Sludge and scum scrapers for rectangular tanks: bridge or chain?

A settling tank only works if the sludge that settles and the scum that floats are removed regularly. Left in place, sludge turns septic, gas lifts it back to the surface, and the quality of the outflow falls. In rectangular tanks, two mechanisms do this work: the travelling bridge scraper and the chain scraper.

Travelling bridge scraper

A travelling bridge scraper spans the tank and runs along the top of the walls on driven wheels. A bottom blade pushes settled sludge towards the hopper. A surface blade moves scum and grease to a collection point. On the return run the blades are raised so they do not push material back. The drive and wheels stay above the water, and a walkway with handrails gives safe access. Set up as a grit and grease removal bridge, the same design serves aerated grit channels at a plant inlet.

Chain scraper

A chain scraper system uses flights fixed between two loops of chain that run the length of the tank. Along the floor, the flights push sludge to the hopper. On the return run near the surface they push scum towards a scum pipe or trough. A cross collector can be fitted in the hopper to move sludge to the removal point. Because nothing travels along the top of the walls, chain scrapers suit narrow tanks and covered tanks.

Comparison

Point	Travelling bridge	Chain scraper
Parts under water	Blades only; drive and wheels above water	Chains, sprockets and flights
Maintenance access	From the walkway, usually without draining the tank	Major work on submerged parts usually needs the tank drained
Tank type	Open tanks with walls that can carry the wheels	Narrow tanks, covered tanks
Scum removal	Surface blade	Flights on the return run
Grit and grease duty	Available as a grit and grease removal bridge	Less common

Oil and floating debris

In industrial clarifiers, scum often carries oil. Scraping it to one end is only half the job: it still has to be lifted out. An oil skimmer at the scum collection point recovers the oil continuously, and a trash skimmer handles floating debris in open channels and basins.

Checklist: what to send us

• Tank length, width and water depth, and the number of tanks

• Position of the sludge hopper and scum collection point

• Type and quantity of sludge, and whether oil or grease is present

• Open or covered tank, indoor or outdoor

• Power supply and control requirements

• Preferred material for wetted parts

Frequently asked questions

How fast does a scraper move?

Slowly, so it does not stir settled sludge back into the water. The speed is set for each tank.

Can a scraper be fitted to an existing tank?

Usually, yes. We need a survey of the walls, the floor and the hopper to check the fit and the wheel track or chain supports.

What about circular clarifiers?

Circular tanks use a rotating mechanism rather than one of these designs. Send us the details and we will advise.

Scrapers are built to suit each tank. Send us your details and we will come back with a proposal. Customers in the European Union can also order through our European company, VDT Europa OÜ in Tallinn, Estonia.