Aquaculture Potential in Nutrient Runoff

The Diary Effluent Conversion Project is jointly funded by the Bay of Plenty Regional Council and Ministry for Primary Industries’ Sustainable Farming Fund, and also involves NIWA, BOP Polytech and Raglan Eels. It has been funded for three years (from November 2014 to November 2017).

The idea was inspired by the work of freshwater scientist the late Charles Mitchell, who was well-respected for his 40 years of freshwater fisheries research.

Charles’s vision was to develop wetland and riparian buffers to intercept, trap and remove nitrate. His idea was to come up with a system to collect and manage nutrient-rich farm drainage so that it supports healthy, productive and valuable fish life before it leaves the farm.

While spreading dairy effluent on pastures is the accepted use, nutrient leaching is still an issue. This project investigates other uses for the effluent. Rivers and Drainage Manager for the Bay of Plenty Regional Council, Bruce Crabbe says the potential implications of this project for farmers around the country are significant. He says that if the trial is successful it 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.

An initial trial funded by the Regional Council’s Innovation Fund saw the concept proven in tanks, allowing the project to go to the next stage – trialing the system on an operational dairy farm.

In mid-2015, the trial began with the construction of special algae ponds at a cost of about $115,000. The trial site is on the dairy farm of Ian Noble. He was approached to do the trial on his property because it has on its boundaries the Aongatete River, two roads and kiwifruit orchards, making it effectively isolated from other dairy farms.

Dr. Rupert Craggs heads a team at NIWA that has been looking at using enhanced pond systems to treat and recover resources (energy, nutrients and water) from wastewater. The NIWA system incorporates anaerobic ponds to recover energy from wastewater solids as biogas, and high rate algal ponds to recover wastewater nutrients by growing algae. The algae are harvested in settling ponds and have potential to be used as a fertiliser, feed or biofuel.

This project will demonstrate the use of high rate algal ponds as a cost-effective way to remove nutrients prior to irrigation of farm dairy effluent and to beneficially use the algae as a food source for freshwater zooplankton to promote native fish in farm drains.

The key part of the project is two purpose-built high rate algal ponds into which some of the effluent is pumped. These ponds have been set up to encourage the growth of algae.

Two small paddlewheels, one in each raceway-shaped pond, keep the pond water mixed, which encourages the growth of ‘good’ green algae.

During the process the algae take up ammonia, phosphate and carbon dioxide and the ultraviolet light in sunlight provides disinfection, helping to improve water quality.

The high rate algal pond effluent flows by gravity into small inverted funnel-shaped algal harvest ponds, where it is settled and harvested and then pumped in slurry form to a separate zooplankton pond constructed within a section of farm drain that has been cleaned and widened. Zooplankton are minute shrimp-like crustaceans which feed on the algae and grow to provide a live food source for fish.

There’s a control structure at the end of the zooplankton pond where it meets the farm drain, which controls its water levels and aerates water as it passes. Screens within the structure also allow zooplankton to pass through to the drain while stopping fish, other than the occasional eel travelling overland, from entering the zooplankton pond.

Central to Charlie Mitchell’s vision was the consideration that nutrient loads were not the only factor in unhealthy farm drains. Oxygen depletion has a significant impact and the zooplankton ponds are wider and include aerators to keep oxygen levels healthy.

By continuing the process in the farm waterways, it is hoped that the process will have the additional benefit of capturing the diffuse run-off from paddocks (in addition to the direct collection from the dairy shed).

Fish-friendly floodgates, at the drain’s outlet to the Aongatete River, enable native fish species to enter the farm drain to find food.
The quality of the water entering the river should not be impacted by the project. It is hoped that providing extra food for native fish should help boost fish stocks.

One concern the scientists have about the project is that it takes some of the dairy shed effluent that would normally be spread back across the farm and uses it to grow the algae. This should not be a concern as the effluent can still be irrigated, but the nutrients will have been removed. This will be beneficial for farms trying to reduce farm nutrient losses to meet nutrient limits