Rangeland Offers Solution for Biosolids Application

The city of Fort Collins, Colo., uses land application for the beneficial reuse of biosolids generated at the city?s two water reclamation facilities, which have a combined capacity of 30 mgd. The Resource Recovery Farm, located just across the Cache La Poudre River from the main treatment plant, has been the base of operations for the biosolids management program since 1983.

In the late 1980s and early 1990s, the utility began studying future biosolids management options in response to an increase in urban growth and the proposed EPA Part 503 biosolids regulations. The results of these studies indicated that the application of biosolids to rangeland for beneficial use would provide the utility with a long-term economical and environmentally sound disposal option.

The utility subsequently purchased the remote, 25,680-acre Meadow Springs Ranch as a potential site for beneficial use. The property is approximately 22 miles north of Fort Collins, just south of the Wyoming border. The terrain at the ranch consists of rolling grasslands typical of the dry Great Plains environment just east of the Rocky Mountains. The elevation at the property is approximately 6,000 feet above sea level. Both the ranch and the solids processing facilities are located close to Interstate 25. This makes hauling between the two sites relatively quick and easy.

Demonstration Phase

In 1993, the utility began a demonstration phase to evaluate the impact of properly controlled biosolids application on the ranch?s environment. The demonstration phase consisted of applying Class B biosolids to three demonstration cells over a six-year period beginning in 1994. The goal of the demonstration phase was to determine:

Optimal biosolids application rates and biosolids composition
Groundwater and surface water impacts
Air quality impacts
Human health and environmental risks
Wildlife impacts
Wetlands identification
Endangered species identification
Cultural heritage zones
Exclusion zones
Operational and facility requirements

Demonstration phase facilities and equipment at the ranch consisted of:

1.6 acre storage pad with detention pond
Land application equipment
Land application cells
Equipment storage garage and operations trailer
Cattle grazing facilities (stock ponds, fencing)
Access roads and bridges

Application to the three demonstration cells used a three-year rotational cycle. Following application of biosolids, a site was allowed to rest for a year, and then the next year it was grazed. Over the last five years the utility has generated an average of 1,640 dry tons per year of biosolids. Approximately one-third of the annual production was applied to the ranch, while the remainder was applied at the farm or on private farms under contract with the utility.

To date, no apparent impacts to wildlife, the environment, soils, surface water or groundwater have been observed, indicating that the demonstration phase has been a success. Wetlands, endangered species, cultural heritage zones, and exclusion zones have been identified and areas containing these elements will be avoided. Procedures also have been implemented to mitigate impacts to wildlife. The grazing association that leases the ranch property reports that pastures with biosolids are more productive than the fields without biosolids.

The demonstration phase has provided valuable experience that will be carried into the operational phase. Some of the lessons learned are:

The optimal biosolids application rate is two dry tons per acre per year. Nitrogen is the limiting nutrient when determining loading rates.
The biosolids composition can be wet solids or dry solids. Although the drying process introduces an extra handling step, allowing solids to dry reduces the volume of material that has to be land-applied. Furthermore, dry solids are much less noticeable after application.
Dry solids should be distributed with a low profile spreader.
Biosolids application can have a noticeable impact on the fields, especially under wet conditions. To minimize land impacts, apply biosolids immediately after grazing or when the grass is dormant.
Cell size should be determined based upon the topography of the cell and the capacity of distribution equipment. Ideally, each cell should be kept under 320 acres to minimize the number of soil samples required by the State Health Department.
It is not necessary to wait an entire year before grazing. The State Biosolids Regulations (Colorado regulations are consistent with EPA 503 regulations) only require a 30-day waiting period. The state criteria should be used as a minimum and then operational considerations, such as the time of year or the amount of growth, should determine the length of waiting period.
A sludge screening process should be added at the water reclamation facility to remove aesthetically displeasing materials in the final biosolids product. To this end, installation of sludge straining equipment on the primary sludge pipelines at the main plant is currently under way.
The annual high groundwater depth beneath the land application sites should be at least 10 feet to avoid groundwater impact. This is double the 5-foot minimum depth required by the biosolids regulations.

Full-Scale Operations

To date, operations at the ranch have been based on fulfilling the needs of the demonstration phase. An updated Ranch Management Plan is currently being developed to address full-scale operations. As part of this overall program, all farm activities will be phased out, land application on privately owned farm land will be minimized and ultimately discontinued, and all biosolids beneficial reuse activities will be moved to the ranch.

For full-scale operations, these facility and equipment needs are anticipated:

Spreading and mixing equipment that can handle both wet and dry solids
A covered dry solids storage area
A 5-acre pad, with run-off detention pond, for air-drying of solids
New application cells

Full-scale operations at the ranch will begin in the year 2000 using the recommended ranch management practices that incorporate lessons learned during the demonstration phase.

Typically, wet cake sludge will be dried at the ranch prior to land application. Wet solids application will only occur on sloped areas where runoff is of concern or as a back-up mode of operation.

During the warmer summer months, air-drying of biosolids will occur by spreading a thin layer of wet cake (6-inch depth) on the drying pad and turning it several times a day. After three or four days the incoming 20 to 25 percent wet cake typically can be dried to 75 or 80 percent.

During colder times of the year, static drying will be utilized. This requires mixing previously dried solids with wet cake at a one to one ratio and wind-rowing the mixture in 3-foot high piles. Biological action in the piles promotes drying and prevents the biosolids from freezing. After 14 to 20 days, depending on the weather conditions, the mixed solids can be dried to approximately 60 percent or better with static drying. The Ranch drying pads have been sized to accommodate the seasonal drying protocols.

To minimize impacts to the land, dried solids will be stored under cover until the winter months when they will be land- applied. Solids will be applied at two dry tons per acre, however, the economics and efficiencies of employing higher loading rates combined with longer rotational cycles (e.g. utilize a field every five years instead of every three years) will be investigated to see if any benefits, operational or financial, are provided.

Summary

Rangeland application has proven to be a viable, cost effective and environmentally sound method of beneficially reusing biosolids for the city of Fort Collins. Purchasing the remotely located property enabled the city to obtain the land at a very reasonable price and also moved the biosolids disposal operation well away from the rapidly growing town.

Grazing on the ranch property has not only improved cattle production, but has allowed a traditional use of the land to be preserved. Because the ranch property is entirely owned by the utility and the property is fenced, only Class B biosolids need to be produced. The present practices, however, will likely produce a Class A product most of the time. Eliminating disposal at privately owned-sites also eliminates the utility?s liability on these properties.

Projections indicate that ultimately, the utility will produce 4,600 dry tons per year of biosolids. Due to size of the ranch, the program to rotate rangeland application cells, and the minimal metals concentrations found in the biosolids, it is anticipated that the ranch has an infinite site life for biosolids application.

About the Authors: Cliff Hoelscher is a Project Manager with Fort Collins Utilities. Dan Peters is a Principal Engineer with Montgomery Watson in Denver, Colo. Bill Tomerlin is a Process and Systems Specialist with Fort Collins Utilities.