WET News

12 WET NEWS SEPTEMBER 2014 ONSITE ADVANCED DIGESTION was going to need even more power. That needed to be offset. Then adding to the mix was UU's carbon reduction commit- ments. By digesting the sludge, UU could significantly reduce the amount of energy it takes. UU required an innovative solution that would deliver its strategy and mitigate its risk. The solution, says Lancaster, also had to enhance quality and significantly increase the renew- able energy generated. The existing asset base had to be maximised and the amount of investment minimised. The answer came in the shape of SBAP – Sludge Bal- anced Asset Programme – involving the the construction of an advanced digestion facility at Davyhulme. The aim of the plant was to reduce quantities for disposal and produce an enhanced qual- ity, pathogen free product Before settling on thermal hydrolysis, UU and Black & Veatch did consider raw incin- eration, which was looked at back in 2010. UU already had an incineration facility 29 miles away from Davyhulme at Shell Green, Widnes. The incineration option would have cost more than £170M. Lancaster says: "What we chose to do is maximise the use of our existing incineration facility at Shell Green by build- ing advanced digestion here [at Davyhulme]. "That sounds crazy but by building advance digestion here we could double the throughput of this facility." A transfer main enables the Davyhulme facility to feed the incineration plant. As a result Davyhulme can take sludge in, digest it and take it to agricul- ture as an enhanced treated product or send it down the pipeline where it is incinerated. Lancaster continues: "It's given us some flexibility. It's the absolute balance in our strategy, and it's also stopped us from having to build a huge raw incineration facility to hit that strategy in the centre of our region. "It's allowed us to generate loads more energy and it's allowed us to use the existing assets here." Issues The contract for the project was awarded in 2009 to Black & Veatch, with construction start- ing in 2010. The project was completed in 2013. During work on the scheme there were "half a million hours" where there were no reports of accidents. Lancaster says: "That's a huge amount of time without any issues. I think it was ultimately broken by someone twisting their ankle." The completed project has resulted in the largest thermal hydrolysis plant in the world so far. A larger facility is currently being built in Blue Sands, Wash- ington in the US but until that is commissioned, Davyhulme lays claim to being the largest. Explaining the process, John Tattersall, Black & Veatch's global director of water technol- ogy, says: "Thermal hydrolysis sounds really complicated, but it's not. We're basically break- ing the sludge apart with the heat to make it soluble. Basi- cally, we're pressure cooking it." Digesters Davyhulme already had eight enormous digesters. By putting the thermal hydrolysis plant there as well, UU has been able to use the eight digesters with double the throughput. Biosolids at Davyhulme are dealt with through two routes. Liquid biosolids are dewatered to about 25% dry solids (DS) using four decanter centrifuges and fed to a storage silo feeding the thermal hydrolysis plant. For imported cake that is brought to the site from other sites, two sludge storage silos feed the sludge to the thermal hydrolysis plant. Tattersall says four thermal hydrolysis streams feed the eight digesters. Each thermal hydrolysis plant stream can be fed with dewatered biosolids from Davyhulme, imported bio- solids or a combination of the two. From the digesters, the bio- solids flow by gravity to into a degassing tank and from there the sludge transfers to digested biosolids storage tanks. Once the process is complete the digested biosolids can either be dewatered at Davyhulme and exported for disposal as an agri- cultural fertiliser or pumped through the transfer main to the incineration facility at Shell Green. The biogas from the digesters is kept in two 9,000m3 double membrane gas holders. The bulk of the biogas is put into the CHP engines to generate the power. Tattersall says: "The configu- ration is pre-dewatering to get it up to 16% DS, cook it, put it in the digesters then squeeze the water out at the end. The stuff Left: Jenbacher CHP engines are fed by the biogas from the gas holders. Below: Richard Lancaster (left) and John Tattersall The thermal hydrolysis process means UU is able to get a 6log reduction in sludge pathogens being taken to land gets dewa- tered here the rest gets put as a liquid in the pipeline to Shell Green." He says the process makes it easier to squeeze the water out of the material. "Instead of get- ting 20% from conventional digestion you get about 25-28%. We're doing about 28% here. That's a good material. That's about a third less water and who wants to pay to take water onto fields." Tattersall says: "We're get- ting more than a 6log reduction in pathogens in the sludge, that's its enhanced treated qual- ity. For every million that come in there's less than one going out, that's what 6log means." He is full of praise for the Cambi technology: "This is a great piece of engineering. So much of the work was done on site in the workshop. It's safer, it's quicker, it's got a whole pile of advantages." The project has resulted in maximum capacity increasing from 39,000 tonnes to 121,000, although Davyhulme is cur- rently averaging 90,000. The solution has saved UU a lot of capital investment, and massively reduced its carbon footprint, says Lancaster. All of Manchester's sludge goes through the facility, plus a huge part of UU's central region. Average flows Lancaster says UU also has flex- ibility in that if it has to shut down one of the streams it can still put its average flows through without turning it off. "The other water companies do not have that capability. They have to have shut-down plans and manage their sludge via alternative routes during those shut-down periods," says Lancaster. The plant also means that up to 32,000tds goes to agriculture from Davyhulme. "It's the first time we have ever taken enhanced treated sludge to agri- culture from Manchester. The farmers have been very pleased to take it." n The two double-membrane gas holders, at 9,000m3 each, are the largest in Europe "This is a great piece of engineering. ...it's got a pile of advantages" John Tattersall

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