Utilize the input text as a context reference, choose the right relationship between digester and biogas from the options. Options: product/material produced, manufacturer, distributed by, industry, position held, original broadcaster, owned by, founded by, distribution format, headquarters location, stock exchange, currency, parent organization, chief executive officer, director/manager, owner of, operator, member of, employer, chairperson, platform, subsidiary, legal form, publisher, developer, brand, business division, location of formation, creator.

The biogas digester has improved their health by providing a safe, renewable and cheap source of fuel that's easily available and sustainable than firewood.

Answer: product_or_material_produced