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Hemp saves sea from sewage dumps

Posted on February 12, 2004

Lismore, Australia — Hemp crops can be used to mop up troublesome sewage effluent that would otherwise be dumped at sea, say Australian researchers.

A team led by eco-technologist Dr Keith Bolton from Southern Cross University in Lismore, New South Wales, is analysing hemp from a one hectare crop harvested from land next to a public sewage treatment works.

His team is testing the hemp to see how well it acts as a ‘mop crop’, a crop that absorbs sewage once it has been treated at a sewage treatment plant.

Mop crops have become an increasingly popular alternative to dumping sewage effluent in the ocean, said Bolton.

Byron Bay residents had been strongly opposed to a proposed local ocean outfall, he said. In response, Bolton’s team worked with Byron Shire Council to plant a wetland of half a million paperbark trees next to the sewage treatment works as an alternative to dumping the effluent to sea.

“That really prevented seven million litres per day discharging into the ocean,” Bolton said.

According to Bolton, a good mop crop is one that grows and takes up nutrients rapidly and provides a valuable product: “One that can use a lot of the effluent and make money.”

His earlier experiments involving artificial effluent had suggested hemp would be a very effective mop crop, he said.

“It was one of the best performers when we looked at the amount of nutrients taken up per unit area per unit time,” he said, having compared it with other mop crops reported in the international literature.

“Hemp grows like the clappers. It just loves the nutrients and also it’s got a particularly valuable product. You can certainly get more money growing hemp than most other crops.”

Bolton is now testing the hemp grown on real sewage effluent. As well as testing the amount of nutrients such as nitrogen and phosphorus absorbed, his team will look at the impact on groundwater and soil quality.

“We want to check there are no salty residues accumulating which can affect the structure and permeability of the soil,” he said. The researchers expect results in a month.

“I visualise in the future there’ll be large mop crop projects using hemp; tens of thousands of hectares.”

The current trial will also compare how well kenaf, another fibre crop related to hibiscus, and clumping bamboo act as mop crops.

Bolton said the type of crop used for any particular sewage plant will depend on the environment.

In Byron Bay, paperbark trees were best suited to the wetland environment and also solved a number of other environmental problems too, he said. But hemp would be “a winner” as a cash crop on high quality agricultural land.

The hemp Bolton grows is low in tetrahydrocannabinol or THC, the active ingredient that gives people a ‘high’. The hemp is grown under strict licence from the state’s health department.

His team worked in collaboration with Ecofibre Industries Ltd, which provided hemp seed and agronomic support, and the Byron Shire Council.

Copyright © 2004, ABC Science Online. All rights reserved.

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