The Llobregat Drinking Water Treatment Plant, located in Abrera (Barcelona), in operation since 1980, is one of the basic elements of the drinking water system serving the Barcelona metropolitan area, thanks to a daily capacity that exceeds 200,000 m3.
Water to the plant comes from the Llobregat river, and is difficult to treat. The river flow is irregular, and the basin has high population density and industrial activity, as well as saline run-off, which impact on the river. Treating the river's water has always been a challenge for the Ens d'Abastament d'Aigua Ter-Llobregat (ATL) company. The difficulty lies in the pollution present in the raw water, which contains microorganisms, a conductivity of 1000–2050 µS/cm, TOC levels of 5-10 mg/L and a bromide concentration of 0.5–1.2 mg/L; in addition, the temperature ranges from 2 to 27 ºC.
The plant has been designed to treat 3,2 m3/s (nominal), and the treatment line comprises the following: pre-oxidation with KMnO4, pH adjustment with CO2, coagulation-flocculation, settling, oxidation with ClO2, sand filtration, GAC filtration, electrodialysis reversal and disinfection with sodium hypochlorite.
- Filtralite® has been key to solve problems that were difficult to address with other filtration technologies
Over the last decade, ATL has done important investments to improve treatment processes at this plant. ATL has always opted for innovation and adaptation to new standards, and an approach that focuses on sustainable energy use and reducing operating costs. Proof of that are the investments in building an electrodialysis stage, the largest in the world, or the fitting of photovoltaic solar panels to satisfy in part the power needs of the plant.
Within this vision of innovation and improvement, the research, development and innovation (RDI) department of ATL and the plant's management studied how to optimise the sand filtration process. Traditional sand filtration, the same as in other drinking water treatment plants, is one of the process bottlenecks in regard to the elasticity of production capacity, due to constant stops to backwash filters and the high consumption of water and energy to backwash the filters. Moreover, the high backwash rate complicates potential increases in the production capacity, especially during times when the water quality is worse. Filtralite® is a synonym of more efficient operations, because the backwash water does not compete with product water to reduce the backwash rate, and the carbon footprint per m3 produced decreases significantly.
Filtro bicapa with Filtralite Mono Multi®.
Filtralite Mono Multi® is a dual media expanded clay product manufactured by Saint Gobain, with a top low-density ground media layer (NC 1.5-2.5) and a bottom high-density ground media layer (HC 0.8-1.6). This composition, achieved thanks to high industrial standards, optimises the filtration bed. On top of being a dual media, there are a series of technical characteristics that ensure the filtration performance of the material is outstanding: high specific area, high porosity, high resistance to wear and tear, and a light weight which facilitates washing.
Thanks to a low head loss, together with a high capacity to filter suspended solids (SS) before clogging, the filter media can operate at very fast filtration rates. The Oset drinking water treatment plant in Oslo is an example of this, working at a filtration rate of 20 m/h with open filters. A fast filtration rate entails greater water production with the same filtration surface. With this approach, Filtralite® has been key to solve problems that were difficult to address with other filtration technologies.
When the water inflow has higher turbidity and SS, the relative difference increases, with Filtralite® performing better
Based on these references, ATL started in November 2018 an industrial-scale pilot test in one of the 137 m3 sand filters of the Llobregat drinking water treatment plant. The purpose was to assess the performance of Filtralite Mono Multi® in situ versus current sand filtration. The goals of the pilot test were to increase the length of filter runs, ensure the quality of the filtration effluent and validate the performance of Filtralite in critical scenarios, including high levels of SS and high turbidity.
After more than 7 months of testing, the data obtained allow an assessment to be carried out. Filtralite® has been used in parallel with a sand filter, using the same operating strategy, keeping the same rate of flow in both cases and the same backwash scheme. During this period, Filtralite® has been operating for 3,900 hours, processing a total of 2.78 Hm3. To filter that volume, the filter with Filtralite® has been backwashed 16 times, whereas the control sand filter needed to be backwashed 45 times. The sand filter had to be backwashed three times as often as the Filtralite® filter (3/1 ratio). Furthermore, when the water inflow has higher turbidity and SS, the relative difference increases, with Filtralite® performing better. That is, the more necessary Filtralite® is, the more the filter media features resilience to clogging.
On the other hand, the adaptation of the Filtralite® backwash requirements to the current systems, blowers and pumps, was optimal. According to ATL data, energy use, equipment handling, and consumption of treated water during backwash decrease three-fold with Filtralite®. If we add to this the high durability of Filtralite®, which is more than 20 years, we have a very short payback time for the investment; according to the estimations of Saint Gobain, less than three years.
In conclusion, Filtralite Mono Multi® shows once again a huge operating advantage versus sand filtration at a drinking water treatment plant. It is a fast and simple solution to improve the process, both in terms of OPEX and production capacity. In other words, with Filtralite® you can produce more water with less money.