If you want to set a strategy for proper separation distances for odor control, a good rule of thumb is to first balance the nutrient needs of the land.

When you allow for adequate cropland to apply the proper amount of nitrogen and phosphorus, you may come close to providing adequate separation distance to protect against manure odor nuisances.

Those are key research results of a project entitled, "Separation Distances for Swine Odor Control in Relation to Manure Nutrient Balances."

The goal of this research project by Texas A&M's John Sweeten was to improve manure odor dispersion by ensuring adequate separation distances. Sweeten is resident director, Texas Agricultural Experiment Station at Amarillo.

He came up with minimum separation distances based on two approaches. One method involved actual field observations of odor concentration at two commercial hog farms to evaluate the impact of distance downwind from the odor source. The second was formulation of minimum separation distance standards based on creating a nutrient balance for nitrogen (N) and phosphorus (P).

That formulation uses typical annual estimates to match N and P crop requirements to achieve nutrient balance in land applications. Common loading rates that meet soil needs are 200 lb. PAN (plant available nitrogen)/acre/year and 75 lb. phosphorus/acre/year.

Table 1 on page 51 compares estimated land area needed for nutrient balance with two types of waste management systems. System A calls for flushing with fresh water in farrowing and nursery units and using recycled effluent to flush finishing buildings. Flushed water runs through screened mesh to a two-stage lagoon system and is irrigated onto cropland. System B employs a manure scraper with limited flush assist to flow manure into an underground pit. Manure is pumpedout to a covered, above-ground storage pit and injected into the soil in the spring. Table 1 indicates that the land area needs to be three times as large for System B over System A to produce a nitrogen balance.

For example, in terms of nitrogen needs, in a 10,000-head operation with PAN application rate of 200 lb./acre/year, a minimum buffer zone or radius of 2,360 ft. around the facility is needed, along with 403 acres for distribution of lagoon effluent using System A (See Table 1). A radius of 3,960 ft. is indicated for a 10,000-head operation using System B with 1,130 acres. Buffer distances are generally 67% larger for System B than for System A for nitrogen balance considerations.

For phosphorus balance, larger acreages will be needed than for nitrogen balance. At a rate of 75 lb./acre/year as shown in Table 1, 2,600 acres of cropland is needed per 10,000 hogs in System A for phosphorus balance and 3,000 acres with 10,000 hogs in System B. The minimum radius or buffer zone required for phosphorus per 10,000 hogs is 6,030 ft. and 6,450 ft., respectively, for Systems A and B.

Sweeten used a sensory odor concentration device known as the Scentometer to calculate minimum buffer distances for odor dispersion. He tested two farrow-to-finish operations. One, an 8,400-sow unit, uses System A for waste management, while a 200-sow operation uses System B. Odor observations were summarized according to location proximity to the hog facilities. The larger operation recorded 66 dilutions to threshold (DT) for all buildings and lagoons, the highest concentrations coming from the primary lagoon serving grow-finish barns.

By comparison, odor concentrations were 25 and 31 DT for swine barns and the manure storage tank at the 200-sow operation.

The standard odor concentration of 2 DT is regarded as a low-odor level that generally does not cause nuisance conditions. Sweeten says a regression model indicates that a distance of 2,600 ft. from the odor source produced 2 DT at the 200-sow farm using System B. Greater distance of 7,580 ft. was necessary for the larger operation using System A to achieve 2 DT.

For both farms, odor levels were related to distances downwind of facilities.

In short, Sweeten calculates that the 8,400-sow farm with all buildings attached would require a minimum separation distance of 6,850 ft. to achieve a nitrogen balance and 17,500 ft. to achieve a phosphorus balance (See Table 2).

By comparison, reaching a 2 DT odor concentration would take 7,580 ft. of buffer distance.

Therefore, the phosphorus balance would be the limiting distance for facility design. It would provide the widest margin of safety for odor control.

Adequate separation distances for the 200-sow operation include 1,770 ft. for nitrogen balance, 2,880 ft. for phosphorus balance and 2, 600 ft. for odor control. Site selection and facility design must be based on information that will provide adequate separation distance to sustain plant and water quality and protect against nuisance odor. Sweeten suggests that these objectives can be met by selecting the greater of the distances.

Researcher: John Sweeten, Texas Agricultural Experiment Station, Texas A&M University, Amarillo, TX. Phone Sweeten at (806) 359-5401; e-mail Sweeten: j-sweeten@tamu.edu.