Methane exiting upflow anaerobic sludge blanket (UASB) reactors (operating at standard conditions) dissolved in the liquid phase constitutes a significant proportion of the total quantity of methane that is evolved in the reactors. This significantly affects the energy balance and greenhouse gas (GHG) emissions of the reactors.
Initially a method for direct measurement of the liquid phase concentration of methane in the wastewater exiting the UASBs at Hammarby Sjöstads Reningsverk pilot plant is developed. Results from the measurements show that the wastewater has a methane concentration of between 2.5x10-5 and 3.8x10-5 [mol fraction], corresponding to about 120 to 180 % of the theoretically predicted saturation concentration given conditions in the UASB headspace.
Systems are also designed to recover and oxidise the methane from wastewaterexiting UASB reactors in a proposed full-sized local wastewater treatment plant for Hammarby Sjöstad, handling wastewater from 15,000 person equivalents, and with a COD equal to that of wastewater treated at Hammarby Sjöstads Reningsverk pilotplant.
Dimensioning, costing and performance analysis procedures are developed for two main types of system; packed tower counter-current cascades and bubble column counter-current cascades. For each type of system, two distinct cases are considered: One where the methane concentration in the gas phase exiting the respective cascadesis 0.0125 [mol fraction]. For this case, a regenerative thermal oxidiser (RTO) with high heat exchange efficiency is used to oxidise the methane. Secondly, systems are designed where the methane concentration in the gas phase exiting the respective cascades is 0.28 [mol fraction] up to 0.41 [mol fraction]. For this latter case, no specific final oxidation step is considered.
It is shown that all systems considered are capable of reducing the quantity of methane released to the atmosphere by at least 97%, compared to the situation where no methane recovery and oxidation system is considered. It is furthermore shown that it is technically and economically feasible to do so and that a high proportion of the energy content of the recovered methane can be utilised.
Packed tower cascades desorbing to an exiting gas phase methane concentration of 0.0125 [mol fraction] with subsequent oxidation in an RTO are capable of performing the entire process at a total cost of 0.47 [SEK/m3 wastewater] and process energy use of 4.6 [kJel/s] or 127 [kJel/m3 wastewater]. Total cost for equivalent systems desorbing with bubble column cascades are very similar, though process energy use is roughly an order of magnitude greater.
Combustion of recovered methane at the low exiting gas phase concentrations considered in these systems in the RTO yields an energy output of 47 [kJ thermal/s] or 1.3 [MJthermal/m3wastewater] in the form of flue gas at 340 [oC]. This energy output maybe used for process or space heating purposes.
Packed tower and bubble column systems for desorption to high exiting gas phase methane concentrations are at least 20% more expensive and have a process energy demand about an order of magnitude greater than the respective low exiting gas phase concentration systems described above. These systems have an advantage over low exiting gas phase methane concentration systems because the high concentration methane output has the possibility to be upgraded to vehicle fuel, increasing the total environmental benefit of the system by using evolved methane as a direct replacement for a fossil fuel.
Stockholm, 2006. , 174 p.