Taking guesswork out of food processing wastewater disposal
For more than a century, cheesemakers and vegetable processors have applied water remaining after production to open fields and farmland. Producers have removed most impurities from this wastewater. But there was no definitive answer on whether the remaining nitrogen (in the form of nitrate) was polluting groundwater.
The report of a three-year study, jointly funded by industry and the Wisconsin Department of Natural Resources, was released late last year. The results offer some support for the long-held belief that much of the nitrogen in the wastewater leaves the soil and harmlessly enters the atmosphere.
Wastewater disposal is a daily need at Del Monte’s canning factory in Cambria (a village in Columbia County, Wisconsin, US), which processes green beans, peas and sweet corn for about five months every year. Each day, the Cambria plant pumps about over a million litres of wastewater, mainly from washing, blanching and cleaning, to a nearby hayfield, where a centre-pivot irrigator drips it onto the soil.
Producing 1.5 billion kg of cheese in America’s Dairyland creates about 21 billion litres of an effluent that carries a higher concentration of nitrogen than cannery wastewater, and is typically disposed of in furrows. Although some large cheese facilities use in-plant wastewater treatment systems, a solution that is expensive for vegetable processors who operate seasonally and for small and mid-size cheese plants.
In both the vegetable and cheese industries, the study results could affect the cost of disposal, by raising or lowering the need for land and equipment.
As well as providing some support to the belief that much of the nitrogen in the wastewater leaves the soil and harmlessly enters the atmosphere, the groundbreaking study also showed that rates of this process, called denitrification, vary depending on season, soil type, crop cover and application rates, said Geoffrey Siemering, a UW–Madison soil contamination specialist.
Because the atmosphere contains about 78% nitrogen, measuring transfers from the soil is extremely difficult, said study leader Francisco Arriaga, a professor of soil science. The automated technology developed in his lab captures and measures nitrogen as it escapes to the atmosphere. Efforts by PhD candidate Clay Vanderleest were instrumental in developing this technology for the study.
“We expected to be surprised, as this important measurement has never, to our knowledge, been possible in real time,” said Siemering, who works with Arriaga. “What we found was reassuring, on the whole, but also suggests that better system management could improve how the water is treated and further reduce the possibility of nitrogen entering groundwater.”
The study made repeated measurements of nitrogen release at disposal systems used by three cheesemakers and three vegetable processors in Wisconsin. Data were gathered from probes buried in the soil. Gas was collected from the soil surface during several one-week periods at different times of the year.
The heart of the system was a series of automated sampling chambers connected to a photoacoustic analyser. The analyser, based on an 1880s invention by Alexander Graham Bell, identifies elements by exposing samples to specific wavelengths of light, and then measuring the soundwaves that are generated. This system measured nitrogen gas exiting the soil every few minutes.
The new results should help to refine disposal permits so vegetable processors and cheesemakers can maximise the amount of wastewater disposal while still protecting the environment, Siemering said.
The $255,000 project was funded by the Midwest Food Processors Association, Wisconsin Cheese Makers Association and the state DNR.
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