In this article we explain a great way to solve food waste AD plant pipe blockage problems, by using pumps which are highly efficient in pumping difficult to pump fluids.
Pipe Blockages – An Increasing Problem for Food Waste Biogas Plants
This topic came up at the RWM 2014 Exhibition at the NEC Birmingham, UK, last week, while I was talking to Anaerobic Digestion service providers.
A recurring topic which came up was the significant number of UK biogas plants which have recently been afflicted by pipe blockages. These were in many cases proving to result in long plant outages, and substantial lost biogas production.
Pipework systems in which substrate flow has become blocked, may have to be dismantled to physically clean them. That can be difficult unless process design pipe layouts are designed for this to be done. Watch our video in which we took this to heart – just like Verder!
(Please try not to get too emotional while watching!!)
It seems that it has been at those biogas plants which digest solely food waste, or those where the operators have added food waste to their substrate, that this problem has been most acute. What was also surprising to hear was that expert AD consultants have also been slow to appreciate the greater demands on pumping systems which the addition of food waste as a substrate imposes, and they have in many cases failed to anticipate these problems.
2014 UK Government-led Push Toward Reducing Food Waste Raises Number of Food Waste Pump Blockage Incidents
The recent UK government-led push toward reducing food waste, and digesting what remains, rather than sending it to landfill, has resulted in a lot of additional biogas plants accepting this type of waste in the last year.
The result has been some very highly stressed UK biogas plant operators! Food waste varies enormously in viscosity, and in the UK tends to arrive at AD Plants with a lot more packaging still present, or so it seems, than has been the experience previously in elsewhere in Europe.
So, it is not only UK based experts that have been caught out! There are two main problems which have been causing blockages. These are foreign matter, and the high viscosity of much food waste.
De-packaging equipment isn’t totally successful in removing all foreign matter and this, particularly if fibrous in nature, may build-up on pumps reducing efficiency, leading to blockages. Centrifugal/ impeller type pumps are sensitive to the viscosity of the liquid being pumped and their efficiency tails-off when viscosity rises, so the solution has often been to replace these with positive replacement pumps. Positive replacement pumps can also pump solids within certain sizes.
Biogas plant operators have naturally been reluctant to pay for such pumps due to their greater cost, nevertheless the cost of plant outages can be huge. This is especially damaging when Anaerobic Digestion Plant operators are unable to process their waste feed stocks for a period and may need to tell their suppliers that they are unable to accept more deliveries. The value of the power produced from biogas plants to the electricity grid operators is also dependent upon high plant operational availability.
So, this is always an important consideration, beyond simple equipment capital cost considerations. There are a number of types of positive replacement pumps on the market, but suppliers inform me that the one being used most often nowadays for these plants in the AD industry is the peristaltic pump designed for pumping solids and slurries.
such system is that manufactured by our sponsor Verder Pumps. They have produced details of case studies for sites where their Verderflex peristaltic pumps have recently been installed by them, and are operating successfully.
Food Waste AD Pipe Blockage Prevention Case Study
Verderflex peristaltic pumps were installed at prestigious brewery, based in Norfolk. This brewer produces fine quality real ales, which result in organic residues which are treated in their anaerobic digestion (AD) facility, where they are producing biogas to be fed into the national gas grid. The anaerobic digestion facility is also fed with kerbside-collected food waste, food processing waste, and supermarket food waste in addition to their own brewery waste.
During the process the substrate enters a feed inlet at the start where a rotor chopper pump breaks down the raw material after which it is stored in a holding tank.
Verderflex peristaltic pumps transport the chopped slurry from holding tanks and through a heat exchanger to raise the substrate’s temperature to a level suitable for efficient digestion, prior to its entering the main AD plant digester tanks.
As this hot organic matter enters the main AD facility it is dosed accurately at a pre-set rate, by a Verderflex Dura Pump with a pH buffer solution to accelerate and maximize the breakdown of the material.
Maximising the Surface Area of Microorganisms Available as Food for Higher Biogas Yield
The large mass of organic material is ‘churned’/ recirculated by Verderflex pumps located in a plant room connected through a series of pipes into the digester tanks. This maximises the surface area of the incoming material available for use as food by the bacteria, and thereby accelerates the AD process.
In fact, this is described as being instrumental in raising the yield of heat and biogas from this plant. After completion of the AD process, the digestate and waste water is pumped from the tank by another Verderflex peristaltic pump for further use as fertilizer.