Intelligent sewage handling

Pump and control system failures are creating major problems for water companies. Peter Costello offers a potential solution

The small- to medium-sized pumping station, usually located below innocuous town centre car parks and used as a holding tank for sewage before it is transferred to the treatment plant, offer financial challenges that need to be balanced with the need to protect the public.

There are thousands of these type of sewage stations around the UK, which service small estates, public-sector and private premises, pumping sewage into the existing system. The water industry it self often calls them "package" units because they form a complete system, including sump, pumps, valves and controls.

It is in these stations that threats to the surrounding urban environment are increasing, primarily because of the changing nature of society.

Modern society thinks nothing of flushing and pouring all sorts of waste down into the sewage system - everything from inorganic solid through to sanitary ware and synthetic fibre bags. And, the growth of fast-food restaurants, hotels and restaurants in town centres has given rise to increasing amounts of inorganic solids, such as fat, grease and detergents within sewage systems.

Due to the small size of these ''wet wells'' and the requirement to empty the vessel to prevent settlement which usually leads to odours, it is becoming increasingly difficult to locate the ultrasonic transducer in its optimal position.

The Siemens range of ultrasonic level controllers contain software features developed from more than 30 years of experience within the water industry. These increase the likelihood that the correct level is going to be reported. With the addition of digital overrides, it is also possible to account for scenarios such as foam in the well, which has an adverse effect on the ultrasonic signal.

A problem is detecting when the pumps are poorly performing due to inefficiency, rather than a genuine fault that will prevent the pump from functioning. Pumps therefore fail to push sewage out into the main sewage system with nowhere to go but out into the environment. Because of the location of many holding stations, that environment can often be town centres, retail premises and domestic properties.

Simultaneously, the Environment Agency is placing increasingly stringent demands on water companies to increase the water quality flowing from their pumping stations as well as limiting overflow and spillage issues. The European Community Directive on Urban Waste Water Treatment, in particular, has an objective to protect the environment from the adverse effects of sewage discharges. The directive sets treatment levels on the basis of sizes of sewage discharges and the sensitivity of waters receiving the discharges.

If Environment Agency regulations and consents are not met, water and sewerage companies can face substantial fines. As a result, entire sewage station networks are now regulated to ensure assets are operated responsibly and within the framework of the directive.

To enable assets to meet these standards, particularly where pumping stations are visited and checked less frequently, intelligent motor controllers and level measurement devices offer a reduction in costs by reducing maintenance budgets and identifying potential problems before failure occurs.

Operating costs benefit from intelligent devices, particularly by the clarity and speed of information back to the programmable logic controller (PLC). A PLC is effectively a computer with input/output modules (I/O). I/O are method of getting signals from the outside world into the computer and back again. Accurate data on motor protection can be relayed and motor load can also be detected to quickly identify dry running conditions and overall motor efficiency.

Due to local recording capabilities, the information captured can be accessed at any time, increasing the detection time of faults or leaks. Communication is increased as the device can now be operated independently of the central control system, which cuts unnecessary call outs.

The type of information communicated via telemetry link is also improved through intelligent devices. The current lack of ability for equipment to differentiate between spurious operation and a more serious fault becomes less of a problem with more sophisticated receptors and I/O modules.

Intelligent systems can offer a whole package solution which rivals those placed in larger more sophisticated pumping stations.

The potential for environmental damage from pumping stations remains a growing concern. Intelligent motor control systems could offer a simple solution for the UK's water industry, making pumping station failures a thing of the past.

Peter Costello is from Siemens Automation & Drives

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