Scrubbing out odour nuisance
Richard Hanson reflects on ERG’s experience of catalytically enhanced chemical scrubbing and how the benefits of Odorgard have received the endorsement of Northumbrian Water
Typically the odour emission rate from the odour control stack is reduced until the model shows that no lines of constant odour, of greater than 5 odour units/m³ (based on 98th percentile hourly average or similar), stray outside the boundary of the sewage works. As a result of this modelling stack emission limits as low as 500 odour units/m³ (ouE/m³) equivalent to 235ppb of hydrogen sulphide (H2S) are now becoming commonplace.
By way of explanation, an odour unit is an empirical estimate of the strength of an odour, and its value represents the number of times a sample of gas has to be diluted before half an odour panel cannot detect it.
One solution for meeting these stringent requirements is chemical scrubbing which has traditionally involved a two or three-stage process. In the first tower scrubbing is with dilute sulphuric acid to remove ammonia and amine-based odours. Then dilute caustic and bleach are used in the second scrubbing tower to remove sulphur-based odours like H2S. Finally a third tower, scrubbing with dilute caustic often at high pH, removes any less soluble mercaptans and any carry-over smell of chlorine from the second tower. This approach is proven and has worked successfully for many years, but is expensive both in capital and running costs.
About 15 years ago the late Dr Fred Valentin was commissioning a caustic bleach scrubber on a meat rendering plant. He decided to back a hunch and poured a cupful of nickel sulphate crystals into the sump. As he suspected, he found odour removal performance improved significantly. Instead of the bleach taking several minutes to oxidise the absorbed sulphurous compounds, the reaction was happening in seconds. The net effect was the scrubbing liquor being re-circulated back to the top of the tower was free of odorous sulphur contaminants, so it scrubbed more efficiently and less odour was exhausted.
Dr Valentin then entered into a partnership with ICI Synetix and produced a nickel-based catalyst called Odorgard. Unlike nickel sulphate crystals the catalyst ensures no nickel, a heavy metal, is able to leach into the scrubbing liquor.
The basic design ICI Synetix began with is explained in Figure One and remains largely unchanged. The company set about producing a comprehensive computer programme for designing Odorgard catalytically enhanced scrubbing systems, based on a number of pilot trial studies. This research led to important modifications to the accepted design practices. The key ones were:
- scrubber sump sizing
Traditionally designed caustic bleach scrubbers need a long residence time (5-10min) in the scrubber’s sump to allow the oxidation process to complete. However, highly odorous chloroamine formation, from the reaction between ammonia and hypochlorite, is encouraged by long liquor residence times in the scrubber sump, and so it is critical ammonia and amines are removed prior to the caustic bleach stage. Odorgard systems however need the opposite: catalytic oxidation is rapid so small sump volumes with small residence times (2-3min) are required. In addition catalytic oxidation with bleach oxidises ammonia and amines thus preventing unwanted side reactions.
Odorgard scrubber sumps are therefore small and there is no need for pretreatment with an acid tower provided ammonia and amine levels are typically less than 5-10ppm.
- liquor quality control
Catalytically enhanced oxidation also means the scrubbing liquor can scrub at pH9-9.5 instead of pH10-10.5. This results in reduced chemical usage, as the scrubbing liquor no longer wastes its efforts capturing odourless carbon dioxide to form sodium carbonate, which is the case if the pH rises above ten.
A further advantage of catalytic oxidation is the scrubbing liquor can operate at lower bleach concentrations, so there is no release of chlorine into the scrubbed gas, further reducing chemical usage and avoiding an un-wanted chlorine smell. Five years on ERG (Air Pollution Control) has installed more than 22 fully operational Odorgard scrubbers for the wastewater industry.
- significant amounts
Whether Odorgard scrubbers remove more total odour than equivalent non-Odorgard scrubbers is difficult to prove. What does seem clear is that H2S removal is increased, typically from 99.5%-99.75%. At first this seems only a minor improvement, however for sensitive sites this is significant as the H2S odour being released to atmosphere is halved by using the catalyst. Enhanced H2S removal however is only a contributing factor to total odour removal.
In the case of sludge processing, odours are typically a cocktail of sulphurous contaminants, which can contain up to about 1ppm of mercaptans to every 10ppm of H2S. This is a critical factor to successful odour control design because mercaptans, a term that embraces a large number of sulphur-based odorous contaminants, are 30-1,000 times more odorous than H2S. So removing mercaptans is just as important on sludge processing odour control system as removing H2S.
With this important design requirement in mind, the Synetix Odorgard design requirements have considerably improved caustic bleach scrubber design for total odour control.
- more liquor and packing
Over the years, certain rules of thumb with regard to caustic bleach scrubber design have become accepted. For Odorgard however Synetix insists on about a third more residence time for the contaminated gas in the packed section of the scrubber and about a third higher scrubbing liquor rate. Synetix recognises that to achieve total odour removal it is important to capture compounds such as mercaptans by improving the mass transfer of the scrubbing system, as well as rapidly oxidising them.
Olfactometric (odour panel) testing is a human judgement and the statistical validity of results is limited. For example, using an odour panel of eight testers, a concentration of 500ouE/m³ can only be considered as an odour concentration in the range of 240ouE/m³ -1,045ouE/m³. Equally, measuring mercaptan removal using gas chromatography mass spectometry (GCMS) is complex and expensive.
ERG recently carried out tests on an installed Odorgard scrubber treating highly odorous gas from sludge storage tanks and drum thickeners at Howdon STW. The purpose of the test was to confirm the advantages of catalytically treating the scrubbing liquor. The results are shown in Table One as sampled by AES and olfactometrically tested by OdourNet (UK).
Between tests one and two the inlet odour strength increased considerably, however, with the catalyst on-line the outlet remained at a level unlikely to generate complaints. For tests three and four the effect of the catalyst is more marked with the outlet emission reduced by 75% while the inlet loading increased by 15%.
In all cases, the total reduction in odour is impressive and these tests certainly seem to show the advantages of catalytically treating the scrubbing liquor.
ERG’s opinion is that Odorgard scrubbing can not be considered the wonder-cure for odour control, nevertheless the technology does deliver considerable benefits:
- Odorgard has enabled a single-tower scrubbing system to replace a three-tower one,
- the system appears to give improved total odour removal while ensuring contaminants are captured and treated using the minimum chemicals.
In his recent paper Alan Thompson, a project manager with Northumbrian Water, noted that Odorgard scrubbing was selected “because of its ability to respond to the odour loads of varying intensity which could be expected” and that across ten Northumbrian sites with the Odorguard system “few odour complaints had been received” during the first year of operation.
Over the last five years ten Odorgard scrubbers have been installed at Howdon STW in Newcastle-upon-Tyne, a further endorsement of the system’s capabilities.
The technology is not without its limitations. For light loadings where the probability of ammonia or amines is low, it offers little advantage over single tower, un-catalysed scrubbing. Equally for very high loadings (>300ppm H2S) a single tower design results in excessively large pumps being required and the benefits of the technology are reduced. For applications where mercaptans and other organic sulphides are unusually high, adequate mass transfer is far more critical than rapid catalytic oxidation of the scrubbing liquor and a multi-tower approach is likely to be more appropriate.
However, for most sewage and sludge processing applications, the Odorgard system is an efficient method of odour abatement, which offers enhanced odour removal at lower operational and capital cost.
- Synetix Odorgard Process Design Guide, revision B, 1998
- International Conference on Odour Management and Treatment ODOUR 2, Cranfield, March 2002