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Covering up the smell

Negative air pressure systems offer other benefits besides odor reduction.

You might think that big piece of black plastic fabric was floating on fresh water. That's how fresh the air really is near the negative air pressure (NAP) covers floating on a handful of lagoons in Canada and the U.S.

Odor control is the big selling point, though it's not the only one, says Doug Small, Canadian co-developer of the NAP system. The cover also:

  • Reduces greenhouse gas emissions.
  • Prevents nitrogen losses.
  • Keeps rain out of the storage.
  • Prevents erosion of the slopes.

A secondary feature, a compressed-air agitation system, was added in 2003, which permits pump-out without removing the cover.

Small and his partner, Dennis Hodgkinson in Winnipeg-based DGH Engineering, developed the NAP technology after first trying an opposite-air supported dome approach.

“Instead of blowing a cover up and pressurizing, we turned the fans around, sucked the air out and sucked the cover down to the surface,” Small says. “It worked so well that we commercialized it and formed Encon Technologies to market it.”

The prototype, placed on a primary cell in Manitoba in 1996, is still functioning. Since then, DGH Engineering has found an improved, 20-ml UV-inhibited fabric. “We rate the cover to last at least 10 years,” says Small.

One-Piece Installation

On site, an installation crew welds the plastic material into a single sheet. They dig a trench around the perimeter of the cell, fold out the plastic and place it in the trench. A small air duct goes in at the same time. Several one-third horsepower fans and centrifugal blowers are connected to the duct, to draw out biogas and any air that seeps in through the soil.

The trench is refilled and the fans are turned on. As air is sucked out, a static pressure develops that presses the fabric to the liquid surface. Even high winds can't get a grip on it. Operating cost is less than $500 Canadian/year for the fans.

Odor, evaporation and nitrogen loss stop immediately. Rain can be pumped off before it gets into the lagoon.

Bubbles of biogas in summer gradually move to the perimeter to be sucked out. It's a concentrated ‘burp’ of biogas, but very small in relation to the size of the storage.

Small estimates nitrogen losses with the NAP system at less than 2%, compared to around 40% on earthen manure storages that have straw or other covers.

“If nitrogen value is 30-35¢/lb., what you save in nitrogen content virtually pays for the cover in about seven years,” he says.

Independently, he adds, Encon Technologies and the Prairie Agricultural Machinery Institute have reached very similar conclusions on cost/pig for the floating cover.

“If you have a covered storage, it adds 24¢ to the cost of raising that pig,” says Small.

Air agitation system

The second part of the system, compressed air agitation, solves a perennial problem with solids in liquid manure storages and offers some damage prevention.

Conventional methods agitate only one part of the storage at a time. “Solids are settling 15 minutes after you move the nozzle,” says Small. “You end up chasing the solids around the storage.”

As well, clay-lined earthen manure storage requires special care. “People keep the agitator in one position too long and erode a hole in the clay liner. Our system eliminates that risk,” he says.

Encon's solution is some ¾-in. PVC pipe, a set of proprietary air diffusers on a 39.4 in. grid at the bottom of the lagoon, and a wet well beside the lagoon.

“We hook up an industrial air compressor and blow air through the diffusers while we're pumping from the wet well,” says Small. “The bubbles create enough turbulence to stir up the solids — under the cover.

“The diffusers are a very simple design and not expensive. They can lay at the bottom without any operation for 12 months or more while solids build up on top — but they won't plug up over that period,” he explains.

Cost of the air agitation system, which can be used without a cover on the lagoon, is similar to the cost of conventional agitation.

Independent study results

Nazim Cicek and Qiang Zhang, University of Manitoba Biosystems Engineering professors, launched a two-year independent study of odor emissions from different types of farm operations in early 2003. Biogas or greenhouse gas emissions have been measured from open hog manure storage, straw-covered storage and NAP-covered storage.

“Preliminary results from the negative air pressure site show that the synthetic cover had a very significant effect on how much odor was emitted,” says Cicek. “Odor is reduced by almost 99.9%. It is high in concentration, but very, very little is coming out of those pipes and it is very low in relation to the overall mass.

“The preliminary results indicate carbon dioxide was reduced by 97% and methane was reduced by 87%, so there's a dual benefit from the negative air pressure system,” he adds.

NAP availability

A joint strategic alliance to market, supply and install negative air pressure floating cover systems in North America was announced in April 2003 between Encon Technologies and Layfield Geosynthetics & Industrial Fabrics.

The covers are patented, lightweight systems using geomembrane material produced by Layfield. The company, with offices in Seattle, WA, and Edmonton, Alberta, is ISO 9002 registered, with over 25 years of experience in supplying flexible membrane liners.