Pumping systems at sewage treatment works are always being challenged by items which arrive via the sewerage network and which cause serious maintenance issues. Two novel pumping solutions are aimed at tackling these issues. Sean Ottewell reports.
Plastics, rags, baby wipes, straws, false teeth and even frozen chickens are among the objects that find their way into sewage treatment works via the sewerage system. The challenge has always been how best to remove such objects efficiently, before they have chance to block and damage pumping systems.
Flygt's solution is to release an adaptive impeller version of its Flygt N-Pump. The new enhanced design allows the N-impeller to move axially to allow easy passage of trash flow-virtually eliminating clogging even under the most difficult conditions. This advanced design is now available for small Flygt N-series pumps from 2.2-hp through 10-hp.
"This is a giant step in the further development of our N-series of wastewater pumps", said Ulf Arbeus, R&D director of ITT Water & Wastewater (ITT WWW-Flygt). "This patented solution is quite simple but, the benefit provided to the operators of pump stations is remarkable. Reliability and energy efficiency can now be significantly improved compared to their existing operations. We are very excited to launch this design to the market."
The new technology works like this: when larger pieces of trash enter the volute, the patented adaptive version of the N-impeller allows the impeller to rise momentarily, axially upwards enabling very large debris to pass the impeller. After passage of the debris, the N-impeller immediately returns to the normal operating position in close proximity to the volute insert ring.
Adaptive impeller design now takes the wastewater industry one giant step further. "Extensive field trials in Europe and North America have proven that Flygt brand N-pumps equipped with adaptive impellers not only eliminate major clogging problems but also deliver extended service intervals," says Bob Domkowski, business development manager for transport pumping with ITT WWW-Flygt.
In New Mexico for instance, the new enhanced N-technology completely eliminated clogging at the main Waste water treatment plant serving the capital city of Santa Fe. Not one instance of clogging has occurred at the clarifier scum pit.
A Flygt N-pump equipped with the adaptive impeller design easily coped with huge amounts of trash like plastics, rags, baby wipes, straws and much else.
"We are more than impressed," said Santa Fe plant superintendant Luis Orozzo. "My team no longer has to deal with the messiness of removing the pump and unclogging it and summoning a vacuum truck. The adaptive impeller saves lots of time, money and headaches."
Solids-in-suspension
In a similar vein, the company now offers its FLYGT A-C Series with its novel 'Shearpeller' pump designed to handle large solids-in-suspension, slurries, heavy viscous material and gas entrained liquids to 25 per cent air vapour.
The Shearpeller, or model NSX, was originally designed by A-C pump as a solution to handling large solids while not becoming air-bound. The cut-away inlet edge of the impeller vanes permit the air to enter unobstructed and the regeneration within the impeller does not allow the air to collect, thus preventing the air binding so frequently experienced in conventional centrifugal pumps.
Endless hours of shutdowns and impeller clean out are eliminated while pumping raw un-comminuted sludge from a wet well of primary settling tank to digesters. The heterogeneous mass of the sludge is easily handled using a 4x4x11 inch pump with ITT's innovative NSX impeller design.
According to the company, the FLYGT A-C Series Shearpeller pump can be operated in either the vertical or horizontal configurations and will often fit into existing systems without costly modifications.
Maceration too
Mono Mutrators are packaged pumping systems that combine a Mono macerator and Mono progressing cavity pump to macerate any solids within a flow and protect the pump from blockages.
They were specified by UK company Severn Trent Water to replace its existing CMD40 Mutrators, which had continued to perform efficiently for nearly 30 years, and now needed replacing. The new equipment has been installed for the transfer of raw sewage at a capacity of 13m3/h to a high level rotating biological contactor.
Neil Horton, project engineer for Adroit Construction Services, the framework contractor on the project, commented: "We were very happy with the performance of the existing units at Burnhill Green and therefore wanted a 'like-for-like' replacement. Mono's service was excellent. We simply informed Mono of the model number of the existing Mutrators and they quickly supplied us with the latest design. The new Mono Mutrators are not only saving us energy but also reducing our maintenance costs."
By combining a Mono progressing cavity pump together with a tri-hammer macerator fitted in the suction line, the Mono Mutrator is able to lift the raw sewage from the collection sump into the cutting chamber to finely macerate any solids prior to entry into the pump (Fig. 1). Blockages are therefore eliminated and when the pumping cycle ends, large unmacerated solids fall back into the sump, clearing the suction pipe for the next duty run.
Reduced power consumption
The Mono Mutrator is available in various design options to suit different wastewater handling applications and can be installed either vertically or horizontally to give the lowest possible design/construction costs. It has a number of drive options for optimum pump speed and reduced power consumption, and is also able to pump long distances and high heads.
The positive displacement action of the Mono pump offers the benefits of smooth and controlled transfer of the sewage through small bore pipes into the main sewage system for further treatment. Being a positive displacement pump, head is independent of speed. This, combined with correct pump selection, leads to low wear rates, minimal spares usage, lower running costs and less downtime, minimising least whole life costs. On many high head pumping duties, Mono Mutrators can consume less than 50 per cent of the energy of conventional sewage pumps.
Mono has also helped United Utilities with its sludge handling problems at a plant based in the south of England.
Here four pumps have been installed vertically mounted to automatically transfer primary sludge, with a dry solids concentration of up to 6 per cent, in a consistent and controlled flow from the bottom of the primary sedimentation tanks to the secondary treatment process, at the sewage treatment works.
Stuart Merson, mechanical project engineer for the Gallifordtry-Costain-Atkins joint venture - the framework contractor for capital schemes in the South for United Utilities - commented: "Mono was able to provide us with a far better technical solution than our existing equipment offered. The Mono pumps deliver excellent performance and could be installed to meet our specific plant requirements."
The plant needed to maximise space in the sedimentation tanks, so Mono's in-house CAD team designed the pumps to be mounted vertically at the side of the tanks. Special pre-fabricated pipe adaptors were also designed for the pump inlet to allow easy connection to the existing discharge pipework at the base of the tanks.
Davit points also eliminate the need for maintenance staff to enter into a confined space to disconnect the pumps for any routine maintenance.
As a result of the design recommendations at this site, a number of other waste water treatment works are now using Mono pumps for this type of application.
The remaining two Compact C pumps were installed to transfer sludge liquor. Once sewage has received sufficient treatment, excess liquor, a combination of sewage and biological mass, is discharged into settling tanks and the treated supernatant is run off to undergo further treatment before discharge. Mono's pumps were installed to transfer this sludge liquor from the top of the sludge settling tank, for further storage, before returning it back to the inlet works.
Steve Merson continued: "Liquors are very concentrated and problems can be experienced in periods of low flow. Mono was able to offer variable speed control so that the amount of liquor pumped out was dependant on the flow coming in."
The Mono pumps were installed inside the bunded walled dry well area of the plant's pump house and, because the pumps were available on a short lead time, the customer was able to order the equipment to suit project timings.
