Severn Trent Reduces Digester Cleaning Costs by 35%
LSM Pump-based system from Atlantic Pumps reduces anaerobic digestor (AD) emptying costs and cleaning downtime by over a third.
Severn Trent treat 1.4 billion litres of raw sewage waste per day. Their Bioresources team is a key part of their sustainability drive; turning the final sludge into renewable energy and biofertiliser via anaerobic digestion (AD), whilst preventing GHGs (greenhouse gasses) from escaping into the atmosphere.
One of the most pressing challenges that the AD process brings is grit, rag and solids build-up in the digester tanks. Over time, this reduces the capacity and efficiency of the facilities, causing extra load on the recirculation equipment, increases cost and reduces throughput capacity.
This means digester tanks need routine emptying and de-gritting in order to maintain their design capabilities. Traditionally this is a long, labour-intensive task which involves a specialist vacuum tanker truck. This alternates between vacuuming out the sludge from the bottom of the AD silo, then blowing it back out into a wash-plant and dewatering rig.
Background
During AMP6, Severn Trent had invested in a mobile wash-plant and dewatering screen to separate solids, sending the recovered ‘activated’ water back into the AD system and the solids into biofertiliser production.
This turned out a wise move, reducing road miles by processing as much as possible onsite and recycling water and recovered nutrients effectively.
Severn Trent has been using industry-leading DISAB Vacloaders to empty digestor tanks for a number of years. These super-powerful tanker trucks have a rotary piston vacuum / blower that is directly driven by a 9 litre diesel Caterpillar engine. They represented a significant breakthrough when first launched to the market, and there’s still not really a close rival to them when it comes to suction power. They became synonymous with flexibility, have over 90% vacuum efficiency and can reach a whooping 200kWh of power.
The challenges of vacuum transfer
However, even these vac truck giants experience limitations when emptying AD digesters in sewage works. In the case of Severn Trent, the batch-processing approach of suction in, then blowing out didn’t match the higher volume potential of the dewatering unit.
The mismatch between the sludge delivery and dewatering screen capacity meant that the later unit was running without feed for most of the time and then receiving a batch to cope with. Furthermore, when a blockage occurred, it took a time-consuming intervention from the staff to clear it.
Effective solution for improving digester clearance
Ever on the look-out for improving wastewater treatment capacity, efficiency, resilience and sustainability, Severn Trent’s Andy Kerr spotted potential in LSM, a range of large industrial peristaltic-type pumps, supplied by Atlantic Pumps. These pumps could be his answer to providing a steady stream of sludge that could potentially match the wash-plant and dewatering screen’s capacity.
Partnering with Atlantic Pumps’ engineering department, a trial installation of a built-up LSM pump and pipework going directly from digester to the wash-plant was conducted.
The ultimate solution to speed up digester clearing was an LSM-150 pump fitted with lifting hooks, an inverter (VFD) drive, Bauer quick-connect couplings and a 3-phase plug. This gives Severn Trent the assets and ability in-house to empty their AD tanks, easily moving the pump and from tank to tank as needed.
Outcomes
This system drives efficiency as it requires only a single operator and takes less than half the time of the previous process, allowing the digester to be back in operation days earlier than before.
Whereas the previous sludge remover could only manage 50m3 an hour, the LSM based solution keeps apace with the dewatering screen at around 130m3 an hour. This constant feed rate suits the wash-plant and dewatering screen, and the variable speed controller helps synchronise supply and demand for best throughput.
“The LSM pump can handle rope, rag and large solids up to 100mm meaning it hardly ever gets blocked. But even if it does, the single operator can just flick the pump into reverse until the blockage has cleared” explains Atlantic Pumps Project Manager Ed Smith.
The LSM-150 pump is renown for it’s reliability, longevity, ease of service and low cost and time-saving when it comes to servicing. It’s energy efficient 18kW electric motor delivers up to 130m3/h with 4.5 - 10 bar (max) pressure and can come with a vacuum pump to support suction lifts of up to 8m (the basic pump has a 4m suction lift).
The steady stream of sludge throughout the process creates an energy-saving and time effective way to keep digestor tanks at their optimum capacity.
Following successful trials at two of Severn Trent’s waste water works, they are partnering with Atlantic Pumps to roll out this same system to other sites throughout 2024.
For further information regarding this digestor clearance and de-gritting application or to discuss LSM pumps for your site, please contact Atlantic Pumps Project Lead Deryck Harmer on 0800 118 2500.
