Biofertiliser Management

13th December 2016

This report examines the potential links between digestate stability and digestate odour potential.

Key points
Anaerobic treatment of biodegradable wastes was shown to be capable of producing digestates with low odour potentials
Separating the digestate into liquor and fibre fractions generally had little impact on the nutrient profiles of the different fractions
Subjecting whole digestates to sequencing batch reactor (SBR) tests successfully converted the ammoniacalnitrogen to nitrate-nitrogen

Overview

Overview

The PAS110 specification includes a compulsory digestate stability test – developed to demonstrate that input materials have genuinely been subjected to a digestion process and have not simply been passed through the system. This Residual Biogas Potential (RBP) test is time consuming to perform, and feedback from industry has questioned whether the current RBP (stability) limit is set at an appropriate level.

In addition, there have been documented complaints about odours associated with land-spreading of digestate. This could be due to use of inappropriate spreading practise, or spreading of digestates that might be deemed "unstable", but this was difficult to determine without a clear understanding of the links between stability and odour. The impacts of separating digestate (into separate fibre and liquor fractions) on stability and nutrient content were also unknown.

Possible links between odour potential and stability, as well as odour potential and nutrient partitioning associated with separation of whole digestate into liquor and fibre fractions were investigated. The potential for a Sequencing Batch Reactor (SBR) approach to increase stability, reduce odour potential and convert ammonium to nitrate was also investigated.

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Findings

Findings

  • Anaerobic treatment of biodegradable wastes was shown to be capable of producing digestates with low odour potentials. However, the relationship between digestate stability and odour potential was not clear since some anomalies were found, in which some stabilised digestates passed the RBP test, but still had elevated odour potentials. Best practice (for example: Defra, 20101 ) suggests that digestates be band-spread at the soil surface or injected into the soil. These measures will reduce ammonia losses and minimise potential odour emissions.
  • It was recommended that the current RBP threshold be maintained at the level of 0.25 L/g VS, which was shown to be comparable with the range of RBP values found for cattle slurries, and which seemed to be best associated with digestates with low to medium odour potentials.
  • Separating the digestate into liquor and fibre fractions generally had little impact on the nutrient profiles of the different fractions. However, since the liquor fraction comprised by far the majority of the total digestate mass, this represents the most significant reserve of nutrients that could be recovered for agronomic benefit.
  • Subjecting whole digestates to sequencing batch reactor (SBR) tests successfully converted the ammoniacalnitrogen to nitrate-nitrogen. Digestate stability was also greatly increased, and the BOD (Biological Oxygen Demand), COD (Chemical Oxygen Demand), total VFAs (Volatile Fatty Acids)2 and dissolved carbon lowered. The odour potential and hydrogen sulphide were eliminated – demonstrating the potential of this technique to reduce digestate handling issues.

Related

Related

To find information related to that contained in this report, please use the following links:

In vessel composting (IVC) >>
Open windrow composting >>
Anaerobic digestion >>
Thermophillic aerobic digestion >>

Where next?

Want to know more about composting? Take a look at our reports, guides, tools and case studies for further information:

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