In 2016, Canal de Isabel II commissioned Spain’s first industrial scale plant for the recovery of phosphorus in the form of struvite at the Sur WWTP (Madrid). The plant was supplied and assembled on a turnkey basis by the Spanish subsidiary of Veolia Water Technologies. It is sized to treat up to 260 kg of phosphorus per day from the WWTP’s Ntwo return streams. The controlled formation of struvite is carried out in an upflow fluidised bed reactor. Canal de Isabel II has invested €2.3 million in this initiative, which is in line with the company’s commitment to the circular economy, environmental care, sustainable management and investment in R&D&i.
The Sur de Madrid Wastewater Treatment Plant (WWTP) is located on the left bank of the River Manzanares and receives wastewater from the Sur sewer (left bank interceptor sewer). This interceptor sewer receives the water from the sewers of La Gavia, Valdebernardo, Pavones, Palomeras, Yeseros, Tomateros, etc., which collect the wastewater from the districts of Vicálvaro, Puente de Vallecas and Villa de Vallecas.
The existing sewer network enables the wastewater from other plants situated upstream from the Sur WWTP (Viveros de la Villa, La China, La Gavia and some of the wastewater from the right bank interceptor sewer) to be diverted to the Sur facility in the event that further treatment is necessary to guarantee the quality of the water returned to the receiving waterway. This also endows the system with flexibility.
Project objective and scope
Uncontrolled struvite precipitation is a problem at wastewater treatment plants because it causes clogging of pipes, generally in the sludge line, downstream from anaerobic digestion. Struvite is a crystal made up of magnesium, ammonium and phosphate (NH4MgPO4·6(H2O)) which forms when these three ions are present in the solution above their saturation points.
Magnesium is the limiting reactant in wastewater treatment and it must be provided in the form of magnesium chloride or magnesium oxide in order to balance the concentration of phosphorus, generating a product that can be used at industrial level for certain applications.
Eliminating this compound significantly reduces struvite precipitations in sludge line pipes.
This project saw the implementation of a struvite production system. Struvite is generated at different points of the sludge line, hindering operation of the facility. The Sur WWTP has suffered from periodic episodes of uncontrolled struvite precipitation in the digested sludge outlet pipe, in the elbows of the centrifuge feed pipes and inside the anaerobic digesters.
The rate of struvite formation is controlled in the process in such a way as to obtain a high-quality product with the desired physical properties. Phosphorus removal efficiencies of around 90% in the treated stream can be achieved and the product obtained is sold as fertiliser.
Plant Report published in: FuturENVIRO June 2017