digestion requires effective source segregation of organic waste from other types of waste. Improved collection of waste with increased focus of business and communities on separation at source has internationally proven to increase the level of recycling and reuse of organic waste which would otherwise have gone to landfill hence underpinning the goal of the waste hierarchy.
Internationally most food waste AD plants produce biogas for heat, electricity and use as a vehicle fuel, and biofertiliser from the digestate.
AD or composting
Anaerobic digestion sits alongside composting as alternative processing technologies. Composting and AD processing of organic waste are both two well proven, extensively used and often complimentary technologies used world wide. The choice of processing pathway depends on the composition of the feedstock and the desired products produced in response to the local market demand and opportunities. In some applications with specific feedstock, composting will be the optimal processing path, while in other applications and feedstocks AD processing will be optimal. In general, high energy feedstocks such as food waste are better suited for AD whereas high fibre feedstocks such as greenwaste are more suited to composting. Most importantly, they both keep waste out of landfills and hence substantially reduce greenhouse gas emissions and other environmental risks and impacts associated with that processing practice .
In general terms compost is an organic soil improver and AD biofertiliser is an organic fertiliser.
Dispersal of biofertiliser to land
Digestate contains high levels of macro- and micro-nutrients and as such presents an environmentally sound alternative to mineral and synthetic fertilisers. Nevertheless, the use of digestate as biofertiliser has been limited by the perception of farmers, food wholesalers, food retailers, politicians, decision makers and the general public as being un-safe due to its origin from waste materials and animal by-products. This is generally because all residues from anaerobic digestion of organic waste have been considered as being a biosolid and thus within the regulatory limitations on its use. Separate consideration of the attributes of digestate and biosolids will assist in maximising the value of the products from anaerobic digestion of organic wastes.
Technical Guide 8 - The production and use of digestate as fertiliser provides guidance on the production of high-quality, safe and healthy digestate for use as biofertiliser. The goal is to provide incentives for further investment in anaerobic digestion applications for processing of source-segregated organic waste in New Zealand, Australia and the South Pacific, by improving the awareness of the monetary, social and environmental benefits this technology offers for the treatment of organic waste.
The Guide does not cover the disposal of biosolids to land. These are covered extensively in the Guidelines for Beneficial use of Organic Materials on land (Water NZ, 2020).