Excellent results with Filtralite® Mono Multi at an industrial scale pilot in Llobregat ATL DWTP

Located in Abrera (Barcelona) and in operation since 1980, the Drinking Water Treatment Plant of Llobregat is one of the three treatment plants supplying drinking water to the Barcelona metropolitan area. It has a daily capacity of more than 200.000 m3.

A challenging surface water quality

The treatment plant takes its water from the river Llobregat, which is a complex inlet water to render potable. The river has irregular flow, and is under strain with high demographic and industrial pressure including salt water runoff. Treating this inlet water has always been a challenge for the Abastament d'Aigua Ter-Llobregat (ATL) water utility. The main difficulties come from the high content of microorganisms, a conductivity between 1000 and 2050 µS/cm, a total organic carbon (TOC) level of 5-10 mg/L and the presence of bromide between 0.5 and 1.2 mg/L. Additionally, the water temperature varies greatly, from 2 to 27 ºC.

The plant treatment line is designed for a 3,2 m3/s capacity, with the following steps: pre-oxidation with KMnO4, pH adjustment with CO2, coagulation-flocculation, decantation, oxidation with ClO2, sand filtration, filtration with GAC, polarity reversal electrodialysis and disinfection with sodium hypochlorite (bleach).

ATL improves continuously the treatment line

During the last 10 years, ATL has made significant investments to improve the treatment line in this plant. ATL has always opted for innovation and adaptation to new standards and also to develop sustainability and reduce operating costs. For example, the plant now includes the largest electrodialysis step in the world and photovoltaic solar panels supply part of the plant's required energy.

Within this vision of innovation and improvement, the R & D department of ATL and the management of the plant have suggested an optimization of sand filtration step. Conventional sand filtration, like in many other water treatment plants, is one of the process bottlenecks for production capacity, including an important washing water and energy consumption. Moreover, the high frequency of backwashing challenges production capacity increases, when at the same the outlet water quality worsens. Filtralite is designed to improve the existing process as the number of backwashes is reduced and less water is wasted, furthermore Filtralite will decrease the carbon footprint per m3 produced.

How Filtralite® Mono-Multi can improve the filtration bottleneck

Filtralite Mono Multi is a two-layer Filtralite media manufactured by Saint Gobain. It is made of expanded clay and consists of an upper layer of low density and half crushed (NC 1.5-2.5) and a lower layer of high density and also half crushed (HC 0.8- 1.6). This composition, manufactured with high industrial standards, optimizes the configuration of the filter bed. Additionally, Filtralite material has technical singularities like high specific surface, high porosity, great resistance to wear and lightness that facilitates backwashing.

With a low headloss and a high suspended solids (SS) retention capacity, Filtralite can operate at very high speeds. The Oset Drinking Water Treatment Plant in Oslo is an example of this, working at a filtration of 20 m / h with open filters. A filtration at high speed is equivalent to a greater production of water for the same filtering surface. With this approach, Filtralite has been decisive for solving problems that are difficult to tackle with other filtration technologies.

ATL started an industrial scale test in November 2018 in one of the 137 m3 sand filters of the Llobregat Drinking Water Treatment Plant. The objective was to validate, in situ, the operation of Filtralite Mono Multi in comparison with sand filtration. This pilot tested filter cycles increase, filtration effluent quality and Filtralite performance in critical scenarios: high levels of SS, high values of turbidity…

Outstanding results after 7 months of testing

After more than 7 months of testing, Filtralite has worked in parallel with a sand filter following the same operating strategy, maintaining the same flow rate filtered in both cases and backwashing according to the same procedure. During this period, Filtralite has been in operation for 3,900 hours treating a total of 2.78 Hm3. To filter this volume, the Filtralite filter has been washed 16 times while the control sand filter has required 45 washes. A backwash ratio of 1 to 3 was recorded in favor of Filtralite. In addition, when the inlet water quality is critical, more turbidity and SS, Filtralite performs better, the media has a greater resilience to clogging.

No extra specific requirement is needed for backwashing

On the other hand, Filtralite backwashing requirements didn’t bring any changes to the current systems; blowers and pumps were still optimal. According to the ATL data, energy consumption, equipment maneuvers and water consumption for backwashing are reduced by three with Filtralite. Added to its high durability, which is more than 20 years, Filtralite, brings a very short pay-back; according to Saint Gobain's calculations it is less than three years.

In conclusion, Filtralite Mono Multi again demonstrates the enormous operational advantage over sand filtration in a Drinking Water Treatment Plant. It is a quick and simple solution for process improvement in both OPEX and production capacity. In short, Filtralite® allows you to produce more water for less money.