Anaerobic Digestion Economic Feasibility Study: Generating energy from waste, sewage and sargassum seaweed in the OECS. Small island developing states in the Caribbean face serious challenges relating to 1) waste management; 2) sewage treatment and; 3) energy sustainability. At present, all too often, inefficient and poorly planned delivery of these key services is having a negative impact on the quality of people’s lives and the environment. Action across the public and private sector is needed regionwide to address these issues in a cost-effective, controlled and sustainable manner.
Anaerobic Digestion (AD) is a technology which can provide real benefits to all three areas of concern and which is currently not being deployed in the Caribbean. This report seeks to study how anaerobic digestion could be used in a Caribbean context to treat biogenic waste, including sewage sludge and in the process, generate renewable energy in the forms of electricity, heat and biomethane gas for fuel. AD can be implemented at all scales and there are existing and well proven technologies available at all scales of delivery.
For the purposes of this study, St Lucia and Grenada were taken as examples of developing island states. They both exhibit all three problems and could benefit from an AD solution. Available information on St Lucia has been used to provide an initial estimate of the available tonnage of potential AD feedstocks; 66,000 tpa of biogenic waste (not including agricultural, food or drinks processing, brewery/distillery or slaughterhouse waste) could yield between 46,400,000 and 72,300,000 MJ of energy, the equivalent of 13 GWh of power. Corresponding data on Grenada indicate that there could be 46,000 tonnes per annum of biomass waste available for treatment through AD which could yield 81,500,000 MJ of energy or the equivalent of 22.5 GWh of power.
AD could also provide a waste treatment solution for beached Sargassum, a growing problem throughout the Caribbean. Available information on Sargassum natans and fluitans, the two species of primary concern across the Caribbean, is sparse. The small BMP assessment carried out showed that ‘old’, beached Sargassum, when milled to a powder and digested, had a very low BMP at 61 m3/tonne VS added (compare with food waste at 421 m3/tonne VS added). In spite of Sargassum’s low SMY, it could still be treated through AD, as an amendment to a plant taking other wastes as its primary feed.
Regarding economic viability of an AD approach to energy generation, modelling suggested that it should be possible to make a financial return on implementing AD technology in St Lucia and Grenada. The percentage contribution to electricity and/or heat supply made by AD would be relatively small but significant. In St Lucia, AD could provide up to 6%, possibly more if all potential feedstocks for AD were treated. In Grenada, that rises to around 11%.
Varying levels of financial interventions would be required, to help AD investment yield positive returns. Small scale projects need more incentives to be financially viable, such as generation tariffs and gate fees (where possible), plus state aid or capital grants. Large scale plants could make a return with less incentives and no gate fee for waste, depending on the energy content of the waste. Small community scale projects may be the answer in hard-to-reach communities and could double up as both energy generation and sewage treatment. Technology at the back end to dewater and purify the digestate could also deliver grey water for non-potable uses.
Taken together with other renewable sources, such as solar PV and thermal, plus wind, these renewables have high potential to replace a large fraction of fossil fuel derived energy, creating more sustainable and resilient island nations, not completely dependent on, or at the mercy of wildly fluctuating energy markets.
Policy development would need to look at how to structure an energy generation tariff system and remuneration levels for electricity and heat sales. Examination of existing systems and their faults would be useful. This may require thought on how best to capture the organic fraction of municipal and commercial waste. As a potentially valuable source of energy, a source-segregated approach would be the ideal option but this is not without its challenges. There will need to be some policy and regulation in place for the use of digestate (on land), to ensure environmentally responsible application. Education of the public is also a major challenge, together with the logistics of collection. A Food Waste Study could help identify the issues and prepare options for moving forward. In this respect, there are European examples to draw on, both in terms of the technologies involved, and the associated public education campaigns.
As explored in Section 10 (Next Steps), further research, experimentation and investment is required to take forward the use of waste-to-energy anaerobic digestion solutions in the Caribbean. This includes additional experimentation with samples of Sargassum and more in-depth techno-economic studies into the economic viability of AD plants in the region. Finally, supplementary research on the politico-economic landscape in individual OECS states would enhance the policy making process, and help identify which of the Scenarios covered in this report are most likely to gain support among civil society.