Ultimate Recycling: Ash From Combustion of Swine Manure Used as a Mineral Source for Pigs

Traditionally, pigs were fed household waste, while their manure was used as a fertilizer for crops. Although it may not have been realized at the time, this system resulted in a continuous reuse of minerals such as phosphorus, and thus it prevented environmental prob-lems with these minerals.

In modern production practices, where feedstuffs are brought in from afar, this cycle is often broken. Minerals are allowed to accumulate somewhere in the swine production system-for example, as precipitated phospho-rus on the bottom of a lagoon. Although this practice is not in itself harmful, a system in which minerals are reused is more sustainable.

One method of waste disposal that is receiving a lot of attention is to combust (or thermally decompose) dried fecal material for recovery of energy, a method that can also be used to dispose of mortalities. A benefit of this technology is that energy is recovered which can be used to heat buildings, drive a generator, or possibly even be used for the generation of a liquid fuel such as diesel. Another benefit is that bacteria, viruses, and antibiotic residues are destroyed.

This combustion process, though, does not really solve the mineral problem. Minerals like phosphorus and copper are harvested as ash. Typically, for every 1000 pounds of dry fecal material, approximately 130 pounds of ash are formed. This ash is rich in minerals, such as phosphorus, calcium, potassium, and magnesium. Currently, it is still legal to dispose of the ash in a landfill, but from an environmental standpoint, this process is not a preferred solution. Other options are to use the ash as a fertilizer high in P and K.

Ash also can be used as a building material. It can be used in the production of concrete, drywall, and similar products. However, the color of the ash is a concern, as it may affect the color of the building products. Color seems to depend strongly on the method of combustion used, and incinerators seem to yield ash of suitable color. One other solution that has received little attention is simply to feed the ash back to the pigs. Pig diets are commonly fortified with phosphorus, calcium, and micro-minerals such as copper and zinc, and thus there is a need for these minerals in swine nutrition. To make this solution practical, though, the minerals in the ash must be usable (digestible) by the animal, and the ash should not contain any mineral at undesirable levels.

To test how practical this method might be, a trial was done with grower pigs. These pigs were fed a typical grower diet, and their feces were collected and dried using our belt collection system. Subsequently, the dried feces were shipped to Idaho, where they were combusted for energy recovery. The ash obtained was shipped back and tested for feed use.

The ash contained high levels of calcium (11.5%) and phosphorus (13.3%), two minerals of great interest in swine production. Phosphorus is especially important, as it is a mineral that is expensive to add to swine diets. The ash also contained micro-minerals, including zinc (0.18%), copper (0.05%), iron (0.73%), and boron (0.01%).

Minerals that are a potential swine health concern are potassium and selenium. Selenium, though, was found in very low concentrations and does not seem a concern. Potassium was present in high concentrations (12%) and should be watched when formulating diets. Another mineral that came in at high levels was magne-sium (5.8%). This mineral is not toxic, though, and recent research actually suggests that higher levels in swine diets may have benefits for meat quality.

Minerals that were notably low were sodium, chloride, sulfur, and nitrogen. The reasons for this are twofold:

For nitrogen, this is fortunate, as it would have no beneficial effect when included in a diet. Chloride, however, will have to be added to the diet, and this can be easily achieved by adding hydrochloric acid (which can be extracted from the flue gas of the combustion system). This has another benefit as it reduces the high pH of the ash. In addition, some salt will have to be added to supply extra sodium.

Thus, if the animal can utilize the minerals in the ash, it could be a valuable supplement to swine diets. This was first evaluated in vitro by assessing the solubility of ash minerals in water adjusted to pH 2 with hydro-chloric acid, mimicking the environment in the stomach. The results of this test showed that calcium, phospho-rus, potassium, and magnesium are all highly soluble, suggesting that they are also highly digestible.

The next test was to evaluate digestibility in animals. For this, a basal diet was formulated with ingredients very low in minerals (corn starch and casein). This basal diet was supplemented with commercial sources of minerals, such as dicalcium phosphate, limestone, potassium chloride, magnesium acetate, and salt, or with ash, hydro-chloric acid, and some salt and limestone in order to generate diets that were nutritionally adequate. These diets were tested for mineral digestibility in grower pigs. In these tests, the percentage of a mineral taken up with the feed but excreted with the feces is considered indigestible; the remainder is digestible. This assay, though, is only relevant if a mineral is fed below the animal's requirement, as other-wise, uptake of the mineral in the intestines may be inhibited, leading to an artificially lowered digestibility. Minerals digested are either excreted in the urine or accreted by the animals (part of tissue growth).

Two mistakes were made in this experiment:

The results of the trial, nonetheless, were very promising. Mineral digestibility of the ash is very close to that of the commercial mineral sources (see Table 1), even though high levels of phosphorus in the diet are known to decrease digestibility of both phosphorus and calcium. Thus, if properly formulated diets were used, a digestibility at least as high as that observed in this trial would be expected. The only surprise is magnesium: the magnesium acetate used as a reference was effectively not digestible, while magnesium in the ash had a digestibility of only 39 percent, possibly as a result of high levels of magnesium in the diet inhibiting magnesium uptake in the intestines (as in-vitro solubility was excellent). This digestibility, though, is more than adequate from a nutritional standpoint.

 

These data suggest that ash is an excellent source of minerals for pigs, and that feeding ash back to pigs may be the ultimate method for recycling minerals. One important question remaining is whether feeding ash to pigs over time will lead to a large excess of a certain mineral in the feed that results in an even larger excess of this mineral in ash to the point that the ash is not suited for use as a feed ingredient. For potassium (and other minerals mainly excreted in urine), this is an unlikely problem, as excess dietary potassium is excreted in urine. For magnesium, this may become a concern, but one that may be alleviated through selective extraction of magnesium (note that commercial diets will contain much less ash than our test diets; thus, they are less likely to cause a mineral excess).

Our next step is to gather ash from different sources and evaluate it for its nutrient content and digestibility. Provided results are as encouraging as these data, feeding ash is a real solution that not only solves the environmental problems with minerals, but also reduces the need for supplemental and often expensive minerals.