Janet is looking for an alternative to having to join her local council’s proposed new sewage scheme. Are there any other options?
Niwa’s Chris Tanner says putting in an alternative is more than possible, it’s a sensible idea. He says there are already examples of people working together to use an alternative system in New Zealand.
“It depends on the particular situation as to what will work best but there is a whole range of options.” Chris and his team have been studying the nutrient-capturing potential of artificially constructed wetlands for more than seven years now. Recently they have been looking at developing a wetland system that could be sized to suit many situations. “It might take up a bit more space, but it would be hardy.”
He pointed me in the direction of Fraser Thomas, an engineering and consultancy company in Auckland which includes specialist environmental, geotechnical and structural engineering sections.
Director Ray Hedgland says the environmental section has specialised in small town sewerage, especially in environmentally sensitive, touristy areas.
Recently they have come to the realisation that about 70 percent of the cost of a sewerage system lies in the pipe network, with the remainder being treatment and disposal, although most public interest is in the treatment and disposal side.
Ray and his associate Greg Maddren are now actively promoting decentralised sewerage (DS) technology, having inspected several small towns in the West Coast of America where these concepts have been in operation for over 25 years and, he says, continue to operate successfully.
The key to the systems is small diameter reticulation, preferably retaining a septic tank at each house with an effluent filter and pump added. Flow from each house is buffered in the septic tank, and grease and bulk solids are removed. Fluids from the septic tank are pumped out slowly into the pipe. If the pipe is full, the pump stops and waits until later, thereby further balancing peak flows.
The pipeline is a pressure system, he says, and only about a third of the diameter of a gravity-fed sewer. Because of this, stormwater cannot leak into it as it does in conventional systems. A conventional sewer is sized so that 50 percent of the pipe is available for the stormwater infiltration that always occurs. The DS pressure pipeline is not only smaller but it has no costly manholes and can be laid shallow, using a simple chain digger trenching system, and following the contour of the land. Gravity sewers, on the other hand, have to slope downhill all the time, reaching some five metres in depth before having to be pumped. “So you can see where the cost savings occur,” he says.
A range of treatment options is available, depending on housing density and the local environment. All systems are monitored by a single service provider so that all alarm conditions, maintenance, and operation are attended to when required without homeowners having to worry or do anything. “The outcome then is the same as for a conventional sewerage system, ie ‘flush and forget’ which is what all homeowners tend to want.”
But wait, there’s more
Niwa scientist Rupert is looking at producing energy from the biogas that comes off farm effluent ponds.
“There’s nothing new about the technology, most large domestic wastewater treatment plants capture biogas. Last year, using a converted generator running on gas from a farm pond, we plugged in two fan heaters, three spotlights, and a three-phase motor – they ran for over two hours. At full throttle it generates 13kW, we ran it at nine or 10.”
The four-year project has demonstrated that farmers can easily and cost-effectively convert biogas emissions to power. In tests on a Waikato farm, the generator ran for more than two hours using biogas collected under a small (25 sq m) trial cover – “and this was in the middle of winter, with no waste going into the pond.”
It took just two hours to convert the generator used in this trial.
“We wanted to show how easy and simple it is to operate for on-farm electricity generation. More recently we have been looking at a large-scale trial, on a reasonably sized farm. The data we’ve collected over the last year are very promising and backs up the smaller study, with good if not better gas production.”
But can the technology be applied by life-stylers with a smaller operation?
It all comes down, says Rupert, to economies of scale. “There are certainly not insignificant costs involved but it could be worth considering if you are able to join in with others in the area, and share the costs and rewards.”
The technology has been in place in small villages in Japan and China where communities collected their rubbish and use one large digester to cook.
“So getting together in a small group might be a valid way to approach this. In this research, we’ve been focusing on farmers with fairly large operations. But we have shown it is cost-effective, using simple pond-based digestion technology and off-the-shelf equipment.”
It helps if some of the infrastructures are in place already. “Many lifestylers have a generator on site, a diesel-run one can be good as well. There’s also some animal and plant waste around and all kinds of things you can look at doing. You’d need to bear in mind it’s a bit smelly, and there’s some maintenance involved. Although anyone with a small amount of mechanical skill can convert a generator to run on biogas.”
Everyone, from large-scale farmers to lifestylers, has to manage their wastewater. “The simple technology is a septic tank and a soakage field.
An anaerobic digester is essentially a septic tank with a lid on it, that collects the gas – it does the same kind of thing.
“You have to be a bit careful with what you put into septic tanks in the way of household cleaning products, and this is also the case for anaerobic digestion, the bacteria are pretty sensitive. That’s just one thing to keep in mind.”
Producing power from biogas has many advantages. “The neat thing about anaerobic digestion is that you don’t lose the nutrients. So after you recover the energy, you’re still left with a nutrient-rich effluent that you can irrigate. As power prices go up, and the value of these advantages is realised, the technology will become increasingly attractive.”
For more information visit www.fraserthomas.co.nz
www.mfe.govt.nz/publications/waste/wastewater-mgmt-jun03/html/part3-section7.html
