WET News

NEED TO KNOW • Minworth is Severn Trent's largest sewage treatment works and serves an equivalent population of 1.75 million from Birmingham • The plant treats sludge from an equivalent population of 2.5 million • Severn Trent aims to generate 50 per cent of the energy it uses from renewables by 2020 wwtonline.co.uk | MAY 2019 WET NEWS 13 Installing the thermal hydroly- sis process upstream of the exist- ing digesters would see sludge screened, dewatered first to 22 per cent then diluted to 16 per cent before being hydrolysed. Then, inside reactor vessels for between 22 and 30 minutes, the sludge is held at a pressure of 6 bar and a temperature of 175 degrees to destroy pathogens. To further break up the cell structure, the sludge is depressur- ised into flash tanks. This vigorous disintegration process produces a hot hydrolysed sludge that is diluted 3-to-1 with sludge from the digesters and cooled in heat exchangers to bring its tempera- ture down to 42 degrees. "It is at this point where the crucial role of the pumps come in," Thomson adds. "We knew that, especially for the first few months, taking legacy sludge directly [at a flowrate of 20m3/hr, total head 5 bar] from the 12 digest- ers, all the way back through to the hydrolysis plant, would see the pumps face an arduous duty. "As it was, the decision on which manufacturer to work with the digester galleries to allow for easy servicing at waist height. "It has been crucial to be able to get the digester sludge back to the hydrolysis plant so that it can be blended with the hydro- lysed sludge and put through the h e a t e x c h a n g e r s , a n d t h e n pumped back to the digesters," he adds. "Diluting the hydro- lysed sludge improves the char- acteristics 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 trouble- some filamentous material. From a process perspective, the chopper pump's continuous maceration is very advantageous." Thomson explained that, to cope with the backlog of exist- ing sludge, flushing sequences were introduced to control the flows and mitigate the risk of blocking of plug valves from being too frequent. The plan worked – and now the flushing is no longer required, which is a remarkable achieve- ment in the course of three reten- tions to convert traditional digest- ers to new hydrolysed sludge tanks. On a two-year build pro- gramme, the three-to-six months' clean and shutdown required for 5000m3 digesters just wasn't viable, which made it all the more critical that the Landia pumps worked well. "On our SCADA we've been able to see the trending data on the pumps' performance, includ- ing current, flows, head – demon- strating just how hard the duty was in those first three months," Thom- son says. "As the sludge has gradu- ally improved, so has the efficiency of the pumps, with fewer rags meaning far less energy required. "Not surprisingly, the pumps haven't come through completely unscathed, but a couple of bear- ings and just one cutter replace- ment is nothing when one consid- ers the sheer scale of the task in such an important and statement- making upgrade in energy recov- ery for the water industry." was made easy in the end by Cambi who recommended Landia due to good performance on vari- ous sites. This led to a healthy collaboration as we designed the most efficient solution for this very arduous application. In fact, Lan- dia was the only chopper pump t h a t co u l d a ch i e ve wh a t we needed." During the selection stage, MWH Treatment and Landia looked long and hard at impeller sizes for the 30kW, 3,000rpm chopper pumps, choosing a mid- dle of the range size to allow for flexibility. "This design feature from Lan- dia showed how versatile its pumps are – giving us the oppor- tunity to change in the future if necessary," Thomson says. The small footprint of the Lan- dia pumps was also an important factor as space at Minworth STW is limited. Thomson and his team at MWH set about designing a series of pump-tables that fitted in with existing pipework configu- rations – and with removable wheels so that the units can be moved around in the confines of The digester galleries at Minworth, where 12 Landia chopper pumps have been tackling legacy sludge MWH Treatment's Richard Thomson with one of the chopper pumps The chopper pumps' continuous maceration is proving advantageous

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