Abstract

A portable micro-biogas digester was designed to convert household organic waste into biogas, using biological enzyme additives to boost gas yield. In this project, small-scale digesters were built from sealed containers, fed with a slurry of organic waste and inoculum, and fitted with a gas-collection system. Three enzyme boosters (yeast, cellulase enzyme, and lime) were added to separate digesters to test their effect on gas production. Biogas volume was measured daily by water displacement. It is expected that the enzyme-treated digesters will produce gas faster and in greater volume than a control without additives, based on literature showing that bio-enzymatic additives can increase methane yield by 20–60%mdpi.comlink.springer.com. This study demonstrates a simple renewable energy solution: a compact digester that turns waste into fuel, benefiting both the environment and small-scale users.

Background Information

Biogas is a renewable fuel produced by anaerobic digestion (AD) of organic waste. In AD, bacteria break down materials like food scraps, manure or plant waste in an oxygen-free environment, producing a gas mixture rich in methane (CH₄) and carbon dioxidemdpi.com. Methane is the main combustible component of biogasmdpi.com. Biogas can be burned directly for heat or electricity, and using it for cooking or heating reduces reliance on wood and fossil fuelsmdpi.commdpi.com. For example, substituting biogas for firewood closes the carbon cycle and cuts greenhouse gas emissionsmdpi.commdpi.com.

Enzymes and microbes can speed up digestion. Cellulase enzymes hydrolyze (break) the tough cellulose in plant fiber into simple sugarspmc.ncbi.nlm.nih.gov. These sugars are then more easily fermented by bacteria into biogas. Yeast (a fermenting microbe) can also help by converting some sugars into carbon dioxide and alcohol, which bacteria further convert into methane. Studies show that adding biological additives (like enzyme-rich cultures) to the digester can significantly boost methane output. For example, one study reported a ~22% increase in methane yield when a biological enzyme mix was added to a maize silage digestermdpi.com. Another found fungal hydrolytic enzymes improved cellulose breakdown and biogas qualitymdpi.com. Lime (calcium oxide or hydroxide) is not an enzyme but it adds alkalinity, buffering the digester. A stable AD needs nearly neutral pH (around 6.8–7.8), and lime prevents acid buildup from volatile fatty acids. In one experiment, mixing cow dung with limewater (1:1 ratio) increased methane content to about 61%link.springer.com. These facts suggest that a small digester with added yeast, cellulase, or lime should produce more biogas more stably than one without additives.

Problem Statement

Many rural homes and farms generate organic waste (food scraps, crop residue, animal manure) that can pollute the environment if not managed properly. Converting this waste into biogas provides a clean cooking/heating fuel and reduces greenhouse gases. However, traditional biogas plants are large and expensive. This project addresses the challenge of making a portable, low-cost micro-digester for household use, and investigates whether enzyme “boosters” (yeast, cellulase, lime) can improve gas production. The problem is: Can simple enzyme additives significantly increase biogas yield in a small-scale digester?

Hypothesis

Adding enzyme boosters will increase biogas production. Specifically: Digesters treated with yeast, cellulase enzyme, or lime will produce more total biogas (and likely more methane) than an identical digester without these additives. This is based on reports that enzymatic and microbial additives improve the breakdown of organics and raise gas yieldmdpi.comlink.springer.com.

Objectives

• Build a compact, airtight micro-biogas digester suitable for a small farm or household.
• Test the effect of adding yeast, cellulase enzyme, and lime on biogas production.
• Measure and compare the volume of biogas produced under different treatments (with boosters vs. control).
• Evaluate the practical and environmental benefits of the system (waste treatment, renewable fuel).

Variables

• Independent Variable: Type of additive used in the digester. Four treatments are planned: (1) Control (no additives), (2) Yeast booster, (3) Cellulase enzyme, (4) Lime addition.
• Dependent Variable: Volume of biogas produced (measured in milliliters). Optionally, one could test methane content qualitatively (e.g. flammability) or measure pH, but volume is primary.
• Controlled Variables: To ensure a fair comparison, keep these constant for all digesters: same amount and type of organic substrate (e.g. cow dung or kitchen waste), same water content and volume of slurry, same inoculum (starter sludge), same digestion temperature (ambient), same digester size, and same observation period. For example, each digester could use 1 L cow dung and 1 L water (1:1 slurry) with 100 mL starter inoculum, stirred and maintained at room temperature (25°C).

