Technical Publications

A Successful Conversion of a Chemical Scrubber to a Biotrickling Filter – Some Experiences

Lawrence Koe, Lin Wu, Yong Ying Loo, Yan Wu, Jurn Wei Chai, Yih Ming Koh

EXECUTIVE SUMMARY

Obnoxious sewage air at sewerage facilities in Singapore is currently treated by a combination of chemical scrubbers and activated carbon towers. These traditional methods of odour control have been found to be very expensive in terms of recurrent chemical and carbon usage as well as dangerous in view of the need to handle vast quantities of chemical solutions.

Over the last decade, research has shown that a special blend of bio-culture of the genus Thiobacillus is effective in adsorbing odorous hydrogen sulphide, a principal component of sewage air. By immobilising the bacteria onto the surface of packing material in a biological reactor, it is possible to develop an efficient biotrickling filter for treatment of sewage air. When sewage air passes through the biotrickling filter, the biofilm of pollutant degrading microorganisms will aerobically degrade the absorbed pollutants. The technology has been experimented under lab conditions. Some field trials have also been tested and the results have been promising.

This paper will share some experiences gained in the conversion of an existing multi stage chemical scrubber with airflow of 200 m3/min to a biotrickling filter at a large water reclamation plant in Singapore. The paper will discuss a number of operational challenges that were encountered during the conversion project. Currently, the chemical scrubber has been fully converted to a biotrickling filter operating at full flow capacity. It is able to achieve a consistent high performance of exceeding 97% H2S removal at a short gas residence time of 7 seconds. The influent sewage off-gas contains high levels of H2S ranging from 50 to 60 ppm.

During the conversion process, it was discovered that scrubber packing material had significant influence on the performance of the biotrickling filter. The original packing material used in the chemical scrubber was found unsuitable for bioscrubbing applications because of its low specific surface area. It was then replaced with a new composite packing material with much higher specific surface area. After replacement, the composite packing outperformed the original packing with an H2S removal efficiency of more than 90% compared to the previous efficiency of 44% at 9 seconds GRT.

During a normal field performance operation, the biotrickling filter encountered system shocks. It was due to rapid build-up of undesirable metabolic products and an increase of acidity in the recirculation liquid. By flushing the packing in the horizontal biotrickling filter with filtered effluent water and reducing the H2S loading (higher GRT) for a period of one day, the biotrickling filter could recover from the shock in less than 48 hours.

Further investigations showed that regular discharge and replacement of recirculation liquid were essential in maintaining stable and high performance of the biotrickling filter. This helped in reducing build-up of metabolic products that would affect stability of the biotrickling filter.

The converted biotrickling filter has been subjected to various testing scenarios such as shock H2S loading, system overhaul, interruption of feeds, and interruption of liquid recirculation. The study has also shown that the converted biotrickling filter was robust, with only 2 – 3 days of recovery period from system upsets and downtimes.

In short, conversion of chemical scrubber to biotrickling filters has been shown feasible. The converted system has a good and stable performance with low operating costs and maintenance efforts.