Le Marin: an exemplary waste treatment plant
Situated in the south of Martinique, out in the Caribbean, the new Le Marin Sainte-Anne waste treatment plant is a model of green technology. Waste water, sewage sludge and human waste are processed here whilst showing the greatest respect for the environment.
The new waste treatment plant for the towns of Le Marin and Sainte-Anne responds to a major challenge for the south of the island: restoring the quality of coastal waters and preserving marine biodiversity. As a result of an ecological assessment of the area, the two towns (which are situated 5 km apart and with populations of 9 000 and 5 000 respectively) have been linked by a ‘bay contract’ since 2006. The town councillors drew up a three-point action plan: reduce pollution levels, preserve and maintain the natural environment and encourage an integrated and sustainable management.
Protecting the bay
Built by SICSM (Syndicat Intercommunal du Centre et du Sud de la Martinique), the Le Marin and Sainte-Anne treatment plant has a capacity of 12 500 population equivalent (PE), which is due to be doubled by 2025. The sewage treatment plant is currently the most efficient in the Caribbean. It is equipped with state-of-the-art technology, most notably a membrane filtration system allowing collection of reusable sanitary quality discharge water, which significantly reduces the amount of polluted water released into the bay. Another environmentally-friendly innovation is the solar sludge drying process.
“We needed to choose a solution which was compatible with sustainable development. The councillors chose a sludge treatment process based on a range of solutions – spreading directly on land, composting, use in thermal power plants or, if necessary, incineration and landfill,” said Frédéric L'Etang, director of the SICSM Community Sanitation department.
The process followed is conventionally used at municipal activated sludge treatment plants. It includes a primary pre-treatment stage (screening, de-sanding and de-greasing). Firstly, the sewage passes through a 6 mm mesh sieve to remove fibres, hair and detritus. Sewage is then cleaned of any grease, oil or sand that it contains (the latter two elements are stored in tanks) before it is sifted through a 0.75 mm mesh which provides another opportunity to remove specific particles.
The sewage is then treated in three biological tanks. It undergoes a pre-anoxic process which partially breaks down nitrates, an anaerobic process to remove phosphates by biological methods, and the sewage is then mixed in an aeration tank to remove nitrogen and carbon completely.
After the biological treatment, sewage undergoes an extremely complex filtration process. A cell membrane system separates treated water from sludge (which will again go through the biological treatment stage). The treated water can then be used for cleaning or irrigation in industry. This last point is particularly important in an area which experiences severe drought.
Sludge treatment process
Sludge is firstly processed using a centrifuge which separates the solid from the liquid sludge which will be processed again using the water treatment cycle. Following the dewatering stage, the solid residue which has a dry-solid content of 20 % (or contains 80 % water) is moved into a greenhouse. A heating plate powered by five 600-litre solar heaters dries out the sludge. The sludge dry-solid content reaches 90 % at the end of the process (bearing in mind that the manufacturer’s guarantee is from 70 %). The dried, solid sludge is in granular form. It can be used in various ways: anaerobic digestion, incineration, landfill, composting or land spreading.
“Solar drying provides the great advantage of being able to store sludge on site, which in turn reduces the need to transport this material”, added Frédéric L'Etang. “The volume is four to seven times less when compared with output from systems traditionally used in Martinique. Costs of transporting and disposing of sewage sludge are directly proportional to the amount of sludge requiring disposal. Moreover, a greenhouse uses 13 times less electricity compared to a similar system.”
Human waste and Purification process
Material from septic tanks, known as holding tanks, are treated in a separate system, using oxidation and hydrolysis, before undergoing a pre-treatment cycle. The plant has a capacity of 1 000 tanks per year.
The treatment plant is equipped with two air purification systems. The sewage treatment process itself has chemical purification equipment. In addition, the contaminated air from the greenhouse is treated using specific biological purification equipment.
Results and effects
The treatment plant has a capacity of 12 500 population equivalent. The use of innovative processes has a positive impact on several spending areas, as well as a negligible environmental impact. the reuse of water by industry, for cleaning or irrigation, is a particularly crucial factor in an area which experiences severe drought; drying sludge on-site and in greenhouses limits the need to transport this material, and it uses less electricity (13 times less electricity is used by greenhouses than when using a similar system).
The treatment plant encourages the restoration of the quality of coastal waters, and the preservation of marine biodiversity, whilst following an integrated and sustainable management through the use of innovative processes (membrane filtration system to separate treated water from sludge, and on-site solar sludge drying).
Total cost and EU involvement
The estimated cost of the project “Construction de la nouvelle Station d'Épuration du Marin (STEP)” (Construction of the new Le Marin Treatment Plant) is EUR 11 100 000. Several phases have been included in this undertaking, as the first project to build the treatment plant was co-funded under the 2000-2006 programming period (a total cost of EUR 7 300 000), and another phase for sludge treatment and effluent transfer to the new treatment plant is co-funded under the 2007-2013 operational programme, with aid of EUR 3 800 000.
The European Regional Development Fund contribution amounts to during the 2000-2006 and 2007-2013 periods. The contribution from the State is during the two programming periods. The initial planning for the project started in 2003 and work was finished at the end of July 2009.