Scottish Water has invested £10M at its Troqueer wastewater treatment works near Dumfries, as part of its commitment to deliver a cleaner environment and enhance the service it provides to customers and communities across Scotland. An essential element of this investment, which will increase overall treatment capacity for the town and its environs, is a new £330,000 odour control plant to sit alongside the new sludge treatment system.

Designed and installed by odour control specialist ERG Air Pollution Control, the plant removes the nuisance odour emissions that previously came from the sludge handling process and its out-dated water-based odour control system.

Housed in a large new portal frame building, the upgraded sludge treatment centre and odour control system are no longer sensitive to changes in weather conditions and particularly variations in temperature.

ERG’s brief from Scottish Water was to design and install an odour control system that would extract and treat the odours from the plant and ensure that any odour emissions from the plant crossing the boundary would be at a barely detectable level of less than 0.6 parts per billion.

To meet the brief, the company designed two air extraction networks. These are made up from hundreds of meters of ductwork that effectively ‘vacuum up’ the foul air for treatment. The first network extracts highly odorous foul air from the ‘hot spots’ that had been identified by the two companies in and around the key sludge treatment processes and from sludge handling tanks. This gives rise to a highly odorous air stream with a flow of 2,250m3/hour.

The second is a network of ducting that collects low-level odorous air from the general building ventilation. This low-level foul air stream has a much higher volume at 17,850m3/hour.

Because sound can be as much of a nuisance as odour, the fans transferring the first of the foul air streams into a new 40m3 biofilter are housed in specially soundproofed enclosures. This filter is charged with woodchip biomedia along with calcified seaweed fertiliser that acts as a support structure for the bacteria bioculture that builds up as it feeds on the H2S in the foul air. The expected life of the biomedia before it needs replacement is four years.

The now much cleaner air exiting the biofilter is then mixed with the 17,850m3/hour of low level contaminated extracted air from the second network.

This gives a combined flow of 20,100m3/hour that is then passed into a 3m diameter 4m high dual bed annular carbon filter. This is charged with 1.9 tonnes of caustic impregnated activated carbon. The residence time in this unit is roughly one second, during which time the highly efficient carbon extracts 99% of the residual odour.

According to the design rating the expected life of the carbon media between changes is four years. The performance delivered over the two filtration stages results in a combined odour removal efficiency under normal operation of 99.8%.

Finally, the air vents to atmosphere exits from the top of the carbon filter via a 15m high exhaust stack. The odour concentration at the stack outlet is less than 300 parts per billion but because it has a velocity of 15m/s, odour dispersion modelling based on 98 percentile hourly average weather data shows that odour from the stack will give concentrations at the plant boundary of less than the 0.6 part per billion specified by Scottish Water.

A benefit of the new plant from an environmental point of view is that sludge can be treated to European directives before being used for soil improvement and fertiliser.

The project, managed by Scottish Water Solutions in partnership with Carillion Capital Projects, gives the region an enhanced capacity to grow and thrive.

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