Water and Effluent Treatment Magazine
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equipment – pipes, compres- sors, storage tanks, engines, etc – and reduce the life of a CHP unit, the heart of any AD operation. The average lifetime of a CHP unit is around 60,000 hours. If it is operated without any desulphurisation technol- ogy, the unit will require main- tenance a• er 20,000 hours of operation. However, the introduction of a step to remove H2S can increase uptime by 50% to 30,000 hours. The AD industry encom- passes a wide range of plant types and a 'one size ‡ ts all' approach should not be applied to desulphurisation. In‰ uencing factors including feedstock type, site conditions, plant uptime, capex and opex, and end use of the biogas, 16 WET NEWS DECEMBER 2017 Top tips on toxic gas treatment to prolong AD plant lifespan D eveloping an anaerobic digestion (AD) plant does not come cheap. Costs range from £100,000 up to as much as £24M. When calculat- ing their costings and potential return on investment (ROI), developers o• en plan for a plant lifespan of 20 years, so it is essential plants are well main- tained to avoid unexpected cost. This is particularly impor- tant as there are a number of things which can adversely a— ect a plant's longevity, such as large deposits of grit, the absence of a pressure relief valve, foaming and process instability, for example. In addition, there is an invisible threat to AD plants which, if le• untreated, can seriously impact performance, pro‡ t and plant lifespan – sulphide. Biogas produced through beverage industry. And in the case of sewage AD plants, sul- phates present in wastewater lead to the production of sul- phide under anaerobic condi- tions. Concentrations above 6mg/l of sulphate will start inhibiting the production of methane (methanogenesis) – above 320mg/l and methano- genesis will not be possible.š H2S also has a strong odour of rotten eggs. If it is not removed, this unpleasant smell is retained within the digestate (the nutrient-rich biofertiliser which remains at the end of the digestion process), causing o— ence to neighbouring com- munities and making the digestate impossible to sell to farmers and growers. The most damaging side e— ect of H2S though, is its ability to corrode essential plant need to be assessed before introducing a desulphurisa- tion process. H2S can be removed in-situ during digestion by air injec- tion or through the addition of iron salts/oxides. It can also be removed from the raw biogas by physical-chemical separa- tion processes or biotechno- logical methods (or a combina- tion of the two). For high concentrations of H2S, chemical scrubbers are usually recommended, while for biogas upgrading, acti- vated carbon ‡ lters are typi- cally used as a pre-treatment. For medium or high loads, alkaline scrubbers are pre- ferred, while upgrading tech- nologies based on physical absorption (such as water scrubbers or non-water sorb- ents) can simultaneously DMT's Sulfurex CR desulphurisation technology at a Manchester AD plan the decomposition of organic matter contains hydrogen sul- phide (H2S), a colourless, toxic gas which is one of the main inhibitors of micro-organic growth; if the bugs cannot get to work within the digester to break down the organic mat- ter, then the volume of biogas produced per tonne of feed- stock will be dramatically reduced, a— ecting an AD oper- ator's pro‡ t margins. This is particularly relevant for certain types of plants, as di— erent feedstocks contain di— erent levels of sulphates. Examples of feedstock rich in sulphur-containing proteins include cattle, pig and poultry manure, so on-farm plant operators should be especially vigilant. In addition, there is a high sulphate content in wastewaters from the beer and ¡ INSIGHT Sludge treatment & disposal Sulphide is an invisible threat to anaerobic digestion plants. If left untreated, this can have an impact not only on performance, but pro‚ t and the facility's lifespan as well.