Packaged for a safer option

Traditional submersible sewage pumping stations can be dirty and dangerous to work in. Ian Hallows takes a look at packaged systems, which, as they are mounted on the surface, successfully avoid a whole raft of health and safety issues.

Traditional submersible sewage pumping stations tend to be difficult and dirty to maintain. The work involves lifting contaminated sewage pumps out of the sump, which means lifting gear, access, and personnel are all subject to stringent safety measures.

Sewage pumping stations commonly use centrifugal pumps in the wet-pumping chamber, an approach that presents a number of challenges. For example, it requires that the chamber is always kept flooded to allow the pump to prime and lift liquid for discharge.

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. This means further implications - damage can take place if the pump draws in heavy objects, blocks and requires subsequent manual cleaning.

The interior of a submersible sewage pumping station also poses a number of health risks, not least because methane gas can accumulate in the sump. Sump entry and the associated problems caused by confined-space restrictions, mean that working in these environments can be hazardous, and all too easily conflict with health and safety legislation. This typically results, for instance, in requirements such as the use of certified chains.

Sump entry
The Health and Safety at Work Act 1974, the Control of Substances Hazardous to Health Regulations 2002 (COSHH), the Confined Spaces Regulation 1997, and the Lifting Operations and Lifting Equipment Regulation (LOLER) 1998, must all be adhered to when dealing with submersible pumps and sump entry.

For example, COSHH outlines that, while sewage treatment reduces the water content and removes debris, it does not kill or remove all harmful micro-organisms. The regulations recognise that exposure to sewage or its products may result in illness, and aims to reduce the risk.

The Confined Spaces Regulation applies when assessment identifies serious risk of injury. It states that entry to confined spaces must be avoided by conducting the work from outside. And, if entry is unavoidable, a safe system of work should be strictly followed.

If a plant wishes to avoid sump entry, lifting chains are required. These are also covered by legislation that impacts on procedures. LOLER aims to reduce risks to health and safety from lifting equipment provided for use at work.

It requires this equipment is strong and stable, and marked to indicate safe working loads. Chains must also be positioned and installed to minimise risks, and are subject to ongoing rigorous examinations.

To provide a solution that enables wastewater treatment operators to avoid these health and safety issues, packaged pumping systems have been developed by leading pump manufacturers.

These innovative packaged systems enable sewage collection to satisfy health and safety requirements and legislation. The systems also control cost and energy use by delivering effective sewage transfer, combined with ease of access, installation and maintenance.

Packaged pumping systems pump and macerate waste in a single operation. This has proved more effective and safer than operating and maintaining submersible pumps in a sump. Typically they use less than 50% of the energy consumed by conventional centrifugal sewage pumps.

Packaged pumping systems are mounted on the surface, avoiding the need for lifting chains or sump entry. As the equipment is installed in a dry, accessible and clean environment, the inspection and maintenance of the system is far safer and more hygienic.

Also, the systems do not require constant priming, and are not affected by damage from large solids.

The packages can be installed into existing pump stations or pre-assembled into an insulated glass reinforced plastic kiosk. This contains the required pump and grinder control equipment, and the necessary pipework for rapid installation.

With a capacity range of up to 500m3/h raw sewage, packages typically combine a positive displacement pump with a solids grinder, which enables the use of a small-bore rising main system. This means lower installation costs compared with a traditional 100mm rising main sewer - up to 75% savings can be achieved.
The positive displacement action of the pump in the packaged system lifts the raw sewage from the sump into the cutting chamber of the grinder.

The sewage, and any textile and fibrous matter, is macerated and then pumped away into the main gravity sewer. When the pumping cycle ends, unmacerated material falls back into the sump, clearing the suction pipe for the next duty run.

Final effluent
The reduced pipe size also increases the velocity of the sewage, providing an optimum flow rate. This eliminates 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. This minimises shock loading and downstream blockages.

Centrifugal pumps have traditionally been a familiar part of the internal workings of wastewater treatment transfers. But progressing cavity pumps in packaged systems have proved invaluable when it comes to handling the more viscous media such as filter cake and primary, secondary and dewatered sludges.

The packaged systems are made up of a hardened-steel rotor and resilient stator, which form discrete cavities within the pump. It is this design that enables them to transport difficult-to-handle media, even types that appear to not be flowing.

They have also been designed so that flow volume is effectively linear to running speed, which means they are easy to adjust to required changes in flow demands. Unlike centrifugal pumps, progressing cavity pumps are not greatly affected by pressure, making them versatile.

They can also pull suction and vacuum without the need for constant priming, which is why they can be used above vessels and sumps. They are ideally suited for high suction lifts through the grinder because pressure drop is minimal. The most efficient packaged pump systems also feature the latest generation of twin-shaft grinder, which is designed for the treatment of heavy solids and abrasive sludges.

Precision cutters
The grinder features precision cutters, mounted on contra-rotating shafts, which enable it to deliver highly effective size reduction of solid particles in incoming wastewater.

The differential speed of the cutter stacks pulls apart fibrous material, while the leading edge and sides of the cutter teeth crop and shear other materials such as plastic into small pieces. The circumferential land of the cutter crushes friable or brittle material.

High-performance packaged system designs feature grinders with layback cutter shafts set at an angle to the incoming flow. This more effective design allows grinders to capture irregular shaped objects and, if a reverse cycle occurs, the object is automatically dropped into the built-in trash trap, which is set clear of and below the cutter stacks, retaining the solids away from the cutters.

Packaged pumping systems offer plants a consistently safer, more hygienic and reliable solution to submersible sewage pumping stations. They easily meet health and safety requirements, and deliver far lower whole-life costs.

Leading manufacturers have the detailed knowledge and practical experience to devise individual solutions for systems that can provide optimum efficiency, and deliver significant cost and energy savings, a valuable service that is generally not offered by supply-only sources.

As such, they provide a very valuable alternative for wastewater treatment plants, helping them to avoid the health and safety issues that submersible sewage pumping stations present, while minimising energy and maintenance costs and, importantly, improving efficiency.

Ian Hallows is Mono's business manager for Waste-Tec products. www.mono-pumps.com

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