Packaged pumps stem wastewater woes
01 May 2009
Dealing with wastewater is a difficult business, particularly when it comes to issues like health and safety. But pump manufacturers are developing new products to help treatment plants meet the legislation. Ian Hallows explains how packaged pumping systems can make a measurable difference
As well as being difficult and dirty to maintain, traditional submersible sewage pumping stations are subject to strict safety measures because of the associated access and lifting requirements. Sewage pumping stations commonly use centrifugal pumps in a wet pumping chamber, an approach that presents a number of challenges, not least being a need to keep the chamber flooded to allow the pump to prime.
This places constraints on plant design because the centrifugal pump must be located below liquid levels to ensure the inlet and discharge ports are submerged. As a consequence, damage can occur if the pump draws in heavy objects, blocks and requires subsequent manual cleaning.
To help wastewater treatment facilities avoid health and safety issues, pump manufacturers have developed packaged pumping systems, which deliver effective sewage transfer, combined with ease of access, installation and maintenance. Packaged systems pump and macerate waste in a single operation, which is far more effective and energy efficient than operating and maintaining submersible pumps in a sump. Indeed, they will typically consume less than 50% of the energy of conventional centrifugal sewage pumps.
Because packaged pumping systems are surface mounted, lifting and access to sumps is avoided. And as equipment is installed in a dry, accessible and clean environment, inspection and maintenance is far safer, more hygienic and user friendly. Packages typically combine a progressing cavity pump with a solids grinder, which enables the use of a less costly ‘small bore’ (as opposed to a 100mm bore) rising main system.
The positive displacement action of the pump lifts the raw sewage, plus any textile and fibrous matter, from the sump into the cutting chamber of the grinder where it is macerated and then pumped away into the main gravity sewer. When the pumping cycle ends, material that has not been macerated falls back into the sump, clearing the suction pipe for the next duty run.
The reduced pipe size also increases the velocity of the sewage, providing an optimum flow rate, eliminating the risk of solids settlement and septicity. With this method of solids handling, the final effluent being discharged to the treatment works arrives in constant, short discharges, minimising shock loading and downstream blockages.
Progressing cavity pumps are capable of handling viscous media such as filter cake and primary, secondary and dewatered sludges. Hardened steel rotors and resilient stators form discrete cavities within the pump, enabling it to transport difficult-to-handle media, even those that appear non-flowing. Moreover, suction and vacuum can be drawn without the need for priming, which is why they can be used above vessels and sumps.
A useful feature of progressing cavity pumps is the linear relationship between flow volume and running speed, which makes them easy to adjust as flow demands vary. And unlike centrifugal pumps, progressing cavity pumps are not greatly affected by pressure, adding to their versatility.
The most efficient packaged pump systems for this type of duty feature a twin shaft grinder to treat heavy solids and abrasive sludges. Equipped with precision cutters mounted on contra-rotating shafts, these grinders are able to pull apart fibrous material, while the leading edge and sides of the cutter teeth crop and shear other materials, such as plastics, into small pieces. Meanwhile, the circumferential ‘land’ of the cutter crushes friable or brittle material.
There is no doubt that packaged pumping systems equipped with progressing cavity pumps offer a consistently safer, more hygienic and reliable alternative to traditional submersible centrifugal sewage pumping systems. They easily meet health and safety requirements and, what’s more, deliver far lower whole life costs.
Ian Hallows is business manager, Waste-Tec products at Mono NOV
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