Introduction to the Commercialized Household Fiberglass Reinforced Plastic Digester

During the International Training Workshop on Biogas Technology for Developing Countries, I joined the 17 other international participants to visit a company in Kunming, Yunnan that produces fiberglass reinforced plastic digesters.

Kunming Blue Flame Science and Technology of Biogas Ltd.Co. has installed over 100,000 fiberglass reinforced plastic digesters in Yunnan Province including Kunming City, Yuxi City, Hong He Prefecture, Chuxiong Prefecture, Bao Shan City and fourteen prefectures, He Bei Province, Jiang Xi Province, Zhe Jiang Province, Sichuan Province, Vietnam, and Myanmar. Yunnan Normal University and Yunnan Provincial Rural Energy Engineering Bureau provided technical support for the project.

There are three different volume capacities: 4 cubic meters, 6 cubic meters, and 8 cubic meters. The weight of the digesters are 95 kg, 120 kg, and 145 kg respectively. For a 4 cubic meter digester, the manure from 3-4 pigs or 1 cow is sufficient for gas production. For a 6 cubic meter digester, the manure from 4-6 pigs or 1-2 cows is sufficient for gas production. For a 8 cubic meter digester, the manure from 6-8 pigs or 2-3 cows is sufficient. The product life for each digester is 20 years.

In most parts of the developing world where biogas digesters are used, digesters are constructed out of cement, stone, and sand. Compared to fiberglass digesters, cement biogas digesters are resource and labor intensive, and can be 50% more expensive than fiberglass digesters. For example, constructing a 6 cubic meter cement biogas digester would range from $1,000-$1,800 (including material and labor cost) in Africa, whereas constructing a 6 cubic meter fiberglass biogas digester in China would only cost $400. If the Chinese fiberglass digester were to be shipped to Africa (shipping cost would be $500, and the total cost would be $900), the final price would still be cheaper than constructing a cement biogas digester, depending on each country’s material and labor cost.

Moreover, the fiberglass biogas digester is less resource intensive, lighter (enabling convenient transportation), and requires less construction time. The fiberglass digester is composed of four parts, inlet chamber, upper dome, lower dome, and outlet chamber. These four parts can be separated during the transportation process and then easily put together during construction. The product is already manufactured in a standardized process, from production to installation, so the construction time is only 3-5 hours, whereas the construction time for a cement biogas digester is 7-10 days. Additionally, fiberglass biogas digesters are well sealed and airproof, while cement biogas digesters are prone to methane leakage due to the molecular spacing of cement and the molecular diameter of methane.

The fiberglass digesters have been fairly successful due to the superior design and material. During one of our field visits to a villager’s home using a fiberglass biogas digester, the villager told us there is sufficient gas supply to cook three meals a day and that a service personnel is readily available in the village should a problem arise. However, it is unclear if maintenance staff is available at each location site where the company’s biogas digesters are constructed. For biogas to be successful, farmers should learn the many benefits of using biogas, be properly trained on how to use and maintain a biogas system, and have a designated person in the village to be responsible for repairing biogas digester.

One of the workshop participant commented that families in his home country are larger than Chinese families, and an 8 cubic meter digester would be too small. Professor Zhang Wu Di, the head professor of the Bioenergy Department at Yunnan Normal University, said two 8 cubic meter digesters can be connected together to achieve a larger digester size. The fiberglass digester is only suitable for household use, and not for large scale industrial sized biogas digesters.

There is no one perfect biogas digester, but different environments and conditions must be considered before designing the appropriate digester.

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