Materials and Equipment

• Digester vessels: 4 airtight plastic containers (e.g. 5–10 L buckets or bottles) with lids.
• Substrate: Organic waste (e.g. chopped vegetable/fruit scraps or cow manure).
• Inoculum: Starter culture from cow dung slurry or existing biogas slurry.
• Water: Clean water to mix with waste.
• Additives:
  • Yeast: Baker’s yeast (Saccharomyces cerevisiae) or yeast culture.
  • Cellulase: Powdered cellulase enzyme (available from biotech supply or fermentation store).
  • Lime: Hydrated lime (calcium hydroxide) or limewater solution.
• Gas collection: Plastic tubing, a plastic pitcher or bucket of water (for water displacement), and an inverted graduated cylinder or gas collection tube to measure gas volume.
• Other: Measuring scale (for substrates), measuring jugs, stirrer or stick, funnel, rubber gloves, safety goggles, pH test strips (optional), thermometer (for ambient temperature).

Methodology / Procedure

1. Prepare Slurry: Chop the organic waste into small pieces. In a bucket, mix a fixed quantity of waste (e.g. 1 kg of chopped kitchen scraps or cow dung) with an equal volume of water (1:1 ratio) and about 10% inoculum (e.g. 100 mL). Stir well to make a uniform slurry.
2. Set Up Digesters: Label four identical airtight containers as Control, Yeast, Cellulase, and Lime. Fill each with an equal volume of the slurry (e.g. 2 L of slurry). Seal each lid tightly.
3. Add Treatments: Immediately add additives to three digesters:
   • Yeast digester: Add a measured dose of yeast (e.g. 5 g baker’s yeast or 1 sachet). Stir gently.
   • Cellulase digester: Add a measured dose of cellulase (e.g. 1–2 g of cellulase powder, as per manufacturer’s instructions). Stir gently.
   • Lime digester: Add enough lime to raise alkalinity (e.g. 5 g of hydrated lime). If using solid lime, dissolve in a little water first to make limewater. Stir gently.
   • Control digester: Add nothing extra.
4. Attach Gas Outlets: Drill or poke a small hole in each lid and insert a short plastic tube (airtight). Lead the tube into an upside-down water-filled container (or attach a balloon) so that any gas produced will bubble into the water and be collected under the inverted measuring cylinder. Ensure no air can escape from the digester except through the tube.
5. Start Digestion: Place all digesters in a warm location (ideally 25–35°C). If ambient temperature is low, use insulation or sunlight to keep them warm, as AD works best in a consistent warm rangemdpi.com.
6. Observe and Measure: Each day for 2–3 weeks:
   • Record Gas Volume: Note the volume of gas collected in each inverted cylinder (in milliliters). Empty the cylinder into water after reading so it is ready for the next day. Also record the daily gas for each digester.
   • Check pH (optional): Measure the pH of each slurry using test strips to see how it changes (especially for lime vs control).
   • Maintain Mix: Every few days, gently stir each slurry to homogenize it (be careful not to let air in).
7. Data Recording: Log daily gas volumes in a table. If possible, plot gas volume (y-axis) versus time (x-axis) for each treatment. The slope and total area under each curve will show production rates and total output.
8. End Experiment: After gas production slows (peak passed), stop. You will have final total gas volumes for each digester.

Safety Considerations

• Biogas Flammability: Biogas contains methane, which is highly flammable. Do not allow open flames or sparks near the digesters or gas collection area. When testing gas quality (if done), do so cautiously.
• Lime Handling: Lime is caustic and can irritate skin or eyes. Wear gloves and goggles when measuring or mixing lime.
• Waste Handling: Wear gloves when handling manure or waste slurry to avoid pathogens. Wash hands thoroughly after the experiment.
• Gas Pressure: Ensure the digester lids are not completely sealed shut to avoid pressure buildup; gas should escape through the outlet only.
• Ventilation: Conduct the experiment in a well-ventilated area to prevent any dangerous build-up of gases (especially hydrogen sulfide, which can form in digesters).

