As water resources become increasingly important, the use of onsite wastewater treatment systems which recharge aquifers are gaining in popularity worldwide. NZ Engineering News takes a look at an established solution now available here…
One of the latest to reach New Zealand is Advanced Enviro-Septic (AES) wastewater treatment systems.
AES systems are designed and produced by David Presby of Presby Environmental in New Hampshire, USA where his desire to protect the environment drove him to develop the AES system.
The system has been in use overseas for over 25 years and there are more than 250,000 installations, and now it is available here through Environment Technology.
The essential design component for the passive advanced secondary standard AES system is the Advanced Enviro-Septi pipe. A traditional primary septic system has wastewater influent entering a septic tank where heavier solids settle, with the liquid effluent and suspended solids moving on to a soil disposal/leaching field, where they are broken down by bacteria as they soak into soil.
Mr Presby felt the piping in a conventional leaching field did not lend itself to a healthy bacterial community; he saw a lack of circulating oxygen and
too little surface area for bacteria to colonise and grow on. The AES design relies on a 300mm diameter central plastic pipe with two distinctive features: deep ridges on the outside to increase the surface area and provide additional bacterial growth areas, and plastic skimmer tabs on the inside
of the pipe to prevent grease and suspended solids from exiting the pipe before bacteria have the opportunity to break them down.
A 6mm thick mat of plastic fibres then surrounds the pipe, creating a large area for colonisation and then a geotextile type synthetic fabric encases
everything, allowing liquids to leave and air to enter while providing a protected surface for microbial activity and treatment.
Because treatment occurs inside this multi-layered system, his design eliminated the need for the thick layer of filtering crushed stone under pipes as in a traditional leaching field. Instead, the AES pipes are surrounded by as little as 150mm of sand that wicks wastewater away from the pipes while allowing oxygen to be drawn in. This provides far superior treatment and eliminates the inevitable clogging of the pore spaces in the receiving soil.
Less fill material also means cost savings and easier installation. The smaller, adjustable footprint of the treatment bed also provides more flexibility; an AES system can go in on smaller house sites and can be installed on considerable slopes.
As Mr Presby began putting his idea into production, he initially relied on his construction workers to customise pipe bought from a company in Canada. But after the Canadian company threatened a major price hike, he brought pipe production in-house.
Ever the inventor, he put together his own plastics extruder, in which heated plastic is passed through mould blocks to create the desired shape, then cooled in a water tank before being cut to the specified length. To slice his pipe into three metre sections, Mr Presby got the perfect blade from a place where cutting is a specialty, an old sawmill.
He admits some things have not changed since those early days; a sawmill blade is still doing the job of cutting his pipe, though he now has two extruders so production can continue if either one of the hulking machines goes down. And while he still owns the construction company, his workers there no longer build the wastewater treatment systems.
In 2001, he bought land in Whitefield, New Hampshire and a year later, began making pipe at the site. Presby Environmental now has some 30 employees of its own, and a manufacturing capability that shows how far the company has come.
“We can produce 50,000 feet of pipe a day easily, and sometimes we’re running the extruders five days a week,” Mr Presby says. “That’s a lot of septic systems.”
On a recent visit to the plant, Environment Technology’s Dick Lamb witnessed the truck and trailer dispatch the AES pipe systems needed for 800 three-bedroom houses, in one day.
Outside the manufacturing building, six giant silos store the recycled plastic used in the fibre mats. This use of recycled material is just one of the company’s sustainable practices; others include recycling the plastic in any pipe section rejected as substandard, and repeatedly reusing the water used for cooling in the extruders.
The pipe contains a 300mm diameter, high-density plastic pipe which is corrugated and perforated with skimmer tabs that extend into the pipe at the point of each perforation. Warmer effluent from the septic or Imhoff tank enters the pipe and is cooled to ground temperature.
Suspended solids and grease that remain in the effluent separate from the liquid effluent as it cools. The skimmer tabs assist in separation of solids and scum from the liquid layer in the corrugated pipe.
A further key element in the Advanced Enviro-Septic system is the Bio-Accelerator fabric, which runs the length of the bottom of the pipe, between a layer of coarse plastic fibres and the pipe itself.
This screens solids from the effluent and develops a biomat which accelerates start-up bacterial growth, reducing the invert basal pipe permeability and further protecting the outer layers and the receiving surfaces of the sand and soil so they remain permeable.
A non-woven geo-textile layer holds all the components in place and provides a protected surface on which another layer of biomat develops. Liquid exiting the geo-textile fabric is wicked away from the piping by the surrounding system sand. This assists in cooling the effluent and enables air to transfer to the bacterial surfaces.
Anaerobic bacteria utilise the effluent to form biomat layers on the provided surfaces during high flows and during low flows, aerobic bacteria consume the biomat. Bacterial efficiency is increased by the large air supply and fluctuating liquid levels which provide large food supplies.
Advanced Enviro-Septic can handle large variations in daily loading without chemical or mechanical intervention which makes it a practical solution for community facilities, show grounds, caravan parks, camping grounds and small towns.
In regards to capital, maintenance and operating costs, councils and rural communities can benefit from the passive nature of the Advanced Enviro-Septic system.
It can be used to refurbish sand filtration beds by bacterially treating primary effluent and protecting the infiltration bed against progressive failure, or for completely new sewerage treatment plants with massive reductions in capital and operating costs over standard mechanical and chemical plants.
An example of a larger scale application is the 198,000 litres/day installation at the Municipal Treatment Plant at Blodgett Landing in the town of Newbury, New Hampshire. The existing sand filtration beds were upgraded to AES beds in 2011, due to a growing community and higher treatment standards. With a small budget and limited space, the town had to find a product to handle 50,000 gallons per day that would be easy to maintain with minimal upfront and ongoing costs. The town decided to upgrade its system to a series of passive Enviro-Septic beds.
The simple treatment process not only provides Advanced-Secondary treatment but also reduces Total Nitrogen levels by 69 percent. After influent screening, Imhoff tanks are used for the initial primary treatment. Following this, the Multi-Level AES system provides secondary treatment. Fifty percent of the treated effluent is recycled through the treatment system for further nitrate reduction.
In August 2014, when the plant renewed its license, the testing interval was increased to six months as the testing regime for the previous two years had produced consistent and high quality results to the following levels: TSS = 5.0mg/L, BOD = 6.0mg/L, TN = 7.1mg/L.
According to plant manager Tim Mulder, “The upfront cost saving with minimal ongoing cost and maintenance is exceptional. We are very pleased.”
In New Zealand, over 60 AES systems are now installed from Te Anau to Kaipara with numerous others going through the consent process.