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Caption if needed sdkvjb sdkvjnsdlk dvsdvdvsdvsdvsdv dv skjbdv lskdjvbsdkljvbsdkv ONSITE ANAEROBIC DIGESTION 10 WET NEWS AUGUST 2018 | wwtonline.co.uk (l-r) Bob King, Gary Davies and Landia key account manager Paul Davies lime that caused a distinctive rotten fish odour was used to enhance the treatment of sludge at a cost of £80,000 per year, but that expenditure – and the smell generated by the lime – has gone. Now there is Doosan Enpure's established pasteuri- sation technology, which holds the sludge at 55°C for a mini- mum of four hours, prior to passing to the anaerobic digesters. "As well as the pasteurisers, the investment in our CHP sees minimal flaring of gas," Davies says. "It all goes to the CHP – and we can now use the excess heat to heat the sludge. The whole focus of our operation has changed, because we look at gas production every day and see how, if possible, we can tweak it to make it better. "We might see if we can thicken more, thicken less, change feed rates – look at what is coming in, look at what is being washed away. We'll never stop wanting to learn and improve." He adds: "Doosan Enpure are very good process people to work with, helping us achieve up to 500kW of electricity (and the same again in heat) – about £800 to £1,100 per day. The pas- teurisers make a big positive dif- ference in presenting the digest- ers with much better material. Further on in the process, we now have far less grit/silt and rag to deal with. Before we had to take a digester out of service, bring in costly tankers to remove two to three metres of grit/silt and rag – but now we're very much heading in the right direction." One area that did require much more than mere fine-tun- ing was the continuous furring up of the new plant's heat exchangers, which were having to be cleaned every week – going against the quest for min- imal maintenance. "Initially we blamed the exchangers, as perhaps most operators do!" King says. "We tried linking them in series, in parallel, but eventually we found the remedy by introduc- ing an air-line. Our heat exchangers now have no trouble reaching their required temper- atures and require very little in the way of cleaning. The down- side to this was additional wear to the pasteuriser mixing pumps, caused by aerated sludge. This issue was addressed by upgrading to hardened impellers to improve service life." Bellozanne and Doosan Enpure also chose Landia pumps for three separate tanks containing digested sludge, thickened sludge (fed to the digesters via pasteurisers) and unthickened sludge that comes off the final clarifiers, which is then thickened and added to the thickened tank to maximise overall residence time. As with the Landia chopper pumps on the GasMix systems of the three digesters, none has required any spare parts since being installed three years ago. Bellozanne is achieving high volatile solids (VS) destruction at an average of around 60%. "The pasteurisers must help, but good mixing is critical with the high temperature feed to the digesters to make sure that everything is distributed prop- erly," Dave Garnett, technical specialist (wastewater) at Doo- san Enpure, says. "Landia's Gas- Mix is working very well indeed because we're seeing plenty of energy produced by the CHP – which has already more than paid for itself – so making a big reduction in Opex." He adds: "It is interesting that many digesters in the industry run on 2 to 2.5 per cent solids, when with a simple ret- rofit to a superior mixing sys- tem, they could increase to 4 per cent and see an immediate pay off." PROJECT SPECS • Replace original Bellozanne sludge plant in St Helier, which had been built in 1959 • Meet 2035 design horizon where works will need to fully treat 830l/s or 71,700m3/d • Ensure efficiency, ease of maintenance and high performance THE VERDICT "Overall, we now have a far more complex plant – yet our investment in quality equipment is saving us money." Bellozanne STW principal engineer Gary Davies Davies agrees: "From a pro- cess point of view, the good mix- ing we have here with the Lan- dia equipment is very important for consistency. It has a very positive knock-on effect all the way through, including that big reduction in grit/silt, because we can see that the digesters are being mixed properly." Further evidence of the plant's enviable performance comes in the shape of the sludge cake. Without lime, there was some concern initially about how consistent it would be but, according to King, the plant now produces a far better product. "It is much more appealing to farmers, for whom we now provide technical assistance to help them with spreading com- pliance, testing their fields for them," King says. "We still have more we can achieve in this area, but we've already come a long way since we used to have to pay farmers to take the treated sludge away!" Davies adds: "Overall, we now have a far more complex plant – yet our investment in quality equipment is saving us money. Not everybody under- stands the drivers when being on an island. No point suppos- edly saving £1 million on Capex if it's then going to cost you an extra £3 million in maintenance. This plant's performance proves that we are winning the argument." MacLeod agrees: "I've never seen such a clean, tidy and well-maintained site, which is highly beneficial for the equip- ment, run by people who are very knowledgeable about the process and care about it."

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