3.9 Using the Products of Anaerobic Digestion

3.9 Using the Products of Anaerobic Digestion


hi again in this last module on anaerobic digestion we’ll talk about using the products of AD the gas and the digestate I will try to explain what needs to be considered so that a biogas facility can be sustained maintained before we do this however let me quickly sum up what we have heard about ad until now so we have looked at some issues of the substrate chain we have talked about particle size we had talked about volatile solids of different waste materials I also described the biochemical process stages and the main parameters which are contained in here and some typical digester technologies so what now is left to do is to cover the product chain the gas and the digestate we will also discuss the issues around sustaining the whole system and answering the question when can anaerobic digestion be an appropriate treatment option for waste management and when not so now with an appropriate digester and good operating conditions the reactor will produce biogas this biogas is saturated with water vapor and in the first instance the water vapor moving through the pipes will condensate and moisture will collect as water in the pipes we will need to remove this moisture we call this dewatering we can remove the water at the lowest point in the pipes if this water is not removed it can block the pipes hindering flow of gas that is why you will need to install a water trap at the lowest point in the gas lines there are different types of water traps you can install here are two examples an automatic one and a manual one both have T joints at the lowest point in the gas pipes and a water storage unit below the T joint where the condensate can flow into the automatic trap empties itself when full by using a u-bend overflow whereas the manual water trap is emptied by opening a valve the biogas produced in the reactor will vary during the day according to feeding patterns and also ambient temperature changes biogas production also continues during nighttime when less gas is used and therefore there are periods when the gas has to be stored this can either be in the digester itself as in the top part of the fixed dome digesters as we had seen before or in bags or balloons like in this unit in indonesia gas can also be stored with medium or high pressure in gas tanks this can be done with compressors but of course this requires energy the Indian Institute of Technology in Delhi is engaged in research on bottling of biogas for use as fuel in three-wheeled vehicles they combine conditioning and cleaning of biogas and compression in an uncompressed form the energy content of one cubic meter of biogas corresponds to around a half a liter of diesel as a rough rule of thumb about ten kilograms of bio waste are needed to produce this one cubic meter of biogas for all use in machines engines or generators biogas needs to be conditioned we could also call this cleaned the goal is to remove water vapor which we’ve already talked about but more importantly also hydrogen sulfide which is very corrosive and co2 which has no energy value removing hydrogen sulfide is what we call these sulfur ization we can do this with ferrous oxide which then transform the hydrogen sulfide into iron sulfide we can also do this with water scrubbers which at the same time also can remove co2 if we think about biogas tation and appliances actually the most simple way is to use the biogas close to the anaerobic digestion facility for direct combustion for this purpose conditioning of biogas is not necessary a household cooking stove only read needs 202 around 450 litres per our frequently when no direct use of gas is obvious methane is fed into gas generators to produce electricity however with the conversion losses we can only retain about thirty percent of the energy as electricity the remaining is lost as heat besides the gas another product from anaerobic digestion is the digestate in wet digestive systems this is a nutritious slurry with nitrogen phosphorus potassium as well as trace elements fifty percent of this nitrogen is available as ammonium nitrogen that can be directly assimilated by plants so all in all it’s a good fertilizer that can be used in agriculture but remember if it’s in the mesophilic range we’re not having high temperatures so if we have pathogenic substances these are not hygienist if it is not possible to use this digestate in agriculture then the digestate has to be treated before disposal as it still has a high organic load and based on regulations would not be allowed for discharge into surface waters remember no system works without maintenance and care what does that mean for a biogas system we need to regularly check for leaks we can do this with a pressure test looking if the gas pipes are okay we also need to check and unblock pipes if required we don’t need to forget to empty water traps and we need to regularly observe the slurry level in the outlet chamber it gives us a lot of information on the pros remember to not overload the digester otherwise we will have acidification and now and then we need to dislodge the digester typically recommended is every 5 to 10 years and this will be mostly sand and grit which accumulates at the bottom of the digester so now we’ve heard a lot about anaerobic digestion as an option for waste management but when should we actually choose this option well one aspect is that we need access to a well-sorted organic waste source what we also need to think about is that we need local skills and experience in construction and operation then need and demand for gas is a requirement or we need to have a small power grid which we can feed into we also need a competitive chance to compete with other energy sources and this is an economic issue finally we need an institutional environment allowing feeding into the grid especially if we’re converting gas to electricity and ideally we need a demand for the digestate for reuse in agriculture so let me summarize what we covered in this module we looked at biogas dewatering and conditioning we looked at bio gas storage options we looked at gas use and energy content we discussed what issues are necessary to maintain the system and finally we also looked at criteria favoring the choice of using anaerobic digestion for organic solid waste treatment thanks for listening

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