Sewage sludge in the dock
Tackling the loading and unloading of Northumbrian's sludge tankersAs a regional sludge treatment facility, Northumbrian Water's Bran Sands WwTW gave rise to the usual problems of sludge transportation from other sites. But in an unusual move, alongside road tankers and collection pipelines, sludge from major WwTWs serving Teeside, Tyneside and Wearside - which also act as collection centres from local works - is also transported to Bran Sands by sea.
Following a technical assessment, Northumbrian Water selected pumps from Hidrostal for both loading and off-loading duties. The Newbury based specialist supplier has delivered 24 units for this project, each fitted with the company's non-clog screw centrifugal impeller. The pumps are located at the loading terminals at Hendon and Howden, and on-board MV Northumbrian (1,250 tonnes) and MV Bran Sands (3,000 tonnes), the two vessels work in tandem, seven days a week, to transport the mixed process, domestic and industrial sludges.
Six 90Kw variable speed cargo pumps are mounted on each ship together with six 7.5KW fixed output-stripping pumps for washing down. The pumps, which are suitable for total immersion in seawater, were all delivered as vertically-suspended, deck-mounted units ready for immediate installation. The order included a spare pump set of each size. The cargo transfer units have to handle sludge of up to 5% DS and are rated at 110 l/s against a head of 40.5m, however the stripping units that are used mostly for cleaning duties have a capacity of 8.4 l/s at a head of 20.3m. All units have a cast iron casing with a nodular iron impeller and a 25% chrome iron liner. Motors were to IP56 deck watertight TENV specification and Cardan drive shafts were used to allow for movement. At the loading terminal at Hendon, Hidrostal provided two 185Kw fixed-speed pumps able to deliver 375 l/s at a total head of 28m. The baseplate-mounted units are flexibly coupled. As sludge density can be as high as 8% DS, the pumps are mounted in an open well 3m below ground level. This ensures a positive suction head from the dockside sludge storage tanks located adjacent to the tanker berth. As hard, abrasion-resistant linings, were needed the casings were fitted with high chrome iron linings which are adjustable, to maintain peak efficiency during the lifetime of the pump. At Howden, a similar loading facility has been provided, equipped with three 250Kw fixed speed units. Cargo handling times are a key factor in the efficient operation of Northumbrian Water's fleet, and pump reliability was a key feature in the selection of pumps.