Ensuring the use of reservoir flows in the current drought is becoming a titanic task, especially because the drop in their levels forces us to face the challenge of a worsening of the quality of these waters. The work is particularly complex in the Sau and Susqueda reservoirs (in the Ter), which supply the region of Barcelona, ??part of Girona and the Costa Brava. 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 dispose of the water from these reservoirs, which are key to this supply, are entering an unknown territory, since the technicians have never had to fight with such an extreme situation.
Continued monitoring 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 predictable drop in levels, requires extreme controls to prevent non-potable flows from draining.
The problem is that the Catalan Water Agency (ACA), which carries out these controls, is losing its ability to select the best quality flows that can be released to be transferred to the Cardedeu water treatment plant (from the public company ATL ). Because? Sau and Susqueda have catchment towers with several sluice gates at different depths that allow selecting at any time, according to the sluice gate chosen, the transfer of better quality resources from one reservoir to the other and, finally, the flow that will be potable. These continuous quality analyzes that are carried out make it possible to determine and choose the water layers that will require the least chemical reagents when it comes to potability. However, this possibility of choice is being lost due to the progressive reduction of Sau and Susqueda levels, which entails a loss of decision-making and management capacity.
In addition, all control can be even more complicated in the summer, when the reservoir waters are stratified in different layers of quality, due to solar radiation and rising temperatures. Thus, the upper layer, which is warmer, can suffer episodes of algae proliferation and an increase in organic matter, while the lower part, which is much colder, would be left with less oxygen and subject to the risk of the redissolution of the compounds that are in the bottom sediments and bring up precursor nutrients for more algae, as well as heavy metals and other unwanted products.
Another fearsome factor is the accumulation of organic matter, from livestock waste that has entered Sau for many years and has been deposited in the bottom sediments. For this reason, last year the fish were removed from Sau, to avoid that, when they die from lack of oxygen, the load of organic matter increases and the drinking process is complicated.
Water in a bad state that comes out of the reservoirs puts potability to the test at the Cardedeu station and may force the dosing of chlorine to be intensified in Cardedeu. This reagent is effective against pathogens but, in contact with organic matter, 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 mark whether the 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 ACA’s Environmental Management area. Algae, if they reach the potabilizer, are retained in the activated carbon, while heavy metals are precipitated in the potabilization process.
Munné points out that the situation is different in the case of the Llobregat reservoirs, since while in the Ter the water is collected 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 course of the river, and a self-purification process takes place and is diluted before being made potable in Abrera or Sant Joan Despí, kilometers downstream. However, it is also true that throughout this route the water circulating in the river is not out of danger from other possible risks and discharges. However, the Llobregat stations in Abrera and Sant Joan Despí have more advanced drinking water systems.
Meanwhile, the public company ATL, owner of the Cardedeu water treatment plant, is intensifying measures to ensure the correct treatment of the water coming 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 so that they act as filters against the possible arrival of contaminated water in the reservoirs. “We have introduced several improvements and made investments due to the possibility of a deterioration in the quality of these waters. All this makes us better prepared to deal with this deterioration in water quality”, says Fernando Valero, head of R D I and process control at ATL.
To this end, active carbon has been regenerated, the main filter/barrier used to retain pollutants; 3 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 Prat desalination plants, which are mixed with river water in the network supply “The desalination plants are not only key to guaranteeing the quantity and continuity of the service, but from the point of view of quality”, he argues.
The mixture mainly provides an improvement in the organoleptic quality (taste). “This gives us the 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, but it does admit the challenge of dealing with its lower layers. “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 water treatment plant of Ter in Cardedeu is an old plant, from 1966, and an investment of 130 million is underway to remodel and modernize it, so new stages of filtration will be introduced and optimize the active carbon filter. With the change in arrangement, the Ter water will have to overcome a triple barrier (sand filters, ozonation and activated carbon). But we will have to wait for the works to culminate around 2025 or 2026.
