Effect of wood biochar dosage and re-use on high-solids anaerobic digestion of chicken litter

Highlights

• High dosage of wood-pellet biochar increases methane yield and VFA degradation.

• Addition of re-used biochar improved performance compared with control digesters.

• Biochar addition increases potential annual throughput of batch digesters.

• Lower dosages increases concentration of methanogens on biochar.

Abstract

The use of biochar (pyrolysed biomass) in anaerobic digesters can improve the efficiency of methane production at a low cost. However, the biochar dosage and the ability to re-use biochar affects the viability of this process improvement. This study shows the effects of high dosages of both pristine and re-used wood-pellet biochar on the performance of high-solids anaerobic digesters processing chicken litter. Performance was assessed on a basis of methane yield per gram of volatile solids and the volumetric efficiency. The 90-day methane yield increased by 39% with a pristine biochar dosage of 1 g_TS−char/g_TS−feed but unchanged under pristine biochar at lower dosages and re-used biochar at all three dosages. The use of pristine biochar at the highest dosage increased propionate degradation compared with other treatments. In addition, the highest dosage of pristine biochar causes the largest reduction in lag time compared with the control, a reduction of 35%. Lowering the biochar dosage affects the concentration of methanogens on the biochar. Methanosaetaceae and hydrogenotrophic methanogens had greater concentrations on the biochar at dosages of 0.25 and 0.5 g_TS−char/g_TS−feed. The addition of both pristine and re-used biochar at all three dosages investigated allows for shorter retention times and a greater number of batches to be processed per annum compared with digesters without biochar.

Introduction

Anaerobic digestion is a biological process used for energy generation from organic material. The process produces energy in the form of biogas. Biogas contains methane, carbon dioxide and trace amounts of water vapour, hydrogen sulphide and hydrogen. Biogas can be combusted for heat and electricity generation or upgraded and used for energy storage [1]. Anaerobic digestion is also an appropriate technology for energy generation in rural areas or resource-constrained communities [2].

A variety of organic material can be used as feedstocks for the process. One highly abundant type of organic material is chicken litter. The volume of chicken litter produced worldwide is also increasing. Chicken-meat consumption is growing faster than any other meat [3]. Despite its abundance, chicken litter is not a common feedstock for anaerobic digesters [4]. Problems include a potential to form high concentrations of ammonia which can inhibit the activity of methane-generating microorganisms [5]. In addition, a bedding material is mixed with the manure from farms using deep litter manure management systems such as meat-chicken farms. Piggeries, horse stables and cattle farms can also use deep litter systems [6]. This bedding material can be straw, wood-shavings or rice husk and can clog stirred tank anaerobic digesters [7]. Variations to the anaerobic digestion process and technological advances are needed to allow for methane production from this highly abundant resource.

High-solids anaerobic digestion is a process configuration which has been used for processing manures with bedding material [8,9]. These digesters operate with a total solids (TS) content, a measure of the moisture content in the bulk sludge, $\gtrsim$ 20%. Due to lower water requirements, these digesters are smaller in volume compared with low-solids anaerobic digesters which use a total solids content $\lesssim$ 10%. As a result, a high-solids anaerobic digester has a greater volumetric efficiency (volume-methane/digester volume) than a low-solid anaerobic digester when the process is not inhibited [10]. Problems with high-solids anaerobic digestion include lower methane yields and longer lag times before methane production starts [11]. Efficient methane production from chicken litter in high-solids anaerobic digestion remains a major challenge. Appropriate solutions suitable for high-solids anaerobic digesters in resource-constrained communities are needed.

The addition of biochar to anaerobic digesters decreases lag times before methane production starts, increases cumulative methane yields and increases peak daily methane yields [[12], [13], [14]]. Using biochar is an attractive process improvement, especially for resource-constrained communities, as it can be produced using robust technology such as pyrolysis kilns, gasifiers or earth pits [15,16]. It can also be produced from a wide variety of biomass types such as wood, manure or crop-based agricultural wastes [17]. Furthermore, the use of biochar in anaerobic digesters, in addition to its traditional use as a soil amendment, may make thermal bioenergy production systems such as gasifiers and pyrolysis kilns more economically viable.

One proposed mechanism for biochar enhanced anaerobic digestion occurs through the attachment of methane-generating microorganisms (methanogens) and electron-donating bacteria to biochar. This results in an efficient form of electron transfer between these microorganisms called direct interspecies electron transfer (DIET) [18]. DIET is an alternative to interspecies hydrogen/formate transfer. Both of these are pathways for syntrophic anaerobic oxidation of key intermediate products such as volatile fatty acids, alcohols, and aromatics [19]. Hydrogen/formate transfer is also enhanced due to reduced interspecies distances when cells aggregate [20]. Increased rates of hydrogen/formate transfer through attachment of syntrophic (partnering) microorganisms on biochar is an alternative mechanism for biochar enhanced anaerobic degradation. Both DIET and hydrogen/formate transfer rely on attachment of microorganisms on the biochar and contact with the organic material. Therefore, the beneficial effects of biochar addition is likely to vary with the biochar dosage.

Biochar dosage in an anaerobic digester is defined as the mass of biochar per volume bulk sludge. Alternatively, a dosage on a mass-basis, given as a dry weight biochar to the dry weight feedstock, is used. The dosages used in low-solids anaerobic digesters range from 2 to 15 g/l or 0.4–6.25 g_TS−char/g_TS−feed [13,[21], [22], [23]]. Studies of biochar addition in high-solids anaerobic digesters have used dosages of 2–30 g/l [24,25]. These dosages used in high-solids conditions are expected to be low on a mass-basis due to the substantially lower working volumes. Since performance of anaerobic digesters increases with wood biochar dosage up to 3 g_TS−char/g_TS−feed in low solids conditions [22], there is potential for improvements in high-solids anaerobic digesters by using higher than previously used biochar dosages.

The addition of biochar into anaerobic digesters will have an associated cost. This cost will increase with higher biochar dosages. A method to lower the associated costs in batch-phase high-solids anaerobic digesters is by recovering biochar from the digestate at the end of the batch. The biochar can then be re-used in a subsequent batch. In both high and low-solids anaerobic digesters, the re-use of biochar can increase methane yields and reduce lag times to a greater degree compared with the addition of pristine biochar. In HSAD, re-used biochar dosages of 1 g_TS−char/g_TS−feed has improved methane yields by up to 69% [26]. While in LSAD, re-used biochar dosages of 15–20 g/L has a similar performance to the use of pristine biochar [27]. However, this has been previously only shown for one biochar dosage. It is unclear if lowering the dosage of re-used biochar will decrease its beneficial effects on methane production performance from high-solids anaerobic digesters. It is possible that inactive microorganisms attached to the re-used biochar can prevent attachment of active methanogens and limit the effectiveness of biochar re-use. This detrimental effect may be exacerbated at lower re-used biochar dosages. However, this hypothesis is yet to be investigated. A benefit of using lower quantities of re-used biochar is this will lower the required efficiency of the biochar recovery system.

This study reports the impact of dosage levels of both pristine and re-used biochar produced from wood-pellets on the performance of high-solids anaerobic digesters processing chicken litter. The objectives are to determine the effects of; (1) the dosage of both pristine and re-used biochar on methane yield, methane production rate and volumetric efficiency and (2) pristine or re-used biochar dosage on the chemical conditions and population of methanogens in the bulk sludge or attached to biochar.