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.
Denitrification bed
Wastewater from the glasshouses passes through a denitrification bed before being discharged onto land. Denitrifying bacteria within the bed remove nitrate by converting it to atmospheric gases.
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.
Testing types of denitrification beds
An experimental trial at the Taupo District Council wastewater treatment plant uses different types of denitrification beds for removing nitrate from wastewater.
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.
Earth’s water distribution
A number of human activities can impact on the water cycle: damming rivers for hydroelectricity, using water for farming, deforestation and the burning of fossil fuels. When people use water for irrigation, they are taking water from streams or from the ground and, as a result, the water table drops. It can take a long time for groundwater to be recharged. The flow-on effect is a reduction in the amount of water in rivers or lakes, which then impacts on the environment – animals living in the water may find themselves affected.
Water that seeps through the soil carries particles of nutrients, and these particles will eventually reach the groundwater. Too many nutrients can pollute freshwater storages. Also, the removal of too many nutrients means farmers have to add more chemicals to their soils to keep the plants growing.
However, not all water that falls on the ground will seep into the soil. Some of the water will run off overland and transport soil particles with the water into streams and eventually into lakes. This can affect the stability of the land.
Deforestation or removal of trees changes water take-up and can lead, in extreme cases, to an increase in over-land run-off and erosion of land.
Points of interest
Students could consider what farmers could do to reduce over-land run-off. Think about what David Hamilton means when he talks about how we take water for human consumption without considering the water needs for our lakes, streams and groundwater.
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.
Nature of Science
Imagination and creativity play an important part in science. Louis saw how effectively riparian wetlands removed nitrate, so he came up with an innovative design to mimic natural processes.
Useful links
Visit Louis Schipper’s website to learn more about denitrification beds.
http://waiber.wordpress.com/projects/denitrifying-bioreactors/
