Date:31 December 2007
‘If you enjoyed your meal, please express your gratitude by visiting our toilet.’ This may sound a bit weird, but to devotees of home biogas digesters, it makes perfect sense
Some people believe humanity’s profligate consumption of fossil fuels is an insignificant contributor to global warming when measured against “perfectly natural” cyclical phenomena. The same people think Al Gore (“An Inconvenient Truth”) has lost the plot, and doesn’t deserve a Nobel Prize for his work.
For the record, most scientists believe no such thing. But no matter what our stand point on the issue, just about everyone agrees on one thing: global warming is bad news. Greenhouse gases are affecting our planet’s weather patterns in ways we have yet to fully understand, and there’s strong evidence to suggest that we won’t like the long-term effects.
US president George W Bush wrote to senators in 2001 explaining his administration’s views on global climate change: “As you know, I oppose the Kyoto Protocol because it exempts 80 per cent of the world, including major population centres such as China and India, from compliance, and would cause serious harm to the US economy. The Senate’s vote, 95-0, shows that there is a clear consensus that the Kyoto Protocol is an unfair and ineffective means of addressing global climate change concerns.” www.whitehouse.gov
Because America is such a huge producer of greenhouse gases, that constitutes more bad news. However, President Bush has recently showed signs of softening his stance on environmental protection (without actually going along with the Kyoto thing, of course), and less than two months ago, British Prime Minister Gordon Brown committed the UK to reducing carbon dioxide emissions by 60 per cent before 2050. The Climate Change Bill will make Britain the first country to put carbon emissions reduction targets into law.
Here in South Africa, some forward-thinking individuals are adopting new, planetfriendly technologies with an enthusiasm that occasionally borders on obsession – and they make us proud. Starting small, and doing much of the work themselves, they’re designing, installing and exploiting myriad forms of “alternative” energy with the express intention of minimising their carbon footprints and living off the grid.
PM has showcased quite a few of these technologies over the years, but none has been quite as illuminating – or satisfying – as biogas digesters. In effect, the technology provides environmentally savvy homeowners with a three-pronged solution to their power and waste management needs – energy production, a waterborne sanitation system, and an organic kitchen and garden waste recycling opportunity.
Renewable power options such as solar arrays and wind turbines make sense in the domestic environment if the requirement is primarily for lighting and suffi cient power for entertainment devices such as a radio or TV. But solar and wind power have serious limitations when it comes to power-hungry applications such as water heating and cooking. Consequently, most homeowners striving for independence rely on bottled liquid petroleum (LP) gas, as it’s the most practical option.
But let’s get real here: LPG is still a fossil fuel, and with an escalating oil price, it’s an increasingly expensive option. Biogas, unlike LPG, is considered a green energy source because it’s not derived from crude oil. Instead, it’s created in the here-and-now from black water (raw sewage), kitchen scraps, garden waste and animal dung. The average domestic biogas digester is – on a technological level, anyway – simplicity itself. It comprises an airtight container with an inlet for the solid waste, an outlet for the nutrient-rich outflow, and a gas line to the kitchen. Inside, anaerobic requiring no oxygen) bacteria break down nutrients in the organic matter. The final stage in the process is the production of methane and carbon dioxide, making up two thirds and one third respectively of the total gas produced. One cubic metre of biogas provides at least two hours’ cooking time, or an electrical output of about 1,5 kWh.
Biogas digesters are nothing new, having been developed in the 1950s. Today, over 14 million biogas systems are operating worldwide, with China, India and Nepal leading the way. But they are less common in the West, and virtually unknownin South Africa. AGAMA Energy’s Greg
Austin wants to change all that, explaining: “The bigger picture is about climate change and how we manage resources. We cannot just keep pushing buttons and expect our power requirements to be magically resolved anymore.” Austin is also concerned by the amount of food that ends up in landfill sites. Once all the recyclable materials have been removed, he says, about 47 per cent of what remains is food waste – and as it decomposes, it releases methane into the atmosphere. Says Austin: “Pure methane released into the atmosphere is a greenhouse gas 21 times more harmful than carbon dioxide, but when burned, its impact is significantly reduced.” Johannesburg-based Caryl and Peter Richmond, having reached a stage in their lives where they needed to make practical retirement plans, decided they wanted to live the green life in style. So they bought a 5-acre plot in the De Goede Hoop Estate in Noordhoek, Cape Town. They cut down the large trees situated where they wanted their house to stand, and incorporated the wood into their home. The design features a number of innovative elements such as solar underfloor heating and a natural swimming pool with a gravel and reed filter.
But it’s their biogas digester that really gets them excited.
Caryl presently spends about 10 days a month on the property, during which time the system produces enough gas for her to cook every night and boil the kettle whenever she feels like it. Their 6 m digester was “designed for four bottoms”, as she puts it. When the whole family (including two “hungry men”) moves in, she says, they’ll generate about four hours’ worth of gas each day.
