Patrick Maurel aims to make a significant contribution to the sustainability of Mauritius. At La Chaumière, on the edge of La Ferme reservoir, an enormous structure is taking form. It is not Gamma Energy’s controversial waste-to-energy incinerator but an industrial scale composting plant. Just like the proposed incinerator, it will process unsegregated household waste that would normally go directly to the landfill site at Mare Chicose. But instead of generating electricity, it will produce compost – an organic fertiliser and soil conditioner. Unlike the incinerator, it has attracted little attention, was offered no Government subsidy and is now just a couple of months from completion.
The high humidity and organic content of household waste in Mauritius means that it is more suitable for composting than incineration. However, handling it and removing the non-compostable material is normally a difficult, unpleasant and potentially hazardous task. Patrick turned to India for the solution where a company has developed a cost-effective technology and installed it in some sixteen locations around the country. The plant in Mauritius will be the first outside India.
The process is relatively simple. As soon as unsegregated waste arrives at the site it is weighed and sprayed with a cocktail of biological organisms, enzymes and herbal extracts. It is claimed they eliminate foul odours and hazardous pathogens in just a couple of hours. The waste is visually inspected and large items are removed before it is piled into long rows (called windrows) in the open air on a massive platform of 25,000 square metres. The platform is a thick sandwich of concrete and waterproof membranes to prevent leakage of any liquids into the ground. It has a gentle slope and undulations that carry rainwater run-off to a large storage tank from where it can be reused to maintain the humidity of the waste at an optimum 25%.
Microbes decompose the waste aerobically, generating heat which raises the core temperature up to 70oC, hot enough to destroy any remaining pathogens. As the microbes require oxygen, a crane grab is used to turn over the windrows every week to aerate them; this also mixes waste on the outside of the pile with that in the middle. The process is similar to the one used at Medine sugar estate to produce fertiliser from cane trash (sugar cane leaves). After 4-6 weeks, the composted waste is moved under a roof where it dries for about fifteen days before being loaded into an automated separator.
Although most of the organic matter in the waste has been transformed into compost, it still contains inorganic items which must be removed. It is fed by a conveyor belt into a rotating screened cylinder (trommel) that removes any objects larger than 35mm which can be sorted and recycled or sent to the landfill. The compost and smaller items fall through the screens onto a conveyor below and pass under a magnet that removes any ferrous objects. A second trommel removes items larger than 16mm, while the compost falls through and under another magnet. It is then stored for about 15 days and tested to confirm that the biological activity has stabilised.
Once stable, the compost passes through a third trommel with 6mm screens. This time the removed objects are crushed, passed through a cyclone, which filters out the dust, and returned to the compost stream. Additional microbes are also mixed in that take nitrogen from the air and fix it as compounds that can be used by plants, increasing the value of the compost as a fertiliser. It is then packaged into 25kg bags for easy handling or placed in bulk storage for large customers.
Although the plant accepts unsegregated domestic waste, certain items are not permitted because they affect the decomposition process or contaminate the compost. They include batteries, paints, oils, electronic components, pesticides and herbicides. In any case, samples of the compost are tested at an on-site laboratory to ensure it is free from contamination, for example, by heavy metals, in which case the batch is discarded to the landfill.
The simplicity of the process is mirrored by the components of the automated separator. It uses permanent magnets instead of electro-magnets and hydraulic motors instead of electric ones. This should make it very reliable and easy to maintain.
Even before operations have commenced, samples of waste are being composted and separated manually. Peanut, lettuce and tomato plants are being grown on site to compare the benefits of the compost with synthetic fertilisers. A visitors’ centre will be constructed for educational purposes and a restaurant may be added later.
When operational, the plant will receive 400 tonnes of waste per day, about one third of all that we generate. However, since the automated separator will only be operating 8-10 hours out of 24, potential capacity is significantly greater. It will produce about 30,000 tonnes of compost per year, enough to replace 25% of all the synthetic fertilisers we currently import, which require significant amounts of fossil fuels for their production. This will reduce our carbon footprint, improve our trade balance, restore the natural fertility of our soils and contribute to food security.
In the absence of comprehensive waste segregation at source, i.e. by us in our homes, Patrick Maurel’s project provides a pragmatic solution to our problem of waste disposal as well as generating a very useful product. Hence, we think he deserves the title of “Mr Ile Durable 2011”. Don’t you?