Sludge Screening - Improving Efficiencies

With over 624,000 kilometres of sewers in the UK collecting over 11 billion litres of waste water per day, the UK water agencies have the huge responsibility of treating this material in the most efficient manner possible.

Whilst bio degradable products and materials are now common place in our daily lives, these and other non bio degradable materials increasingly find their way into the UK sewerage systems. As a result man-made products are causing significant fluctuations in the volumes of dry solids in raw sludge. Managing the removal of these products from the overall water processing system at an early stage can massively increase efficiency, in some cases by up to 300%.

Technological advances are making the biggest changes particularly during the Preliminary Treatment stage which greatly affects efficiency. In turn these create savings in operating costs, decrease overall processing time and reduce the cost of maintenance and servicing.

One large UK water agency in the Midlands that is committed to making significant improvements to its efficiency in waste screening chose CDEnviro to provide and install its sludge screening solutions at one of its sewage treatment works. The site had been previously spec'ed to handle around 3% of dry solid waste but it was now experiencing significant problems handling sludge build-up within the system. Within weeks of installing a CDEnviro S:MAX screening system, an analysis of the material processed showed an average dry solids content of 4.1% with the highest level being 10.5% - much higher than had expected and clear proof of the size of the problem they had been facing.

"It is this variation in dry solids content that leads to inefficiencies with some sludge screening systems as they are not built to cope with this percentage swing", explains Marc Jennings, Programme Manager at CDEnviro. "The volume of dry solids found in sludge may vary, so in order for the treatment process to operate as efficiently as possible, it is crucial that these variations are accommodated. The S:MAX has been designed to comfortably handle 10% dry solids within the raw sludge giving operators a great sense of comfort when it comes to specifying a new sludge screening plant."

CDEnviro won the contract to supply eight sludge screens to three sites and is now working with a number of water agencies in the wider vicinity as part of the E5 alliance. In responding to this project brief CDEnviro specifically developed the S:MAX SRU:15 sludge screen for this application. Equipped with the same design features as the larger S:MAX SRU:25, the SRU:15 allows reduced capacities to be efficiently processed. The S:MAX SRU:15 can accept waste from two streams; the imported waste collected from tankers in the surrounding areas and sludge generated by the local sewage treatment works.

The S:MAX sludge screen effectively removes screenings before the material reaches the anaerobic digestion phase. This maximises the potential for energy generation from bio solids by reducing the level of contamination within the digesters. Eliminating blockages during the screening process also means that rag does not accumulate, which would otherwise cause down time. The S:Max can also be provided with an odour enclosure allowing for the safe operation and placement of the system in an internal space
Underpinning the whole premise of this programme was the expectation that a step change in efficiency would be delivered. Through the tender process CDEnviro proposed innovation which would 'challenge existing standards, expectations and requirements' and with the S:Max sludge screen it provided substantial benefits, challenging and improving existing specifications plus reducing costs across the life of the plant.

The S:MAX solution can cope with variable rates and consistency of sludge plus allows tankers to discharge directly into the system without the need for a reception tank. This provides both substantial operational and maintenance savings and minimises capital expenditure as buffering tanks and pumps are not required. The S:MAX can accept sludge from the sewage treatment works or sludge imported from smaller works with no sludge handling capacity. In addition there was an increase in up-time of equipment downstream with the S:MAX processing sludge early in the process as the increased screening and capture rate helped avoid the accumulation of rag throughout the processes.

The virtual elimination of blockages during screening is undoubtedly one of the most significant reasons for these cost reductions. Water authorities often experience problems when blockages occur which in turn causes damage to the screening plant. Extra costs are then incurred when replacement parts are required to get the plant operational again. Hidden costs, such as diverting tankers containing the raw sludge for screening to alternative sites for processing, has a further impact.

Marc Jennings, Programme Manager at CDEnviro concluded: "This installation has proved that with effective sewage sludge screening comes improved efficiency and many other long term benefits that will both quickly help recoup the capital investment and lead to further productivity savings moving forward."

For further information please email CDEnviro

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