“Biofilter” and “Bioreactor” are terms used by the Natural Resources and Conservation Service for filter beds built of wood chips and similar natural materials. They’re intended to extract nitrates and other pollutants in runoff water from field tile and surface drainage.
The challenge at our farm is the surge of storm runoff from 100 acres of a neighbor’s watershed. For years the floods have silted our pond and dumped algae-forming nutrients into it.
Nov. 22, 2017 — Our grandson Blake Carlson is working on a way to solving our pond scum problem and keep our pond water more clear. He negotiated a connection with a local source of wood chips and leaf compost. And he’s getting truckloads of “biofilter” blend, Using our tractor loader, he’s spreading a foot-thick blanket of this material around the slope above our pond, which is close to a half-acre in size.
The ground just above the pond has always been too wet to farm, even though we tiled heavily. After pulling out our combine a few times, we gave up on farming that 10,000 square foot area. Because the soil was wet, willow roots had a way of finding and plugging drain tile. The owner of Hershberger Tiling told us when he installed our pattern tile that “willows are the worst enemy of field tile.”
The mulch mix here, about half chips and half leaf compost, is more than able to smother willows and other weeds.
This barrier should also filter the bursts of sediment-loaded runoff water which sometimes roar across our field from the large, untiled neighboring field. Since we’re very conservative in nitrogen application in our own field, water from drain tiles hasn’t been the source of algae-feeding nutrients.
In past years, we’ve tried to keep the green pond algae in check with at least 10 grass carp, which grow to the size of small submarines. But they all died in a bitterly cold freeze four winters ago. Restocking with carp and giving them time to grow big enough to harvest the algae is still underway.
There’s a large amount of research on wood-chip biofilters, often used to clean gray water and nitrate-laden water. The wood and leaf material has a high carbon-nitrogen ratio, which is what you want for trapping nitrogen. The chips have huge pore spaces for rapid absorption, so the biofilter doesn’t clog readily.
Blake says: “My main reason for putting in wood chips is to plant food crops into that low spot. I’ll never have to water it, and almost never have to weed it.
“The mulch enhances soil microbiology. Most important, this combination allows growing incredibly nutrient-dense food to heal people. With a deep wood chip mulch, soggy soil can become prime productive land.
“Next summer with a polyculture of vegetables and plants, you’ll see how productive that spot will be. Keeping the pond clean is just an extra gift.”
Blake is already planting permanent food crops under mulch on other areas of the family’s ground.
Another minor benefit: We’ve found that the soil covered with such mulch beds is an easy place to quickly find dozens of earthworms for fishing in the pond.
We suggest that you check with staff members of your nearby city’s Park Service or street department to see what they’re doing with wood chips and leaf waste. Often, towns have a substantial disposal problem, and could be very eager to help you put their waste to a biologically beneficial use. Your county’s NRCS conservationist could also have some good technical information on “biofilter reactors.”