With an investment of 60 million euros aimed at producing 30% more green energy from its largest wastewater treatment works, Severn Trent Water’s upgrade of its 12 anaerobic digesters has put people and pumps to a stern test.

Severn Trent, one of the largest water and wastewater companies in England and Wales, had experience in advanced digestion of sludges, but conversion of its conventional digesters to a new thermal hydrolysis process involved a difficult transition phase requiring a specialist pump selection. 

The Minworth Sewage Treatment Works (280 mgd design, 120 mgd average) serves a population equivalent of 2.5 million in the Birmingham area of England. Each of its dozen 1.32 million-gallon digesters had built up an amount of settled grit and rags that made the legacy solids difficult to pump.

The solution was a set of chopper pumps with custom-sized impellers. The pumps were used to remove diluted solids, macerate the rags and deliver the material to the new digestion process.  

Renewable energy

Severn Trent Water was founded in 1974 as a regional, state-owned authority responsible for water supply management and wastewater treatment. It is located in the catchment areas of the Severn and Trent rivers, from which it takes its name. All told, the company provides services to more than 4.3 million homes and businesses.

Severn Trent generates about 34% of the energy it uses from renewables. In seeking to reach 50% by 2020, the company engaged MWH Treatment to design and build a three-stream thermal hydrolysis plant using Cambi technology.

Making the anaerobic digestion process more efficient through hydrolysis provides the added benefit of producing biosolids cake classified as enhanced product fertilizer with potential use in growing crops for human consumption. The Minworth site already had a combined heat and power plant able to generate over 8 MW continuously and a gas-to-grid process producing up to 26,500 cubic feet per hour of biomethane.

Cambi’s advanced thermal hydrolysis process reactors destroy pathogens before digestion. They are designed to treat 77,000 dry tons of biosolids per year. For many years at the Minworth plant, solids were moved from holding tanks to the digesters for about 15 days of retention time and then on to secondary digesters before dewatering.

Challenging material

Richard Thomson, project director at MWH Treatment, notes that upgrading to the new digestion process meant sourcing pumps capable of a demanding duty point.

“We knew of other treatment plants that had issues with sludge recirculation,” Thomson says. “The head and the pipework distance were a concern in the design, not to mention that the pumps would have to be capable of handling that very challenging legacy sludge without blockages or breakdowns.”

Installing the thermal hydrolysis process upstream of the existing digesters means screening the sludge, dewatering it to 22% solids and then diluting it to 16% solids before hydrolysis. Then, inside reactor vessels for 22 to 30 minutes, the material is held at a pressure of 87 psi at a temperature of 359 degrees F to destroy pathogens.

To further break up the cell structure, the material is depressurized into flash tanks. This produces a hot hydrolysed material that is diluted 3-to-1 with sludge from the digesters and cooled in heat exchangers to bring its temperature down to 108 degrees F. “This is where the crucial role of the pumps comes in,” Thomson says.

Collaborative selection

“We knew that, especially for the first few months, taking legacy sludge directly (at 5,300 gallons per, total head 72 psi) from the 12 digesters all the way back through to the hydrolysis plant would see the pumps face an arduous duty,” Thomson says.

“The decision on which manufacturer to work with was made by Cambi, which recommended Landia based on good performance on various sites. This led to a healthy collaboration as we designed the most efficient solution for the application. In fact, Landia was the only chopper pump that could achieve what we needed.”

During selection, MWH Treatment and Landia looked long and hard at impeller sizes for the 40 hp, 3,000 rpm chopper pumps, choosing one in the middle of the range to allow for flexibility. “This design feature showed how versatile its pumps are, giving us the opportunity to change in the future if necessary,” Thomson says.

The small footprint of the pumps was also important, as space at the Minworth plant is limited. Thomson and his team at MWH Treatment set about designing a series of pump tables that fit in with existing pipework configurations and with removable wheels so the units could be moved around in the digester galleries to enable easy servicing at waist height.

Continuous duty

“It has been crucial to be able to get the digester sludge back to the hydrolysis plant so it can be blended with the hydrolysed sludge, put through the heat exchangers and then pumped back to the digesters,” Thomson says. “Diluting the hydrolysed sludge improves the characteristics of it for the success of the whole process.

“Since going online in March, April and July last year — four pumps at a time, one on each digester — they’ve been very good operationally, especially having to work under such duress for those first three months. You could hear them chopping the troublesome filamentous material. From a process perspective, the pumps’ continuous maceration is very advantageous.”

To cope with the backlog of legacy sludge, flushing sequences were introduced to control the flows and mitigate the risk of blocking plug valves. The plan worked; now flushing is no longer required. On a two-year build program to convert the traditional digesters to new hydrolysed sludge tanks, a three- to six-month clean and shutdown required for the digesters wasn’t viable. That made it all the more critical for the pumps to work well.

Working hard

“On our SCADA, we’ve been able to see the trending data on the pumps’ performance, including current, flows and head,” Thomson says. “It demonstrates just how hard the duty was in those first three months. As the sludge has gradually improved, so has the efficiency of the pumps. Less rags mean far less energy required.

“Not surprisingly, the pumps haven’t come through completely unscathed, but a couple of bearings and just one cutter replacement means nothing when one considers the sheer scale of the task in such an important and statement-making upgrade in energy recovery for the water industry.”