Understanding sludge improves pump systems
A combination of well thought-out design, a knowledge of fluid dynamics and regular refurbishment are key to balancing energy-efficiency and capital outlay in wastewater treatment, writes Jeanette King, senior consultant engineer at the BHR Group
Safe and effective operation of pumping stations remains the primary requirement but calls for energy reduction are growing in importance. Designers face the challenge of delivering new stations with the specified performance but which minimise capital and operating costs. The need for refurbishment and continued improvement of existing facilities remains. And demand for additional or upgraded pumping stations will increase, driven by climate change fears.
The need to save energy is not a new issue, however. Climate change and global sustainability aside, the need to reduce operating costs (to which electricity use is a major contributor in pumping applications) and increase profits is also fundamental to a sustainable business. The fact that pumps are the largest single users of electricity in the EU also highlights the opportunity within the water industry for energy savings.
In an existing plant, energy use is heavily influenced by design. The design factors employed and future capacity estimates can all be integral parts of a successful design, but equally have the capability to contribute to energy inefficiency. As energy prices rise, balancing the efficiency with capital expenditure and future capacity needs is increasingly important.
There are several aspects of pumps that could be more energy efficient. The design itself is an obvious but relevant place to start, as losses can originate from both the motor and the pump design.
High-efficiency motors and hard-wearing components can be expensive. But, as operating costs are often several times that of the initial costs for pumping applications, investment can bring significant benefit, particularly for longer-term investments. In fact, motors offering a 2-3% increase in efficiency can have payback periods of about one year if the pump is on continuous duty. Considering the life of a pump is about 20-25 years, the potential savings could be significant. Factors such as increased reliability and replaceable components can also present a case for using higher-efficiency pumps.
Design aside, pump size and specification is inherently linked to the pressure loss in the system. In this respect, pipeline designs can present an even greater opportunity at the design stage of a project. Simply using wider-diameter piping can reduce the losses in a system caused by friction, resulting in smaller pumping duties. It is also true, that a well thought-out plant layout can save long-term operating costs, by minimising pipe friction and using low-loss fittings, all of which contribute to system head loss.
Care should also be taken in the selection of the pump operating point. Any potential small gains in motor and pipework efficiency may be easily outweighed by inappropriate pump selection. Although pumps may have to cope with a variety of demands, designing according to the majority of the pump duty could still be beneficial. If demands are to change at a later date, then retrofitting additional pumps (again at their optimum conditions) is likely to be a better option than installing one oversized pump from the start.
If extra capacity must be built into the initial design, an alternative to one large pump could be two smaller pumps, one that could be turned on only for peak demand, or potentially the use of a variable motor if the economics were appropriate. Comparing operating costs with capital costs, the higher initial cost may be justified.
Everything discussed so far implies energy saving is a direct result of careful and tailored designs that balance the capital costs with the long-term operating costs. But, if such designs are to be implemented, systems must be appropriately maintained to ensure design work is still relevant. Figure 1 shows that regular refurbishment of pumps can lead to sustained performance and hence better energy efficiency, compared with an unmaintained system.
Ideally, understanding your system and designing in line with that knowledge to give the lowest cost across the total lifespan of a project, is the way to keep energy costs low.
Knowledge is power
It could be argued that the biggest energy challenge to the water industry is not knowledge of energy efficient design, and appropriate caution with design factors, but is partially being able to predict the behaviour of the fluids well enough to design accurately.
For clean-water systems using standard pumps this should be straightforward as there is widespread knowledge of these systems available. But the water industry, especially when dealing with wastewater treatment, has to cope with an increasingly varied and unpredictable feed. The water companies continue to innovate and employ new methods for dealing with this material, resulting in new types of sludge (eg hydrolysed, rehydrated, thickened and unthickened surplus activated sludge). But each new process creates another type of fluid with potentially different behaviour. In this case, it is necessary to fully understand the flow characteristics of the new fluid, in order to design an energy-efficient system (or adapt an existing one).
Maintenance also becomes a key issue in the efficiency of wastewater treatment. The build up of solids and fat deposits etc can alter flow characteristics. The pump may initially perform optimally, but build-up can reduce the diameter of the pipe and potentially increase the frictional losses. Conversely, the build up of some deposits may increase wall slip and give better than expected flow, which again may be outside the optimum design point. Such factors make designing the optimum system difficult.
One way to provide this necessary information is to employ appropriate rheological measurement techniques to the fluid to be pumped. This should highlight key characteristics such as yield stress, shear thinning and shear thickening behaviour. Rheology experts have published work on how these measurements can be obtained accurately.
For systems such as sewage sludge, there is an increasing amount of work to understand these fluids and the systems that are designed around them. In the last 15 years, BHR Group has built up a body of research into sludges and their rheology to aid prediction of system behaviour. This knowledge can be used in the design or redesign of pumps and pipe systems to characterise how the fluid will flow and what kind of systems are needed to handle it. As part of the current phase of water and wastewater mixing research consortium project led by BHR Group, a tool is being developed to predict the anticipated losses in a pump and pipeline system for sewage sludges.
There is still work to be done in this area, as the properties of sludges become more wide and varied. But continued building of knowledge in these areas will help design to be more specific and in turn more energy efficient.
*BHR has called for papers for the group's Water and Wastewater Pumping Stations Conference, to be held in Cranfield, June 17-19, 2008 (see Diary page 12).