Which is better, composting or anaerobic digestion?

Composting actually complements anaerobic digestion (AD), according to a 2011 study by WRAP.

The Relative benefits of compost and AD digestate
Views differ as to the benefits from composting and AD as applied to the recycling of food, garden waste and manure, and WRAP has recently (September 2011) published a report entitled Compost and Anaerobic Digestate Quality for Welsh Agriculture.

This report goes some way towards clarifying the position. It explains that green and green/food composts were sampled from six Welsh facilities on November 2009 and March 2010 and tested (in triplicate) for a wide range of determinands. The report presents the results alongside data for three English anaerobic digestates and a range of livestock manures / slurries.

Executive summary extract
With Welsh Assembly Government (WAG) targets for collection and beneficial recycling of food wastes, there is a need to develop recycling infrastructure, and – most importantly – sustainable markets for the resulting composts and digestates. The recent leap in use of compost by farmers and growers has resulted in substantially increased scrutiny of compost quality and a need to understand the suitability of composts for long-term, sustainable application to agricultural land. Furthermore, while in some respects long-established (through the agricultural use of digested sewage sludge – biosolids), anaerobic digestates are considered new to market by farmers and growers in Wales – and there is a considerable lack of certainty around their undoubted agricultural value.

This report was commissioned to close gaps in understanding of compost and digestate quality, to build confidence amongst potential compost and digestate users, and to identify any specific areas where improvements in quality might be considered desirable. The objective of this project was to quantify the quality of source-segregated composts and digestates to determine their suitability for use in a range of agricultural sectors in Wales, with particular emphasis on their potential for application to pasture land.

Triplicate compost and digestate samples were taken during November 2009 and March 2010 from nine different sites, giving a total of 36 compost samples and 18 digestate samples. Three in-vessel (ABPR-compliant), three open windrow (non-ABPR) composts, two ABPR-compliant (food-based) digestates and one non-ABPR (manure and purpose-grown crop) digestates were sampled. In November 2009, of the six composts sampled, three were from PAS 100:2005 accredited grades, two had applied for PAS 100 accreditation and the sixth site had previously been PAS 100 accredited (but the certificate had expired recently). In March 2010, three of the six sites were PAS 100 accredited for the grades sampled, with the other three sites working towards PAS 100 accreditation.

The two food-based digestate sites were working towards PAS 110:2010 compliance; the manure-based digestate site was not seeking PAS 110 accreditation. All of the compost and digestate samples were analysed for nutrient and heavy metal contents, chemical and physical properties, microbial pathogens, physical contaminant and organic compound contaminant concentrations at accredited laboratories.

The main findings of the project are summarised below:

  • Nutrients and organic matter: The food-based digestates contained c.7.4 kg total nitrogen-N/m3 and the manure digestate 4.4 kg total N/m3 (compared with =typical‘ pig and cattle slurry values of 3.6 kg total N/m3 and 2.6 kg total N/m3, respectively). The readily available N content of the food-based digestates was c.80% compared with c.60% for the manure-based digestate (compared with =typical‘ values of c.70% for pig slurry and c.45% for cattle slurry). Digestate was shown to be a rich source of N that can replace the need for manufactured N fertiliser use. The digestates also contained agronomically useful amounts of phosphate, potash and sulphur. Given the high N (and other nutrient) requirements of large areas of productive grasslands in Wales, digestate is a particularly valuable fertiliser for use during the spring/summer growth season.
  • Compost was shown to be a good source of phosphate and potash, and stable organic matter (supplying between 5 and 6 t/ha at typical application rates). The organic N content of the compost samples was typically c.95% of the total N content (the mean total N content of the green compost samples was 9.5 kg/t fresh weight (fw) and the green/food compost samples 15.6 kg/t fw), which will slowly mineralise and become available to crops over a period of months to years. Given the large areas of productive grasslands in Wales, which have high potassium offtakes, compost represents an agronomically useful fertiliser and soil fertility enhancer.

(ABPR – Animal By-Products Regulations)

The results confirm that both compost and digestate offer important soil improvements that are mutally beneficial rather than alternatives. Further trials are in progress. See: http://aplus.adas.co.uk/Services/Soils/ADAS-leads-high-profile-farmer-focused-digestate-and-compost-research-project.aspx and http://randd.defra.gov.uk/Default.aspx?Menu=Menu&Module=More&Location=None&Completed=2&ProjectID=17289

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