Giving sewage the stainless steel treatment

August 12, 2016


Sewage treatment is a process that most people in developed countries take for granted. It’s a topic that many of us may choose not to dwell on, yet effective processing of waste water is essential for the health of both humans and the environment. When sewage arrives at a waste water treatment plant, it is a nasty liquid mix of food materials, human waste, detergents, fats, oils and greases, as well as other waste chemicals and debris. At the sewage treatment works (STW) it undergoes a process of screening, settlement, oxidation and disinfection until it is sufficiently clean to be discharged back into the natural environment. Nickel-containing stainless steels play an increasing role in these plants (see table, page 11). Stainless steel Types 304 (UNS S30400) and 316 (S31600) and their low carbon variants are the principal alloys used, with duplex alloy 2205 (S32205) used where particularly severe conditions are anticipated or extra strength/decreased thickness is required. Stainless steel is ideal for numerous applications throughout the process including machinery for screening, washing, compacting, grease and oil separation, thickening and dewatering of various types of screenings and sludge as well as sieve filtration, ozone and UV equipment.

Meadowhead Sewage Treatment Works

The Meadowhead Sewage Treatment Works in Scotland has recently selected a stainless steel solution in its inlet works facility. Inlet screens are used to remove solid materials that would interfere with subsequent downstream processes. After removal, the screenings are treated, often by washing and compacting, and collected for disposal. Meadowhead STW serves a population of 220,000 and the six large pumping stations within the catchment area handle flows for the plant between 1,000-5,500 litres/sec. The existing works inlet had consisted of both fine and coarse screening areas which had to contend with the large variations of flow. With unacceptably high levels of maintenance to the old screens, a new solution was required. Potential solutions were explored to solve issues of high screenings removal rates, improvement to the equipment reliability, and to fit any new equipment within the limited space of the existing building with minimum disruption. Scottish Water chose to install four large, HUBER Multi-Rake Bar Screen RakeMax®, coarse screening units as well as four HUBER Screenings Wash Presses WAPL, all manufactured and designed by Huber Technology in Type 304 stainless steel. The multi-rake coarse bar screens are 6,300mm long,
2,752mm wide with 19mm bar spacing each handling 2,200 litres/sec per screen. These have a high screenings removal capacity and it was possible to retrofit them into the existing inlet works.

To reduce delivery times, the multi-rake screens were shipped in sections and reassembled on site and deployed through the roof of the building. “As Huber Technology are experienced manufacturers of stainless steel machines for waste water treatment, we were confident that the strength and the flexibility of the design would allow reassembly at the treatment works,“ says Steve Morris, MD, of Huber Technology. ”Although a challenging task, the relatively light weight allowed ease of lifting to reposition the screens within the building.” Since the installation in 2014, several large storms have already been experienced and all units are working well.

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