| | Nitrogen losses from snowfluent treatment of liquid hog manure | Phosphorous and potassium losses from snowfluent treatment of liquid hog manure | Nutrient content of the residue
The use of this technology by pork producers raises a issue not typically encountered in wastewater treatment. Treatment systems like Snowfluent focus on maximum water treatment. But since some hog producers use liquid manure as a nutrient source for crops, they are interested in retaining as much of the nutrient content of the manure as possible.
Irrigating with the meltwater is possible if re-use for manure flushing is not feasible. However, since the nutrient content of the water is reduced after the Snowfluent process, the value of the runoff water as a crop input would probably not outweigh the cost of hauling and spreading it. The ideal design for Snowfluent in a hog- operation context, is to maximize the amount of nutrient retention in the solid residue. This leaves a relatively clean runoff fraction for reuse in the barns, and a nutrient rich, lightweight solid which can be spread on cropland.
Table 3. Raw Liquid Manure Characteristics
Test Location | % Total Solids | % Total Nitrogen | % Total Phosphorus | % Total Potassium |
| This Trial | 0.01 | 0.24 | 0.04 | 0.11 |
Typical
(AAFRD, 1995.) | 4 | 0.36 | 0.13 | 0.17 |
Typical
(Turnbull, 1983.) | 6 | 0.45 | 0.25 | 0.15 |
When discussing nutrient retention it is important to remember that the aim of Snowfluent in the context of the hog industry is:
1) odour reduction;
2) production of a much improved meltwater; and
3) separation of liquids from solids.
Nutrient retention is a secondary objective, and given that the bulk of the nitrogen content of liquid hog manure is in the form of ammonia, preventing losses to the air is bound to be problematic for a system that gets most of its treatment from aerosolization. However, there are methods that can be investigated to customize the Snowfluent process to maximize nutrient retention for hog producers.
Nitrogen Losses From Snowfluent Treatment of Liquid Hog Manure
Turnbull (1983) estimates 25% of nitrogen is lost during storage and land spreading for conventional liquid manure systems. Total aeration systems, used when odour control is essential, reduce nitrogen by 62%. Estimates of nitrogen losses from lagoons are 70 to 85%, and mostly from ammonia volatilization (Livestock Wastes Facilities Handbook, 1993). Nitrogen losses within four days of application are 10 to 25% for broadcast spreading of liquid manure without incorporation. With incorporation, broadcast spreading has nitrogen losses of 1 to 5% and with injection, losses are 0 to 2%.
The lagoon water processed in this single test of Snowfluent treatment had a total nitrogen content of 2.4 kg/tonne (Total Nitrogen Content of 0.24%) compared to AAFRD's Code of Practice For the Safe and Economical Handling of Animal Manures (1995) average of 3.6 kg/tonne (Total Nitrogen Content of 0.36%). The lagoon water was drawn off the top of the lagoon without agitation. The solid content of the liquid manure processed in this trial is not typical of liquid hog manure. The solid residue left in the lined plot after snowmelt is only 0.009% of the original mass of the lagoon water processed. This is much less than the 4% to 7% solids typically reported for liquid hog manure (Turnbull, 1983). There were no measurements of the percentage of total solids in the raw liquid manure samples so the estimates of the solids in the water processed in this test may be low.
Losses of nitrogen in this single test are significant. Ideally, nitrogen, primarily in the form of ammonia (ammonia made up 80% of the Total Nitrogen in the lagoon water), would separate at snowmelt from the runoff and remain in the solids.
To calculate loads, it was necessary to estimate the percentage of the water processed into snow, that actually fell within the boundaries of the test plot. This proved to be quite difficult due to the transport caused by moderate winds during the trial. For this reason, the Raw Manure loads are given as a range. The upper limit of the range uses 75% as an estimate of what fell within the plot, while the lower limit represents a 50% capture.
Table 4. Nitrogen Loads and % Total Nitrogen Content in Snowfluent Phases
 | Lagoon Water | Snow | Runoff | Residue |
| Total Nitrogen (kg) | 345-518 | 224 | 105 | 0.7 |
| % of Lagoon Load | 100 | 43-65 | 20-30 | 0.14 - 0.20 |
| % Total Nitrogen | 0.24 | 0.25 | 0.13 | 3.4 |
Nitrogen loads in the runoff water were significantly reduced (70-80%) from the original loads in the lagoon's liquid manure. However, the solids left after runoff in the lined plot retain only one tenth of a percent of the original mass of nitrogen that was present in the lagoon water. The bulk of the nitrogen is lost to the air. If solids are the only material returned to crop land, then the amount of nitrogen recycled to crops would be very small compared to the nitrogen originally available.
