The solid residues from anaerobic digestion of organic matter is referred to as either digestate or biosolids. Biosolids is the residue from anaerobic treatment of municipal sewage, while digestate is the residue from anaerobic digestion (AD) of other organic matter. 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 on land is called biofertiliser.

Processing of residual organic matter in an anaerobic digester also produces biogas

The residual organic matter processed in a digestor is the remaining material from food processing, of from municipal waste after the quantities of waste have been reduced by minimisation, recycling or reuse. Organic residues can have a high reuse or recycling value and are only waste when we decide not to use the organic matter for a beneficial use.

Efficient and trouble free anaerobic digestion requires effective source segregation of organic waste from other types of waste. Improved collection of organic 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.

Anaerobic digestion 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 sewage biosolids use. Because the characteristics are different 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.

Regulations applying to dispersal of organics to land

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).

Supporting documents are:

  • EPA advisory about clopyralid
  • Fertilisers, Plant Biostimulants and Soil Conditioners - Requirements for compliance with conditions of exemption

Certification of digestate as biofertiliser

A scheme for certification of digestate as a biofertiliser is being developed with assistance of funding from the Waste Minimisation Fund.

  • Information on the project is available here

The aim of the project is to have a scheme that provides information to digestate buyers that the digestate purchased can be used as a quality biofertiliser and that it meets specific standards.

With the growing interest in the recycling of residual organic wastes by anaerobic digestion there is a concern that the current disposal of organic waste to landfill is just replaced by disposal of digestate to landfill. Not only would disposal of digestate to landfill be contrary to the policy objective of Zero recyclable organic waste to landfill by 2035 but it ignores the beneficial value of digestate as a fertiliser.

Technical Guide 8 builds on the UK PAS110 Specification for whole digestate, separated liquor and separated fibre derived from the anaerobic digestion of source-segregated biodegradable materials which sets out the foundations for classification and certification of digestate as fertiliser.

With Technical Guide 8 providing the foundations essential for regulating digestate from source segregated feedstock via establishing a robust quality assurance based framework the task now is to broaden the platform to cover all digestate from anaerobic digestion of organic matter and establish the criteria for certification. Once the classification of digestates is established research needs to be undertaken on the various grades of digestate to adequately recognise their attributes as fertiliser.

A key aspect of interest to fertiliser users is that the product has known characteristics and is consistent in composition. The certification scheme will focus on the consistency of the anaerobic digestion process and the quality control on feedstocks, as both these affect the reliability and consistency of produced product.

With digestate classified and its fertiliser characteristics identified the digestate is then ready for market distribution underpinned by the credibility of the certification scheme.

Biofertiliser from recycling of other organic matter such as from agricultural residues will follow.