They say necessity is the mother of invention. Bob Bergquist agrees.

Bergquist is owner of EnviroPork, a 5,000-sow, farrow-to-wean operation near Larimore, ND. When North Dakota's Department of Health found him in violation of state odor restrictions, he found himself up against a wall. Rather ironically, that wall inspired him to develop another wall, the vertical wall biofilter, in an effort to reduce odors.

Engineers he consulted for odor control ideas suggested trying a biofilter system modeled after University of Minnesota trials (see "Biofilters Provide Low-Cost Odor Solution," page 12). Concerned about the effect of a biofilter on fan efficiency in his two, 80 x 560 ft. tunnel ventilated barns, and restricted by space to lay out the horizontal biofilter vegetative strip, Bergquist suggested the engineers modify the idea, incorporating some windbreak ideas that he had heard of.

They built a 9-ft.-high superstructure out of treated lumber, positioning it 8 ft. (two fan diameters) away from the fans at the end of the barn. The superstructure was closed off square with the 80-ft. wide barn, with doors provided to give access to the area between it and the barn. (The accompanying photos show a similar structure built by Premium Standard Farms, Princeton, MO.)

Bergquist built the superstructure about a foot off the ground and spread gravel underneath. A plywood skirting is attached to restrict air flow.

Within the superstructure, two, 4 x 4 ft. chicken wire "cells," one on top of the other, divided by a 2 x 4 in., are lined up straight out from each of the 10, 48-in. fans on the ends of the barn. The chicken wire cells are 2 ft. thick and stuffed with barley straw. They decided to use the two 4-ft. cells rather than an 8-ft. straw wall to limit settling of the straw. The top is left open.

"When we presented the idea to the Health Department people, they laughed and said this would never work," says Dan Stepan, engineer with the University of North Dakota's Energy and Environmental Research Center. "We got it built and low and behold it worked better than anybody had even anticipated."

Hydrogen sulfide emissions, as measured with a Jerome Meter, have been reduced from 50 ppb down to 2 ppb.

Measurements of odor taken with a scentometer dropped to 2 or less. (The scentometer records a 0, 2, 7 or 15.) Frequent readings of 7 and 15 had put EnviroPork in violation.

Stepan agrees more research is needed to explain the lower odors and emissions, but he notes a smoke test provided some clues.

"The smoke test showed the air flow patterns between the fans and the filter wall and what you see is a real good recirculation pattern in the chamber," he says. He believes the mixing in the chamber and the air getting pushed upward where it can be more fully dispersed are key points. He also says the filter wall traps a significant amount of the particulate matter - a large proportion of odor.

Additional trials will be conducted this summer to try and force more air through the filter and to channel more air further upward. Smoke tests indicate only 10-15% of the air gets blown through the wall.

Bergquist says maintenance so far is limited. Rain soaks through the filter and evaporates. Chicken wire can be removed from one side, the old straw taken out and the cell repacked. He expects to repack it every spring.

There was concern about the wall's impact on fan performance. But, Bergquist says, readings before and after biofilter installation show they've improved performance. He speculates the wall serves as a windblock, thus, actually aiding fan efficiency.

Barley straw and wheat straw have both been used as a medium. The barley seems to work better. "There may be some microbial action that does some biodegradation of some of the odor compounds," says Bergquist.

A roof is being considered to help with the recirculation process, and also to prevent the wall from catching snow. A 20-ft. snow drift caused some fans to break off last winter. So far, the project has cost less than than $2,000. Bergquist says that's a price he was willing to pay now that the Health Department standards are being met.

New legislation passed this year in North Dakota gives livestock facilities a half mile setback. Although EnviroPork is a mile and a half from the nearest neighbor, a suit filed by environmental and family farm advocates had forced the Health Department to take measurements at the section road, 50 ft. from the barns.

Producers and researchers elsewhere are tinkering with ideas similar to Bergquist's. Premium Standard Farms (PSF) began experimenting with what they're calling "air dams" last summer. Although no hard scientific data on odor and dust reduction are available yet, PSF environmental affairs director Dave Townsend says field observations have them convinced the air dams work. It convinced them enough that by the end of the summer they'll have built about 200 of them on their tunnel ventilated barns in Missouri, spending an estimated $1,000 on each structure.

The PSF design is similar to Bergquist's. They use primarily 2 x 4 in. lumber, high density polyethylene, chicken wire and straw. The PSF structure is also set 8 ft. out from the barn, but varies in that it uses an angled extension rather than a fully closed off end. PSF only went 3 in. thick with their straw, but ran straw the full length of the building and all the way down to the ground.

Townsend says they based their air dam idea off North Carolina State University agricultural engineer Bob Bottcher's windbreak designs. Bottcher has had success reducing odor with windbreaks built from pipe frames. His design uses tarps connected with grommets and breakaway ties. Modeled after similar designs used in Taiwan's poultry industry, Bottcher says the windbreaks work well on tunnel ventilated barns, especially those that blow out directly over a lagoon.