About the Lisbon - Frielas Nereda WWTP
Frielas wastewater treatment plant (WWTP) serves Lisbon, the capital and largest city in Portugal.
Frielas WWTP receives mainly domestic wastewater from 250,000 people living in the Greater Lisbon area. Since it became operational in 1997, the WWTP experienced huge changes in the nature of the wastewater characteristics that the original plant was designed to accommodate. It was determined that a considerable upgrade was required, to bring the plant into the 21st Century.
SIMTEJO, which operates the site in conjunction with Agua de Portugal, decided to implement Nereda technology combined with the existing Continuous Activated Sludge (CAS) plant, by running a field trial of the two systems side-by-side. One of the six existing CAS reactors was retrofitted into a Nereda demonstration reactor, which operated alongside the remaining CAS reactors. The demo reactor was started up in December 2011, using normal activated sludge from one of the other aeration tanks as seed. By May 2012, the process had stabilised, providing an improved quality and consistency of effluent. In addition to the carbon removal requirements being fully achieved, even during the start-up period the phosphorous levels were reduced due to the biological phosphate removal in the Nereda process. Nitrogen removal efficiency was possible and was obtained without internal recirculation and at a sludge load with an average temperature of 18°C – considerably higher than is usually needed to obtain the same effluent quality with a conventional activated sludge system.
Image: Activated sludge granules from the Nereda Frielas wastewater treatment plant
Over two months, direct aeration energy savings of approximately 30% were assessed. Adding this to the savings of Nereda not needing separate settlers, sludge recirculation and post-filtration, the potential savings of the plant rose to 50%. As a result of the positive performance of the Nereda reactor, SIMTEJO has upgraded the demonstration plant into a full-scale application, operating in parallel with the CAS technology. Since Summer 2014, the retrofitted Nereda has had a treatment capacity of 12,000m³/day, serving 44,000 inhabitants. Since the upgrade of Frielas WWTP, the combined system was not only able to meet treatment objectives but also to operate as a hybrid Nereda plant, which has resulted in a further increase of its hydraulic and biological treatment capacities. And if and when in future stricter effluent requirements would immerse, more CAS reactors can be retrofitted.