The demand from industry for anaerobic digestion (AD) solutions has never been greater in the United Kingdom.
Partly that’s down to the fact that it’s a relatively young market, so while its expansion looks impressive, it has begun from a very low base.
Yet grow it has and to an impressive degree, year on year. As leading AD pumps manufacturer and distributor Verder puts it: “In the UK, we have just passed the halfway mark for the number of tonnes recycled against those that go to landfill. Other reports show that we throw away as much as we buy in terms of tonnage. The race is on then to provide enough non-animal stomachs to digest all this food mechanically.”
Equipment demands have changed in much the same way as every other field; more powerful, more efficient and, above all, more cost-effective
David Rozee, managing director, Triark Pumps
Some of the enthusiasm for AD is generated by economic considerations and some by political constraints: it offers a method of recycling waste material both as fuel and digestate, solving the problem of waste storage and disposal as well as offering useful energy sources.
That has proved to be good news for a sector seeking to consolidate its hold in traditional markets and exploit new income streams. Pumps, in a variety of forms, are intrinsic to AD and biogas production throughout the process of converting waste to useful product.
The challenge is to ensure that maximum efficiencies are in place, in order to ensure the best ratio of savings to expenditure.
That’s sometimes easier said than done when a plant has to contend with challenging material such as food waste – where plastic packaging and other foreign bodies will inevitably be included in the material presented – or when mixtures are harsh or viscous and contain coarse content such as sand.
The challenge is to ensure that maximum efficiencies are in place, in order to ensure the best ratio of savings to expenditure
It has been a learning curve; not every company adapted to the demands that AD and bio placed upon pump technology in as agile a manner as they needed to.
Three years ago, Paul Davies from pumping and mixing solutions supplier Landia cited poor controls over domestic green bin waste as a major source of contaminants that needed addressing.
A reminder that the good intentions in public legislation often take time to filter down into good practices on the home and industry fronts.
Changing public habits is not something firms can directly influence but the net result, said Davies, was something they needed to address.
In short, some pumps being employed – especially with regards to submersible mixers – were not up to the job. Partly because they were of insufficient quality or power, but also because too little was being done to promote proper maintenance.
Replace vs maintain
As Davies explained: “To maintain equipment such as submersible mixers, there’s no option other than to climb up, peel back the lid of the digester and purge the tank, which is extremely costly and time-consuming.”
Given that a small submersible pump can have a life of just two years in such circumstances – way less that the quarter century certain chopper pumps can achieve – there’s a question to be asked about maintenance versus replacement.
Do you spend good money on getting the most robust product, do you go cheaper but devote more effort on maintenance, or do you buy cheap and resign yourself to more regular replacement?
For David Rozee, managing director at Triark Pumps [pictured above], there is no one black and white answer to the replacement versus maintenance debate. It depends on decision-makers taking a call based upon individual circumstances: “There is a point at which the argument moves from for to against. Smaller, cheaper units tend to be replaced rather than repaired as the man hours are not cost effective. Larger pumps are usually more cost effective to repair whenever possible,” says Rozee.
Ultimately the concern should be to limit the greatest threat to efficiency: poorly-controlled downtime.
“The challenges start with the fact that the wrong pump will lead to unnecessary expense, not only in replacement pumps and parts, but also in downtime,” warns Rozee.
“The main issue tends to be particle size and abrasion, both of which must be addressed as a minimum requirement in the first instance. Then you can add efficiency – both energy and production – into the mix, as well as life span.”
It’s a perspective shared by Borger UK MD David Brown who offers a caution to those inclined to determine their choice of pump product according to the quantities of feedstock they are handling.
Rather, what they need to do is consider the size and type of material involved: “Understandably, those with low volumes of waste will want to keep their costs down. They may believe that they do not need industrial-size equipment for their application, but the likelihood is that they will.
“Even if laboratory tests with something bulky might point, for example, to a small 2-inch pump, a 2-inch macerator and a small auger feeder – the scaling down of equipment to suit lower volumes of waste should only go so far.”
Those with low volumes of waste will want to keep their costs down. They may believe that they do not need industrial-size equipment for their application, but the likelihood is that they will
David Brown, managing director, Borger UK
Away from the lab, cautions Brown, the need for sufficient power to feed waste, macerate and then pump it, is an absolute must. You undersize at your peril.
“The front-end of the AD plant is crucial. Typically, size requirements are dictated to us, such as 12, 8 or 6mm.
“However, our advice would be to opt for a pump that can handle 20-30mm, because even if the waste has been through a plate or multi-spindled macerator, it is only macerated in one direction. So, in effect, a feedstock could become a series of long chips that will cause problems if you haven’t sized up your pump correctly.”
Up to the job
Verder’s core brand of Verderflex peristaltic pumps played a role at each pumping stage when a Norfolk brewer installed an AD facility to produce biogas for the National Grid.
Kerbside-collected food, food processing, supermarket food and brewery waste is fed in via a rotor chopper before the slurry is pumped through a heat exchanger.
After entering the main facility it is dosed by the Verderflex Dura pump with an inorganic pH buffer to speed breakdown, then churned and recycled before being pumped for fertiliser use, with the final biogas product processed for the National Grid. In all, coping with six varieties of material, slurry, liquor and solutions.
Borger, by contrast, offers an example of a biogas plant where under-performing competitor rotary lobe pumps were replaced by the insertion of a more efficient alternative.
Existing pumps showed signs of severe wear on the rotors after just a few weeks with frequent damage to the seals and gears.
The operator of the biogas plant conducted a year-long test before opting for 40 of Borger’s Boerger rotary lobe pumps.
Also used were a number of Boerger Premium profile rotors, developed for the biogas sector and in which fibrous matter is captured in the profile grooves of the equipment.
Triark’s option for an abattoir client seeking an eco-friendly approach to waste was to use 230v single phase ABS Piranha lightweight cutter pumps [pictured left] and DAB FEKA sewerage pumps to ensure circulation, aeration and filtration via cesspools to produce a pH neutral liquid.
Pick of the pumps
Dairy and meat products have been notably successful materials for biogas production, but large solids can comprise more than 10% of the entire waste product, with fibrous material that increases clogging and sharp bones that can damage machinery – causing expensive downtime.
All of which highlights the need to ensure one can pick the appropriate pump for every stage of the process journey. Yet, Triark Pumps’ Rozee says, while expert advice is key, there are principles we should all be able to bear in mind: “Basic knowledge of the system and the resulting effluent at each point of the process usually is enough.
“AD and bio pumps tend to be ‘over-specified’ so as to be able to work in a worst case scenario, but it is usual for the first pump in the process to be quite crude (macerator) with the final pumps requiring a little more finesse (dosing or pumping clear fluids).”
AD and bio is a younger sector than most – at least in terms of operating at scale. Its developing demands, though, are not radically different.
“Equipment demands have changed in much the same way as every other field; more powerful, more efficient and, above all, more cost-effective solutions continue to be the opening gambit,” assesses Rozee. As for the expected advances to be made in coming years…
“Without over simplifying things, more of the above; greater efficiency, longer life expectancy, improved remote monitoring and better serviceability.”