Pump blockages are a growing problem for wastewater systems, where they cause unreliable operations, increased energy usage and premature failure.
On a global basis, it is an issue that is estimated to cost the industry and utilities billions of dollars a year.
Businesses that handle wastewater are facing an ever-growing problem with the clogging (often known as ragging) of their electric pumps. The market does offer a potential solution with water-dedicated variable speed drives (VSDs), which integrate pump cleaning functions. James Chalmers, vice president of sales at ABB Drives, unpacks the challenge and the solution.
The problem results from increasing levels of fat, oil and grease
One of the main causes of the problem is changing diets with a trend to increased consumption of fatty foods. That means higher levels of fat, oil and grease (FOG) have to be carried by household and commercial drains. Once they reach the sewerage system they solidify to form floating crusts as well as coating pipe and pump surfaces.
FOG also envelops solid objects such as wet wipes, sanitary products, cotton buds and diapers, carelessly flushed down toilets. The resulting masses continue to gather FOG and other materials, creating “fatbergs” that can reach enormous proportions – one in Belfast, Northern Ireland weighed in at over 200 tonnes. The current trend for water-conserving toilet cisterns that provide lower volume flushes is making the situation worse as they make it easier for FOG to settle, accumulate and aggregate.
If a blockage prevents a pump from operating it will need to be mechanically lifted, opened up and manually cleaned. When the pump is in a remote location the downtime and labour costs involved are particularly high. Even at lower levels, ragging adversely affects the pump’s performance. This is because it produces a lower flow, while an extra strain is placed on the pump, resulting in higher energy consumption and faster wear. In turn, the need for inspection and maintenance activities is increased.
In the very worst cases, pump failure or loss of effectiveness can cause flooding. This damages property and can pollute natural waters, requiring clean-up operations and possibly resulting in fines. Furthermore, whenever floods occur or pumps need to be removed and handled, maintenance teams and communities can be exposed to biohazards.
VSD cleaning sequences set pumps free
Precise control of pump operations is offered by the latest generation of VSDs developed specifically for water industry applications. They offer built-in cleaning sequences that control the pump’s impeller to perform a series of rapid forward and reverse movements to dislodge attached or entangled solids. The number, duration, acceleration rate and pattern of these movements can be adjusted to suit different solid materials or mixes.
The VSD can be programmed to initiate the cleaning action automatically when a possible blockage is indicated by a reduction in flow rate. Operators can also decide how many times the cleaning routine should be activated before a warning of potential trouble is generated.
In some cases, water industry operators have achieved a successful resolution to blockage issues by installing macerator or ‘chopper’ pumps. In some cases, this approach might cause problems downstream, as the materials are broken into smaller pieces which may pass through screens at the treatment works. The resulting fragments are difficult to remove and may disrupt processes. However, automated cleaning by a VSD will keep large objects moving along to the screens, where they can be easily extracted. In some cases, they might even be used as fuel for energy conversion plants.
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VSD makes a practical difference
ABB’s dedicated ACQ580A is a tried and tested example of a VSD with cleaning functionality for the water industry. Using this drive has doubled or even tripled flow rates in some installations, while optimising the pumps’ operation for maximum energy efficiency.
Mittelmärkische Wasser-und Abwasser GmbH (MWA) in Brandenburg, Germany has used the VSD pump cleaning function since 2014. When the current rises due to clogging, the VSD switches the pump into reverse operation or a higher speed. The company hasn’t experienced any clogging since the VSDs were installed.
On a smaller scale, the municipally-owned Pietarsaaren Vesi utility on the west coast of Finland serves a population of 20,000. Around 80% of the area’s wastewater flows through one unmanned pumping station, on its way to a central treatment plant.
Controlling flow to avoid flooding is a priority, as spills into the sea could cause severe ecological damage. Before installing VSDs with integrated cleaning capability, clogging was a major issue that frequently required pumps to be pulled up and manually cleaned. Now, the cleaning function is activated on a weekly basis as a routine precautionary measure, or whenever a spike in current drawn by a pump indicates a blockage.
In Queensland, Australia, Unitywater provides water and sewerage services for more than 820,000 people and operates 17 sewage treatment plants. Its Deception Bay pumping station alone handles 1.2 million litres of wastewater per day, using two pumps. These suffered clogging and needed to be removed regularly using a specialised lifting gear. Simultaneous failure of both pumps would have incurred the additional expense of transporting extra pumping equipment to keep the site operating.
Installing VSDs with cleaning functionality has meant no more lifts and has saved around $20,000 per annum in maintenance costs. Water flow has doubled, without a corresponding increase in pump speed, so component wear and energy consumption are reduced. Electricity bills have been halved. It took less than six months to achieve a full return on investment.
Water dedicated VSDs offer advantages for all stakeholders
Using VSDs with cleaning functionality offers benefits for every stakeholder in the water business. Whatever the priority – saving money, saving energy, protecting the environment of efficient works management, the technology makes good sense.
Not only do these drives allow pumps to operate at the optimum speed for energy efficiency, but they also feed treatment works at consistent rates. This is highly beneficial when it comes to managing biological and chemical processes. The sustainability benefits from reducing blockages relate to overall reduction of carbon footprint as well as power consumption. When considering sustainability, the CO2 emissions of vehicles travelling long distances to deal with blocked pumps can be an important factor. This is especially true when a single incident might require the presence of a crane operator and more than one specialised engineer, each arriving in separate vehicles.
A further key benefit of water dedicated VSDs is their flexibility to adapt to changing needs. As an example, catchment sizes and conditions may alter over time, making initial flow estimates invalid. Oversized pumps are often specified to allow for such variation, but this adds to running costs. With VSDs, the pumps can be adjusted as and when necessary for the best possible match to the latest operating regime.
VSDs can also offer a useful function in the form of flow measurement in installations where a dedicated flow meter is not available. For example, knowledge of sewage flows can be very useful to allow early warning of increased loads, clarification of diurnal patterns, and an indication of leakage. ABB’s own VSD solution has a unique inbuilt flow calculator and is able to make measurements without inserting sensors (which could obstruct and entangle solid waste).
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The critical success factor is specialist installation and commissioning
VSD solutions such as those described here are generally quick and easy to retrofit, often without large civil engineering costs. However, buyers should still factor specialist installation, commissioning and support into their investment. This expertise might be offered directly by the supplier or through its delivery partners. Utilising expert knowledge in this field will ensure that the equipment is correctly specified and set up for the specific application and circumstances.
If this critical expertise is not employed there is a risk that VSDs may not achieve optimum results and could even increase energy use. Most importantly, the experts will ensure that the nature and needs of existing systems, infrastructure and components are understood, and that VSD operation and cleaning is programmed to ensure smooth and reliable pump operation. To show why this is important, the Australian plant referenced above had a nearby non-return valve that was sensitive and vulnerable to disruption and damage. In this case, careful management and fine-tuning averted the problem.
The water companies are carrying out important work to educate consumers on the impact of flushing inappropriate material. However, the challenge of blockages continues to grow. Therefore, variable speed drive technology correctly applied with the right expertise in water systems can play a vital role in keeping pumps and sewage moving.