High-rate anaerobic co-digestion of kraft mill fibre sludge and activated sludge by CSTRs with sludge recirculation

Transcription

1 High-rate anaerobic co-digestion of kraft mill fibre sludge and activated sludge by CSTRs with sludge recirculation Eva-Maria Ekstrand Dept. of Thematic Studies - Environmental Studies, LiU, SWEDEN

2 Outline Background Why? Challenges? Methods Establish process Improve efficiency Results/conclusions Eva-Maria Ekstrand

3 3 Why use anaerobic digestion on pulp and paper mill sludges?? Eva-Maria Ekstrand

4 Anaerobic digestion beneficial for the mills Primary sedimentation Dissolved organic matter Aerobic treatment Fibre sludge Activated sludge + Reduce sludge volumes + Produce energyrich methane gas

5 Pulp and paper production a large and important industry in Sweden After Germany, the largest producer of pulp within the EU Third largest exporter of pulp and paper in the world Pulp Paper Sawn wood prod. Million tons Eva-Maria Ekstrand

6 Million ton/year 6 Production of pulp worldwide million ton /year Mechanical Semi-chemical Kraft Sulphite Dissolving Other Process type FAOSTAT, 2011 Eva-Maria Ekstrand

7 NmL CH 4 / g VS High methane potential in kraft fibers NmL CH 4 / g VS Already pre-treated! Kraft fibre Liquid manure, cow Household waste Eva-Maria Ekstrand

8 Challenges Fibre sludge/activated sludge Large wastewater volumes Conventional methods for anaerobically treating particulate substrate require too long retention times Fibre sludge Nutrient-poor Floating? Low degradability of activated sludge

9 Aim Investigate if co-digestion of fibre sludge and activated sludge is possible, at high rate Biogas Fibre sludge Activated sludge

10 Methods Lab-scale (4L) Completely stirred tank reactor with sludge recirculation Mesophilic (37 C), two parallel reactors (R1, R2) 10 Substrate Biogas Reject Thickening of sludge return to reactor Preventing wash-out of microorganisms Centrifugation Prolonging residence time of fibres Sludge recirculation Eva-Maria Ekstrand

11 Methods Phase I Phase I Both reactors were given fibre sludge OLR 3 g VS/L day, HRT 8 days R1 and R2 800 days Phase II Phase II Activated sludge introduced as co-substrate to R2 TS, mixing, nutrient supplementation Phase III Phase III Both reactors running on fibre sludge + activated sludge Increased organic loading rate tested Phase IV Phase IV Decreasing HRT Increasing OLR Ref: Ekstrand, E.-M., et al. High-rate anaerobic co-digestion of kraft mill fibre sludge and activated sludge by CSTRs with sludge recirculation. Waste Management (2016) Eva-Maria Ekstrand

12 Results, Phase I and II Addition of Mg and K stabilized performance Eva-Maria Ekstrand 12

13 Acetic acid [mmol/l] OLR [g VS/L day] Co-digestion and process stability (Phase III) 40,0 35,0 30,0 25,0 20,0 15,0 10,0 5,0 0,0 5,0 4,5 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 0,0 Days Acetic acid R1 Acetic acid R2 OLR R1 OLR R2 R2 (co-digestion) more robust less accumulation of VFA Eva-Maria Ekstrand 13

14 Foaming abated by frequent mixing Starting at: mixing at 4 times per day, duration 15 minutes Gas accumulating in sludge bed Mixing = quick release of gas bubbles Fibres/sludge accumulating Mixing frequency adjusted 20 times per day, duration 4 minutes Less time between mixing events = less accumulated gas = less accumulation of fibre Eva-Maria Ekstrand 14

15 Results, phase IV Lower HRT + higher ORL = smaller reactor/more substrate per time unit Degradation efficiency and methane production maintained Eva-Maria Ekstrand 15

16 Improved degradation of activated sludge Pilot-scale study at kraft mill Increase loading of wastewater = lower sludge age Macro nutrients Ingoing water Outgoing water Aerated basin Sedimentation basin Sludge return WAS (Biosludge) Biogasproduktion inom svensk pappers- och massaproduktion Syntes av möjligheter och begränsningar samt teknisk utvärdering,

17 Summary Higher production capacity Maintained treatment efficiency Primary sedimentation Increased load Aerobic treatment Fibre sludge Activated sludge Higher degradability Methane production Increased methane production

18 Conclusions High-rate AD of fibre sludge and activated sludge was achieved HRT = 4 days, OLR = 4 g VS/L day CH 4 production: 230 ml/g VS Addition of magnesium and potassium was needed Co-digestion stabilized performance Degradability of activated sludge improved Methane production from the fibre sludge at one mill average co-digestion plant in Sweden (about 2.7 MNm 3 CH 4 per year) Ref: Ekstrand, E.-M., et al. High-rate anaerobic co-digestion of kraft mill fibre sludge and activated sludge by CSTRs with sludge recirculation. Waste Management (2016) Biogasproduktion inom svensk pappers- och massaproduktion Syntes av möjligheter och begränsningar samt teknisk utvärdering,

19 Acknowledgements Bo Svensson Annika Björn Madeleine Larsson Jörgen Ejlertsson Anna Karlsson Marielle Karlsson Xu-Bin Truong Fredrik Nilsson Ylva Borgström Energimyndigheten Swedish pulp and paper mills

20 Thank you for listening! Questions?