A development organisation has installed a community biogas plant in Village M in western Kenya. The plant converts cattle dung and crop waste from local fa...
Question 1 Report
A development organisation has installed a community biogas plant in Village M in western Kenya. The plant converts cattle dung and crop waste from local farms into biogas for cooking and lighting. The village has a population of approximately 3 500 people across 480 households. Before the biogas plant was installed, 85% of households relied on firewood collected from surrounding woodland for cooking. Women and children spent an average of 3 hours per day collecting firewood, and deforestation in the area had increased soil erosion and reduced water quality in the nearby River Ondo.
Fig. 1 shows a simplified diagram of the biogas digester system used in Village M.
Table 1 compares the costs and environmental impacts of three cooking fuel options available to households in Village M.
| Fuel type | Monthly cost per household (USD) | CO2 equivalent emissions (kg per month) | Indoor air pollution level | Availability |
|---|---|---|---|---|
| Firewood | 0 (collected free) | 45 | High | Declining (deforestation) |
| Kerosene | 12 | 38 | Medium | Purchased from town (25 km) |
| Biogas | 3 (maintenance fee) | 8 | Very low | Produced locally from farm waste |
The biogas digester works by anaerobic digestion. Cattle dung and crop waste are loaded into a sealed tank where bacteria break down the organic matter in the absence of oxygen, producing biogas. The gas, which is mainly methane, is collected from the dome above the tank and piped to households. The leftover material, called digestate, is rich in nutrients and is used as fertiliser on crop fields.
The plant currently serves 120 households but has a maximum capacity of 200 households. Some farmers are reluctant to participate because they currently spread cattle dung directly on their fields and are concerned that diverting it to the digester will reduce their soil fertility.
(a) State the main gas produced during anaerobic digestion in the biogas plant. [1]
(b)(i) Using Table 1, state which fuel type produces the highest CO2 equivalent emissions per month. [1]
(b)(ii) Using Table 1, describe one advantage of biogas over kerosene as a cooking fuel. [1]
(c) Explain how switching from firewood to biogas could reduce deforestation in the area around Village M. [2]
(d) Using the diagram and information provided, describe how the biogas digester produces gas from organic waste. [3]
(e) Suggest how the digestate produced by the biogas plant could benefit local farmers. [2]
(f)(i) Explain why some farmers are reluctant to supply cattle dung to the biogas plant. [1]
(f)(ii) Suggest one way this concern could be addressed. [2]
(g) Using Table 1, calculate the difference in CO2 equivalent emissions between a household using firewood and one using biogas. [1]
(h) Using all the information provided, suggest two social benefits that the biogas plant has brought to the people of Village M. [4]
Answer Details
(a) Main gas produced during anaerobic digestion: [1]
Methane (CH\(_4\)) [1]. Methane is the combustible component of biogas that provides the energy for cooking and lighting when burned.
(b)(i) Fuel type with the highest CO\(_2\) equivalent emissions (from Table 1): [1]
Firewood (45 kg CO\(_2\) equivalent per month) [1].
(b)(ii) One advantage of biogas over kerosene: [1]
Biogas costs only $3 per month (a maintenance fee) compared to $12 per month for kerosene, making it significantly cheaper and more affordable for low-income households [1].
(Other valid advantages: biogas produces far lower CO\(_2\) emissions (8 vs. 38 kg/month); produces very low indoor air pollution vs. medium for kerosene; is produced locally from farm waste rather than requiring a 25 km trip to purchase.)
(c) How switching from firewood to biogas could reduce deforestation: [2]
- If households switch from firewood to biogas for cooking, they will no longer need to cut down trees and collect branches from surrounding woodland for fuel. The 85% of households previously relying on firewood represents enormous daily timber extraction [1].
- This reduced demand for firewood would allow the woodland to regenerate naturally, reducing the soil erosion and water quality problems in the River Ondo that deforestation has caused [1].
(d) How the biogas digester produces gas from organic waste (from diagram and text): [3]
- Cattle dung and crop waste are mixed with water in the mixing pit and loaded into the sealed underground digester tank through the input channel [1].
- Inside the sealed tank, bacteria break down the organic matter under anaerobic conditions (without oxygen). This biological decomposition process is called anaerobic digestion [1].
- The decomposition releases biogas (mainly methane), which rises and collects in the dome above the tank. From there, it is piped through a gas line directly to household cooking stoves for use as fuel [1].
(e) How the digestate could benefit local farmers: [2]
- The digestate is rich in plant-available nutrients (nitrogen, phosphorus, potassium) that crops need to grow, making it an effective organic fertiliser [1].
- Farmers can spread the digestate on their fields to improve crop yields while reducing or eliminating the need to purchase expensive chemical fertilisers. The digestion process also kills many weed seeds and pathogens present in raw dung, making it a safer and more effective soil amendment [1].
(f)(i) Why some farmers are reluctant to supply cattle dung: [1]
Farmers currently spread cattle dung directly on their fields as fertiliser and are concerned that diverting it to the biogas digester will reduce the fertility of their soil, leading to lower crop yields [1].
(f)(ii) One way to address this concern: [2]
The project organisers could explain and demonstrate to farmers that the digestate returned from the biogas plant is actually a more effective fertiliser than raw, unprocessed dung [1]. During anaerobic digestion, the nutrients in the dung are converted into chemical forms that are more readily absorbed by plant roots, while harmful pathogens and weed seeds are destroyed. Farmers would receive the digestate back as a superior fertiliser, so their soil fertility would improve rather than decline [1].
(g) Difference in CO\(_2\) equivalent emissions between firewood and biogas: [1]
\[ 45 - 8 = 37 \text{ kg CO}_2 \text{ equivalent per month} \] [1]
A household switching from firewood to biogas would reduce its monthly greenhouse gas emissions by 37 kg CO\(_2\) equivalent - a reduction of over 82%.
(h) Two social benefits of the biogas plant for Village M: [4]
- Women and children previously spent an average of 3 hours per day collecting firewood from surrounding woodland [1]. Switching to biogas, which is piped directly to homes, eliminates this daily burden entirely. The time saved can be used for education (attending school or studying), income-generating activities, childcare, or rest - dramatically improving quality of life, especially for women and girls [1].
- Indoor air pollution from burning firewood is rated as "High" in Table 1, causing respiratory diseases that are a leading cause of illness and death in rural communities worldwide [1]. Biogas produces "Very low" indoor air pollution, so switching significantly improves the respiratory health of household members, particularly women and children who were exposed to firewood smoke for hours every day while cooking [1].
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