Anaerobic Digestion of Corn Stover and Swine Manure
Background and Significance
Large round-baled corn stover, after grain harvest, has potential as a source of energy and chemicals. Combustion of stover and straws in power plants is limited because their high levels of alkali cause problems. The conversion of fibers to ethanol by either acid or microbial processes has not developed significantly, although much effort has been expended in these areas. Processing to fiber board and paper are being studied.
Development of farm-sized anaerobic digesters producing methane or other chemicals from corn stover may be the best method for widespread use of this diffuse supply of energy. Stover residue after harvesting corn is about 10,000 pounds per acre. Baling collects mainly the standing stalk portion (6000 pounds) which is low in fertilizer nutrient value. In Iowa there is about 50 billion pounds of corn stalk residue or about 20,000 pounds per person in Iowa.
Many believe removing baled corn stover would be harmful to soil health and would increase soil erosion. This is true if the field is deep tilted as are most of the fields including soybean fields with levels of residue that do not preclude no-till planting. If corn stover is baled, most farmers could plant no-till, using thrash wipers on the planter, without much risk of delayed planting or weed control. The amount of residue left in the field would usually be adequate for conservation compliance. Soil tilth (health) and percentage of soil organic matter (which continues to decline with tillage) is determined more by what takes place 3-12 inches deep in the soil than by amount of residue on the surface. Tillage returns old organic matter and soil aggregates from deep in the soil to the surface where organic matter is rapidly oxidized and soil aggregates deteriorate. Aggregate stability is a many-year process that occurs as a result of old root organic matter, gums, waxes, soil particles, and microbial activity in undisturbed soil. If soil is not tilled roots produce most of the organic matter in the soil. Crumb structure of soil determines soil aeration for root growth and water permeability. To maintain and improve soil productivity it is imperative that deep soil tillage be reduced. The ideal solution is an economically feasible conversion of stover to a useful fuel as is being attempted in these anaerobic digestion studies.
We are investigating the use of on-farm or small community anaerobic digestion of corn stover or mixtures of stover and swine manure to methane gas that can be combusted for heat or used to fuel engines generating rectified electricity for on-site use or fed into utility power lines. The energy produced is renewable in contrast to coal for these types of fuels. With the present electrical energy market, it is difficult for fuels from anaerobic digestion to be economical. Our goal is to combine benefits to soil productivity from removing corn stover, providing another outlet for farm labor, and for an increase in yield of soybean and corn from applying sludge to soybean in addition to the value of methane as a fuel.
Summary of Work
There are two main types of anaerobic digestion. In single-stage digestion wetted biomass or sewage are pumped at an appropriate rate into a large tank or lagoon. In the lagoon there are two groups of bacteria with different types of metabolism and optimum conditions for growth and metabolism. The first group of bacteria decomposes fibers to sugars and then acids. This group has maximum activity at 125° F and an acid pH of 5.5. The second group that convert acids to methane gas grow best between 75° – 95° F and require a neutral to slightly alkaline pH 7.0-7.4. In single-stage digestion neither of the two groups have their optimum conditions for growth, but the most critical requirement is pH for the methanogenic bacteria so the reactor is usually run so that the pH is about 6.8.
In the laboratory, we have conducted many studies using 24-pint jars as reactors. We studied feeding rate, type of feed (stover, manure, and mixtures of stover and manure), temperature, and amount of stirring. We measured pH changes, excess acidity and alkalinity, ammonium N, rate of gas production, percentage methane in the gas, and percentage solids on a daily basis over a period of 2-5 weeks. From these data we know the conditions for maximum activity and the signs indicating that a reactor is beginning to fail when using these sources of feed. In these studies we obtained the greatest rates of methane production and the most stable digestion with mixtures of stover plus swine manure.
We also have studied the beneficial effects of applying sludge or liquid swine manure to soybean fields during early July. Soybean yield was increased 4-6 bushels per acre and residual N equivalent to about 100 pounds of N per acre for the following corn crop was present. Application of manure to soybean during July, although more difficult than fall or spring applications, provides a time of the year when the farmer is less busy and when soils have the greatest biological activity for absorbing odors and incorporating the manure or sludge. In other related studies we have shown why yield of soybean is increased and how residual organic N continually releases ammonium N which improves efficiency of N use by corn.