WET News

20 WET NEWS APRIL 2016 INSIGHT Odour control SULPHUS for wastewater ordours B iotrickling filters with synthetic media have been successfully applied since 2000 and are now the industrial standard in Europe, North America and the Middle East, they are not yet fully established in the UK. In many other countries pumice stone and organic media nowadays is rejected and not included as an option in the odour control tender specifications that are issued by consultants, however, pumice stone is still being installed in biological reactors in the UK. This is about to change since higher standards on operation, performance, and whole life cost are also becoming more important within the UK wastewater treatment industry. Indeed, in the US at the beginning of 2000 chemical scrubbers were successfully being refurbished into biotrickling filters with plastic synthetic media. Progress Although the operation of biological odour control systems is simple and almost effortless, the combination of different physical, chemical and biological process mechanisms results in a relatively complex system. In the past few years, enormous progress has been made to understand the fundamental aspects (like mass transfer, bio-kinetics, biodiversity, hydrodynamics) necessary for design, operations and application of biological systems, such as biotrickling filters. At Pure Air Solutions, we have been doing intensive research to gain in-depth knowledge of the capabilities of biological reactors, including ground rules and dynamics of the processes and mechanisms that occur in the biotrickling reactors, resulting in an improvement of their biotechnologies as a technically excellent and cost-effective solutions to different markets. Concept The performance of biological reactors is highly dependent on the contact time between the pollutants and the micro- organisms, the higher the better performance as it is well known. To evaluate this the concept of empty bed residence time (EBRT) has always been used, which is referred to the volume of packing media of the reactor without considering the type of media installed. This can lead to great errors, since different types of media have different void fractions, so the higher the void fraction the more similar that the actual bed residence time (ABRT) is to the EBRT. Obviously there is another variable in the equation: specific surface area. This parameter is the support for the development of microorganisms, as well as a high specific surface area enhances the mass transfer of oxygen and pollutants from the gas to the micro-organisms. Packing media So, the higher surface the more micro-organism in the bioreactor, and the higher contact between them and the oxygen and pollutants the higher performance. Therefore, an appropriate packing media should at least have two parameters: high specific surface area and high void space, besides longevity Contrarily to random media (pall rings, pumice stone) these two characteristics converge in structured synthetic packing media, allowing to decrease the volume of the reactor needed, which has a positive influence on both capex and opex. This is partly why different bioreactors are used to treat the same stream, more media is necessary to reach the same performance due to its worse characteristics and impact on the reactor configuration. THE CONCEPT • SULPHUS synthetic media has 2-3 times higher real residence time than pumice stone (=smaller reactor vessels). • SULPHUS has higher mass transfer due to a specific area four times greater than pumice stone • OdourPack comprises syntactic plastic media that is resistant to sulphuric acid and has a life time of >20 years. media that are resistant to sulphuric acid NEED TO KNOW 1 As an example comparisons undertaken on a wet scrubber system identified savings which lead to a return of investment of less than one year 2 The performance of biotrickling filters with syntactic media is so high that either no 2nd stage carbon polishing is required or the lifetime of the carbon media when adopted is significantly increased 3 The combination of different physical, chemical and biological process mechanisms results in a relatively complex system, however nowadays biotrickling filters are extremely reliable, easy to operate and maintain THE VERDICT 'Thanks to the high performance of the SULPHUS system the whole life cost of odour control in the UK can be reduced due to savings in carbon media and chemicals' Felius Sempere Ph.D A solution: Improving performance without cost SULPHUS biotrickling filter system with OdourPack media can have a positive influence on costs. W ith the objective of reducing whole life cost of odour control in mind, Pure Air Solutions is carrying out a research project on a full scale industrial installation with the innovation / cooperation of a major UK water authority. The project has adopted the company's SULPHUS bio- trickling filter for odour abatement without activated carbon as a polishing step for high odorous streams and low outlet odour concentrations. The OdourPack structured synthetic media used in the SULPHUS fulfils the above two characteristics with a high void fraction >92%, and a high and 100% accessible specific surface area for micro-organism colonisation >400 m2/m3. The OdourPack is made of plastic media that are resistant to sulphuric acid, ensuring a life time of more than 20 years. Bacteria The low apparent density and uniform structure enables different possibilities in the design and operation of the SULPHUS, allowing plug flow configuration that enhances the performance of the process. A clear example of a non- appropriate packing media is pumice stone. Pumice stone has a large porosity and seemingly creates a lot of space for bacteria to settle, but much of this surface area is blinded by internal pores and rock to rock contacts limiting the access of nutrients, water and oxygen/ pollutants, being the overall void fraction only around 40-50%. This low void fraction together with the weight of the media implies bigger and more reactors than if OdourPack is used. Pure Air Solutions recently undertook a challenge to upgrade an existing pumice stone biotrickling filter that was treating 1,250-2,500m3/h. Due to the increased presence of the contaminate load in the pumping station it was necessary to increase the extract capacity air flow rate to 8000m3/h, which could not be handled through the existing pumice unit. The characteristics of the OdourPack resulted in that for the same volume of the pumice stone bioreactor, the new SULPHUS installed could treat a 3.2 times higher air flow rate and still maintain an extremely high performance. A normal guarantee is more than 99.5% of the inlet H2S concentration at contact times as low as 8.5 seconds. Investment Upgrading of the existing activated carbon filter was also carried out, but thanks to the high performance of the SULPHUS system the operational costs due to carbon replacement will almost certainly decrease dramatically and the media may not ever need replacing. Also compared to a chemical scrubber the savings adopting a biotrickling filter with synthetic media is significant. Where initial investment costs of a chemical scrubber maybe somewhat lower than a biotrickling filter for an inlet concentration of eg. 50ppm H2S the costs for chemicals such as caustic and sodium hypochlorite are around £36,000 per year without considering the higher pressure drops, logistics, storage and handling of these hazardous chemicals. This means that the difference in capital costs (initial investment) will be a fraction of the savings in operational costs or in other words by replacing a chemical scrubber with a biotrickling filter with synthetic media the possible additional investment costs are usually recovered in less than six months. n Feliu Sempere Nacher Ph.D is senior process engineer of Pure Air Solutions. The challenge: Understanding the ndamentals Great progress has been made in the evolution of biological systems, benefitting whole life costs. Pure Air Solutions' Feliu Sempere Ph.D explains.

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