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31 July 2019
HAWL bursaries

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A2: Biosolids and biosolid products: the way forward

Chair: Christine Watson (SAC)

Discussions about biosolids always lead to intense debate. This session will look at products derived from biosolids for their potential use in organic systems. There will also be a presentation and discussion around the use of food waste in the production of green waste compost. (Organised by SAC)

Session summary

Peter Vale from Severn Trent Water began by giving a presentation on how Severn Trent are transforming sewage sludge into a valuable product. He highlighted that Phosphorus (P) reserves will be depleted within the next 100 years and that steps need to be taken to find alernative sources. Through the use of Struvite, Severn Trent have been able to develop a novel methods of recovering P from sewage- up to 40% can be recovered through using the method developed. The end product is in a pelleted form which allows much more targetted use that Sewage.

Arnie Rainbow from Vital Earth then gave an overview of the composting process that Vital Earth uses. He highlighted the advantage of green waste compost in that it provides all nutrients needed in forms available to the plant except for an N supplement. Peat-free compost is however much maligned because of bad experiences in the past. Arnie also highlighted that using green-waste compost has a triple benefit; reducing peat extraction, reducing waste and helping to cut down on pesticide usage and labour, the latter for the removal/prevention of liverwort, a common weed in peat based nursery stock).

Robin Walter then presented on the use of source separated human urine, highlighting that an average person produces approximately 0.8 to 1.5 litres of urine per day, and that this can provide a valuable resource in terms of closing the nutrient gap on farms. Robin highlighted that this is not a new concept; urine is used frequently in developing countries, but has become less frequent with the advent of cheap fertiliser. Robin pointed out the use of urine as fertiliser often falls outside of current directives (e.g. Nitrates and Waste Water Directive), and it is not on the organic list of permitted fertilisers. To use it within agriculture, it is necessary to guarantee that the product is free from levels of pollutant, pathogens and pharmaceuticals, but this should not exclude its use.

Discussion points:

  • Chris Atkinson (Soil Association) highlighted that if a fertiliser is not on the list within the EC organic regulations, then organic producers in Europe are not able to use it. Currently both human sewage and urine are excluded. There is a conflict here with the organic principles of recycling.
  • There is a need for drivers for change at a grass roots and policy level to encourage use of biosolids and urine.
  • A question was raised as to how the Vital Earth compost products compare to peat based products. About equal was the answer from Arnie Rainbow; for container nursery stock production it is a particularly good alternative (because of liverwort avoidance) but there is a little way to go with smaller scale growers and transplant producers who have special needs.

Individual speaker presentations and abstracts

Peter Vale (Severn Trent): Phosphorus recovery from sewage sludge (PDF 785KB)
Severn Trent Water Ltd produces and treats circa 240,000t (as dry solids) per year of sewage sludge. Through anaerobic digestion this re-source is converted into circa 165,000t of agricultural fertiliser and 175 GWh per year of renewable electricity. Significant research and development activity is going in to further optimizing both energy recovery and nutrient recovery from this valuable resource, and one area of particular focus is recovering the phosphorus present in digested sludge liquors in the form of struvite (magnesium ammonium phosphate). Struvite appears to have a great deal of potential as a sustainable phosphorus fertilizer. This paper discusses the potential of sewage as a phosphorus resource, and outlines the processes available for recovery as struvite.

Arnie Rainbow (Vital Earth): Food waste in green waste composting (PDF 5.4MB)
The ability of compost to transform the chemical, physical and biological properties of over-exploited soils and low quality soil-like substrates is well known. The use of green compost in growing media is less well-known. The UK government aims to end the use of peat in horticulture in England by 2030, with interim target dates of 2015 (local government) and 2020 (retail market). The main driver was protection of biodiversity, but carbon fixation/ global warming/ climate change is now their chief concern. For historic reasons, green compost is often still regarded as having little potential in growing media. Yet considerable progress has been made and several new benefits have been realised in container nursery stock and other high value crops. At present, the potential for green compost to replace peat in growing media can be estimated at approximately one million m3 per annum. Manu-facture of peat-free growing media is limited by availability of suitable low-nutrient diluents (such as bark), not green compost. Vital Earth compost green waste in mobile ventilated containers for one week at 75C, followed by one month in indoor aerated static piles (as per ABPR 2002). Temperature and fan output are monitored and recorded throughout the process. Size-graded green compost is blended with processed forestry co-products (UK FSC sources only) and other diluents, plus a proteinaceous meal which acts as a slow-release source of nitrogen, with excellent storage and in-use life.

Robin Walker (SAC): Source separated human urine (PDF 486KB)
In non-organic cropping systems, fertiliser costs are high and are unlikely to reduce substantially in the near future as energy prices remain elevat-ed. Growers are looking for ways to provide adequate fertility to their crops and yet limit costs to economically acceptable levels. The use of biological N fixation has many benefits, but there can also be problems associated with the use of legumes and other N fixers in terms of optimising their N fixing capacity and release of N in an available form as the crops require it. The use of alternative N (and P and K) sources should be explored beyond those traditionally used. The potential use of source separated human urine for use on UK crops is outlined and illustrated with examples of systems being developed in other countries. The paper discusses legislative issues as well as some of the likely environmental benefits and potential barriers to the broad scale use of source separated human urine in this context.