Proceedings

Evaluating options for upgrading digestate from the Loowatt System

Purves, I.1, Gardiner, V.1, Parker, B.2 and Forte, B.2, 1Loowat Ltd, UK, 2InCrops, UK

(free)

Loowatt is a patented waterless sanitation system which uses a biodegradable sealing film to contain waste in a cartridge before addition to an anaerobic digester. Together with InCrops, Loowatt have been investigating ways of adding value to the process by upgrading the digestate from the system. This study examines process options for digestate containing low solids content. The role of dewatering was examined and several different methods have been assessed for suitability, including composting; struvite precipitation; fortification and pelletisation, as well as biological conversion processes such as microalgae. Each option was evaluated on the basis of scalability, capital costs, maturity of technology and value of end product.

Keywords: Digestate, Struvite, Compost, Algae, Ammonia recovery

 

Introduction: The challenge of developing waterless sanitation and generating energy from waste has prompted designers to rethink how the toilet system may be redesigned. One approach, developed and patented by Loowatt Ltd is to use a biodegradable sealing film to hold waste in a cartridge system, which can then be emptied directly into an anaerobic digester. The Loowatt system is currently deployed in pilot form in Madagascar, and there are demonstrations planned for the UK. In terms of the domestic context, it is desirable from an economic standpoint to evaluate options for upgrading the resulting digestate to add value to the process. This has a number of points of differentiation compared to municipal water treatment works where AD has been employed: firstly, the volume of waste is lower, the concentration of nutrients is higher and the risk of contaminants (e.g. undesirable chemicals from greywater or run-off) is minimised.

The Loowatt system has a nutrient profile that reflects the inputs into the system. Ammonia concentrations are likely to be high, but energy content relatively low compared to dedicated AD feedstocks such as maize. Based on data provided by Loowatt, we will assume the solids concentration is low, estimated to be between 3 – 4%. The NPK profile has been calculated as N 4.95, P 1.64, K 1.83 from feedstock inputs.

Digestate generated from human waste can present a challenge in disposal. If regulatory requirements regarding sanitisation are satisfied, in some cases digestate can be used as a direct fertilizer by spreading on the land. However this presupposes that the biogas plant where the digestate is produced is close to the fields where it can be spread otherwise transport costs would make this operation less interesting. Spreading is usually restricted to a few months a year; in winter time it is normally not permitted as excess precipitation during these months can cause the fertilizer to run off the land and potentially contaminate local water sources, so storage capacity is needed to buffer the digestate. In addition, limits on the application of ammonia may limit the amount of digestate which can be spread in this way. This study examines a number of alternative options for the upgrading of digestate to either reduce the burden of disposal or to generate income from an additional product.

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