The right choice for shifting sludge

Handling sludge efficiently is a challenge for anyone trying to plan and run a waste-processing facility. Finding the right type of pump to cope with these varying demands is critical to the whole operation's success, says Andrew Barry.

An estimated 35 million tonnes of raw sewage is produced in the UK each year, something the general public might not consider too often, but for ITT Flygt it is at the heart of its business.

Sludge defines the viscous, semi-solid mixture of bacteria and virus-laden organic matter, toxic metals, synthetic organic chemicals and settled solids removed from domestic and industrial wastewater at a sewage treatment plant.

It is both difficult to pump and notoriously hard on the pumps doing the work. The problem lies in the fact that, in some processes, the sludge's character changes as it is pumped through a pipe network, therefore creating different head losses. The nature of sludge can change so rapidly, even within a single system.

Added pressure
The subsequent need for differing amounts of pressure and power to shift the sludge cannot be easily overcome. Conventional centrifugal pumps are also prone to clogging in sludge-handling operations.

Meanwhile, there is the need to reduce the volume of sludge produced (and thus the costs of handling) by thickening it, which also helps to answer environmental concerns. All this puts extra pressure on pumping capabilities.

The Holy Grail for companies is an environmentally friendly, efficient and cost-effective solution for pumping sludge. The introduction of the Progressive Cavity (PC) Pump, combined with the growing success of the N Pump from submersible pumps, mixers and aeration systems supplier ITT Flygt, moves that goal a step closer. It is not an easy business to be in, but it is getting easier. Although it is a centrifugal pump, the clog-reducing open design of the N Pump impeller has proved very effective in general sludge handling applications.

In customer sites throughout the UK and Europe, the combination of the N Pump and a variable frequency drive (VFD) is having a dramatic effect in reducing clogging-related pump downtime and subsequent maintenance costs. These success stories will help overcome customers' prejudices against centrifugal pumps for use in sludge handling.
The PC Pump is a new and specialist addition for more challenging sludge situations, designed to complement the work of the N pump. The PC Pump is a self-priming rotary positive displacement pump.

As the rotor turns inside the stator, it forms a series of cavities that progress uniformly from the suction point to the discharge, carrying the liquid plus any suspended solids with them.

The design is very resistant to clogging and produces an accurate, consistent flow. PC Pumps have the added benefit of being able to change easily from duty point to duty point, making them ideal for dealing with the varying properties of sludge. Flygt believes that between 60% and 70% of all sludge-pumping requirements can be successfully met by the N Pump, with the rest being answered by the PC. Mixers also play an important part in the sludge-handling process by keeping solids in suspension in holding tanks and digesters.

Two types of mixing solutions for sludge processes are offered by Flygt - high-efficiency submersible propeller mixers and venture jet mixers, based on the advantages of the N Pump, to provide ease of maintenance external to the sludge tank.

Complete solution
The company's aim is to provide a complete solution to pumping and mixing within the sludge treatment plant and move sludge more efficiently throughout the whole process.
It is always going to be a dirty business but Flygt hopes to help operators avoid getting their hands dirty so often.

Andrew Barry is ITT Flygt's product manager for pumps and process equipment.
E: Andrew.Barry@flygt.com
Enquiry 022

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