As part of the site overhaul at Calder Valley Sewage Treatment site whereby the site was transformed into Huddersfield Energy and Recycling Facility under Murphys Process Engineering Ltd, Greenacre we brought in to install ventilation extraction and odour control systems for both the cake import and the sludge thickening processes to mitigate DSEAR risk, COSHH risk as well as provide odour control.
The larger of the two systems extracts contaminated and odourous air from the drum thickeners, thickener liquors PS and the digester feed tanks. The system has 2 stages of odour treatment; 1 single bed, counter current, trickling biofilter and a single bed activated carbon adsorber polishing unit. The Biofilter has an inert layer of bacterial support media to remove the bulk of the odorous components whereas the carbon polisher allows the system to meet stringent odour consents by removing residual odours in the airstream.
The biofilter bed utilises 37m3 of 10-15mm pumice stone as a support matrix for bacteria. This provides an inert, robust and extensive surface for the required biological reactions to take place. The media is guaranteed for 15 years and can be washed down if solids accumulate from the irrigation liquor or via the inlet air stream.
This media is continuously irrigated with Final Effluent (FE) and on the basis that FE contains enough nutrient to support bacterial growth and hence nutrient supplements are not required. The irrigation liquor is fed on a single pass basis and there is no recirculation. The FE supply is trace heated and thermally insulated. Innovation regarding spray nozzle access and ease of removal for cleaning was incorporated in the design. Operators do not need any tools to clean, service or remove any of these components which has typically been a major issue on other installations.
Quick access to spray bars via quarter turn hatches
Quick release spray nozzle assemblies for cleaning and maintenance
The odorous air passes into the biofilter underneath the media and rises up through the beds in counter-current fashion with respect to the irrigation liquor (which is percolating down the bed).
The biomass on the media surface breakdown the odorous components: H2S to acids and VOCs which are broken down to water and CO2. The Biofilter removes approximately 96% of the Hydrogen Sulphide and 40% of the biodegradable VOCs entering the OCU. The residual H2S and other odorous components are then removed in the activated carbon polisher.
The carbon polisher is a deep, vertical bed type adsorber with approximately 1.85 tonnes of copper oxide impregnated carbon pellets. The air flows up through the adsorber and is efficiently cleaned as it passes through the carbon bed. The process of “polishing” is a combination of adsorption onto the carbon and also chemisorption in the reaction with the copper oxide. The Copper oxide is utilised because it has been found to provide the broadest and most reliable range of odour control for this type of application and it is able to operate in high humidity. Electronic pressure transmitters alarm in the case of a high differential pressure across the filter.
All ductwork is PVC with a GRP coat and steelwork is mild steel hot dipped galvanised in line with Wimes Specification. The exhaust stack is 15m tall with a bespoke support structure installed by crane and MEWP. Two fans are installed in a duty and standby arrangement. The fans are 7.5kw, ATEX rated Zone 2, centrifugal-type manufactured from stainless steel and rated at 4,428m3/hr and each installed with an independent flow sensor. The fans will run continuously; in the case of a low flow alarm from the sensor this will initiate an automatic changeover of the duty fan.
Following the installation, in depth performance testing was completed over a period of 7 days confirming the air volume flow rates and the effectiveness of the odour control filters. Results of the testing confirmed that the primary stage reduced H2S levels by more than 98% with a further reduction achieved by the polishing carbon filter to less than 1000Oe/m3.