Devon County Council and Viridor Waste Management

Viridor operates an in vessel composting (IVC) plant at one of its sites in Devon. The IVC was built for Viridor on behalf of Devon County Council in 2007 under a design and build contract.

Green waste is delivered to the site in bulk, shredded and placed in vessels to compost. The vessels are concrete tunnels approximately 4.5 metres high, 4 metres wide and 10 metres long. Concrete bulkheads are used at the end on each tunnel to maximise throughput and provide an even thickness of waste to compost.

When the tunnel is full of shredded material, and before it is finally sealed, six temperature probes are inserted into the waste. These probes are linked to the process control and are required as part of the process control system to keep the compost within specified limits for effective, even composting and to comply with animal health requirements. The tunnels are sealed at each end with rail-mounted, light-weight doors. On completion, air is supplied through floor vents and extracted through a vent in the vessel roof to drive the composting process.

Once the first stage of the cycle is complete, the doors are opened, the probes removed and the compost removed, turned and loaded into an adjacent vessel for a second cycle. This provides compliance with the Animal By-Products Regulations.

A review of the risk assessment for placing and removing the probes identified that risks could be reduced. The temperature probes are in metal tubes between 1.5 m and 3 m in length with a diameter of around 15 mm. When the vessel is full of compost ready for the probes, the top of the material is some 3 metres deep, leaving a space of about 1.5 metres from the concrete roof.

The system of work originally proposed by the suppliers for installing the probes required operatives to climb a ladder over the bulkheads and carry/drag the probes across the waste. This involved stooping, and the risk of operatives banging their heads against the roof, or walking and crawling across shredded green waste, which does not provide a sound footing and may cause direct injury. The operatives would then push the probes up to 2.5 metres into the compost in a specified grid pattern from a stooping position.

In addition to the direct physical hazards, the green waste releases bio-aerosols, potential allergens and also gasses as it begins to decompose. The risk assessment showed that this combination of biological hazard, direct injury, confined space, work at height and most of the manual handling risks could be eliminated by inserting the probes vertically through the roof of the vessel, as is the case on other designs.

The system of work was revised to use a mobile elevating work platform (MEWP) along with confirmation of oxygen levels and respiratory protective equipment (RPE).

Viridor raised the recommendation to insert the probes through the roof with Devon County Council through existing monthly communication meetings between their respective Operations Departments.

Site visits were made by Devon County Council appointed engineers to assess the context of the problem and they gave a recommendation to proceed with the works. Viridor and Devon County Council shared resources to find a suitable contractor. DCC let the contract and through their appointed engineers with the contract being subject to regular client/contractor review through both normal communication meetings and contract specific meetings.

The concrete roof is a poured reinforced concrete slab and, to avoid warranty issues, damaging the rebar and affecting the long-term integrity of the structure, an X-ray survey of the roof was taken to identify suitable locations to bore holes for the probes.

The work should be completed in January 2010, when all identified risks will be significantly reduced as far as reasonably practicable through the indentified modifications.