Cow shed effluent converted to fish food

By Jim Cornall contact

- Last updated on GMT

A farm trial in New Zealand is looking into using farm effluent to create fish food.
A farm trial in New Zealand is looking into using farm effluent to create fish food.

Related tags: Agriculture, Dairy farmers

DairyNZ and Bay of Plenty Regional Council held a public open day on a dairy farm near the New Zealand coastal community of Katikati to demonstrate the on-farm trial of a process to convert dairy shed effluent into fish food.

The project, jointly funded by the Bay of Plenty Regional Council and Ministry for Primary Industries Sustainable Farming Fund, is working to convert nutrients from dairy shed effluent into algae and zooplankton that can be used to feed fish.

This conversion process continues within the farm drains, with the additional benefit of converting run-off nutrients from pasture.

In mid-2015, the physical works for the three-year trial were completed and the facility started operation in December.

Increase in fish numbers

Regional Council rivers and drainage manager, Bruce Crabbe, said that the potential implications of the project for farmers were huge.

He said that improved water quality in the drainage system would mean an increase in numbers of fish species such as tuna (eel), inanga (whitebait), and mullet.

“Dairy effluent is currently used as a liquid fertilizer around New Zealand and this trial is considering an innovative alternative use. This interesting solution could see dairy farmers, aquaculturalists, and the environment benefit from a new use of existing technology.

“A successful trial will provide options for resilient and sustainable farming systems that reduce nutrient loss, improve water quality and aquatic habitats, and provide additional income from aquaculture,” ​Crabbe said.

Huge economic potential

It is possible that in addition to increasing the number of fish in local rivers and improving water quality, the system could be adapted and developed to create commercial fish food.

Crabbe added that farmers were interested in the potential economic advantages of the project.

“It was great to see visitors catch on to the possibilities that open up if we can show that the technology works in this trial,”​ he added.

Effluent creates algal growth for plankton

The system uses dairy effluent to provide nutrients to promote algal growth in purpose-built High Rate Algal Ponds (HRAPs).

The algae are settled and harvested in Algal Harvest Ponds (AHPs) then piped in slurry form to a separate zooplankton pond constructed within the farm drainage system.

Zooplankton then feed on the algae and grow to provide a live food source for fish.  With the consistent availability of an abundant and nutritious food supply, fish will be encouraged into the farm drainage system from the Aongatete River through a fish-friendly passage.

The NIWA (National Institute of Water and Atmospheric Research) designed both the HRAPs and the AHPs. Raglan EELS, a New Zealand based environmental research company, designed the zooplankton pond, including the aeration devices and level control structures.

Related topics: R&D, Sustainability, Fresh Milk

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1 comment

Cow sheds

Posted by Jack Spratt,

I presume the effluent from the fish can be turned back into cow food. The Bay of Plenty area of New Zealand does, I understand, include the town of Te Puke which is home to the Kiwi fruit industry. I think in the circumstances I will stick to eating kiwi fruit rather than fish fed on cow effluent.

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