Biogas production improvements that raise the yield of methane from any given quantity of biomass fed into an anaerobic digestion plant are the holy grail of AD.
The one thing which, above all else, is holding back on the development of anaerobic digestion is the current level of profitability which, while viable for many businesses in the right set of circumstances, is not adequate for wholesale adoption of AD everywhere and globally.
Biogas production improvements are key to weaning the biogas industry off subsidies.
That’s why we were keen to republish the following article by Seepex.
All biogas plant operators should find this a useful article to read as the application of this pump technology has applications well-beyond Wastewater Treatment by the Water Utility Companies:
Increasing biogas yields from thickened sludge
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.
The biogas production challenge
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.
After consultation with Anglian Water to fully understand the process and the pump performance required, SEEPEX evaluated a series of elastomer immersion tests. These were used 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 better biogas production.
The pump has provided more even heating of sludge to a higher temperature and a well-mixed, less variable sludge for digestion.
These improvements have seen VFAs (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 [in the words of Seepex]:
- 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.
Seepex Article Ends
Use of HPH in Anaerobic Digestion Plants for Better Biogas Production
HPH (heating, pasteurisation, hydrolysis) and similar technologies (such as ultrasonic disintegration) which are being used increasingly in the Wastewater Treatment industry (sewage works application of AD), are not necessarily the way-ahead for all AD plants.
Sewage sludges are notoriously hard to break-down fully, during anaerobic digestion. Thus the degree of biogas production rate improvement in WwTW applications is much higher than for more easily digested biomass.
Biomass such as food waste and farm waste (with the possible exception of manure) break down much more quickly that WwTW sludges, so HPH may not be appropriate in such instances.
Nevertheless, the message is that progressive cavity pumps may be worthy of greater use in the AD industry than has been in the past.
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