Tackling the wet wipe dilemma

Suzanne Gill finds out how pump system providers are helping the utility and wastewater industries to overcome the challenges posed by the increasing quantity of wet wipes and other solid products in systems and networks.

The asymmetrical blade arrangement of the KSB F-M impeller allows solids of different sizes to pass easily.

Every year UK utilities spend around £100 million clearing an estimated 300,000 fatbergs from the sewerage system. These blockages are made up of congealed fats, oils and grease (FOG), mixed with ever-increasing quantities of solid waste that has been deposited into the network, such as wet wipes.

Although promoted as flushable, single-use wipes do not break down in the same way and in the same timeframe as toilet paper and handling these ‘disposable’ products is becoming a serious problem for utilities and wastewater systems. Indeed, wet wipes are believed to be responsible for 80% of blockages in sewer systems today. System failure due to blockages can require costly manual attendance – often in confined spaces. It can lead to the need for emergency tankering, equipment removal, and cleaning, in addition to strip down and repair of the pump.

Attempts to combat the problem can include the installation of macerators and choppers in the system. However, over time, the cutting action of these solutions can become less effective as the wearing parts become blunt and so regular maintenance is often still a necessity. “We believe the answer to the majority of blockages is free passage,” said Paul Walthew, Business Development Manager – Site Services at Caprari. “In our three and four-inch adoptable sewage pumping stations we endeavour to install, where possible, vortex and channel impeller pumps matched to the pipework size to enable what enters the pump to be passed through the pumping station pipework.”

The Caprari KCW range has a large free passage – 80mm & 100mm on 3in and 4in branch sizes respectively. “We have teamed these with other technologies, such as fibre cutting and back impeller cleaning, to ensure that the pump is efficient and fights against blockages effectively. We have also installed telemetry units on pumping stations to monitor performance and combined with regular maintenance, we have seen reactive callout intervals now being measured in months – and in some cases years!”

CUTTING-EDGE DESIGN

NOV, formally Mono Pumps, has been providing heavy-duty wastewater grinders for over 30 years and its Muncher range of twin shaft, slow speed, high torque grinders are used by all the major UK water companies, along with many private operations such as airports, hospitals, hotels, shopping centres and leisure facilities. “In recognition of the increasing use of wet wipes and other one-use disposable products, NOV conducted several operational performance trials to measure the effectiveness of grinding this fibrous product to an acceptable and handleable particle size to significantly reduce pump station blockages,” said Ian Hallows, Business Development Manager at NOV. “Our Muncher grinders are fitted with ETOS cutters which offer positive and effective solids maceration and which have been designed specifically for the effective processing of products such as wet wipes.” The ETOS cutters provide effective maceration with the cutter shafts operating at differential speeds to exert a tearing action on the solids. The off-set tooth design of the cutter reduces impact load and power. The saw-tooth profile of the ETOS cutters offers greater strength, while its close machining tolerances offer less clearance and ‘chatter’ on the shaft. The design includes a serrated tooth profile to snag and hold textiles, and a large circumference gives less clearance and better grinding.

IMPELLER DESIGN

Centrifugal pumps employed in the wastewater treatment industry are characterised by their design features – specifically impeller type, the direction of flow and type of installation. If the pump runs too slowly with a heavy load there can be a problem with suction becoming blocked; if the pump runs at intermediate speeds with a partial load, then the gap between the impeller and the pumping case may become blocked as suspended solids settle out and collect in the pump inlet; it is also possible that the gap between the impeller wear ring and casing wear ring becomes blocked. “These factors make pump selection, and more importantly impeller selection, vital because the impeller must match the nature of the fluids to be pumped and the operating environment,” said Christoph P. Pauly of KSB.

According to Pauly, the most important consideration in impeller selection is its operating reliability, and this can only be obtained by the size of the free passage. “While free-flow or ‘open’ impellers enable suspended solids in wastewater to pass more easily through the pump than closed single-channel or multi-channel impellers, they do not compare favourably when it comes to performance. It is for these reasons that KSB has focused on designing an open impeller that can achieve and even exceed the efficiency levels associated with single-channel impellers,” continued Pauly. “The first stage in finding a solution to clogging is to differentiate between rigid and non-rigid solids. Rigid solids need to have sufficient space in the pump chamber for them to pass through the pump. When it comes to non-rigid solids, it is necessary to ensure that the presence of wet tissues and similar fibrous materials do not form a mass.”

KSB has addressed these issues with the development of the Amarex-KRT F-Max impeller which incorporates varying distances between its blades, arranged in groups with two small and two large distances. This asymmetrical blade arrangement offers wide, free passages ensuring that even larger rigid solids pass easily through the pump. Improved hydraulic system and motor performance are delivered through the design of a new vortex impeller and a motor that offers efficiencies currently calculated according to the same IEC 60034-2 measurement method as that used for motors of dry-installed pumps.

“Blockages involving soft materials start at the hub or ‘eye’ of the impeller, and there is a physical reason for this,” explained Pauly. “The revolving motion of the impeller introduces velocity and the greater the distance from the centre of the impeller is where the velocity is greatest. If there is material at the centre of the impeller and there is insufficient speed to eject the material, this means that a swirl has to be created to remove the material. The swirl motion through a slightly convex profile at the hub of the F-max impeller achieves efficiencies previously only reached by single-channel impellers. When rotating, the impeller keeps solids in suspension and reduces the risk of clogging. Since the radial forces and vibrations created by the new impeller are usually lower than those of single-channel impellers, the service life of shaft seals and rolling element bearings is increased.”

Image courtesy of Caprari Pumps

DESIGN AND SMART CONTROL

Sulzer has taken a multifaceted approach to solving pump blockages using both robust and optimised design coupled with smart pumping control and monitoring. Over a decade ago, Sulzer Pumps established a test method – based on site surveys, material examination and usage analysis – to study samples taken from a site and to decide upon a representative test material. Thishelped the company in the redesign of its Contrablock Plus pump impeller system.

“Blockages involving soft materials start at the hub or ‘eye’ of the impeller, and there is a physical reason for this.”

In addition to its optimised impeller design, recent advances in smart pumping have also helped in the battle to overcome blockage issues. A recent field trial of the latest Sulzer control and monitoring solution, coupled with Contrablock Plus, has resulted in improved reliability and reduced downtimes. Smart sensing of the electrical supply coupled with custom start/ stop cleaning cycles has ensured that any spike in load on the pump results in removal of the potential blockage. “Issues which had once been reported on a test site in the UK every three months, have since disappeared and the site has run without intervention for eight months. This pilot project – a joint venture between Anglian Water, Sulzer Pumps, Vega Controls and ABB – was awarded Project of the Year 2019 by The Pump Centre,” concluded Robert Connolly, Hydraulic Design Engineer at Sulzer.