Ensuring the use of reservoir flows in the current drought is becoming a titanic task, especially because the decrease in their levels forces us to face the challenge of a worsening of the quality of these waters. The work is especially complex in the Sau and Susqueda reservoirs (in the Ter), from which the Barcelona region, part of Girona and the Costa Brava are supplied. Can the waters of Susqueda continue to be used even when water has to be extracted from its swampy bottoms?
The monitoring and control operations to be able to have water from these reservoirs, which are key to this supply, are entering unknown territory, since technicians have never had to deal with such an extreme situation.
Continuous inspection has allowed the quality of the resources in Sau and Susqueda to be good at the beginning of spring. But the scenario of the coming months, and the foreseeable decrease in levels, requires extreme controls to prevent non-potable flows from being drained.
The problem is that the Catalan Water Agency (ACA), which carries out these controls, is losing the capacity to select the best quality flows that can be released to be transferred to the Cardedeu water treatment plant (owned by the public company ATL). Because? Sau and Susqueda have collection towers with various gates at different depths, which allows selecting at any time, depending on the gate chosen, the transfer of best quality resources from one reservoir to another and, finally, the flow that will be be purified. These continuous quality analyzes carried out allow us to determine and choose the water strata that will require fewer chemical reagents when it is made drinkable. However, this possibility of choice is being lost due to the progressive reduction in the levels of Sau and Susqueda, which entails a loss of decision-making and management capacity.
Furthermore, the entire control can become even more complicated in summer, when the dammed waters become stratified into different layers of quality, due to solar radiation and rising temperatures. Thus, the upper layer, which is hotter, may suffer episodes of algae proliferation and an increase in organic matter, while the lower part, much colder, would be left with less oxygen and subject to the risk of redissolution of the compounds that are in it. sediments from the bottom and bring out nutrients that are precursors of more algae, as well as heavy metals and other unwanted products.
Another fearsome factor is the accumulation of organic matter, coming from livestock waste that for many years has entered Sau and has been deposited in its bottom sediments. For this reason, last year the fish were removed from Sau, to prevent them from dying due to lack of oxygen, increasing the load of organic matter and complicating the purification process.
Water in poor condition coming out of the reservoirs puts the purification at the Cardedeu station to the test and may force chlorine dosing at Cardedeu to be intensified. This reagent is effective against pathogens, but, in contact with organic matter, it can produce dangerous byproducts. ACA experts are concerned that excess organic matter reacts with chlorine, as it gives rise to compounds such as trihalomethanes, which could exceed the maximum legal values ??allowed by the Department of Health. Trihalomethanes are one of the indicators that indicates whether water is drinkable.
Another danger is the proliferation of algae, although until now “the episodes that have occurred have never caused us toxins,” explains Antoni Munné, director of the environmental management area of ??the ACA. The algae, if they reach the water treatment plant, are retained in the active carbon, while the heavy metals precipitate in the water treatment process.
Munné highlights that the situation is different in the case of the Llobregat reservoirs, because while in the Ter the water is captured in the reservoir and directed directly with a pipe connected to the water treatment plant, in the Llobregat the water from the reservoirs circulates through the river bed, which produces a self-purification process and is diluted before being made drinkable in Abrera or Sant Joan Despí, kilometers downstream. However, it is also true that throughout this journey the water circulating through the river is not safe from other possible risks and spills. However, the Llobregat stations in Abrera and Sant Joan Despí have more advanced purification systems.
Meanwhile, the public company ATL, owner of the Cardedeu water treatment plant, is intensifying measures to ensure the correct treatment of the water that comes from the Ter, which is purified before being distributed in the Barcelona region. At this station, new reagent dosing equipment has been put into service to act as filters against the possible arrival of contaminated water to the reservoirs. “We have introduced various improvements and made investments in the face of the possibility of a worsening of the quality of these waters. All this makes us better prepared to face this worsening of water quality,” says Fernando Valero, head of R D i and process control at ATL.
To this end, a regeneration of active carbon has been carried out, the main filter/barrier used to retain pollutants; Three million have been invested in 48 new filters; and the dosing capacity of reagents (such as sodium hypochlorite) has been increased.
However, Valero clarifies that when it comes to improving the quality of water from the Ter, a great ally is the flows from the Tordera and El Prat desalination plants, which mix with the river water in the supply network. “Desalination plants are not only key to guaranteeing the quantity and continuity of the service, but also from the point of view of quality,” he argues.
The mixture provides, above all, an improvement in the organoleptic quality (taste). “This gives us peace of mind that we will have quality water both in a normal situation and in an extraordinary situation,” says Valero. ATL does not believe that the Susqueda reservoir will be completely emptied, although it does admit the challenge of treating its lower strata. “If the treatment capacity is exceeded in any plant, we would certainly have problems, but we are prepared to treat even the worst quality,” he reiterates.
In any case, the Ter water treatment plant in Cardedeu is an old plant, from 1966, and an investment of 130 million is underway to remodel and modernize it, for which new filtration stages will be introduced and the active carbon filter will be optimized. . With the change in disposition, the water of the Ter will have to overcome a triple barrier (sand filters, ozonation and active carbon). But we will have to wait for the works to be completed in 2025/2026.