Key points
The changing nature of communities and our lifestyle choices cause havoc for wastewater pumping stations. Chris Harvey, an expert in wastewater pump selection at Xylem, outlines the problems and offers some solutions
The water and wastewater industries face continuing pressure to reduce their impact on the environment through both energy and carbon reductions as well as protecting it from flooding and spillages. However much of what they do, is governed by the specific nature of the wastewater flowing into them.
Our disposable society is a major culprit in this, with many consumers throwing items such as cleaning wipes, cotton buds and even clothing down the toilet. This unnecessary action is wreaking havoc on our wastewater systems and as a result, the focus of pumping station teams has turned to resolving the impact of non-biological solids, as opposed to system hydraulics.
In fact, one of the key problems, is that wastewater is often discharged into old gravity sewer systems – positioned upstream of a pump station. Never designed for high material loads, the resulting effect is blockages. With this in mind, there has been growing acceptance within the market that pump blockages were simply a natural consequence of pumping wastewater. This needn’t be the case.
Factors to consider
For the delivery of an effective pumping system, it is crucial that it is reassessed at various intervals during its working life. Over time, pumping station needs will change with new housing or commercial developments increasing the inflow and with it, potentially the characteristics of the water and solids may alter too.
Pump selection therefore has always depended on some basic key criteria, such as the required forward flow; how high and how far the liquid needs to be pumped, the internal diameter of the pipe (or rising main as it is often referred to) that the liquid travels through and, not forgetting any undulations in the rising main.
What should be taken into consideration, but can be difficult to ascertain, is the internal condition of the rising main. As a consequence of the impact of liquid on the walls of the pipe, it can result in friction loss within the system – the more liquid that is pushed through, the more disproportionate the amount of friction it generates. So a relatively small increment in flow, in long rising mains, can require a significant increase in required pump pressure. However, the more pressure the pump has to generate to pass a given flow, the more power it consumes.
Over-time the internal walls of pipework can become rough; caused by abrasions, corrosion, and/or fat build up, which in turn effectively reduces the pipe diameter. Any increase in roughness, or reduction in the effective pipe diameter, will increase the friction loss / resistance that is being generated at a given flow. With fixed performance pumps, this can result in a reduction of forward flow capability.
Delivering the right solutions to waste water pumping stations is unquestionably easier when the historical and current performance data is available, but to make specification even trickier, this is not always the case. In fact, having access to performance data, can in fact lead to systems being oversized, also resulting in increased power consumption.
In short, wastewater pump selection has multiple variants, many imponderables and elements of risk.
What can be done to reduce these risks and the higher operating costs?
In the UK water industry there are currently around 36,000 wastewater pumping stations. Take a look through the asset base of any water company, and you will see different pump manufacturers and models, each with specific flow and head characteristics. This makes stocking emergency spares a truly herculean prospect.
One of the biggest problems is having access to the right data. In an emergency situation, having a pump in the ground often takes priority over having the right pump to optimise longevity, efficiency and performance.
While the reasoning behind this may be sound, the approach does lead to a number of potential issues. Firstly, in the case where a pump is fitted based on availability rather than correct selection, it will not be delivering optimum pump performance and is therefore at risk of premature failure. At the point of replacement, there is a tendency to replace equipment like-for-like, beginning a constant cycle of having the wrong pump for the application.
Secondly, the reactively selected pump may not meet the performance requirements, resulting in the pump having to be taken out and the impeller changed, at an increased cost and risk, further down the line. That’s not even considering the impact on power consumption the greater risk of clogging – further increasing operational costs.
The good news is that the next generation of pumps have arrived, and they are capable of being much more flexible in operation, equipped with intelligent integrated control systems that can automatically deliver optimal wastewater pumping performance. Flygt’s new Concertor pump, has built-in capabilities to adapt to different duty points, which means that instead of replacing the pump or impeller, the pump will adjust its performance to cope which changing conditions over time.
This a world away from the traditional pumps in place today. Having the potential to automatically adjust performance in-line with operational conditions, is enormous. The development will mean that there is no need for large stocks of back-up inventory because one pump size can be used in multiple applications. What’s more, high energy consumption can be solved by automatic self-optimisation, which has the potential to result in up to 70% improvements in energy savings along with increased product reliability and longevity.
Greater operational efficiencies
When combined with self-cleaning clog-free impellers and blockage detection/pump cleaning protocols that reduce the need for vacuum cleaning, call-outs can be reduced by up to 80%. Over the entire serviceable life of a pump, these factors offer a significant reduction to whole life cost and risks. Planned maintenance can now be resourced effectively thanks to a reduction in stoppage time.
Modern life is impacting the operational efficiency and risks of wastewater pumps which, in turn, is making pump specification during new build and replacement projects, increasingly challenging. The answer however may lie in a new generation of intelligent wastewater pumps that can alter their performance in line with the changing demands of each application.
