As a result of sludge fermentation, biogas is generated, which is mainly comprised of methane. This is recycled and transformed to a generator of energy, which is used to operate part of the wastewater treatment plant. At the Rincón de León WWTP, a total of 2,500,000 kWh are cogenerated annually, which covers approximately 25% of the plant's energy needs.

A total of 37,000 kWh are produced annually via the solar panels installed in the plant. This is a small fraction of the total energy consumed in the WWTP.

The residues generated in the first wastewater treatment process are: solid urban waste of varying sizes, sands and fats. All of these are processed according to current legislation on waste by authorised managers and are transported to the corresponding waste treatment plants.

In addition, other waste is generated (packaging, plastics, etc.) in the process and maintenance operations. This waste is treated by authorised managers according to current waste legislation.

Before sludge is removed, it must be treated to reduce its fermentation capacity and adapted so that it can be reused. In addition, its volume must be reduced for transport.

In the anaerobic digestion process, organic matter is converted into methane and carbon dioxide in two stages. The sludge that comes out of the digesters then passes through a dewatering process to eliminate as much water as possible.

Once dewatered, the appearance of the sludge and its properties are similar to those of organic fertiliser, in whose production it is currently used.

The biogas that is obtained has a methane content in the order of 65% approximately, and can be used in electrical energy production.

Sludge from wastewater treatment is used. Its main applications are heat recovery and agricultural application as organic fertiliser.

The volumes of sludge applied are listed below, according to use:

• Total sludge for heat recovery: 22,000,000 kg.
• Total sludge for use in agriculture (kg): 5,600,000 kg.

Heat recovery from sludge extracted in the wastewater treatment process should be highlighted. It occurs in the thermal drying of the sludge.

The sludge drying plant is located within the premises of the Cemex cement factory in Alicante. There are two parallel lines of treatment via a drying system that is based on circulating hot air through the sludge so that its water content evaporates and is subsequently condensed.

Functioning of the drier tunnels:

The volume of dry sludge obtained is transported to the cement production furnace where it is used as a fuel. Thus it becomes a sustainable alternative to the use of fossil fuels such as coal. Residual heat from this furnace heats the water used to dry the sludge. Hence the environmental importance of the treatment system in this pioneering facility in Spain: it is energy sustainable, as it does not require non-renewable sources of energy (fuel oil, natural gas, coal, etc.), and the thermal energy of the residue is recovered.

Thus, the wastewater treatment cycle is closed by self-management and subsequent recovery of the main residue from the wastewater treatment process.