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Engineers Without Borders Assist Tonga with Biogas

Engineers Without Borders Assist Tonga with Biogas Solution

Engineers Without
Borders New Zealand Tonga Team photo
Team photo at the farm: Anita, Alan, David, Andrew and Peter.

Engineers Without Borders New Zealand (EWBNZ) is an organisation of professional and student engineers who share a vision to confront global challenges of poverty, sustainable development and social inequity by undertaking projects that will directly improve the quality of life in communities within New Zealand and in the South Pacific region. From the 4th-18th December 2008 a team of volunteers from EWBNZ travelled to Tonga to build two biodigesters with locally available materials and labour that would demonstrate the feasibility of this technology to benefit Tongan communities.

The Pacific Islands Trade and Investment Commission received the project proposal from a farm in Tonga and passed it on to EWBNZ. The owners of the farm called upon the skills of EWBNZ to assist in evaluating the technical, environmental and social feasibility of using biodigester technology in Tonga.

Biodigesters are low pressure, warm temperature anaerobic reactors that convert common biodegradable wastes into flammable methane-based off gas dubbed “biogas”. Biodigesters offer a triple benefit: to treat wastes, produce gas that can be used for cooking, lighting, and electricity generation, and to produce nutrient rich fertiliser.

The team that was assembled from EWBNZ’s volunteer base had five months part-time to research and design a suitable biodigester system before travelling to Tonga to implement the design. This groundwork produced excellent results, and the team was able to recommend and develop the detail for two separate designs, a Chinese design and a polyethylene bag design.

The team was greeted at the airport in Tonga and taken to the farm where they would be spending most of their time working. Working time was initially spent modifying the design completed in New Zealand to fit the materials that were confirmed as available to the project. Knowledge of the underlying anaerobic digestion process principles of air-tightness, fluid dynamics and pressure enabled this quick adaptation.

The two digesters presented different challenges during construction. The Chinese design required 2 polyethylene pipes (inlet and outlet pipes) to be heat welded to a 5000L polyethylene tank. The tank was taken to a local heat welder to get this done however the only heat gun in the Kingdom of Tonga was on a different island and would not be returning for a week. Fortunately the team had taken a heat gun with them, so this was used to weld the 2 pipes to the tank.

The polyethylene bag digester required an 8m length of air tight bag however some of the bag got damaged during transit making it unusable. After several unsuccessful attempts at patching the bag the team had to dispose of the damaged plastic and roll out some more. Fortunately 100kg of plastic had been donated by Convex Plastics Ltd for the project so there was plenty to spare.

Once the 2 biodigesters were completed and tested the team spent the last few days educating the owners on the operations and maintenance of the biodigesters. They also provided the farmers with material to give to other Tongan’s interested in implementing a biodigester system as they are very keen to help spread this technology in Tonga.

Chinese biodigester
all complete, Tonga
Chinese biodigester all complete

Polyethylene bag
biodigester all complete, Tonga
Polyethylene bag biodigester all complete.

Once the team EWBNZ returned back home they prepared as-built reports to send to the client in Tonga and have remained in touch as advisors. The last word was that the Chinese digester has started producing gas – good news!

The EWBNZ team consisted of Anita Walbran, and Roy Elliot, both postgraduate students at Auckland University; Alan Collins and James Russell, both undergraduate students of engineering at Auckland University, and Peter Trafford, Senior Process Engineer at Watercare Services Limited of Auckland.

EWBNZ would like to thank the Rotary Club of Auckland East for their financial contribution, Convex Plastics for the donation of a custom run of polyethylene plastic and Pacific Blue for donation of an excess baggage allowance to get materials and tools to Tonga.

EWBNZ have 2 more exciting projects lined up for 2009 – a micro hydro project for Tanna Island, Vanuatu and a solar energies project in Tonga. EWBNZ is always looking for financial assistance for our projects so if you would like to assist please visit


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