Three-stage system scrubs the nasty smells away

When Wessex Water wanted to expand the sludge treatment plant at Avonmouth WwTW, it needed improved odour control measures. A wet scrubber package provided by Parsons Engineering proved to be the perfect solution.

In 2006, Parsons Engineering provided three of Wessex Water’s sewage treatment sites with its innovative service of a scrubber inspection. Involving testing all aspects of the odour control system, the scrubber inspection is an invaluable service to any site with odour control packages.

Following these site inspections and further maintenance works at Wessex Water’s Weymouth, Bridport and Poole sewage treatment sites, Parsons had gained an ongoing and busy rapport with engineering staff across the sites. Struck up in conversation, was talk of Wessex Water’s Avonmouth site expanding its sludge treatment plant.

This would increase the amount of odour on the plant that could no longer be handled by the current odour control on site. Therefore, a further odour control package was required to contain the additional odours created by the sludge treatment plant.

This, of course, was something Parsons could offer and so, after many letters, telephone calls and meetings. Parsons was set the task of the design, manufacture, supply and final installation of the large scale odour control package needed at one of Wessex Water’s largest sewage treatment sites.

After calculations from the Wessex Water team regarding amounts of solids and chemicals passing through the sludge treatment plant and estimations about chemicals and odours, Parsons concluded that the odours from the site were made up of ammonia, VOCs and hydrogen sulphide amounting to 26,000m³ per hour. Parsons’ chemical engineers calculated that this could be processed and eliminated using a wet scrubber package using three stage scrubber system.

The system includes an acid packed column, an alkaline packed column and a polishing activated carbon adsorber, which allows all of the chemicals that the odour is made up of, to be efficiently treated and eliminated. The package included extract fans, pumps, chemical monitoring and dosing, ductwork, chemicals store tanks, and other specialist equipment to ensure optimum performance of the odour control plant.

Following manufacture, Parsons delivered and installed the plant using its team of site workers and engineers. The custom-designed and manufactured piece of equipment was commissioned and doing its job of eliminating odour within three months.

The technology of the odour removal from the air is a complex and difficult procedure. To put it simply, the odours are extracted from the area via a network of ductwork and vents on the ceiling of the odorous building.

The fumes, in this case, are then taken through the first stage scrubber, which is an acid scrubber. This will remove the ammonia from the gas stream. The second stage will be scrubbing with an alkaline sodium hypochlorite solution to remove sulphurous odours.

The Parsons’ process provides improvements to the traditional method of gas scrubbing by using closely controlled low concentrations of hypochlorite, which can be controlled. This precludes the need for a final caustic scrubbing stage to remove any chlorine odour.

To provide removal of any compounds remaining due to their insolubility, there is a final stage treatment of an annular bed carbon adsorber. The carbon adsorber whilst not a practical stand-alone removal device is ideal in this situation as a polisher.

This type of installation is widely used for these types of situations where removal efficiencies of the highest order are demanded. In the instance of an odour control system working incorrectly for a site in a public area, fines could be issued and more drastically sites can be shut down through breach of their environmental contracts with local authorities and the environmental agency.

Upon the commissioning of the Parsons odour control plant at the Avonmouth WwTW, it was clear that all of the hard work and long hours were worth it and the package was a success. The odours from the sludge treatment works were eliminated.

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