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Recommendations for further work:
- To investigate the realistically sustainable gate fees for municipal organic waste streams. Traditional landfill costs have been in the £13-14 per tonne range in the UK. Many sources now quote that anaerobic digestion plants can charge £35-70 per tonne to process municipal waste streams. Although we have seen these magnitudes of gate fees quoted many times, and we know that increases are being driven by the EU Landfill directive, it would be beneficial to understand further the projections of these fees into the future. Might increased competition from more processing plants or other EU directives drive these gate fees higher or lower in the future? What is a sustainable and realistic gate fee for organic waste processing?
- To investigate the potential digestate market in the UK and EU. The Digestate liquor contains nitrate, phosphate and potassium which can be used to offset chemical fertiliser applications, and is available in an easily spreadable liquid format. The digestate fibre contains less nutrients but is a useful compost and soil conditioner. However, the liquor is diluted with water and incurs substantial transport costs when compared to dry chemical fertilisers, and the fibre faces competition for other compost sources and programmes. At present, there appears to be little or only a very low market value for these products. Digestate from UK digestion plants is effectivly shallow landfilled at zero net cost (fertiliser value vs "landfill cost") over very wide agricultural zones (fields!), in amounts limited by the nitrogen loading permissible on each site. If a financial market value for the digestate could be found or created, perhaps by greater taxes on chemical fertilisers or by concentrating the liqour into smaller volumes, the financial viability of centralised rural digestion plants would be greatly enhanced.
- To investigate the potential for standardising digestion plant design. To date, almost every plant design has been different from any other. Differing waste streams at each site, coupled with ever-changing EU regulations governing waste processing and the rapid progression of the technology in recent years have led to high plant design and construction costs. Our cost model, and the costs of plants constructed to date, reflect costs incurred for a one-off type chemical engineering plant. If plants could be stamdardised so that design, risk and overheads could be reduced and construction could be more modular, capital costs might reduced by up to 50%. This would make digestion plants far more viable. It would be interesting to study the major manufacturers of digestion plants in detail, to see if they are able to standardise or modularise the plants. Whether this is possible might depend upon the contracts awarded so municipal authorities would also be part of the equation. A few major cities ordering similar plants in the same timeframe might drive a common design ethic amongst manufacturers.
- To futher understand and optimise the achievable biogas yields from mixed waste feedstocks. The data we gathered and included in our calculator came from many sources. Very rarely were the exact details of the waste quantified; for example cattle manure properties vary widely depending upon the breed of cow, cattle feed, method of collection, dry solids percentage, and the amount and type of bedding material mixed with the manure. The unknowns in our current data are reflected in the sometimes large discrepancies between our "LOW", "MIDDLE" and "HIGH" estimates of waste and biogas yield per animal. It would be useful to have a more detailed database which accounted for at least some of the factors listed above, for all the different waste types. In addition, our knowledge of microbiology allowed us only a high-level understanding of the anaerobic process. While we were able to choose a single-phase, fully-mixed mesophilic process at 37-38°C with an optimum C:N ratio of 20-30 and dry solids proportion of 6-12% as our process of choice, it would be beneficial to understand the microbiology in greater depth. Greater depth of knowledge (or access to a commercial digester!) would allow investigation of such techniques as feeding back some of the digested solids into the digester, which can increase biogas yields due to the recycling of the acidogenic and methanogenic bacteria. These bacteria are otherwise continually flushed from the the system and must be replaced by regrowth at the expense of (biogas) energy.
- Finally, it would be beneficial to maintain a watch over the progress of anaerobic digestion plants in Europe over the next few years, and to measure their success and/or failure. In tandem with this, close attention should be paid to relevant EU and UK directives, legislation and taxes which affect waste disposal/processing, agricultural nitrogen loading, and energy generation. The rapidly changing legislation and incentives concerning these fields have the potential to make rapid changes to the viability of community and centralised anaerobic digestion plants.