Thermophilic anaerobic digestion for increased biogas production and pathogen control

Transcription

1 South Baltic Gas Forum 5-8 September 2011, Gdańsk, Poland Thermophilic anaerobic digestion for increased biogas production and pathogen control Bjarne Paulsrud Beata Szatkowska Aquateam AS

2 Thermophilic anaerobic digestion. Content of presentation Definitions Potential objectives Basic principles for improved patogen inactivation Application of thermophilic anaerobic digestion in different countries Process performance and operational experiences Summary

3 Thermophilic anaerobic digestion. Definitions Mesophilic anaerobic digestion: Operating at o C in the digester Thermophilic anaerobic digestion: Operating at 50 o C in the digester but increasing to 55 o C if sludge hygienisation (pathogen inactivation) is an objective

4 Thermophilic anaerobic digestion. Potential objectives Improved pathogen inactivation (hygienisation), depending on the pathogen control criteria in each country Increased degradation of organic matter in sludge, resulting in: Increased biogas production Reduced amount of sludge to be disposed of Increased capacity of existing digesters or reduced digester volume for new digesters.

5 Thermophilic anaerobic digestion. Basic principles for improved pathogen inactivation (hygienisation) Hygienisation is achieved by exposing every incoming sludge particle to thermophilic temperatures for a certain time period (the exposure time) Using batch or semi-continous operation of thermophilic digesters. Changing the operation mode of the digester(s) from fill-and-draw to drawand-fill will eliminate the risk for short-circuiting Necessary exposure time in the digester for each batch will depend on the operating temperature and the hygienisation requirements. Current requirement in U.S.: 24 hours at 55 o C. Requirement in Norway: 2 hours at 55 o C in order to inactivate helminth eggs (Ascaris suum) When operating digesters in series, only one of the digesters (preferably the first one) needs to be operated thermophilic if the draw-and-fill mode is employed for that digester

6 Thermophilic anaerobic digestion. Process performance and operational experiences Based upon data from U.S., Norway, the Chech Republic, Sweden, Denmark and Germany with a broad range of operating conditions. Most plants have been converted from mesophilic to thermophilic operation, and many plants in the U.S. and Norway are employing the draw-and-fill mode to improve pathogen inactivation and achieve controlled hygienisation. Thermophilic anaerobic digestion (TAD) can increase the reduction of organic matter (volatile solids) by about 20 % and thereby increasing the biogas production by ~20 %, compared to mesophilic anaerobic digestion (MAD).

7 Thermophilic anaerobic digestion. Process performance and operational experiences, continued The amount of total solids (TS) for final disposal can be reduced by % compared to MAD operation, and by increased TS content of dewatered sludge (improved dewaterability), the total sludge volume to be disposed of can be reduced by % The percentage of volatile solids reduction can be further increased by pre-treatment of the sludge before thermophilic digestion, employing: Disintegration of waste activated sludge Enzymatic and/or thermal pre-treatment Chemical and/or thermal hydrolysis Many of the proposed pre-treatment methods are still lacking reliable data from full scale operation

8 Thermophilic anaerobic digestion. Process performance and operational experiences, continued Improved dewaterability and reduced foaming are experienced with most TAD plants. Process stability is not a problem with good process control (frequent analysis of volatile fatty acids (VFA) and alkalinity) Strong odours from TAD sludge at the higher temperatures. Cooling of sludge necessary prior to subsequent treatment, and the first step should be heat exchanging of hot digested sludge with cold raw sludge fed to the digesters. This will also improve the energy balance of the process Increased water content in the biogas from TAD plants may require improved water removal, depending on gas utilization

9 Thermophilic anaerobic digestion. Cost estimates compared to MAD Few data on investment costs of new TAD plants, but they should not differ much from the investment cost of similar MAD plants Converting from MAD to TAD normally involves fairly low investment costs (heat exchangers, boilers, sludge pumps, some piping and valves, etc.) Operation costs nearly unchanged when treating the same amount of sludge. Increased energy consumption is balanced by increased biogas production (provided utilization of all the gas produced) and reduced amounts of sludge for dewatering and final disposal

10 Application of thermophilic anaerobic digestion in some countries U.S.A Many plants converted from mesophilic to thermophilic operation over the last 15 years Hyperion wwtp in Los Angeles Terminal Island wwtp in Los Angeles Columbus wwtp in Columbus,Ga Blue Plains Advanced wwtp in Washington D.C Main objective of conversion is to comply with Class A standards for pathogen control in sludge (biosolids) A lot of digester process configurations have been developed to comply with the Class A standards, including parallel and in series combinations as well as thermophilic and mesophilic combinations

11 Application of thermophilic anaerobic digestion in some countries The Chech Republic Three wastewater treatment plants have converted their digesters to thermophilic operation in the last 10 years Prague Central wwtp is the biggest one (5,8 m³/s) with 12 digesters of 4800 m³ each (6 primary and 6 secondary digesters) and only the primary digesters are heated to 55 C The main objectives of thermophilic operation are Increasing capacity of existing digesters Increasing the biogas production for increased combined heat and power generation

12 Application of thermophilic anaerobic digestion in some countries The Chech Republic Prague Central wwtp Q m³/s PE 12

13 Application of thermophilic anaerobic digestion in some countries The Chech Republic Prague Central wwtp Gasholder II. Stage 52 o C I. Stage Mixed heated 55 o C

14 Application of thermophilic anaerobic digestion in some countries Norway Five wastewater treatment plants have thermophilic operation of their digesters, and the Bekkelaget wwtp in the City of Oslo (max capacity 4,0 m³/s) was designed for thermophilic operation and put in operation 10 years ago with 2 digesters of 4000 m³ volume each Several more wwtp s are in the planning or implementation phase of converting from mesophilic to thermophilic operation of their digesters Main objectives for the conversion are Complying with sludge pathogen standards similar to those in the U.S Increasing the biogas production for increased production of heat and electricity and also the production of biofuels for vehicles (public transport)

15 Application of thermophilic anaerobic digestion in some countries Norway Bekkelaget WWTP, City of Oslo Oslo City Hall

16 Application of thermophilic anaerobic digestion in some countries Norway Bekkelaget WWTP Gas holding tank Administration building, lab., workshops,

17 Application of thermophilic anaerobic digestion in some countries Norway Nordre Follo wwtp

18 Summary Benefits of TAD compared to MAD Biogas production increased by ~ 20% The amount of total solids (TS) for final disposal reduced by 10-15% The sludge amount (tonnes/year) to be disposed of reduced by 25-30% Improved dewaterability (higher TS content in sludge cake) Factors that need to be addressed Cooling of TAD sludge necessary prior to dewatering to avoid odour problems Increased water content in biogas may require improved moisture removal

19 Thermophilic anaerobic digestion for increased biogas production and pathogen control Thank you for the attention!