Pretreatment of straw for enhanced biogas production

Abstract

Wheat straw is known for its complex lignocellulosic structure when it comes to its conversion to biogas using anaerobic digestion (AD). This limitation comes from the restriction of microbial hydrolysis, which results in low methane yields. For this reason, pretreatments have been studied for the purpose of enhancement of biodegradability of straw before proceeding with anaerobic digestion.
This study evaluated and compared the effect of different pretreatments on the biochemical methane potential (BMP) of wheat straw. These include Ca(OH)₂ submerged pretreatment, mechanical pretreatment through briquetting and extrusion, and spray alkaline pretreatment with NaOH and Ca(OH)₂ at low (0.5–2.0%) and high (5–15%) alkali dosage levels based on straw dry matter. All experiments were conducted using the Automatic Methane Potential Test System (AMPTS III) at BPC Instruments, Lund, Sweden, under mesophilic conditions at 37°C.
Ca(OH)₂ submerged pretreatment at 3% concentration enhanced early methane production by approximately 69% at day 5 compared to untreated straw. However, this pretreatment did not improve final methane yield. This is likely because some of the organic matter was lost during the washing step. The most optimal duration of pretreatment was found to be 24 hours, as longer durations did not yield improved results. Mechanical pretreatments like briquetting and extrusion proved to be insufficient, as lignocellulosic recalcitrance cannot be combatted only through size reduction, in the improvement of methane yield compared to untreated straw. Spray alkaline pretreatment at low dosage levels (0.5–2.0%) was showed similar results for both NaOH and Ca(OH)₂, validating the fact that alkali loading is the critical limiting factor under spray conditions, and not duration of pretreatment.
The most remarkable result was given by the NaOH spray pretreatment at 15% dosage. This condition of pretreatment achieved an 89.1% increase in early methane yield compared to untreated straw at day 6. On the other hand, Ca(OH)₂ spray pretreatment at equivalent high dosages showed significantly more limited improvements. These limited results are most likely attributed to the lower solubility of Ca(OH)₂, which is restricting the hydroxide ion availability. Therefore, alkali dosage is the critical factor when it comes to spray pretreatment efficiency. NaOH spray pretreatment represents a promising approach for enhancing wheat straw biodegradability for biogas production when the pretreatment is performed at a sufficiently high dosage.
This study provides practical insights into the selection of pretreatment strategies for wheat straw, a widely available agricultural residue with significant potential for renewable energy generation. Chemical spray pretreatment at sufficient alkali dosage was identified as a simple and scalable approach that can nearly double early methane yields without requiring water, washing equipment, or complex infrastructure, supporting the development of low-cost pretreatment solutions for industrial biogas production from lignocellulosic agricultural waste.

Popular Abstract

Every year, millions of tonnes of wheat straw are left in fields across Europe with little use. What if this agricultural waste could be converted into renewable gas? This study shows that spraying straw with a common household chemical can nearly double its ability to produce biogas.
Biogas is a renewable energy source produced when microorganisms break down organic material in the absence of oxygen. This process is called anaerobic digestion. It can be used to generate heat and electricity, or upgraded to biomethane and injected into the natural gas grid. Agricultural residues like wheat straw are an attractive feedstock for biogas production because they are abundant, cheap, and widely available. However, straw is incredibly tough to break down. Its rigid structure makes it resistant to the microorganisms that produce biogas. This is where pretreatment comes in.
This study tested a wide range of pretreatment strategies to find out which ones are most effective at exposing the energy trapped in wheat straw. The approaches tested ranged from soaking straw in chemical solutions, to physically compressing it into briquettes, to simply spraying it with an alkaline liquid and leaving it to rest.
Most methods showed little to no improvement. Turning straw into briquettes or pushing it through an extruder had almost no effect on how much biogas it produced. But, the real surprise came from the spray pretreatment experiments. When straw was sprayed with sodium hydroxide at a high enough dose (15% based on the dry weight of the straw), methane production nearly doubled compared to untreated straw in the early stages of digestion. The key discovery was that it is not the type of chemical or the duration of treatment that matters most, but simply how much chemical is applied. Low doses achieved almost nothing; high doses delivered dramatic results.
This finding is practically important because the spray approach avoids the need to wash the straw after treatment, which, otherwise, wastes water and washes away valuable organic compounds. It also requires no special equipment beyond a spray bottle and a storage area, making it a potentially simple and scalable solution.
The study was conducted at BPC Instruments in Lund, Sweden, using a specialized automated system that continuously measured methane production.
The results suggest that high-dose sodium hydroxide spray pretreatment could be a practical and effective strategy for increasing biogas production from wheat straw. Future work should focus on optimizing the process at larger scale and evaluating its economic viability for real biogas plants.

• author: Nicolae, Ema Andreea
• supervisor: Mihaela Nistor; Martin Hedström
• organization: Biotechnology (MSc); Biotechnology (M.Sc.Eng.)
• course: KBTM01 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: wheat straw, anaerobic digestion, biochemical methane potential, alkaline pretreatment, spray pretreatment, NaOH, Ca(OH)₂, lignocellulosic biomass, biotechnology
• language: English
• id: 9240695
• date added to LUP: 2026-08-11 09:05:01
• date last changed: 2026-08-11 09:05:01