# Session 2: Sizing Your Digester ## Learning Objectives By the end of this session, you will be able to: 1. Understand why correct sizing is critical. 2. Determine the amount of available feedstock and calculate the daily feeding rate. 3. Understand retention time and its impact on biogas production. 4. Perform basic calculations to determine slurry and gas volume. 5. Select an appropriate digester size. --- ## Case Study: Sizing a Digester *Sanile walks through how to size a biodigester correctly for a typical farm.*
--- ## Why Does Sizing Matter? It is vitally important that the digester is sized correctly: - **Too small** → not enough gas for your needs. - **Too large** → excess gas vents to atmosphere. Methane has **20× the global warming potential of CO₂** — venting it is actively harmful. Start with what the farmer wants to *use* the biogas for, then work backwards to determine the size needed. --- ## Step 1: Assess Your Feedstock and Gas Needs Start by answering two simple questions: **1. How much waste do you produce each day?** (e.g. number of cows × kg of dung per day) **2. How much gas do you want to use?** (e.g. cooking time per day) ```{tip} A simple guide: one gas stove uses about **0.5 m³ of gas per hour**. So cooking for **2 hours per day** requires approximately **1 m³ of gas per day**. ``` Compare what you *have* (available feedstock) against what you *need* (desired gas use). ```{raw} html

⚖️ Supply vs Demand — Does your feedstock meet your gas needs?

Move the sliders to match your situation, then see if supply and demand balance.

🐄 Supply — Daily Dung

≈ 20 kg dung/day → 0.8 m³ gas/day
🔥 Demand — Daily Cooking

Needs: 1.0 m³ gas/day
SupplyDemand
``` --- ## Step 2: Daily Feeding Rate A biogas plant must be fed **daily** with a consistent amount of feedstock mixed with water to form a slurry. **General mixing ratio:** 1 bucket of cow dung : ½ bucket of water (or 1:1 where water is plentiful). The material must remain inside the digester long enough for complete breakdown — typically **40 days** at 30–40°C. The larger the daily feeding amount, the larger the digester must be. --- ## Step 3: Calculate Slurry Volume (Vs) Inside the digester there are two parts: - **Slurry volume (Vs)** — the liquid mixture of waste and water - **Gas volume (Vg)** — the space where biogas collects **Total Volume = Slurry Volume (Vs) + Gas Volume (Vg)** The slurry volume formula is: ``` Vs = Daily slurry input (m³) × Retention time (days) ``` ### Worked Example **Given:** 20 kg of cow dung per day, 1:1 mixing ratio, 40 days retention time. | Step | Calculation | Result | |---|---|---| | Total daily slurry | 20 kg dung + 20 kg water | **40 kg/day** | | Convert to volume (1000 kg = 1 m³) | 40 ÷ 1000 | **0.04 m³/day** | | Multiply by retention time | 0.04 × 40 | **Vs = 1.6 m³** | --- ## Step 4: Calculate Gas Volume (Vg) Cow dung produces approximately **0.04 m³ of biogas per kg of dung added** per day (at 30–40°C with 40-day retention). ``` Vg = 0.04 m³/kg × Daily dung input (kg) ``` For the example above (20 kg/day): ``` Vg = 0.04 × 20 = 0.8 m³/day ``` Because gas is used daily, you only need to store about **⅔ to 1 day's worth of gas production**. ```{figure} ../assets/gas-volume-how-it-works.png :alt: Gas volume — how it works, daily gas production, storage in the digester, and calculating Vg :width: 100% How gas volume works: daily production rate, storage in the bag, and the Vg = 0.04 × daily dung input formula. ``` --- ## Step 5: Total Digester Volume ``` Total Volume = Vs + Vg ``` For our example: **1.6 + 0.8 = 2.4 m³** ```{figure} ../assets/image14.png :alt: Digester cross-section showing Vs, Vg, inlet height (Hi) and outlet height (Ho) :width: 90% Cross-section diagram of the CE Expanding Bag Digester showing slurry volume (Vs), gas volume (Vg), and the critical inlet/outlet height relationship. ``` ```{raw} html

🧮 Interactive Sizing — Steps 3, 4 & 5

Adjust the sliders to see how Vs, Vg and total digester volume change in real time.

🐄 Daily dung input
20 kg/day
💧 Water ratio
1 part water : 1 part dung
⏱ Retention time
40 days
Step 3 — Slurry Volume (Vs)
Dung + water20 + 20 kg
Daily slurry40 kg = 0.040 m³
× retention time× 40 days
Vs = 1.60 m³
Step 4 — Gas Volume (Vg)
Formula0.04 m³ × kg/day
Daily dung20 kg/day
Gas storage~1 day's production
Vg = 0.80 m³
Step 5 — Total Volume = Vs + Vg
Vg Vs gas ↑ ↓ slurry
Vg (gas) = 0.80 m³
Vs (slurry) = 1.60 m³
Total = 2.40 m³
Volume split: 33% gas · 67% slurry
``` ### Quick Reference: Common Digester Sizes | Name | Total Volume | Cows needed | Typical use | |---|---|---|---| | Five Cow | ~2 m³ | ~5 cows | Cooking twice daily on one stove | | Eight Cow | ~4 m³ | ~8 cows | Cooking + some lighting | | Twenty Cow | ~8 m³ | ~20 cows | Cooking + water heating | ```{figure} ../assets/feedstock-worksheet.jpg :alt: Feedstock and Biogas Needs Worksheet — daily feedstock available, current energy use, target biogas use :width: 90% Use this worksheet to record your daily feedstock, current energy use, and target biogas needs before sizing your digester. ``` --- ## Session 2 Quiz ```{raw} html

MC Q1. Why is it harmful to oversize a biogas digester?

Number Q2. How many cubic metres (m³) of biogas does a single stove consume per hour?

Enter a decimal, e.g. 0.5

True / False Q3. The typical retention time for cow dung in a biogas digester is 40 days.

Number Q4. A farmer adds 10 kg dung + 10 kg water daily (1:1 mix) with a 40-day retention time. Calculate Vs in m³.

Vs = daily slurry input (m³/day) × retention time (days). 1 kg liquid ≈ 1 litre = 0.001 m³.

Fill in Q5. Using Vg = 0.04 m³ × kg of dung per day, what is the daily gas volume for 10 kg of dung? (m³/day)

Enter a decimal number.

MC Q6. Why should you start with what the farmer wants to use the biogas for when sizing a digester?

``` --- ## Summary - Size the digester based on **available feedstock** and **desired gas use**. - Use the formula: **Vs = daily slurry input × retention time** - Gas volume: **Vg = 0.04 m³ × daily kg of dung** - Total volume = Vs + Vg In the next session we will learn how to **site** your digester correctly.