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Valorization of biosolids for the production of electric energy by the microsieving-gasification methods

Manali Anthoula

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Summary

The ineffective operation of WasteWater Treatment Plants (WWTP) due to overloading, in combination with the high energy demands of extended aeration plants, as well as the unsatisfactory management of biosolids, are considered as important problems, which could be solved through an innovative system for the removal of suspended solids from wastewater before biological treatment and subsequent valorization of the produced biosolids.Such a pilot plant, with capacity of 5.000 m3/d of incoming wastewater, has been installed at the WWTP of Rethymno, consisting of: partial solids removal upstream of the aeration tank through microsieving, moisture removal of the produced microsieved biosolids through drying, syngas production from the biosolids through gasification, and electric and thermal energy production from syngas combustion in an Internal Combustion Engine (ICE), for the coverage of energy needs of the system.In the framework of the present doctoral thesis, except the highlighting of the energy problem of activated sludge WWTPs, the technologies of the suggested method along with their current applications and the potential of their wide apply in WWTPs are examined and evaluated, investigating thus the valorization of wastewater energy content. Specifically, the major goals of this thesis are: (i) the study of the profile of Greek WWTPs, in order to identify their energy problem, (ii) the design, manufacture, and optimization of the aforementioned pilot plant, (iii) the efficiency examination of microsieving and gasification, (iv) the calculation of the critical size above which a microsieving - gasification plant upstream of a conventional activated sludge WWTP will have a positive energy balance, as well as (v) the characterization of the produced biosolids, the determination of by-products composition and the proposal of their suitable management.The results of the studies conducted lead to the conclusion that the suggested method of wastewater pre-treatment and produced biosolids valorization could be a solution for the environmental and economic malaises which characterize the modern conventional WWTPs. Specifically, about 30 % of solids is removed from wastewater through microsieving, while around 8 kg/h of microsieved biosolids (on dry basis) are produced, with high organic content (TS 36 ± 2 %, VS 89,6 ± 0,69 % of TS) and remarkable energy content (HHV 21,479 ± 1,434 MJ/kg), characteristics which make them ideal feedstock for thermal treatment methods and suitable for further energy valorization. Regarding the energy autonomy of the plant, the calculations of mass and energy balances showed that it may not be possible to achieve complete autonomy in the present application, but there are plenty of alternatives which lead to positive result with minor modifications, easily adaptable to the pilot plant for future applications. Also, based on the balances, it appears that systems with higher capacity have an energy advantage compared to smaller ones due to “economy of scale”, as they consume less per unit of inlet mass. Finally, based on the inputs - outputs determination, the environmental footprint assessment as well as the Life Cycle Assessment (LCA) of the pilot plant, the application of the proposed method showed an environmental improvement compared to usual methods applied to the conventional WWTPs.

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