Tertiary treatment at the Prat de Llobregat WWTP

The Prat de Llobregat Water Reclamation Plant (WRP) went into operation at the beginning of April. The special treatment implemented at the plant results in water suitable for upstream discharge in the event of necessity, for example, in the case of a drought alert. This facility is vital in terms of guaranteeing water supply in the Barcelona metropolitan area.




This scenario makes it necessary to have the Llobregat WRP operating at 100% capacity. The facility was originally built to increase the flows circulating in the
river, to contribute to recharging the aquifers (which are strategic reserves), to provide ecological flows to the lagoons of the lower valley and the Llobregat delta, and to impede the intrusion of saline seawater. The plant can produce up to 3 m3/s, equivalent to the consumption of a population of over one million (or 25% of water demand in the Área Metropolitana de Barcelona).

€4.5 million contribution

This preventive project to mitigate drought episodes is the result of an agreement entered into by the Catalan Water Agency (ACA), which will provide
financing of €4.5 million, and the Àrea Metropolitana de Barcelona (AMB).

The Llobregat plant is the third largest in Spain in terms of volume treated, after the Sur WWTP in Madrid and the Besòs WWTP. It is the only WWTP in Europe equipped with state-of-the-art equipment for complete reclamation of water, including micro-filters, ultraviolet and sodium hypochlorite disinfection, and reverse osmosis membranes for salt removal.

Project objective

In accordance with the studies carried out by Depurbaix and the ACA, it was decided to use the water reclaimed at the Prat de Llobregat EDAR for
both industrial uses and irrigation, to pump it to the headwaters of the Llobregat to maintain an ecological flow in the river and to control the intrusion of seawater into the Llobregat delta aquifer, through the future incorporation of a reverse osmosis facility.

The tertiary treatment line is located on a site preceded by the sludge line. The WWTP site has a length of approximately 1,000 metres and an average width of 300 m. It has a mainly rectangular geometry, although its eastern boundary is more rounded and the tertiary treatment facility is located in this area.

A pilot plant was in operation at the Prat de Llobregat WWTP for a period of 1 month. This plant featured physicochemical treatment with ballasted lamella clarification (Actiflo process) and disc filter microfiltration using screens with a pore size of 10 microns.

The aim of the tests was to evaluate the applicability of these processes for tertiary wastewater treatment. The prime objective was to obtain a low concentration of suspended solids and
parasitic helminth eggs. The tests showed that the combination of the two processes afforded excellent results and efficiencies were well above legal requirements.

General description of tertiary treatment at the Prat de Llobregat WWTP

The plant is designed for a maximum treatment flow of 14,400 m3/h. The water from the secondary settling tanks is sent to a chamber equipped with a spillway. This chamber is connected to the inlet of the existing pumping station that sends the water to the outfall. From this existing connection chamber with the WWTP, the water is sent to a control tank for subsequent pumping to tertiary treatment.

Tertiary treatment consists of a physicochemical process featuring coagulation, flocculation, ballasted clarification, using three lines of Actiflo units housed in a building, and filtration through 10 disc type microscreens with filter cloths featuring a pore size of 10 microns.

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Plant Report published in: #51 FuturENVIRO June 2018