Moroccan city reduces energy consumption with smarter wastewater processing
Marrakech is Morocco’s fourth largest city, home to more than 1 million people. The city has seen rapid urban growth in recent years, causing increased stress on the city’s supporting infrastructure. Prior to 2008, the Marrakech-Tensift El Haouz region’s sewage was disposed of in the Wadi Tensift river with minimal treatment. This practice could lead to damage to the local ecosystem and present health risks to nearby communities.
The local water agency Régie Autonome de Distribution d’Eau et d’Electricité de Marrakech (RADEEMA) joined forces with Waterleau to reduce its impact on the environment and the inhabitants of Marrakech and to operate more sustainably. Waterleau’s customized approach to treating the city’s wastewater included a mix of technologies: a biological water treatment technology that incorporates pretreatment (screening); primary treatment (removal of floating and sedimentary wastes in the grit and grease chamber); secondary treatment (degrading the biodegradable pollution of the wastewater using activated sludge); and tertiary treatment (disinfection) to prepare the water for reuse.
A major component of this project’s success was its sludge digestion process. The key advantage of sludge digestion is the production of biogas, which is used in cogeneration units that produce heat and electricity. The heat is used to bring the digesters to operation temperature, while the electricity is used as a power source. In Marrakech, anaerobic digestion of the municipal wastewater treatment sludge generates up to 70% of the wastewater treatment plant’s energy needs while reducing its carbon footprint and greenhouse gas emissions and meeting the Clean Development Mechanism requirements of the Kyoto Protocol office.
The primary treatment phase includes several steps to prepare the sewage for treatment. First, all floating waste is removed to prepare the water for the biological treatment phase. Large and smaller screens withhold all potentially damaging incoming sewage. Then the grit and grease chamber removes floating and sedimentary waste like oil, fats and sand. After pretreatment, the water flows to the primary sedimentation tanks or primary settlers; the suspended solids settle by gravitation and are collected by a rotating scraper into a hopper in the base of the tank. The clear water is decanted and ready for the secondary, or biological, treatment.
During secondary treatment, the biological pollution is degraded by microorganisms. The Marrakech plant design allows simultaneous nitrification-denitrification and the conversion of the ammonium ion to nitrogen gas in a single bioreactor, as well as the enhanced biological removal of phosphorus. These processes create an efficient use of resources. The waterborne microorganisms eliminating biodegradable soluble organic contaminants are referred to as activated sludge. This activated sludge coming from the secondary treatment is mixed with the sludge from the primary treatment in a closed digester, creating an anaerobic environment that allows the organic matter to degrade and create biogas. The biogas is stored in methane tanks and fuels a cogeneration unit consisting of four engines, producing enough green electricity and heat to power up to 45% of the plant’s energy needs.
Tertiary treatment provides a final treatment stage to raise the effluent quality before it can be used for agriculture and irrigation. The remaining suspended solids are coagulated and filtered using sand filter beds and disinfected using a combination of ultraviolet treatment and chlorination to eliminate the remaining bacteria. Once this step is complete, the water is ready for reuse and is pumped into an irrigation system. The Marrakech wastewater treatment plant allows 30 million cu meters of water to be reused for irrigation.
A plant for solar drying the digested and dewatered sludge is still under construction near the digestion site. Each year, more than 75 tons of digested sludge will be turned by robots in large greenhouses, using vaporization to increase dry matter content from 22% to up to 80%. This solar sludge drying facility will be the largest in the world.
In order to reduce impact for workers on site and neighboring inhabitants, 80.000 cu nanometers of air per hour are treated. Volatile organic compounds and odors are treated using bio-filters and bio-scrubbers, an environmentally neutral process requiring neither chemical treatment nor additional energy.
A Model Example
In Marrakech, anaerobic digestion of the municipal wastewater treatment sludge generates up to 70% of the wastewater treatment plant’s energy needs while reducing the carbon footprint and greenhouse gas emissions. The plant is the first and largest of its kind in Morocco, implementing wastewater treatment, sludge digestion, solar drying and water reuse, as well as air treatment for a fully integrated model. This approach to sustainable development shows that it is in fact possible for major urban centers to reduce their carbon footprint and be smarter with their energy use.