This southern Colorado town uses about 32.5 ha (80 ac) of grass on a private ranch for application of biosolids slurry from its oxidation ditch. Application is approximately 56 dry Mg/yr (62 dry tons/yr) through a sprinkler gun. Other dry land parcels on ranches have also been used.
Average annual precipitation in Gunnison is 28 cm (11 in.); the typical yearly application rate is 5 dry Mg/ha (2.3 dry ton/ac). Grazing on the biosolids-treated land has increased from once to twice per year. Gunnison is also experimenting with applying grass seed with biosolids to increase production.
APPLYING BIOSOLIDS REDUCES SOIL EROSION (Case Study 3)
Scientists from the U.S. Department of Agriculture Rocky Mountain Forest and Range Experiment Station conducted a 5-year study of the effects of biosolids application to degraded semiarid rangeland. Dewatered, anaerobically digested biosolids from Albuquerque, N. Mex., were surface applied to a grass land site in the Rio Puerco Watershed resource area of New Mexico. The Rio Puerco basin is a degraded watershed were livestock have grazed extensively. The project site consisted of a broom snakeweed and blue grama plant community located on a moderately deep, medium textured soil. Mean annual precipitation during the 5-year study was 25 cm. Biosolids were surface applied at one time application rates ofO, 22.5, 25, and 90 Mg/ha (0, 10,20,40 dry ton/acre). Each application rate was replicated four times; plots were in a randomized block design, with a total of 16 plots. Each plot was 3m x 20m (10ft x 65 ft) in size. To minimize disturbance of the existing grassland vegetation, plots were not disked or tilled after biosolids were added. Plant nutrients in the soil, including total nitrogen, available phosphorus, and potassium, increased linearly with increasing biosolids application during the first year of study. Organic matter in soil below the biosolids layer did not increase until after the fifth year because of a lag between increased nutrient availability and subsequent plant productivity and microbial activity belowground. Although soil pH decreases from 7.8 to 7.4 because of biosolids, only DTPA-extractable copper and cadmium increased in concentrations above desirable levels. This occurred after the fifth year and only in plots receiving applications greater than 45 dry Mg/ha (20 dry ton/ac). Blue grama grass, a desirable species for grazing livestock and wildlife, increased during the first and second years, with yield from 1.5 to 2.7 times greater in the treated plots than in the control plots in the treated plots than in the control plots. Throughout the study, blue grama production remained higher in plots receiving 45 to 90 dry Mg/ha (20 and 40 dry ton/ac) of biosolids than in control plots. The application of biosolids also increases the nutritional value of blue grama. Tissue levels of nitrogen, phosphorus, potassium, and crude protein increased to recommended tissue concentrations with biosolids treatments. Trace metals in blue grama grass did not increase during the study, thereby eliminating concerns that toxic amounts of these elements could be transferred to grazing animals.
Results indicated that a one time biosolids treatment ranging from 22.5 to 45 dry Mg/ha (10 to 20 dry ton/ac) yielded positive vegetative response without harm to the environment. An unexpected benefit from the biosolids treatments was a decrease in broom snakeweed, a toxic, non palatable competitive range plant. Following the addition of biosolids, the number of broom snakeweed plants in the biosolids-treated plots decreased over the course of the study.
U.S. AIR FORCE ACADEMY (Case Study 4)
The U.S. Air Force Academy near Colorado Springs, Colo., applies 27 to 36 dry Mg/yr (30 to 40 dry tons/yr) of anarobically digested biosolids to 40.5 ha (100 ac) of native vegitation on the academy property. The program began in 1988 with the permitting of a 405 ha (1000 ac) site. The area, which recieves 39 cm/yr (15 in./yr) of precipitation, consists of highly erodible solids. Through the application of biosolids, the academy has been able to improve vegitative cover, reduce soil erosion, and improve wildlife habitat.
A great deal of our discussions have focused on municipal treatment applications, particularly in this chapter. However, most if not all of the principles throughout the book are readily applicable to industrial water treatment applications. Try to approach each water treatment assignment from a first principles standpoint, and then develop design-specific cases with as much information on the chemistry, physical and thermodynamic properties of the wastewater stream and sludge to be handled. In all assignments, be sensitive to the cost issues. Engineering projects are not complete unless we have evaluated the project economics. Some cost factors for different technologies have been included in our discussions, but no real effort has been made for detailed comparisons between technologies. This really has to be performed on a case specific basis. What we can do before closing this volume is review some of the generalized project cost estimating parameters that are applicable to assessing the investments that may be needed in upgrading and/or installing wastewater treatment facilities and various solid-liquid separation equipment.
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