Waste recycling fuels Severn Trent works

Severn Trent Water has achieved a significant increase in electricity generation and reduced waste for landfill at Minworth sewage works. Peter Craven of CDE Ireland explains how

Severn Trent Water (STW) has made significant progress in its drive to reduce waste to landfill volumes and increase recycling through the introduction of a processing plant from CDE Ireland, based in Co Tyrone.

The plant, which STW has on a rental contract through Biffa, has been processing a variety of waste materials since it was introduced at Minworth Sewage Treatment Works. And the utility has seen a significant reduction in waste to landfill volumes.

Nick Williams, STW's digester fleet said, "We are now sending 50% less waste to landfill than was previously the case. We are recovering a range of materials from the waste which are subsequently used in construction projects and energy generation initiatives within the company."

STW says that generating energy from waste is very much top of its agenda and chief executive, Tony Wray, is very keen to increase the proportion of electricity that the company can generate for its own use.

Wray has a goal of generating 30% of the electricity needs from the Dmax portable plant installation at Minworth internally within five years. This is a significant increase from the figure of 17% recently announced for 2008, but according to Wray, imminent legislative changes now mean that this sort of blue sky thinking is necessary from all water companies. Wray highlighted the fact that water companies will be among those that will be included in the EU's carbon trading scheme, under which they will have to pay for emissions.

He comments, "We're going to get captured in the next phase of the carbon trading scheme, so the efficiency with which we can manage our energy consumption is very important."

Recycling specialist Biffa manages the day to day operation of the processing plant, which processes a variety of waste materials including wet well waste, gully waste, inlet works grit and sewer blockage waste. The plant also processes reed bed material as part of STW's Reed Bed Refurbishment Programme.

The capability of the plant to cope with a variety of feed materials was key at the specification stage, and this was one of the main reasons for the decision to specify a plant from CDE Ireland.

Williams explains: "We had previously purchased a modified version of one of CDE's sand plants and had a good experience with the system. The most important thing from my perspective was ensuring that the system we chose was tried and tested and proven in this type of application. Over the last three years we have seen first hand that the CDE system works."

Mobile unit
The venture into the water sector is relatively recent for CDE, who have traditionally supplied processing systems for quarrying, mining and recycling. However, over the last three years the company's business in the municipal water treatment sector has grown significantly, leading to the establishment of a CDEnviro business unit within the company, specifically charged with further developing business in this sector.

CDEnviro also takes responsibility for the further development of the product offering currently available from CDE to the sector.

"Our initial venture into this industry some three years ago involved the development of a trailer-mounted system designed to process waste from wet wells and digesters," explains Matt Bunting, CDEnviro's newly appointed commercial manager.

One of the company's first customers for this system was Biffa and it was through exposure to the system, as it was applied at the Minworth site that convinced Nick Williams that the system not only offered a means of reducing waste to landfill costs, but also recovering materials for future use.

"The Dmax trailer unit was used by Biffa for over two years as part of our company wide digester cleaning programme and proved very successful," said Williams.

Having seen the trailer unit in action, Williams was keen to find out whether CDE could offer an extension to the system, which would facilitate the processing of a broader range of material.

"The trailer unit was very effective at removing water from the waste and classifying material, but we were limited by the restriction on feed material of 8mm and below," he explains.

After spending considerable time at the Minworth site and discussing the new plant requirements, CDE proposed a plant extension. This would utilise the Dmax trailer unit for processing the minus-8mm material, but would also provide a processing system to be introduced prior to the Dmax unit to accommodate the plus-8mm material.

"Given that this was seen as a new approach to the handling of this waste, there was a 'wait and see' approach at the outset," said Williams. "We were granted a license to process fifty tonnes per day of material through the new CDE system as a pilot scheme and it has been very successful."

Dewatering screen
The process begins with the feed material being delivered to a hopper via telehandler, where it is then conveyed to a ProGrade C-series dewatering screen, with screen mats set to an 8mm aperture.

Throughout the process there is a focus on 'transfer point technology', which ensures spillage is minimised. This not only improves plant efficiency, but also has a significant effect on reducing on-site health and safety concerns.

Once at the primary dewatering stage, all of the plus-8mm material is removed and stockpiled while the minus-8mm material and wastewater is pumped to the Dmax system. The feed is then delivered directly onto the first side of the split, high frequency dewatering screen where the rag is removed and discharged into a skip.

The grit material and wastewater falls through the polyurethane screen deck to the collection sump, where it is then pumped through a hydrocyclone and discharged onto the second side of the split screen. At this stage in the process the grit is dewatered while the wastewater is sent back to the treatment plant for further processing. At the end of the process there are a number of materials available to STW for further use.

Recycled aggregate
The grit removed from the waste is sold as recycled aggregates to the construction market for use in a variety of projects. The other screenings have reuse potential for incineration - an avenue to electricity generation. Similarly the organics are returned to the digestion process to produce methane for use by Severn Trent and furthering the utility's effort to generate 30% of its own electricity within the next five years.

In terms of the impact of the system on waste volumes, the Minworth plant has reduced waste to landfill by 50% since it was introduced. This is the saving made by STW when the full rental and operational costs of the plant have been paid to Biffa.

"In anyone's language the reduction in waste volumes made possible by this plant are extremely significant" said Williams.

Following the successful operation of this pilot scheme, STW recently placed an order with CDE for a new 200 tonnes per day processing system at the Coleshill site, which is very close to the Minworth plant and is also managed by Nick Williams.

"Our order for the Coleshill plant is specific recognition of the successes we have seen at Minworth," boasts Williams. "With the increased capacity of 200 tons per day at Coleshill we will be able to deliver even more significant savings over the coming years."

With a growing focus on reducing waste to landfill and increasing regulation of the industry as whole, Williams believes that major water companies have to step out of their comfort zone and take challenging decisions on how they deal with their waste materials.

"It was only through a top down commitment to exploring new ideas and innovations that we were able to get this system up and running," he explained.

"Through the commitment of ourselves, Biffa and CDE to get this thing right, we have been able to create a successful system, which I believe can be applied throughout the industry to efficiently reduce waste and increase recycling."

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