However, this test represents a worse-case scenario in losses of nitrogen. By not agitating the lagoon and processing a representative sample of the solids with the liquid, this test processed primarily water with 80% of the nitrogen as water soluble ammonia. Considering the amount of aeration that takes place at snowmaking, the loss of 55% of the ammonia just at the snowmaking stage should be anticipated. This was compounded by the fact that the separation process was not as efficient as it could have been, since the plot for this study was deliberately selected to have a steep slope, thereby increasing the chances of resuspension of precipitates and other fine solids into the runoff.
Phosphorous and Potassium Losses from Snowfluent Treatment of Liquid Hog Manure
As with nitrogen, only a small percentage of the original phosphorous and potassium loads were retained in the residue. Unlike nitrogen, it is not likely that much of these loads are lost to the air. The Code of Practice for the Safe and Economic Handling of Animal Manures, AAFRD (1995), mentions specifically that the majority of phosphorous in manure is in the form of P2O5, and is found primarily in the solid fraction. As such the retention of phosphorous in the Snowfluent process, could be improved by using agitation to increase the amount of solids in the raw liquid, and increasing the efficiency of the separation process through site specific engineering
Table 5. Phosphorous Loads and Percent Total Phosphorous Content of Snowfluent Phases
| Lagoon Water | Snow | Runoff | Residue |
| Total Phosphorous (kg) | 60.3 - 90.4 | 30.0 | 14.4 | 1.53 |
| % Lagoon Load | 100 | 33 - 50 | 16 - 24 | 2 - 2.5 |
| % Total Phosphorous | 0.04 | 0.03 | 0.02 | 7.8 |
Table 6. Potassium Loads and Percent Total Potassium Content of Snowfluent Phases
| Lagoon Water | Runoff | Residue |
| Total Potassium (kg) | 160.0 - 240.0 | 5.11 | 0.0062 |
| % Lagoon Load | 100 | 2.1 - 3.2 | 0.003 - 0.004 |
| % Total Potassium | 0.11 | 0.006 | 0.03 |
Some additional treatment possibilities to better retain nitrogen are possible with Snowfluent. For example: "The solubility of ammonia in water is lowered when the pH is raised, as by the addition of lime. Ammonia can be controlled somewhat in liquid manure systems, because much of it dissolves in the water" (Livestock Wastes Facilities Handbook, 1993).
Nutrient Content of the Residue
The residues that were recovered from the liner are a very small portion of the original volume of water that was processed. However, the nutrient concentration of the residue solid is significant (Table 7)
Table 7. Nutrient Content of Solid Residue
| pH | %Total Nitrogen | % P2O5 | % K2O | % Organic Matter | EC |
| Snowfluent Residue | 7.4 | 3.4 | 17.8 | 0.04 | 45.1 | 2.3 |
| Hog Manure* | n/a | 0.36 | 0.15 | 0.20 | n/a | n/a |
| Poultry Manure* | n/a | 1.6 | 2.3 | 0.9 | n/a | n/a |
* Source: AAFRD, 1995. Code of Practice For the Safe and Economical Handling of Animal Manures.
Without agitation, and therefore with uncharacteristically low solids content in the raw manure processed, this trial produced 20 kg (dry weight) of residue from between 213 and 142 cubic metres of raw lagoon water. If it is assumed that most of this solid residue is made up of solids conserved from the lagoon water, it works out to a solid content of around 1/100th of a percent in the raw water. If this is extrapolated to predict the amount of solids retained if agitation produced the expected solids content of 4% in the lagoon water (Turnbull, 1983, and AAFRD, 1995), we could expect to produce 8000 kg of residue.
Since the moist residue samples were oven-dried prior to analysis, it is likely that the actual nitrogen content was higher since there was a strong ammonia smell released as the samples were heated. With little smell, and a dried-granule texture, the solid residue is a good natural fertilizer. |
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