Stormwater Management in Spain

Malfunctions occurred during the emptying phase of the Butarque storm tank in Spain, due to the viscosity and high-solid content of the sludge, with many fibers, rag entanglements and other solids cause problems in the main pumping system. After a pilot test of the Abroñigal storm tank, operated by FCC, the equipment was expanded to other locations in Spain.

By Paul Krampe

Madrid's Council from the Government Area of Environment, Security and Mobility, has promoted a Plan of Water Quality Improvement in Manzanares River during the last few years. The plan includes the construction of 36 storm tanks and more than 34 kilometers of new large dimensions collectors. The main purpose of the plan is to improve the quality of the river water in compliance with the existing regulations.

After an extended or intense rain the Wastewater Treatment Plants (WWTP) are not capable of treating all of this incoming water. In those cases, the storm tank construction allows the retention of overflow from the canalizations. This water volume has to be stored upstream.

Cleaning of the storm tank prior to the equipment was up to €125,000

After studying the systems in Abroñigal, Butarque, Arroyofresno and La China storm tanks, the project was carried out in the Butarque storm tank. This tank is located at Butarque WWTP, south of Madrid. It was built in 2006 for a water storage of 400,000 m3, being one of the four biggest storm tanks in the world. This tank collects the waste and rain water from the right side of the Manzanares River.

Challenges

The storm tank acts as a buffer container constantly loading and unloading. During the loading phase after a storm, for example, the rain water arrives at high speed and with turbulent flow dragging all kinds of heavy solids and sediments, which also cleans the pipes of the sanitary network. This water sweeps the surface waste along with solids in the wastewater and the accumulated sediments in the sanitary network. The water contains organic material; fibers such as hair, rags and wipes; hygienic products; other waste such as plastics, toys and shoes and dragged sediment and aggregates, even small dead animals.

Despite a preliminary treatment like influent screens and grit chamber, a high quantity of solids still flows into the storm tank. Heavy parts of the sludge decant at the bottom of the tank, and some of it forms scum layers. The influent has a dissolved solids content of 1.5% but during the tank emptying and after pumping the decanting water, the DS content can reach 10-20%. The high DS content linked to the huge quantity of fiber entanglements result in the blockage of the centrifugal pumps. The main pumping system was designed to work until a certain level. From that point, where the DS content is higher, the emptying of the tank was expected to be done by the drainage pumps.

The first phase of the emptying, done with the main pumping system, worked usually well during the normal operation and when the DS content is not very high. There were only some punctual blockage problems in the pumps due to the high quantity of solids in the sludge. When the system reaches a certain water level with a much higher DS content, the main pumping system transfers the operation to the drainage pumps. During the operation of the drainage pumps, many malfunctions occur. Because of that, those pumps are not able to exhaust the scum due to its specific characteristics.

IMESAPI Group, the storm tank operator, detected these operational malfunctions and was forced to execute special cleaning and emptying actions to maintain the storm tank's effectiveness.

Proposal solution

Vogelsang delivered a Cut&Pump System. This sludge pumping equipment encompasses a Vogelsang rotary lobe pump VX186-260Q and a Vogelsang RotaCut macerator RCX-58G Inline, and is able to pump very viscous fluids even sludge with high fiber and solids content. The equipment was designed for a capacity of 55-220 m3/h and a pressure of 3 bar.

The sludge pumping equipment was installed in the dry pumping chamber

The rotary lobe pump and the macerator are connected in a special design. Thanks to the suction capability, the rotary lobe pump creates a vacuum that primes the suspension through the macerator. The influent conveys through the macerator screen, where the fibers are precisely and efficiently cut. An integrated heavy material separator was included in the macerator, where the stones and metal pieces are separated by gravity. This separator protects the pump and the rest of the installation against damages and blockages.

The sludge pumping equipment was installed in the dry pumping chamber. The control unit was installed in the plant control room, and analyses in real time every process variable and adapts the operation strategy by condition monitoring.

Economics

The estimated sludge extraction for the Butarque storm tank is between 2100 m3 and 2300 m3, after analysing the operation of the biggest storm tanks in Madrid Community (Butarque, Abroñigal, La China and Arroyofresno).

Vogelsang demonstrating its Cut&Pump system, which the company claimed had a return on investment (ROI) after the first tank cleaning

This extraction is made in an extraordinary actuation with mechanical and human resources. Because of the storm tanks complexity, its internal compartmentalisation and difficult access to the system, a big effort is required by the operating company.

Each cleaning intervention has an estimated cost between €100,000 and €125,000, only for taking out the sludge of the tank. The transport and disposal procedures cause additional costs. For a normal plant operation it would be necessary to completely empty the tank twice a year. Furthermore, the reduction in the operation and maintenance costs, equated to around €50,000. As a result, the Cut&Pump System investment was paid back after the very first tank cleaning.

Paul Krampe is technical director at Vogelsang Spain.