Data Collection Methods

• Volume Measurement: The primary data is the volume of biogas produced each day by each digester. This is measured by the displacement method: when gas bubbles into the inverted cylinder filled with water, it pushes water out. The remaining gas volume is read directly on the graduated cylinder.
• Recording: Use a data table with columns for day number and gas volume for each treatment (Control, Yeast, Cellulase, Lime). For example:

Day	Control (mL)	Yeast (mL)	Cellulase (mL)	Lime (mL)
1	50	80	90	70
2	60	110	130	100
…	…	…	…	…

• Qualitative Observations: Note the time (in days) when gas production begins, peaks, and stops. Record any visible differences in the slurry (smell, texture) if safe to note.
• pH Measurement (optional): If resources allow, use pH strips or meter to check the digester slurry pH at the start and end. Track if lime keeps pH more neutral compared to the control.
• Analysis: From the data, calculate total cumulative gas for each treatment and compare. You may graph daily production curves for visualization.

Expected Results

Based on literature, we expect the enzyme-boosted digesters to outperform the control. Typical results could be:

• Faster Start-Up: Digesters with yeast or cellulase should begin producing noticeable gas sooner, because fermentation of sugars starts early.
• Higher Total Gas: The cumulative biogas volume from the yeast and cellulase treatments should be significantly higher (perhaps 20–50% more) than the controlmdpi.com. This is because yeast and cellulase help break down organics more fully for methane bacteria.
• Lime Effect: The lime-treated digester may produce a steadier rate of gas and avoid early decline (because lime keeps the pH stable). It might also yield gas with a higher methane fraction. One study found cow-dung with limewater gave 58–61% methanelink.springer.com. We expect the total volume might also increase (because bacteria are happier in neutral pH).
• Gas Quality: All biogas should burn with a pale blue flame (confirming methane). The enzyme digesters might show a hotter or steadier flame (indicating higher methane) than control.
  If these expectations hold, the data table will show higher daily and total mL of gas for the treated digesters versus control.

Discussion (Environmental and Practical Impact)

A successful portable micro-digester could have significant benefits. Environmentally, it diverts organic waste from landfills and reduces methane emissions (capturing it for use instead of releasing it into the atmosphere). Using the biogas as cooking or heating fuel cuts down on wood or charcoal use, slowing deforestation and lowering CO₂ emissionsmdpi.commdpi.com. Compared to large biogas plants, a small portable digester is affordable, easy to build, and space-saving, making it practical for rural households or schools.

Adding enzyme boosters makes the system more efficient. If yeast and cellulase greatly increase gas yield, families can get more fuel from the same waste. This could mean cooking fuel for more meals or selling excess gas (or electricity from a generator) locally. Lime buffering ensures the digester runs smoothly, which is important for non-experts.

However, considerations include cost and availability of additives. Baker’s yeast is cheap and widely available; cellulase enzyme might be more expensive and less common, so it would be used only if the extra output is worth it. Lime is also cheap and can be obtained from construction or agricultural stores. The remaining digestate (slurry residue) can be used as a high-quality organic fertilizer, returning nutrients to the soil and closing the nutrient loopmdpi.com.

In summary, a portable digester with enzyme boosters turns waste into clean energy and fertilizer on a small scale. It teaches important lessons about renewable energy, waste management, and microbiology. With careful construction and operation, it provides a sustainable tool to improve energy access and reduce pollution in farming communities.

References

• Mapantsela, Y. et al. (2024). Portable Biogas Digester: A Review. Gases, 4(3), 205–223. (Discusses biogas, AD, and renewable energy benefitsmdpi.commdpi.commdpi.com.)
• Orzechowski, S. et al. (2023). Improving the Energetic Efficiency of Biogas Plants Using Enzymatic Additives. Energies, 16(4), 1845. (Shows enzymes and microbial additives can boost methane yield by 20–62%mdpi.commdpi.com.)
• Tabassum, S. et al. (2025). Investigation on effect of pH level by lime on biogas production. Sustainable Energy Res. (Reports cow-dung mixed 1:1 with limewater gave methane ~61%link.springer.com and improved biogas outputlink.springer.com.)
• Kaur, N. et al. (2021). Cellulases: From Bioactivity to Industrial Applications. Biomolecules, 11, 866. (Explains cellulase enzymes break cellulose into sugars for fermentationpmc.ncbi.nlm.nih.gov.)
• Additional context and data come from biogas and renewable energy sources as cited abovemdpi.commdpi.commdpi.commdpi.compmc.ncbi.nlm.nih.govlink.springer.com.