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Floating wetland created at Coromandel wastewater treatment plant

05 April 2013

A thriving community of organisms is establishing itself in the floating wetland of plants at Coromandel's wastewater treatment facility.

The Coromandel Wastewater Treatment Plant requires a plant upgrade to meet new resource consent conditions and will provide further protection of the environment and aquaculture industry in the township. Work is now complete on the first of three stages over three years.

The technology used will save ratepayers significant money in operating and upgrade costs over time since it puts nature to work in doing what it does best and avoids the need for costly upgrade alternatives.

Some $600,000 is being spent on this first stage of the upgrade which involved laying special 5cm thick material on the pond's surface, planting, a new inlet screen, grit screen and septic receiver that will enhance performance of the whole system.

After the material was laid, the pond was planted out with floating plants that drop roots into the water, playing host to thousands of organisms including snails.

These organisms may not be on the pay roll but with the right working conditions - such as adequate oxygen in the water and enough 'food' - they help reduce nutrients that are toxic to aquatic life.

TCDC Water Services Engineer Steve de Laborde says the new system is working well despite a few teething problems.

"Thousands of organisms live in the plants which introduce extra species, so in effect the plants have established their own eco-systems within the retention pond.

"The pukekos love it, they occasionally pull up the plants but once they get more established we're told the birds don't bother with the plants."

Next year $1.1 million has been budgeted for earthworks to raise the bunds around the WWTP to protect it from flooding from the neighbouring river during heavy rainfall.

Another $1.6 million is budgeted for 2014/15, however the projected expenditure for the year is $400,000 giving a saving in the order of $1.2 million.

"I saw this technology a while ago and am quite excited that we're using it. For me it's like a lifesaver for the pond-based systems that have served our communities well, but due to increased environmental protection standards, require some type of upgrade to performance."

How it works

Constructed wetlands are commonly used to cleanse water of pollutants. They work by exposing water to natural processes - microbial processes - facilitated by plants and organic matter.

In order to expose as much water as possible to the beneficial activity of the wetland, they are created wide and shallow.

This approach is natural and effective, Floating Media Treatment (ETM) act like a constructed wetland but with none of the land requirements. They represent a concentrated wetland effect.

With the huge surface area presented by the individual matrix fibres, every 250 sq ft island equates to 1 acre of wetland surface. Whenever you launch a BioHaven you effectively launch a floating wetland.

The water doesn't have to be shallow for the benefits to be realised - solar-powered pumps can be used to circulate water through the island to increase exposure to microbial activity.

Floating Treatment Wetlands (FTWs) are a relatively new
development. In a recent review of all the offerings on the market today, NIWA (the National Institute of Water and Atmospheric research, of New Zealand) identified BioHaven floating islands as the most innovative and advanced of all.