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Aerial photo of an on-farm wash water treatment system on a potato operation in Alliston, Ont. This system includes sedimentation and aeration basins, treatment wetland cells and a sand filter. Photo: AAFC.

Potato Wastewater

New treatment strategies and technologies being examined in Ontario promise to help Canadian potato operations lower the costs of reducing farm wastewater and also meet future regulatory requirements.

Water availability and water quality are two issues that are gaining a lot of attention these days, and concerns have led to stricter agricultural water-use regulations in some provinces.

In Ontario, for example, farmers will feel the impact of new phosphorus loading targets for Lake Erie that the federal government is expected to announce soon, as part of the Great Lakes Water Quality Agreement between Canada and U.S.

When tubers are moved out of storage for processing, potato operations produce farm wastewater during both the washing process and use of water-filled plumes to move potatoes during handling.

This effluent contains suspended solids from soil and organic debris from damaged and spoiled potatoes, so the question for growers is how to best treat this wash water, in order to meet current and future regulations and not break the bank?

For possible answers, we can turn to three potato wash water treatment research studies from Ontario. One falls under the umbrella of the of the Holland Marsh Growers’ Association Water Project (HMGA WP), now in its third year and funded by Environment Canada.

This project aims to promote whole-farm water quality improvement on all sorts of farms through identifying levels and types of nutrients and sediments in wastewater, evaluating new water treatment technologies, and management strategies for water use and treatment with the focus on Lake Simcoe and Georgian Bay.

Researchers who looked into potato wash water found that there is potential to reuse fluming water after adequate treatment when moving potatoes.

“This water doesn’t need to be kept potable as long as there is a final potable rinse prior to packing for fresh market potatoes,” notes Bridget Visser, HGMA WP communications director. “Soil and nutrients would need to be removed and dosing for oxygen content may be necessary prior to discharge. The starch from potatoes that are cut and peeled is an added challenge.”

Two potato operations were involved in the project. “The first wanted to recycle fluming water to both reduce consumption and eliminate discharge,” Visser explains. “The solution was to pump the used flume water through a filter bag, catching the solids, and clarifying the water for reuse.”

The second operation packs and processes potatoes and other vegetables within the same facility. According to Visser, the water from its potato lines is high in solids and nutrients, but it is successfully treated so it can be discharged into the environment using a multi-celled settling system and experimenting with other technologies such as a filter bag, aeration, and a hydro-sieve.

Contained in these test tubes are: wastewater after sedimentation (left); wastewater after treatment with a coagulant (centre); and wastewater after sand filtration (right). Photo: Vera Bosak, University of Guelph/AAFC.

Contained in these test tubes are: wastewater after sedimentation (left); wastewater after treatment with a coagulant (centre); and wastewater after sand filtration (right). Photo: Vera Bosak, University of Guelph/AAFC.

Sand Filtration

Meanwhile, researchers from Agriculture and Agri-Food Canada in Ottawa (Andy VanderZaag, Denise Chabot, Shea Miller and Vera Bosak) and the School of Environmental Sciences at Ontario’s University of Guelph (Anna Crolla, Chris Kinsley and Robert Gordon) recently examined sand filtration as part of potato farm wastewater treatment.

“Sand filters have been used in wastewater treatment since the early 1800s in many agricultural settings,” VanderZaag explains. “They are an easy, low-cost option for the removal of organic and inorganic solids. We wanted to look at how two different sand filter designs, saturated flow and unsaturated flow, perform for potato wash water treatment specifically, at different loading rates.”

The team found that while both types of sand filters do a good job (with the lowest water flow rate producing the highest suspended solids removal rate), unsaturated flow filters did a better job at total suspended solids removal (89 per cent) compared to saturated flow filters (79 per cent).

Total nitrogen removal was double with unsaturated, because of the recurring cycle of aerobic and anaerobic conditions during sand saturation and drying. Removal of total nitrogen was also higher in unsaturated flow filters.

“Building an unsaturated flow filter, however, takes more engineering and infrastructure to operate and maintain during the winter to prevent freezing,” Bosak notes. “It requires a top-dosing system and bottom outlets for the water. Saturated filters, in contrast, are well-suited to winter operation and are easier to build, similar to building a horizontal subsurface flow wetland, for example.”

Given that a potato farm does not produce wastewater every day, a system with a smaller sand filter size would be suitable, in the research team’s view. However, it should be a system that is operated with flowing water running through it several times a day to keep it functional.

Hydrosieves such as these (seen here in use with carrots) comprise one of the technologies being explored to treat potato wastewater. Photo: Holland Marsh Growers’ Association Water Project.

Hydrosieves such as these (seen here in use with carrots) comprise one of the technologies being explored to treat potato wastewater. Photo: Holland Marsh Growers’ Association Water Project.

Reducing Water Use

Another research project involving many of the same scientists came about after potato farmers were informed by the Ontario Ministry of the Environment and Climate Change that new water regulations would require boosted on-farm wash water treatment.

“This prompted the farmers to contact us, and we suggested an ‘integrated water resources management’ (IWRM) approach,” Bosak explains. “It’s a holistic and inclusive method that results in water being managed efficiently, equitably and sustainably.”

VanderZaag, Bosak, Crolla, Kinsley and Gordon, together with potato farmers and environment ministry staff, followed IWRM guidelines. They began by setting goals, conducted a one-year initial assessment of on-farm water use, brainstormed and planned strategies based on these results, implemented the strategies and monitored over a second year, and then did a final assessment.

The project took place at a large potato storage facility in Alliston, Ont. During shipping time at this operation, potatoes were first transported over finger rollers to remove rotten ones, soil and debris. The tubers were then moved to a grading station (and washed at the same time) using a flume system.

The fluming wash water was pumped from the facility into a primary sedimentation cell of a multi-cell ‘constructed wetland’ treatment system, which eventually discharged into surface water.

“We didn’t have a clear idea how much water was being used for washing, so we installed three flow meters,” VanderZaag explains. “We monitored for a year, and found out that the amount of water being used to prevent sand and debris from damaging the pump bearings and for cleaning the flume was higher than anyone had expected. So we put a focus on reducing these two uses.

“Decreasing the wastewater volume makes treatment easier,” he adds “It’s an important factor in regulatory compliance, and it also translates into lower treatment costs for farmers.”

The existing practice at this facility was to protect pump bearings was to fully open the bearing-lubrication valve at the start of each day and to let it flow all day. To reduce water usage there, the group agreed on a strategy to determine the minimum water required for proper bearing function and then install a flow-control device.

In the second area of targeted water use reduction (washing the flume after each day of potato handling to prevent the build-up of solids that could damage equipment and clog outflow), the chosen strategy was to wash the flume every other day or only when necessary.

The group went even further to reduce water usage. “We could see that we could decrease the need for flume washing by reducing the amount of solids entering the flume in the first place,” VanderZaag notes. “There were 24.5 tonnes of total suspended solids measured over year one.”

The group decided to install new finger rollers, part of a custom-built machine purchased by the farmers with partial cost recovery provided through a provincial government program. The machine had a significantly larger capacity and an improved design with higher capacity, more rollers, and more dirt elimination features.

From implementing the strategies and adding the new machine, total water use for potato washing and fluming at the facility in year two was nearly half that of year one.

“The water used for cleaning the flume was reduced by two-thirds, from 1,481 to 483 cubic metres,” says VanderZaag. “The water used to protect the pump bearings was reduced by half. We are very pleased to say that the changes required relatively little capital investment and resulted in no reduction in yield or disruption of operations.”

 

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