Peter Bysshe stirring the holding tank –
probably the least pleasant part of owning
a biodigester, but not that big a deal.
(Image: Jennifer Stern)
A dome biodigester under construction.
Construction of the reed bed that
performs the final filtration of the
biodigester’s recycled water.
The completed reed bed of Bysshe’s
(Images: Agama Energy)
Global climate change is a reality, and the only way we, as a nation and a species, are going to avoid catastrophe is to utilise our resources more sensibly.
There are lots of ways of decreasing waste, saving water and generating alternative energy. But there’s one piece of technology that does all three at once – a biodigester. And the Overstrand Municipality on the Western Cape’s southern coastline is the first local authority in South Africa to pass plans for one in an urban area.
That’s quite a big step, because biodigesters deal with the unmentionables we all prefer not to think about. But it’s not thinking that creates so many problems. Let’s face it, none of us likes to really reflect on what happens after we flush the toilet, but the reality is that we use litres and litres of perfectly good drinking water to flush away our waste, which is then processed at the cost of quite a bit of energy before being released into the sea or river systems.
More and more people are realising that they are flushing away good quality nutrients and energy, and so are looking at alternative ways of dealing with human waste – and have discovered loads of benefits. It’s all about thinking of it as a resource, not just something we need to get rid of. One such person is Peter Bysshe. When he bought his house in Stanford, he needed to decide whether to install a new septic tank, or to go for a biodigester.
“I took the long view,” he says. “It cost about 15 or 20 grand more, but it was worth it.”
Bysshe’s main motivation was to recycle the water he and his family used in the house to water the large garden. “But I’m not saying no to free gas,” he says with a smile.
The Bysshe family of four, plus domestic staff, do all their cooking on methane produced by the biodigester.
“We came on line during the load shedding in 2007,” Bysshe says, obviously pretty pleased with his timing. In 2007 and early 2008 South Africa was hit by a series of planned power outages after rapid economic growth put a strain on the country’s electricity grid.
“It takes about three or four weeks before you start getting gas. We’ve been going for 16 months, now and it’s been great Once or twice it’s been low, but the next day it’s up again.”
A forward-thinking municipality
Bysshe lives in the pretty country town of Stanford, part of the Overstrand Municipality, which includes the coastal towns of Hermanus and Gansbaai that are famous, respectively, for southern right whales and great white sharks. Country towns can be quite conservative, so Bysshe was pleasantly surprised with the response when he applied for permission to build the biodigester.
“They wrote to DWAF [Department of Water Affairs and Forestry] and asked what the requirements were. We had to put in an additional baffler system before it flows into the reed bed – to purify the water more. They wanted it to get to the point that, when I was watering the garden, I wasn’t contaminating the ground water.
“They took groundwater readings before we built it, and they check it every year.”
How it all works
Biodigesters are pretty simple. The main fermentation tank looks rather like a giant pizza oven buried in the ground. Obviously, gravity plays an important role in the process so the level of the bottom of the tank, or dome, is dictated by the fall necessary to get the waste from all the toilets and drains in the house.
All the waste from the drains and toilets flows into a holding tank, and then into the dome. A separate inlet with a cast-iron cover allows users to add kitchen waste, garden waste and other organic material.
In the dome, all the organic material ferments to form methane, which is tapped off and piped to the stove in the house. Waste water flows from the bottom of the dome through a series of anaerobic baffled reactor tanks, also called expansion tanks, which progressively remove remaining pathogens from the water by simply allowing them to die through lack of oxygen.
The holding tank and all the baffler tanks are accessed by simply removing their cast-iron covers. All of these processes happen underground.
From the last tank, the water flows into a bed of reeds, which do the final purification of the water. The reed bed is an open pond filled with course stones to a level higher than the overflow. So the reeds grow directly in the water but – to all appearances – are growing in course gravel.
Any pollutants remaining in the water nourish the growing reeds so that, once they have utilised all available nutrients, the water is clean enough be used on the garden, thus re-entering the groundwater system.
Maintenance is minimal
Using a biodigester is taking control of the process of waste disposal – unlike the head-in-the-sand approach of most urban people. It requires a certain amount of dedication, but the maintenance is minimal.
“You have to stir it about once a week,” Bysshe says. “Otherwise it cakes up. You want to keep it as liquid as possible. You stir it with a paddle. There is a slight odour when you open it, but it’s not like a honey sucker.” (“Honey sucker” is the rather euphemistic name for the tanker trucks that suck raw sewage from tanker trucks.)
It’s not a very high-tech process. He opens the tank up, shoves in an old scaffolding board and – well – stirs. I watched him doing it and, while it’s certainly not jasmine or orange blossom, the smell isn’t that bad. It’s definitely less offensive than a long drop – not that that’s saying much, I know. And when it is closed there is no smell at all.
The system must be adequately fed. As well as all the toilet discharge and grey water – water used for bathing and washing dishes, for example – Bysshe adds kitchen waste, grass cuttings, bits of paper, and even dog poo.
“Everything in moderation,” he says. “If it hasn’t had grass, for example, you introduce it slowly.”
A diagram of Agama Energy’s domestic biodigester
(Image: Agama Energy)
Saving resources and saving the world
While the primary objective is to effectively and efficiently utilise all the resources at hand, a biodigester also reduces your carbon footprint by burning methane, thereby turning it into carbon dioxide, instead of letting it escape into the atmosphere.
“Methane is 23 times worse than CO2,” says Neil Parker, an engineer working for Agama Energy, the company that designed the biodigester. What that means is methane – the ultimate greenhouse gas – traps 23 times more of the sun’s energy than carbon dioxide.
While Parker’s company have produced Bysshe’s and one or two other domestic units, they aim to focus on larger projects, where economies of scale would make the whole thing more affordable.
“We’re trying to start a rural biogas programme,” he says. “What you want to encourage is setting up small scale biogas plants at individual household level. Guys who have a few cattle, and can get manure, they can use that for fuel.
“It improves their lifestyle tenfold. They’re not walking 10 kilometres a day to get firewood, and it burns cleaner, so there’s less health risk.
Most of the biodigesters set up so far in and around Cape Town have been built from scratch out of bricks – an expensive exercise. But Parker says they’re looking at cheaper ways of mass-producing the domes, which are usually the most expensive parts to build.
“At the moment we are developing a tank. We’re going to make the first ones from fibreglass, and then we’ll start rotomoulding them in plastic.”
Bysshe says it’s a win-win situation.
“The benefits are huge. The water and the gas, and ultimately, it’s sustainable. We’re creating a positive impact on the environment. It’s the difference between being green and being sustainable. This is sustainable.”
Do you have queries or comments about this article? Email Mary Alexander at firstname.lastname@example.org