At the Upper-Middle Arroyo Culebro (UMAC) wastewater treatment plant in Spain, preventing contaminated solids from being washed down into the river during times of heavy rainfall is a priority. The facility, the largest of its kind in Europe, cost 77 million to build and was financed by the Spanish Ministry of Environment, utilising EU cohesion funds to cover 85% of the cost. Looking to tackle the problem of contaminated storm water, the UMAC plant decided to install stormscreens and pumps from designer and manufacturer Mono NOV.
Preparing for a storm
These stormscreens, which have been designed to offer a more economic alternative to many mesh and powered screens for handling storm overflows, were supplied to the plant through Mono NOV’s Spanish distributor, Albosa. Designed to handle a maximum flow of 129,600 m3/day and meet the needs of a population of 1,225,000 inhabitants, Albosa installed 32 stormscreens at the UMAC wastewater treatment plant.
Sara Modrego, assistant manager at Albosa said: “The Mono stormscreens were specified because, as well as being self cleaning and offering low whole life costs, the screens are self powered, offer efficient and effective screening, and continue to operate in the event of a disruption in electrical power.”
Working with stormscreens
Each of the stormscreens’ mesh drums incorporates two arms, fitted with adjustable brushes. As wastewater flows into the channel and upwards through the unit, solids above 15 mm are captured on the mesh. The screened flow powers a paddle wheel, which is mounted below and behind the mesh drum. This drives a brush wheel which subsequently cleans the mesh drum, returning trapped solids back into the main flow for treatment. An integrated scumboard protrudes into the main flow and deflects large floating solids away from the screen, increasing the screens’ self-cleaning capability.
The stormscreen has a modular construction that is adaptable to existing civil works, with a range of diameters and length options to suit different overflow weir configurations. An optional data monitoring system is also available that incorporates status reports and event logging capabilities.
Up until the 1990s, standard inlet screens were required to have 20 mm apertures which resulted in a relatively coarse screening process. However new legislations have led to this being reduced to 5 mm in each direction, improving screening effectiveness, but adding to the loads a much larger volume of screened out material.
As such, 5 mm screens are much more susceptible to blinding and become less efficient much quicker unless cleaning equipment is included into the design, which adds to the cost, maintenance and energy outputs of the system. However, the Mono screens installed at the UMAC wastewater treatment plant are self-cleaning, making them much more efficient than traditional methods.
Transferring sludge
In addition to the stormscreens, the UMAC plant also requested four Mono Widethroat pumps and 42 progressing cavity pumps. These pumps transfer sludge generated from 15 km of collection channels in the southern districts of Madrid, which the plant serves. They were installed to pump the sludge after it has been through the dewatering and sludge thickening process.
Designed to handle highly viscous material, the widethroat pumps can manage sludges, slurries, thick non-flowing pastes and dewatered sludge cake with viscosities up to 48 bar and capacities up to 215 m3/h. The standard design consists of an enlarged rectangular inlet with a screw conveyor to assist the product into the pumping element.
Handling viscous media
Progressing cavity pumps, also in use at the plant, were increasingly being used throughout treatment plants to handle media more viscous than water, as well as where there are solid inclusions. The pumps use a positive displacement action, which provides excellent suction capabilities at the inlet, so, unlike centrifugal alternatives, submersing the pump for medium or full priming is not required. The characteristic pumping action of progressing cavity pumps, which involves a cavity being moved from inlet to discharge by the rotor, allows effortless transportation of materials of varying viscosities which otherwise could be seen as non-flowing.
A specific benefit of progressing cavity pumps is that flow volume is effectively linear to running speed, so it is easy to respond to changes in flow demand, ensuring more energy efficient pumping and processing versatility. They can also run at a range of rotational speeds, without wasting energy.
Mono’s pumps have been designed to minimise whole life costs and deliver reduced spares requirements in wastewater applications. The pumps feature the Flexishaft®, a joint design which links the drive shaft to the helical rotor, solving the problem of connecting the pump drive shaft to the eccentrically orbiting rotor. This design aims to eliminate wearing between the drive end and the pumping element.