Technical Report

Nutrient Recovery by Biogas Digestate Processing

August 2015

Executive Summary

Improved yields from farming practice and food processing are necessary in order to sustain positive growth around the world. A key element to achieve this aim is adequate supply of fertiliser.

Biogas plants produce, along with biogas, digestate, which is an excellent plant fertiliser, rich in both organic matter and in macro- and micronutrients. The physicochemical characteristics of digestate vary, strongly depending on the nature and composition of the digested substrates as well as on the operational parameters of the biogas processes.

Digestate is normally used as fertiliser for crops without any further processing, substituting industrially produced mineral fertilisers. However, the need for efficient nutrient management, required by restrictions on manure application in areas with high livestock density, along with depletion of the global natural reserves of phosphorous and potassium, make recovery and recycling of plant nutrients from manure, waste streams, and other resources increasingly important for farmers, technology providers, investors, and decision makers. This report focuses on digestate from biogas plants, where animal manures and slurries, crop residues, organic wastes and residues from agri-food processing industries and from other industrial processes are the principal substrates.

The nutrients contained in digestate can be extracted and concentrated through application of a range of technologies and processes, although there is no unified approach in the published literature about what defines digestate processing for nutrient recovery. The present report presents the technologies that are commercially available. Nutrient recovery technologies are those that result in an end-product with higher concentrations of plant nutrients than the unprocessed digestate, or technologies that are capable of separating out nutrients in mineral form, or of creating another marketable endproduct, suitable for recycling as biofertiliser, and closing the nutrient cycle.

Digestate processing can be partial, primarily for the purpose of volume reduction, or it can be complete, refining digestate to pure water, a solid biofertiliser fraction and fertiliser concentrates. While partial processing uses relatively simple and cheap technologies, for complete processing complex methods and technologies are currently available with various degrees of technical maturity, higher energy input, and higher investment and operating costs.

The first step in digestate processing is to separate the solid phase from the liquid phase. The solid fraction can subsequently be directly applied as biofertiliser in agriculture or it can be composted or dried for intermediate storage and transport. Depending on the consistency of the digestate, screw presses or centrifuges are most commonly used for solid–liquid separation.

For complete digestate processing for nutrient recovery, membrane technology can be used, such as nanofiltration and ultrafiltration followed by reverse osmosis. Membrane filtration produces a nutrient concentrate and purified (process) water. A further possibility for concentrating digestate is evaporation utilising excess heat from the biogas plant CHP unit. Nitrogen recovery from digestate can be carried out by ammonia stripping, ion exchange or struvite precipitation. Whatever the technology applied, complete digestate processing requires high energy input, use of chemical reagents which, along with high investment costs for appropriate machinery, imply considerable costs for such applications. Due to diversified support schemes in different countries, it is not easy to generalise whether a specific technology always is feasible in connection with a biogas plant. Some countries stimulate biogas plant owners to utilise excess heat through subsidies, while others have not implemented such fiscal instruments.

The techniques for nutrient recovery from digestate are developing rapidly, aiming to improve nutrient management in agriculture and in waste treatment systems. In parallel, there is a general need to increase the degree of commercialisation of organic fertilisers from digestate processing through product standardisation.

The information contained in this report should be of interest to biogas and digestate producers, to livestock and crop farmers supplying manure to biogas plants or using digestate as fertiliser, to agro-food industries and other organic waste producers, that supply organic wastes to biogas plants as well as to policy makers, regulators, investors and end-consumers.