The biological management of a singular water reservoir

Published in 15 de November de 2018

Everyone who has walked calmly at the Agulla’s Park will realize that there is a huge artificial lake. The Agulla’s Lake occupies 6 hectares and allows to store about 200,000 cubic meters of water that is used for human consumption. The lake receives the water from the Sèquia and was conceived as a large reservoir to guarantee the supply to the population when the water flow to the Sèquia is interrupted temporarily for maintenance work. The lake is connected to the Potable Water Treatment Station of Manresa, responsible for purifying the water and providing it with all technical and healthcare guarantees to the population.

But Agulla’s Lake is much more than a reservoir of water! More than a decade ago, it has ceased to be a passive accumulation of water and is actively managed as a phase of pretreatment of water before being purified.

The biological management of the lake means control and interaction in different natural processes, but also detect, anticipate and anticipate new challenges. In the lake, it is striking, that there are aquatic plants in a floating bed. Its function is to capture and retain nutrients (nitrogen and phosphorus) of water. Aquatic plants compete with microscopic algae for nutrients, resulting in a reduction in the organic matter of water that reaches the water purification plant. This fact, together with the addition of permanganate to the water in the traffic towards the water purifier, contributes to improve the taste and the smell of the water and reduces the formation of trihalomethane compounds.

The aquatic plants are given periodically to extract from the lake the plant biomass that contains the nutrients and the trapped organic matter. With this same goal we can collect floating filamentous algae. The diversity of plants is high, it exceeds 25 species, and the dam can be selective. Aquatic plants provide biodiversity in space and allow, for example, the presence of an important population of dragonflies and birds.

Underwater control of the lake is carried out using an echo sounder that records images of the bottom for the control of sediments and the counting of fish. Control of fishing activities and the evolution of the fish population allow foreseeable mortality episodes related to an unsustainable population increase. The lake sediment tank stays stabilized by means of a mobile suction system that avoids excessive accumulation. In the sediments of the lake there is an important population of anodontic naias, an autochthonous freshwater mollusc, filtering, measuring up to 20 cm, which was detected in 2011 and is a good indicator of the quality of the water.

Bird populations that are especially sensitive to avian influenza are also monitored. The large flock of silver-plated gulls, long before frequented by the lake, are wound up with an acoustic system. In short, multiple biological actions aimed at improving the quality of water and the sustainable management of the resource.

Josep Illa, technical director of Phragmites SL