The use of biochar as co-feedstock in anaerobic digestion: Effects on digestate separation performance

© Luke The use of biochar as co-feedstock in anaerobic digestion: Effects on digestate separation performance Elina Tampio, Niina Honkala, Johanna Laakso, Ville Pyykkönen (Natural Resources Institute Finland, Luke) Satu Tiainen, Katja Heikkinen, Elli Hakala (Häme University of Applied Sciences, HAMK) RAMIRAN 2025 –19th international conference, 15– 17 October 2025, Wageningen, NL © Luke Background •Anaerobic digestion at farm-scale enhances energy sufficiency and nutrient balance •Treatment of the digestate with e.g., mechanical separation is often required due to regional nutrient surpluses and to facilitate digestate handling •There is a need for improved solutions to enhance digestate separation •Commonly used flocculants, such as synthetic polymers, have low biodegradability, and are primarily fossil-based •This raises concerns about the potential spread of microplastics and highlights the need for bio-based solutions •Biochar has been reported to enhance anaerobic digestion performance, but its effects on digestate characteristics and separability remain underexplored •Our aim: To investigate the use of locally-sourced biochar as a co-feedstock in anaerobic digestion, with a focus on its impact on digestate separability. © Luke Materials & methods (1/2) •Horse manure was pyrolyzed in a small-scale mobile unit •350–550 °C for 3.5 hours •Ground biochar was used as a co-feedstock in anaerobic digesters processing cow slurry and grass •5 L lab-scale reactors •Mesophilic conditions (37 °C) •One reactor as control •Biochar added in two mass concentrations •1% and 3% •OLR increased from 4.5 to 7.8 kgVS/m3d Horse manure (TS 33%) Pyrolysis Anaerobic digestion Cow slurry (TS 7%) & grass (TS 28%) Sieving & centrifugation Solid fraction Liquid fraction Digestate Biochar © Luke Materials & methods (2/2) •Separation of the digestate •Sieving with sieve sizes 6.3 mm, 4 mm, 3.15 mm, 2 mm •Centrifugation of sieved liquid fraction with Lemitec MD centrifuge •10,000 rpm, differential speed 200 rpm, weir disc 52 •Produced solid and liquid fractions analysed for their chemical characteristics (nutrients, carbon) •Carbon stability of the solid fraction also assessed •Mass balances calculated based on the TS content of the fractions -> nutrient separation efficiencies Horse manure (TS 33%) Pyrolysis Anaerobic digestion Cow slurry (TS 7%) & grass (TS 28%) Sieving & centrifugation Solid fraction Liquid fraction Digestate Biochar © Luke Pyrolysis and anaerobic digestion with biochar •The slow-pyrolysis of horse manure produced biochar with 62% of C •H/C ratio was 0.40, O/C ratio 0.13 TS (%) VS (%) C (%TS) H (%TS) N (%TS) S (%TS) O (%TS) Biochar 90.6 69.1 61.9 2.1 1.2 0.2 10.9 Grass 27.8 25.0 46.2 5.8 2.5 0.2 35.4 Cow slurry 7.4 6.1 45.1 5.5 3.1 0.4 27.6 Biochar (0%,1% or 3% of feed) Grass (4-15% of feed) Cow slurry 0 50 100 150 200 250 300 350 050 100 150 200 litre biogas /kgVS time, days beginning Biogas production Control 1% biochar 3% biochar •The digestion process was not affected by the biochar use •No effects on gas production •H2S levels remained lower with biochar-added digesters © Luke Biochar effects on the digestate •Addition of biochar as a co-feedstock increased the total solids (TS) and carbon content of the digestate. •Milder effect on nutrients TS (%) C (g/kg) N (g/kg) P (g/kg) Digestate, control 8.3 ± 0.04 34.5 ± 0.56 3.9 ± 0 0.8 ± 0 Digestate, 1% biochar 9.0 ± 0.03 40.2 ± 0.27 4.1 ± 0.07 0.82 ± 0.03 Digestate, 3% biochar 10.3 ± 0.06 50.1 ± 0.49 4.1 ± 0.07 0.77 ± 0.07 0% 20% 40% 60% 80% 100% Digestate, 3% biochar Digestate, 1% biochar Digestate, control Carbon stability A W E N •Carbon fractions were analyzed using AWEN-fractionation (acid, water, ethanol, non-soluble fractions) •Results indicate reduced carbon degradation in soil -> higher C sequestration potential •The share of stable (non-soluble) C in the digestate increased by up to 15% due to biochar addition © Luke Separation of the digestates with sieving and centrifuging •2-phase separation •Sieves & centrifuging •The first processing step removed majority (51-61%) of solids and carbon •Material passing the sieves was further centrifuged Mass 39-45% TS 16-17% N 32-34% P 8-12% C 13-17% Mass 12-15% TS 22-33% N 19-27% P 37-52% C 23-36% Mass 39-49% TS 51-61% N 40-49% P 42-49% C 51-63% Sieving Centrifuging Solid fraction from sieves Liquid fraction from centrifuge Solid fraction from centrifuge © Luke 0 10 20 30 40 50 60 70 Mass TS N NH4-N P PO4-P C Solid fraction from sieves Separation efficiency (%) Digestate, control Digestate, 1% biochar Digestate, 3% biochar Sieved solid fraction •In sieved solid fractions, the mass, total solids (TS), and nutrient recovery were lower in digestates containing biochar compared to the control •A larger proportion of the digestate passed through the sieve, ending up in the liquid fraction •This result was contrary to expectations, as biochar was anticipated to enhance solid separation •Biochar was ground into a fine powder, allowing it to pass through the sieve and accumulate in the liquid fraction •This contributed to the lower mass, TS, and nutrient recovery in the solid fraction of digestates with biochar •Additionally, biochar may have enhanced the degradation of particulate matter in the feedstocks during anaerobic digestion •->leading to finer particles and greater solubilization © Luke Centrifuged solid fraction •Centrifuging the sieved liquid fraction enabled recovery of total solids (TS), nutrients, and carbon into the solid phase •Biochar acted as a separation aid •Especially soluble-P recovery was improved •A higher share of nitrogen (N) was also retained in the solid fraction 0 20 40 60 80 100 Mass TS N NH4-N P PO4-P C Solid fraction from centrifuge Separation efficiency (%) Digestate, control Digestate, biochar 1% Digestate, biochar 3% •The use of biochar decreased the density of the centrifuged solid fraction 0 200 400 600 800 Digestate, control Digestate, 1% biochar Digestate, 3% biochar Density (g/L)