According to Landia, the increasing amount of improperly shredded food waste and inevitable contaminants in domestic green bin waste mean that only the very toughest pumping and mixing equipment – designed for purpose – can meet the industry’s demands.
“Mass-produced, bog-standard propeller mixers don’t last very long at all in AD environments”, said Landia’s Paul Davies. “We now see some operators just running their pumps into the ground because it’s actually cheaper to replace them completely than get at them to carry out maintenance. This can be the case for any type of mixing system that doesn’t allow you to carry out a service from outside the tank. Externally-mounted units have huge operational and safety benefits. Everything is at ground level, so there’s no working at heights. Nothing is in the gas-zoned area, so there’s no need for breathing apparatus. 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”.
He added: “Small submersible mixers will often be running flat out, lasting two years or much less – compared to a chopper pump that can last 25 years. You’d think energy savings would be at least part of most AD operators’ decision making, but that’s not always so. Three 30kW pumps doesn’t automatically mean 90kW hours because they might only need to run for a maximum of fifteen minutes per hour”.
Davies also pointed to the rising amount of acidic food waste being used as feedstock that corrodes cast iron equipment – and food waste that contains plastics from packaging that can also cause problems without the right processing or sufficiently strong pumps and mixers to handle it.
“Over-shredding of plastics will make particles too small to be screened out”, he said, “But under-shredding can create a floating mattress of plastic in the digester, which will see gas production reduce and then eventually have to stop so that someone has to go in manually and remove it all. AD downtime isn’t something you want. Lost revenue can amount to thousands of pounds per day – and a problem with fulfilling your contract to the energy provider”.