Keeping it clean

Less than 1% of Earth’s water is directly usable for human consumption, and Dr Louis Schipper’s goal is to keep this water clean. Instead of using high-tech or costly approaches to remove contaminants, Louis’s approach is to mimic nature.

Nitrate and denitrification

Nitrate, a form of nitrogen, is a significant surface water and groundwater contaminant. Nitrate is derived from animal wastes, nitrogen-fixing plants, fertilisers and wastewater. It is water-soluble and is readily leached into groundwater or carried into surface water. Increased nitrate levels contribute to eutrophication, algal growth and habitat loss in lakes and rivers.

Under certain conditions, bacteria can convert nitrate to nitrogen gas. Known as denitrification, this process removes nitrate from soil and water and returns it to the atmosphere, completing the nitrogen cycle. Denitrifying bacteria occur naturally in the soil. Under anaerobic conditions, like waterlogged soil, the bacteria use nitrate instead of oxygen for respiration. Using organic material as a food source, the bacteria are capable of removing significant amounts of nitrate.

Replicating nature

Louis saw how effectively natural riparian wetlands removed nitrate so he decided to replicate these conditions by building denitrification walls. The walls are actually trenches dug into the ground, which are back-filled with a mixture of organic material (woodchips) and soil. The walls intercept shallow groundwater and provide anaerobic conditions ideal for denitrification. As nitrate-laden groundwater passes through the porous walls, denitrifying bacteria use the wood chips to convert the nitrate to nitrogen gas. Once built, the walls look after themselves. Louis isn’t certain how long the walls will remain effective, but one wall he built in 1996 is still removing nitrate.

Louis and his colleagues at GNS Science and Landcare Research are also investigating denitrification beds. Louis describes the beds as large lined containers filled with wood chips. Wastewater is pumped through the beds removing substantial amounts of nitrate before being discharged onto land or into ditches.

Louis is a ‘big picture’ scientist. He enjoys the challenge of seeing how individual pieces of a research puzzle can come together. He also likes the teamwork science involves. This enables Louis and his colleagues to work to each other’s strengths in keeping our waterways clean and productive.