Meeting sludge obligations

Purac's improvements to SWW's digester plant are set to ensure the water company meets new sewage sludge treatment regulations

Purac has been undertaking a project with South West Water (SWW) to help the utility apply Hazards Analysis Critical Control Point (HACCP) to meet the Revised Sludge (Use in Agriculture) Regulations. The requirement to meet the revised regulations is currently voluntary but revisions to the regulations will become law in the near future. The project has involved Purac helping SWW implement its sewage sludge strategy and HACCP monitoring requirements. Purac has been responsible for looking closely at selected SWW sites in Devon - eight in all, five of which have sludge digestion plants, while the others are drying, composting or lime stabilisation plants.

SWW recognised the digesters, some of which were not in the first flush of youth, needed attention to bring them up to an acceptable level of efficiency. Purac visited all the sites covered by the project and made recommendations it believed would bring the sites up to the required standards to meet the new regulations. Before the process began, SWW's project team had anticipated a number of activities that would need to be carried out but once monitoring of the sites had begun, it was discovered several of the actions anticipated were unnecessary because the plants complied already.

On all the sites Purac installed a HACCP monitoring panel - an electronic signal marshalling station that collected all the critical data. The data is passed on to the site Paragon SCADA system, which displays measurements, alarms and logged data for the critical control points (CCPs). The Paragon system then passes the required data on to the SWW measured data archive (MDA). The process is currently a self-certification one, with figures taken internally by SWW personnel in conjunction with sampling and analysis carried out by Pell Frischmann on behalf of SWW.

Reports are made monthly and annually from daily average figures taken at each digester site. These figures include the temperature in the primary digesters, the residence time in the primary digesters and the residence time in the secondary digesters. Part of the recommendation by the Purac team was the need to provide additional temperature probes and flowmeters at some of the sites in order to achieve more accurate and higher quality data to meet the likely requirements of the regulations.

The first site Purac looked at closely was the site at Kilmington, near Axminster. Here, they replaced all temperature probes associated with the digesters. One of the existing probes on each digester was in a pipe containing both recirculating digested sludge and raw feed sludge and as it is necessary to measure the bulk temperature in the digester - not 'contaminated' with cold feed sludge - it was getting an artificially low reading. This highlights how critical it is to get the temp-erature probes in digesters, in the right places.

Over a period of time, digesters that are fed with sludge from WwTWs with no/an inefficient grit removal process can become overrun with grit that settles at the bottom of the digester. This settled grit reduces the effective volume of the digester and can lead to the throughput having to be reduced in order to maintain treatment efficiency. It is a major job to clean out but Purac recommended the primary digesters at Kilmington were both taken out of commission to get rid of the grit. With 700m3 of sludge in each of the two primary digesters you will get an idea of the job this entailed.

In addition, at the Kilmington site, Purac recommended the replacement of the heat exchangers, which heat the sludge up and keep it close to 35°C to allow the digestion process to work efficiently. The existing heat exchangers were struggling to keep the sludge at the required temperature and in winter SWW had had to reduce the throughput of sludge by up to a third to allow the heat exchangers to cope. Replacement of the heat exchangers is currently taking place. The new heat exchangers to be installed are extremely efficient and unlike the old ones they replace, easy to maintain, which ensures the digesters can always be operating at maximum efficiency.

Also as part of the work Purac instigated at Kilmington, it was essential to optimise the throughput of the sludge digesters, which allowed an increased proportion of imported sludges to be treated at the site. A knock-on effect of this is the production of more sludge liquors from the sludge treatment plant that, in this case, had to be treated by the existing WwTW.

Purac is at present helping to optimise this process, improving control of the liquors by automating the process to allow them to be returned to the plant overnight, when the flow rate and associated load reduces. This helps to balance loads over the 24h cycle, while optimising the throughput.

At the SWW Crediton plant, Purac found a fairly old concrete digester tank. Its roof was cracked and structurally unsound so Purac instigated activity to replace the roof totally. As part of the HACCP project, Purac installed three new temperature probes to replace the single probe in place previously, through the roof of the digester at three different depths so sludge temperature could be monitored throughout the tank.

As well as telling you that you are meeting the necessary regulations, when the readings from three probes at different levels in the tank are all within 1°C of each other, the indications are the digester is mixing well. Purac also installed a flowmeter in the feed line so SWW would know exactly how much sludge is going into the tank. Previously, the pump was run for a given time each day but no actual totalised flow figure was known.

In addition, Purac changed the heating and mixing system at Crediton. Not a change in design philosophy but simply a recognition the existing system was around 20 years old and needed replacing. It was a like-for-like replacement, utilising a gas lift pump to ensure circulation and mixing within the tank. Two new secondary digester tanks were added to the site to enable SWW to store digested sludge for the required 14 days - they simply needed more capacity.

Again this was not a design fault but merely recognition that with population growth and higher sludge volumes, new tanks were needed to meet the local requirements. It was necessary to decommission the digester to clear out the substantial amount of rags that were building up in the system. Purac is at present installing an in-line screen that addresses the problem before the sludge makes it into the digester. In addition, Purac is in the process of installing a buffer tank and associated digester feed pumps between the new sludge screen and the digester that allows the sludge to be fed into the digester evenly over 24h.

controlling flow

Prior to this installation, there was no means of controlling flow - because of design constraints, SWW was often forced to put a day's flow into the digester in half an hour, cooling down the temperature of the sludge and significantly reducing the efficiency of the process in the digester. The new feed pump will operate for 4/5min every half an hour, keeping the digester fed with new sludge but allowing the heat exchangers to maintain the ideal temperature for digestion to take place. The Countess Wear site near Exeter benefited from a major upgrade to its sludge treatment facilities two years ago so there was very little work required in this project.

A new raw sludge belt thickening plant was installed to increase the throughput of sludge and a new odour control unit for the whole raw sludge treatment centre was added. At Kingsbridge, a large amount of work would have been required in order to ensure the works could meet the requirements of the sludge strategy. In this instance it was determined the most cost-effective solution was to modify the sludge strategy rather than the Kingsbridge works.

This modification entailed diverting the sludge imports that were going to be taken to Kingsbridge, to be processed at another sludge treatment centre where excess capacity already existed. Consequently, only minor modifications were required at Kingsbridge. The gasholder is currently being replaced. The old glass-coated steel holder had reached the end of its natural life and was replaced by a 'gas balloon' gasholder. In addition, Purac relocated the digester feed flowmeters. They were previously placed in such a way that they were measuring both raw feed sludge and recirculated digested sludge - they were repositioned to measure just the feed sludge.

The final site with a digester plant that Purac monitored was at Totnes. Again the firm needed to replace the 'heatamix' units that had reached the end of their useful life. It involved a like-for-like switch with new externally mounted units. In addition, Purac added a fourth secondary digester - there had been three at the site but the optimum number in order to accommodate the required 14-day batch storage is four, so a fourth was added to bring the site up to spec. Purac's Steve Cooper, principal process engineer, commented: "It was an interesting project that gave us the opportunity of taking a close look at the SWW sites, monitoring them for a period of time, before recommending the remedial actions required to bring them up to a standard, where we believe they will meet the HACCP regulations.

"We feel we were able to recommend and implement a series of cost-effective actions that will see improved quality services for the people of Devon and a vastly improved quality of facilities for SWW without a massive capital outlay." Colin Mather, project manager for SWW agreed: "We were pleased with Purac's sensible approach to the problems we needed to address and the work has been undertaken to our satisfaction and the benefits are already feeding through on a day-to-day basis. We are confident our Devon sites will now meet the revised sludge regulations once they come into force."


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