Sludge treatment upgrade provides winning recipe

Osberstown near Naas hosts a wastewater treatment plant that services much of county Kildare in the Republic of Ireland.

Water utility company Veolia, which manages the site, undertook to carry out a substantial upgrade of the facility including the key sludge treatment works in order that the site could fully service the whole county.

The modernisation project for Osberstown STF managed by Veolia involved the upgrade from a very energy intensive thermal drying plant to advanced anaerobic digestion using their Exelys™ thermal hydrolysis process. The scope of supply also included a new dewatering plant with three belt presses and three sludge cake pumps to transfer the dewatered sludge into the THP feed silos.

Upgrade project presented an immediate challenge: how to fit the new THP and dewatering plant into the restricted space of the existing building. The compact design of Veolia’s Exelys™ process significantly reduced the installation space required for the THP, yet problems remained.

Space restraints meant that using a conventional multi-stage pumping system required to transfer 18-27% DS sludge cake through a total of 50m including vertical pipework with long-radius 90° bends would not be possible. 

Therefore, the dewatered sludge handling system needed to have the operational flexibility to accommodate a fully automated 24/7 operation with one, two or all three presses in operation at the same time, discharging into either of the two existing sludge cake silos through common discharge pipework.

Having worked with SEEPEX on successive projects and being aware of its expertise in sludge treatment, Veolia engaged the contractor to explore how to install a sludge cake handling system within the reduced footprint.

The decision was made to employ SEEPEX’s Smart Air Injection system. SAI utilises a progressive cavity pump and pneumatic dense phase conveying technology and is able to transport 16-40% DS dewatered sludge with a significantly lower operating pressure requirement.

SEEPEX supplied and commissioned threesludge cake pumps with 2.5m custom-made hoppers to suit the dimensions of the dewatered belt presses, the associated BLI pumps as well as the SAI controls.

The SAI system offered considerable savings by allowing the new plant to be installed within the existing sludge dryer building. By using dense phase conveying, Veolia was able to lower the discharge pressure from 16-18 bar to 3-4 bar, enabling smaller cake pumps compared to the 4-stage pumps otherwise required for a conventional system.

Veolia Principal Mechanical Engineer Alan Whitty, described the installation as a revolutionary change in approach, commenting: “I would have to say that the SAI installation has fundamentally changed how I would approach sludge cake transfer design / plant layout in future.”

SEEPEX says the system power requirement is nearly as low as 50% of that needed by conventional pumping, thereby offering vital energy cost savings. Not only did it boast the operational flexibility required by the process and enable the pipework to be emptied when idle, claims the company; it also eliminating the risk of cake line blockages and is easy to operate as well as highly reliable.

“I have worked with sludge cake pumping systems for 17 years, but I have never seen anything like the SAI installation at Osberstown – I am extremely impressed with it.” added Whitty.