Seaweed could soon be used to clear pollution from waterways after a successful trial in Thames that tested the aquatic plant’s ability to soak up freshwater contaminants.
The study by AgriSea and Waikato University took water from the Waihou Estuary and pumped it into tanks on land filled with seedings of sea lettuce, Ulva australis.
The seedlings feed on the nutrients in the water, acting like a sponge in the process as they grow.
The resulting filtered water has 90% less nitrates and 70% less phosphorus, according to measurements by Waikato University.
AgriSea chief innovation officer Tane Bradley says the results far exceeded their expectations, compared to the initial nutrient-removal modelling they had calculated.
The Paeroa-based company set up a testing site on the outskirts of Thames close to the Waihou River using three custom-made high-rate algal ponds – tanks that resemble oversized bathtubs.
Each tank has a different weight of seaweed and water flow rate to determine what the optimal amount of seaweed is required versus the flow rate to result in the best nutrient removal.
The water is also constantly in motion to ensure a steady supply of nutrients for the seaweed and to prevent the seaweed from floating to the bottom of the tank, where it would be deprived of light.
The testing site will help formulate what will be needed if the project takes its next step, which is to be scaled up.
Overseeing the day-to-day handling of the project is AgriSea research technician Taylor Moore.
Every week, the tanks are stocked with seaweed, and water samples are collected twice a week and analysed by scientists at Waikato University to determine the rate of nutrient removal using a before-and-after comparison.
Moore says he was initially skeptical of the likelihood of seaweed growing in such contaminated water, which he described as “the rankest water ever”, but the results shocked him with the lettuce growing almost twice as well as what was hypothesized at the beginning because of the nutrient availability in the water.
Moore says the pilot system is projected to have an average biomass production of 15g per square meter per day, yielding around 328kg of dry weight biomass annually.
With the sea lettuce biomass containing about 4-5% nitrogen in nutrient-rich conditions, harvesting 328kg of biomass would eliminate around 16kg of nitrogen from the environment. “Scaling this up, using the same productivity assumption, annual production would reach upwards of 54.8t of dry weight per hectare, removing 2.7t of nitrogen – equivalent to 12t of nitrate per hectare annually.”
For AgriSea, the project is not just about research, it is also about determining if it is scalable, which Bradley believes they have achieved.
“We wanted to see if this system could work in open environments to soak up nutrients and provide a model for others to pick up and upscale across the country. We are happy with the results of this pilot phase.”
The next steps will be more modelling around the cost to upscale the project so it will work in a full-scale commercial setting. There is an opportunity to partner with others who are interested in the technology, including councils, water treatment companies, local iwi, farmers and landowners – anyone wanting to clean up water and create opportunities.
The pilot was funded using just under $700,000 from the government’s Sustainable Food and Fibre Futures fund (SFF Futures). AgriSea contributed $108,000 and the Agricultural and Marketing Research and Development Trust (AGMARDT) is investing $150,000.
Thames-Coromandel District Council gifted the land lease for the project term worth $40,000, with support from Ngāti Maru and Ngāti Hako. Hauraki District Council, PwC and Te Waka are also assisted with the consent and application process.
Bradley says the collaboration between all these partners has been key to the success and outreach of this project.
“We’ve had local school groups, film crews, national and even international interest for this work – for us it’s about providing solutions and opportunities to the nutrient challenges we have.”
AgriSea met with members of local government and scientists and community recently to plan the project’s next steps.
“There’s a genuine opportunity here and it’s not about pointing fingers.
“Our hope is that it can be adapted and scaled to fit alongside any waterway.”