And that's what the Solvi Group of companies, which owns and manages 39 landfill sites in Brazil, used to do, in the process earning carbon credits in the country's carbon trading scheme. Marcelo Camargo, manager of Termoverde Caieiras, Brazil's largest biogas (methane) energy plant, on the outskirts of Sao Paulo. Credit:Lia Timson "But we saw an opportunity," says Camargo, who's been in the business for more than a decade and oversaw the plant's construction. "We thought it's a pity, we're throwing energy away. We're taking methane, which has calorific value, and only generating credits. We thought, let's generate energy." In 2011 Termoverde, a Solvi subsidiary, opened a small thermoelectric plant in Salvador, in the north-east state of Bahia, as a pilot, and in 2016 used lessons from that site to build a 100 million reais ($39 million) modern, modular, green power plant in Caieiras.

Nestled in lush subtropical hills just 33 kilometres out of the Sao Paulo megametropolis, Termoverde Caieiras is surrounded by an eucalyptus plantation and a environmental reserve belt that has the added bonus of shielding it from the road. The power plant itself is not much bigger than a couple of football fields and most Paulistas don't even know it is there. Trucks work on the 15,000-square-metre rubbish tip in Caieiras, Brazil, where black pipes take methane away to the power plant. Credit:Lia Timson Every day, hundreds of garbage trucks come and go, bringing 10,000 tonnes of rubbish from half of Sao Paulo's households as well as from surrounding towns. Their loads, 55 per cent organic on average, are dumped onto sections of the plant's 15,000-square-metre tip operated by sister company Essencis. The loads are then compacted and landscaped onto terraces, and left to decompose. A network of perforated pipes allows the methane generated by the decomposing rubbish to travel upwards naturally. In the same pipes, the slurry, the compost's liquid residue, travels downwards into a dam and later by truck to a sewage treatment plant. The methane is captured and piped into thermo engines in modular containers. Credit:Lia Timson

Then, instead of burning aimlessly through small flares, the gas is captured and piped into 21 containers housing individual generators. From here, it leaves as high-voltage electricity transmitted by cables to the site's substation, before feeding the state's power grid. Any excess methane that can't be processed by the power plant is control-burnt in large flares. The plant produces enough energy to power a city of 300,000 – say, Wollongong in NSW or Geelong in Victoria – all year around. That's 29.5 megawatts of energy that would otherwise be wasted, and worse, released into the atmosphere. Five of the 21 modular generators at Termoverde Caieiras. Credit:Lia Timson The waste-to-energy technology is known in Australia as anaerobic digestion, that is, without oxygen, and is currently deployed in a modest site in Wollert, Victoria, where it powers the Yarra Valley Water sewage treatment plant. The $27-million 18-month-old site has a capacity to process up to 33,000 tonnes of organic waste each year, or 100 tonnes per day. According to Yarra Valley Water, any surplus energy from the plant is exported to the grid. That's what Termoverde does too. It sells its energy to large commercial clients, and any surplus it sells on the open market. It has proven such a hit the company is thinking of converting all its sites into such private green-energy engines.

Electricity from the containers is fed into the site's substation and then the state's grid. Credit:Lia Timson Key to the technology is that it doesn't burn or heat the waste – as opposed to combustion and gasification technology, which use incinerators and boilers respectively. Victoria is planning a gasification plant for Laverton North that would take up to 200,000 tonnes a year of residual household waste. NSW excludes biological processes, such as anaerobic digestion and composting of waste from its Energy from Waste Policy Statement, in favour of thermal treatments such as combustion, oxidation and gasification. Loading

A planned $700-million waste-to-energy incinerator plant by Dial A Dump Industries in Western Sydney was dumped in July amid uncertainty over the project’s human health risks and impact on air and water quality. "Incinerators are only used where there's no room for landfill, like in Germany or Japan," Camargo says, adding that in Brazilian terms such solutions cost twice as much as anaerobic digestion because of the cost of heating the waste. "Here that technology is superseded, and it has the added problem of burning dioxins from things like PVC which are much more damaging to the environment. “And by not burning the rubbish to create energy, I’m making clean energy from a renewable source," Camargo says.

"I'm also helping reduce the amount of odour emissions from the landfill site." Loading In order to obtain the permits necessary to operate in Brazil, where more than 80 per cent of all energy generation is from renewable sources, the company had to build the substation and transfer its ownership to the state electricity transmission company, which maintains it. "There are very stringent requirements, but the environmental advantages are undisputable. "I'm transforming rubbish into energy and contributing to the national policy of reduction of solid waste. That's what matters."

Loading Sergio Leitao, director of the not-for-profit Instituto Escolhas and former Greenpeace Brazil executive, told Sao Paulo daily newspaper Folha de SP this month that no single renewable energy source can be relied upon to supply an entire country's electricity system. "We shouldn't necessarily select the option with lowest cost or the most competitive," he said, referring also to wind, solar and hydro energy (the latter supplies 68 per cent of Brazil's needs). "It's how they complement each other that provides the best and most efficient mix." NB: This story has been adjusted to reflect the correct flow of methane and CO² in the plant's energy production.