Biobeds for sprayer rinse
By John Dietz
Researchers used polyethylene tanks meant for fish, at Simpson, Sask. Note the grass growth on top and the drip line. Photo by Larry Braul, AAFC.
Thank the Swedes for this idea: “biobeds” that promise to protect water quality for generations to come. The concept represents a low cost, environmentally friendly way to deal with the rinse water flushed out of agricultural field sprayers.
According to Larry Braul, Agriculture and Agri-Food Canada water quality engineer in Regina, the biobed is an organic filter for pesticides, using conventional low value material. The use of biobeds has become an accepted practice in Europe in the past 15 years.
Braul and Claudia Sheedy, research scientist with AAFC at Lethbridge, Alta., are co-leading the project to develop a biobed model to support Canadian farmers. Starting with one biobed at Outlook, Sask. in 2014, AAFC expanded the project in 2015 to sites at Simpson, Sask., and Grande Prairie and Vegreville, Alta. An additional biobed was constructed in fall of 2015 and will be monitored in 2016 at Lethbridge. “At the end of 2016, we expect to have enough data to produce a construction, operation and maintenance manual for biobeds,” Braul notes.
Initial results promising
“The first year at Outlook, it was highly effective. It removed more than 98 per cent and up to 100 per cent of the pesticides it received. That was very positive, and the results we just got back for 2015 are very similar,” Braul says.
“Our climate is much colder than Europe and we have more intense rainfall events. We are working to address those issues with designs revised for the Prairies,” he adds.
In principle, a biobed is relatively inexpensive, easy to use and significantly accelerates the natural breakdown processes for pesticides. The most challenging aspect at this point is in finding or developing an inexpensive method to easily collect the sprayer rinse water. On most farms when rinsing, the sprayer arms are fully extended while water is pumped through the system. As a result, a catch basin for that spray would need to be up to 120 feet long by about 20 feet wide and would need to drain the spray to a point where it can be collected.
The contained biobed for the rinse water uses a mixture of topsoil, compost and straw. It provides an ideal habitat for microbes to break down the pesticides carried in the rinse water, to the point they pose no threat to the environment.
In the project’s first year, Braul and Sheedy discovered the biobed at Outlook was still frozen a few inches below the surface in May, when they hoped to use it. It needed to be warmed to about 10 C, so that microbes could process the rinse water.
They resolved that issue for 2015. Braul says, “Microorganisms like warm conditions. In a new biobed, we put heat tape at the bottom. We can get them up to almost 30 C at the end of May, so they can really start breaking down the pesticides. With a little heat application at the right time, we are probably doubling the decomposition rate they’re getting in Europe.”
European research found that half and up to 90 per cent of pesticide contamination in groundwater could be traced to the places where sprayers were rinsed, Braul says. Two factors go into that: there’s a concentration of pesticides in one place, and a lot of water washing it down. It’s too much for the microorganisms to process.
Often the topsoil is stripped off and replaced with gravel at the site where the farm sprayer is rinsed. This removes the organic matter that absorbs pesticides and allows the pesticide to leach through the soil zone. Often, it’s fairly close to the well that supplies the water.
“That’s the worst situation for managing the site,” Braul says. “It becomes quite a significant source of contamination. Instead, if we capture that rinsate, contain it and treat it, we can make a significant impact on the contamination problem.”
The Swedes were first to address the problem. They collected rinsate and applied it to the top of a simple hole in the ground filled with the biomix material. “The Swedes applied the rinsate to the top of the biomix and let it seep through into the ground. It was the standard for six or seven years. It was a heck of a lot better than putting it on gravel, because it absorbed a lot of the pesticide. Now, with more sensitive instruments, we know that model doesn’t remove all the pesticides,” Braul says.
Current practice is to build a contained biobed up to a metre deep. In the UK, that would be lined at the bottom with clay or plastic, and drained with weeping tile.
For their first project, Braul and Sheedy built a wood frame structure. On later projects they also used open polyethylene tanks meant for fish. Plans call for putting the biomix into big tote bags already used for storing granular fertilizer or pesticide. “Really, you can use anything as a container for the biomix,” Braul says.
The biomix material needs three basic components: topsoil (from a field is best, because it will already have microbes adapted to degrading pesticides); woodchips or straw (to provide the lignin for microbial food and structure); and, compost or peat (to provide the organic matter that absorbs the pesticides).
Among design variations tried in 2015, the most efficient was a two-cell system about a half-metre deep. Each cell has a six-inch layer of crushed rock at the bottom. A sump pump collects leachate from below the crushed rock in the first cell and pumps it to the surface of the second cell. “Two cells remove a much higher percentage of the pesticide than single cell biobeds,” Braul notes.
Although literature from the European experience suggests that nearly all the microbial activity happens in the top six inches of the biobed, most beds are one metre thick to provide additional absorption capacity. At the University of Regina, microbiologist Chris Yost is using DNA testing to determine the type and number of microbes at various depths. Yost hopes to determine the region of greatest microbial activity.
At Outlook, a two-cell biobed only a half-metre deep worked better than expected, Braul says. In practice, degradation of pesticides in the biomix can take three to six months, he adds.
There’s still a need to deal with the reasonably clean leachate coming from the bottom of the biomix, and a need for eventual disposal of the biomix itself. “Effluent has an extremely low level of remaining pesticide. We recommend spraying it on an area that has some organic matter and lots of microorganisms, and allow nature to do its work. One option is to put it into a tank and spray it someplace, or you can sprinkle it safely on grass or drip it along a row of trees. The little amount of remaining pesticide will be degraded in the topsoil,” he says.
Setting up a collection pad for the sprayer rinsate would be the biggest single cost. It can be constructed from heavy plastic but a concrete pad is ideal. “If you want to collect everything you rinse out, you have a fairly large concrete pad. Depending on where you are, it probably could cost $5,000 to $10,000. That’s a big challenge – but some inexpensive creative options are possible,” Braul says.