[ASC-media] Aquatic weeds can be safely recycled

joanne.finlay at dpi.nsw.gov.au joanne.finlay at dpi.nsw.gov.au
Wed Sep 6 04:27:19 CEST 2006

NSW Department of Primary Industries
Wednesday 7 September 2006

Aquatic weeds can be safely recycled

Quality compost that is safe to use and beneficial for farmland, can be 
produced from aquatic weeds, according to the findings of Australia's 
first large-scale scientific trial of the risks associated with recycling 
noxious weed material.

The trial was undertaken on 35,000 cubic metres of aquatic weeds 
mechanically harvested from the Hawkesbury River after a major infestation 
occurred in the summer of 2004.

NSW Department of Primary Industries (DPI) research scientist, Dr Chris 
Dorahy, said harvesting of the weeds had created a significant disposal 
problem and led to an 18 month research project to examine ways to recycle 

"The weeds included Alligator Weed, which is invasive on land as well as 

"One of the big questions was whether an environmental risk remained from 
using this material, even though it had been composted."

DPI conducted the research in collaboration with the NSW Department of 
Environment and Conservation (DEC) and Hawkesbury-Nepean Catchment 
Management Authority to evaluate the feasibility of compost-recycling 
aquatic weeds harvested from the Hawkesbury.

Dr Dorahy said the trial found it was critical to ensure the material is 
heated for long enough at a sufficiently high temperature ? and that it is 
mixed and turned sufficiently.

"The Australian Standard for composted soil conditioners and mulches 
states that the whole mass must be exposed to a minimum of 55 degrees 
Celsius for three consecutive days.

"To ensure Alligator and any other terrestrial weeds don't survive, it is 
important to monitor compost windrow temperatures, as well as the site 
itself, to make sure none establish in or around the compost."

Tests on the quality of the compost were also undertaken.

It was found to be comparable to compost made from organic garden 
material, though with lower concentrations of nitrogen, phosphorus and 
calcium and higher amounts of inorganic material such as sand. 

The concentrations of heavy metals and chemical residues in the composted 
material were very low, indicating a low risk from other contaminants .

Dr Dorahy said erosion control trials at NSW DPI's Centre for Recycled 
Organics in Agriculture (CROA) have shown that the compost is effective in 
controlling soil erosion and improving water quality. 

In addition, pasture establishment in the plots treated with the aquatic 
weed compost was very good.

"This indicates that the compost is likely to be a useful medium for 
re-establishing vegetation, particularly on sites denuded of topsoil. 

Dr Dorahy said that environmentally sensitive areas in the Sydney 
catchment can be affected by a high level of erosion, which impacts on the 
quality of waterways, and the productivity of agricultural land.

"Using good-quality compost to reduce erosion and reduce soil washing into 
waterways can help to improve water quality.

"There is potential to recycle aquatic weeds such as Salvinia and 
Alligator Weed, provided they are composted appropriately."

Other contributors to the project were Hawkesbury City Council, who 
provided their site at South Windsor for the composting operations and 
Bettergrow Pty Ltd, which conducted the composting.

Joanne Finlay
Science Communication Specialist
PH: 63913171
Mobile: 0428 491813
Fax: 6391 3199

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