In fact, the system’s demand for certain, um, essential ingredients has introduced a new twist on post-prandial exchanges with guests, says Caryl. Rather than say “It’s been fun… we must do this again”, she’ll end off the evening with an invitation: “If you enjoyed your meal, please express your gratitude by visiting our toilet.”
Operating the digester involves no fuss whatsoever. The effluent and grey water are plumbed in directly, and it takes only five minutes in the morning to add the finely chopped kitchen waste and give everything a quick stir. Says Caryl: “It hasn’t altered my family’s lifestyle at all, and it’s by no means inconvenient. In fact, I get a satisfied feeling because I’m not having to buy gas. Plus, no water is wasted, as once the outflow has passed through the reed bed, it is gravity-fed to our fruit trees.”
Another benefit the digester brought into the Richmond household was a greater environmental awareness. Caryl explains: “The digester makes you very aware of everything you put down the drain. It alters your view in a positive way, and makes you want to use natural products.”
Consequently, she’s now cleaning her home as her granny did, using products such as borax, vinegar, lemon juice and bicarbonate of soda instead of harsh commercial detergents. Says Caryl: “Now I want to install one in my Johannesburg home, too.”
When Peter Bysshe bought a historic home in Stanford, near Hermanus, he realised he’d taken on a challenge. His new home was in a water-stressed area – but his wife loved gardening.
“Initially, I had no interest in biogas; my motivation was the shortage of water. I figured that it would only get more expensive, and I wanted to recycle what we used back into the garden.” Bysshe then turned his attention to his conservancy tank (the town has no sewage infrastructure), deciding he didn’t like having to pay good money to get it emptied on a regular basis, ei
Once the benefits of the gas had been explained to him, installing a digester made perfect sense. Says Bysshe: “In the beginning, frankly, the finances came first and the environment second. But realising we could generate our own gas appealed to me… I like stuff for free.”
Stanford’s dependence on aquifers for drinking water meant Bysshe’s installation had to meet rigorous water quality standards. The council also specified that the dome should be capable of being isolated and used as a normal conservancy tank, just in case the property was sold and the new owners weren’t interested in the biogas approach.
Says Neil Parker, AGAMA Energy’s resident site engineer: “Because it was one of the first domestic installations we’d tackled, we had to show that the water coming out was of a better quality than that going in.” To achieve this, an upflow anaerobic baffle reactor was included, along with an impressive number of inline settlement and gravel chambers – all of which added to the cost and complexity of the design, turning it into a pretty high-tech affair. The outflow passes through a gravelly reed bed into a sump before being pumped into the garden.
Another Noordhoek resident, Anthea Torr, only has good things to say about biogas. She lives on a smallholding and already uses solar and wind power to help run her home. To her, biogas represents a real opportunity to live completely off the grid. Her two horses definitely help boost gas production, because the more solid matter (and horses make plenty of it) you put into the digester, the more energy you get out.
Pressed on how much gas her system generated, Torr replied: “I couldn’t tell you, but it does what I want; it’s a nonissue. Actually, I’ve ordered a methane generator and water heater, so hopefully we can get completely off the grid soon.”
Biogas digesters are remarkably efficient at eliminating pathogens from waste, their superior performance outstripping septic tanks by far – that’s aside from the fact that septic tanks also leak methane. The main difference between the two designs lies in the retention times of the effluent. Septic tanks tend to have a retention time of only one day, so there’s little time for the biological breakdown of the pathogens, which means solids can leave with the fluid and contaminate the ground.
Biogas digesters, on the other hand, have a minimum retention time of five days. This extra time allows the bacteria to tackle the solids much more efficiently. Plus, as the water flow is much slower than in a septic tank, solids don’t leave the system.
By the time the fluid exits the digester, at least 95 per cent of pathogens and about 75 per cent of the pollutants (as measured by the chemical oxygen demand or COD) have already been removed. Once it has passed through an aerobic process, such as a reed bed lined with gravel, the remaining pathogens are reduced even further. The fluid then becomes a highly nutritious liquid fertiliser for your garden.
Says Austin: “The more water treatment modules you add to your system, the better the water quality becomes; it all depends on what you want to use it for.” For example, by adding one or two “polishing” ponds, you increase retention times and allow ultraviolet light a chance to kill any surviving E.coli bacteria. It’s possible to achieve the Department of Water Affairs and Forestry’s general discharge requirements, allowing the water to be used anywhere.
Every digester is specifically designed around the quality and volume of the available feedstock, as well as its intended purpose. So if you’re only interested in producing gas, and have no interest in gardening, it’s possible to direct the outflow directly into the sewage system.
Although digesters produce a volatile gas, says Parker, they are completely safe. He explains: “These are self-venting systems. If, for whatever reason, excess pressure builds up in the dome, it just forces the water level down and vents into the expansion chamber to prevent any untreated waste from escaping.”
However, he has a word of warning for interested homebuilders and DIY enthusiasts: although the technology involved is simple, you have to get it right. If the digester is incorrectly built, or you don’t stick to the design, it simply won’t work.
For more information visit AGAMA Energy’s website