Sewage sludge in the dock

As 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.