Tonga Biogas Project

From the 4th to the 18th of December 2008, 4 volunteers from Engineers Without
Borders New Zealand (EWBNZ) travelled to Tonga to implement two biodigesters for a farm on Tongatapu. The goal of the project was to research, design and implement a sustainable pilot scale biodigester system that could be later applied and introduced to small island communities that lack a centralised municipal waste system. The biodigesters were to be made in Tonga with locally available materials and labour.

Team photo at the farm: Anita, Alan, David, Andrew and Peter.

The biodigester project was identified for EWBNZ by the Pacific Islands Trade and Investment Commission (PITIC). They received the project proposal from Waste Management Ltd (WML) based in Tonga. WML called upon the skills of EWBNZ to assist in evaluating the technical, environmental and social feasibility of using biodigester technology in Tonga. The EWBNZ biogas team consisted of Anita Walbran, Roy Elliot, Peter Trafford, Alan Collins and James Russell (James was unable to travel to Tonga). 

Prior to departing for Tonga the team worked for 5 months researching and designing 2 biodigesters – a Chinese type digester for larger scale applications and a polyethylene bag digester more suited for small scale, rural applications. The team prepared design reports and submitted them to WML for their comments and WML informed EWBNZ that they would like to implement the Chinese type design. When EWBNZ informed WML that they had sourced the polyethylene bag in New Zealand for the other design, WML informed them that they would like that design implemented also.

The team was greeted at the airport by Lee from WML 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. The team learnt to persist with a search for suitable materials in unlikely places. Once a new way to construct the old designs was worked out, activities were better planned and the Tongan support able to be guided with more confidence. Knowledge of the underlying anaerobic digestion process principles of air-tightness, fluid dynamics and pressure enabled this quick adaptation.

Chinese biodigester all completed.

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.

Polyethylene bag digester all completed.

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 this project so there was plenty to spare. Thank you to Convex Plastics!

Once the 2 biodigesters were completed and tested the team spent the last few day

• Educating the owners on the operations and maintenance of the biodigesters

• Speaking to local Tongans for 2 days at a National Youth Conference on biodigester technology

• Visiting Tonga College to obtain design information for an upcoming EWBNZ project in renewable energies.

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

Supporting this project

Engineers Without Borders New Zealand 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. Thanks to Waste Management Ltd for providing accommodation, transport and food while we were on Tonga and to Peter Trafford for acting as engineering mentor throughout the project.