Water & Wastewater Treatment

In the know Digging deeper: wastewater pumps 34 |NOVEMBER 2015 | WWT | www.wwtonline.co.uk come across larger solids, which complicates the free passage comparison. Beyond these basic impeller types, there are designs which claim to be more clog-resistant than others. One reason that wet wipes and similar fibres are seen as such as major challenge is that they cling to impeller blades, so that even if the throughlet is not totally blocked, efficiency is reduced. Backswept leading edges and relief grooves on impellers are seen by many as beneficial features, while some pumps boast self-cleaning mechanisms. "Standard pumps can deal with standard sewage, but they are no match for the stringy, tear-resistant synthetic fibres of products such as wet wipes," warns Hugh Vaughan of Landia UK. "Buyers should beware of claims of so-called 'non-clogging' impellers, which rags have no trouble in wrapping themselves around." Landia's 'chopper' pumps, which use a knife system to chop and cut materials before they enter the impeller, are an example of another strand of pumping technology which combats tricky waste by grinding or chewing it up. Other variations include pumps that use the impeller itself as part of the cutting mechanism (although this "inevitably leads to problems" according to Vaughan) or pumping systems which include a separate macerator. Hard objects and rags Grinder pumps are seen by the industry as a solution for sites where ragging is a particular problem, although high energy use typically makes them less suitable for universal use. Hard objects can also pose them more of a problem depending on the level of free passage available. A challenging scenario for all pumps comes when hard objects combine with rags, according to Hidrostal's Wilson. "Yes the rag is a problem, but quite o‹en you find that the rag has fixed itself to a solid object," he says. "Kids in particular play around toilets and little action men go down, ninja turtles, all these things go in, and then you find that the rag will start to build around that solid lump. So if you've got a pump with a small throughlet that's able to handle fibres well, and you get a bunch of fibres that have got hard objects in the middle, you get hard blockage and that will churn that pump up. That's why we always think we would rather get the whole lot through if we can." Of course, the design of the pump itself is only one consideration in determining which pumping stations end up being problematic for blockages. The design and arrangement of the wider pumping station and rising main is just as big a factor in many cases. "We know that there are certain pumping stations that are problematic, and most of them are problematic because of a design issue," says Andrew Ball of Grundfos. "It could be that it's the wrong pump - the pump duty isn't right for the pumping requirement – but it could be the civils design, for example the pump chamber could be too small, or too big, or there isn't any benching, so the wastewater sits in the chamber too long or not long enough." Grundfos's top wastewater pump Examples of hard objects found in wastewater at Ness Point, an Anglian Water treatment works Landia's Chopper pumps (above le , and above right) shred rags with an in-built knife system before they enter the impeller

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