Increasing biogas yields from thickened sludge
High-performance pumping was crucial to an Anglian Water project to improve the yield of biogas from sludge at a wastewater treatment works in Essex, writes Lesley Eaton, Business Development and Marketing Manager, SEEPEX UK
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Pumps are critical to the water industry, whether for sludge handling, for use in anaerobic digestion plants, for metering chemicals, or anything in between. However, in today’s squeezed economy, maximising productivity is essential if water companies are to remain competitive, and correct pump selection is key to this. Follow these five steps to make sure the pump you choose is not only right for the job, but also the best value for money.
- Pay attention to the TCO, not the purchase price: Don’t be swayed by the initial purchase price as this can be a false economy. The total cost of ownership (TCO) is a much better way to assess value for money. For example, the lifecycle cost of a progressive cavity (PC) pump is typically ten per cent capital purchase cost, ten per cent maintenance/spare parts, and eighty per cent energy costs
- Check for energy efficiency: With such a high percentage of a pump’s TCO coming from its running costs, selecting an energy-efficient pump is key. Unlike piston, diaphragm and peristaltic pumps, PC pumps produce low pulsation flow and require smaller drives and motors for the same duty. And unlike air-operated pumps, they run on a direct energy source, without the conversion of electricity to compressed air. This helps to reduce the energy required to run the pump. Some of the more intelligent pumping solutions can be up to 20 per cent more energy efficient than traditional PC pump designs
- Consider ease of maintenance: Finding a solution that is easy to monitor and maintain is also important. SEEPEX’s SCT range of PC pumps significantly improves whole life costs due to the ease of replacing parts and clearing blockages. The fact that they can be remotely monitored also makes it easier to anticipate problems, enabling predictive maintenance
- Beware hidden costs: Check whether the pump you choose will require the purchasing of ancillary equipment, such as pulsation dampeners and check valves, which are also liable to blockages. PC pumps do not require these, saving operators money and reducing downtime
- Ask for advice: Operators can reduce their initial purchase price by talking to their supplier about the different options available. A responsible pump supplier will be able to work with an operator to specify the most cost-effective solution for their needs
As the largest water and wastewater treatment company in the UK by geographical area, Anglian Water supplies water and recycling services to over six million domestic customers. Since the provision of these services is energy intensive and Anglian Water is committed to a sustainability philosophy, the company has used innovative technology known as HPH to increase the generation of energy from its sewage sludge.
HPH (heating, pasteurisation, hydrolysis) is a unique advanced digestion process that pre-conditions indigenous and imported thickened sludge before anaerobic digestion to produce biogas and biosolids. This treatment leads to better quality biosolids for agricultural use and a five per cent increase in the volume of biogas generated. The system has been developed, trialled and up-scaled to full operational capacity by Anglian Water.
HPH requires thickened sludge at a dry solids (ds) content of 4-8 per cent to be pumped through heat exchangers to raise the temperature to 42oC before pasteurisation. However, at Anglian Water’s Basildon WwTW, the original lobe pumps installed had a lower than expected flow rate and were unable to feed the heat exchanger efficiently when the ds content was over six per cent.
The reason for this is that lobe pumps have a clearance between the rotating lobes and the pump casing, resulting in ‘slip’ when pumping and a reduction in volumetric efficiency, particularly when pumping high viscosity products. For Anglian Water, this resulted in blockages in the heat exchanger, poor sludge mixing, variable sludge quality, and a reduction in the expected gas yield from the digesters. In comparison, progressive cavity pumps have an interference fit between the rotor and stator meaning that slip does not occur. This leads to improved volumetric efficiency which is not affected by the viscosity of the product.
The lobe pumps also had a high maintenance overhead, with full maintenance required every three months, as well as complete overhauls of the heat exchanger when blockages occurred. The cost of the pump overhauls alone was more than £40,000 per year.
Anglian Water engineers commissioned SEEPEX to supply reliable pump technology which would increase the thickened sludge throughput of the heat exchanger. After consultation with Anglian Water to fully understand the process and the pump performance required, SEEPEX evaluated a series of elastomer immersion tests to determine the optimum stator material for the thickened sludge product and for HPH process conditions. Progressive cavity (PC) pumps were supplied which could deliver up to 198m3/hr through the heat exchanger to the digesters, even when faced with a variable sludge ds content.
The improved pump performance has resulted in more even heating of sludge to a higher temperature and a well-mixed, less variable sludge for digestion. These improvements have seen VFA (volatile fatty acids) in sludge being fed into the digesters increase by up to 100 per cent compared to the VFA in sludge pumped by lobe pumps. As VFA is an indicator of final gas yields, the SEEPEX pumps have improved energy production as well as delivering reliable performance.
The pumps have been installed for over 16 months to date (July 2018), requiring no service interventions during this time. In addition, the heat exchangers have experienced no blockages, reducing downtime as well as saving Anglian Water over £40,000 in maintenance costs.
To date, the SEEPEX PC pumps are delivering 400 per cent longer service life than lobe pumps; increased heat exchanger uptime; increased gas yield per litre of sludge; and increased throughput, resulting in more efficient energy production. Combined with reduced maintenance costs and a payback of just one year, they are already providing tangible benefits to Anglian Water.
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