At the West Sussex mechanical biological treatment (MBT) facility in Horsham, which is operated by the Biffa Energy Division, developments in the odour control process have resulted in significant improvements.
The odour management plan will also help other facilities to minimise the risk of odour escaping and become a nuisance to neighbours. Measures include odour control features on the building and delivery vehicles, along with air extraction, treatment and monitoring.
When the facility was constructed, the odour control equipment comprised a fluidised bed sulphuric acid scrubber to reduce ammonia in air extracted from tanks, followed by a set of five bioreactor towers to treat the exhaust air biologically.
During the commissioning phase, while no noticeable odour was found at the boundary, the average odour concentration at the stack exceeded the contract limit. Biffa needed to improve the system, so first it tested all air flows and ensured the equipment was working at its optimum level.
Exhaust air from the tank system represented around only 4% of the overall air volume treated, but is highly odorous. On investigation, the odours from this stream were found to be dominated by highly volatile organic compounds (VOCs) rather than the high ammonia loading anticipated at design stage.
As a result, the decision was made to convert the ammonia scrubber into a chilled water scrubber, which targets the removal of such VOCs. The extraction system to the scrubber was modified to encompass pasteurisation tanks as well as the existing tank farm.
Inspiration was taken from a similar design that Biffa developed at the Poplars anaerobic digestion plant, which was operating at very high removal efficiencies (see MRW.co.uk/8681003.article).
Highly odorous air from the storage tanks is now treated through the chilled water scrubber. Air is blown through the scrubber where the VOCs are scrubbed with chilled water, which contains a low-level biocide to inhibit biological growth. The water is periodically blown down to control the level of organic material within the process.
The chilled scrubber removed 40% of the total VOCs that were fed to the main odour control system, resulting in a reduction in the loading to the bioreactor system and carbon filter. The new system reduced the odour concentration in the air exiting the scrubber from around 500,000 European odour units per cubic metre (ouE/cu m) to less than 200,000 ouE/cu m.
Biffa also improved the performance of the bioreactors, ensuring equal distribution of air and water, and improving control of the levels of irrigation, heat and pH. Meanwhile, the control system software was rewritten to maintain more accurate control of conditions within the bioreactors.
While work was carried out on the bioreactor system, the overall odour at the stack was, as expected, found to be dominated by low-solubility VOCs, which cannot be removed effectively by biofiltration. So it was decided that a carbon filter polishing stage was required after the biological odour treatment of the air.
Several technology partners were invited to propose solutions. It was complicated by the fact that the plant was already operational and receiving waste. There were also space, energy consumption and noise restrictions to contend with.
Biffa selected ERG (Air Pollution Control) to provide the carbon filter and associated equipment. The final design required an investment of almost £1m and comprised installation of an annular bed carbon filter, a heat exchanger to reduce the relative humidity of the inlet air, ductwork and dampers to direct the flow and a fan with new pressure transmitters and acoustic cladding.
The plant went through a second acceptance test period in June 2015, during which time the odour limit of ≤1,000ouE/cu m was easily achieved. Biffa is now confident that the measures in place will prevent the escape of odour comprehensively, ensuring neighbours are not affected by the plant’s operations.
Plant by Numbers
- Biffa’s West Sussex MBT facility is designed to treat up to 327,000 tonnes of municipal waste a year under a 25-year contract with West Sussex Council.
- The plant recovers recyclable materials from residual waste, and can generate up to 4.5MW of electricity from biogas and produce refuse-derived fuel for onward treatment.
- It is currently ramping up throughput and is expected to be processing more than 200,000 tonnes of waste this year.
Mandip Kalsi is Biffa divisional engineering manager and Andy Burgess is general manager for Biffa West Sussex