Biosolids photo identifier

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Site Selection

Biosolids application can benefit almost any site that produces crops using normal farming practices. Evaluate the site thoroughly before proceeding and you can save money, improve public acceptance, and gather information to guide site design and management.

Factors that help determine site suitability include:

Site & Soil Characteristics

Evaluating topography will quickly tell
you whether a site is usable.


The steeper the slope, the less desirable
the site.

Site Continuity

Land that is not broken up by waterways,
cliffs, steep slopes which require buffers.
By evaluating the texture, structure, color, and depth of the soil, scientists can estimate other properties, including:
  • Infiltration,
  • Drainage,
  • Permeability,
  • Productivity,
  • Potential for leaching, runoff, and erosion.
These soil properties may limit the:
  • Type of biosolids applied,
  • Method of application,
  • How many months of the year biosolids can be applied (timing of application).
Sites with limiting soil characteristics are more expensive to manage because:
  • Biosolids may need to be stored when the conditions for application are unfavorable.
  • Special practices may be required for soils with seasonal high water tables, shallow bedrock or restrictive layers, or steep slopes.

Soil Color

Color is a good indicator of soil drainage. Well-drained soils have horizons (layers) that are uniformly red, brown or black. Poorly-drained soil horizons are gray, often with brownish or reddish mottles.

In a soil profile, the depth to gray or mottled gray soil is often the depth to the seasonal high water table. Seasonal water tables play a role in site suitability for biosolids because:
  • Pathogens tend to survive longer in saturated soils.
  • Soil wetness limits vehicle traffic on poorly drained sites.

Soil Depth

Soil depth affects site productivity. Very shallow soils have low crop yields and can accept only low rates of biosolids. The risk of runoff is increased on some very shallow soils. Soil depth is limited by bedrock or by restrictive layers (compacted, cemented, or dense clay layers).

Applying biosolids to steep slopes increases the risk of runoff. The table below lists the maximum recommended slope for forested sites under different conditions.

Conditions Maximum recommended slope
Liquid Dried or dewatered
Dry season
Good vegetative cover 30% 60%
Poor vegetative cover 15% 30%
Wet season
Good vegetative cover 15% 30%
Poor vegetative cover 8% 8%

Water Resources

Water resource issues include depth to the water table, distance to aquifers, relationship to surface water, and whether or not the watershed supplies public water. If a site is near a waterway, buffers are required to protect water bodies from contaminated runoff or erosion. This reduces the usable area of the site. If there are several drainage ways within or near the site, each one reduces the area that can be used for application.

The primary concern for ground water is nitrate leaching. Biosolids applications are designed to have low enough amounts of nitrate to avoid this problem. Since nutrient uptake and other transformations take place in the top layers of soil, it is best to maintain a distance of at least 2 feet to the water table at the time the biosolids are present. Sites with standing water or flooding potential may not be suitable or may require special management practices. Where the seasonal high groundwater table is within three feet of the surface, a groundwater management plan is a required component of the Site Specific land Application Plan as outlined in the General Permit.

Soil Surveys

The tables provide more information about
soil properties and how they may affect
various land uses:

Physical and Chemical Properties
Clay content, permeability, water holding
capacity, pH, shrink-swell potential and,
in some cases, organic matter content.

Soil and Water Features
Runoff potential (hydrologic group),
flooding, water tables, and depth to bedrock.

Engineering Properties
Additional data on texture and coarse

Yields Per Acre of Crops and Pasture
Crops that are commonly grown and the
relative yields that can be obtained
under good management.
Most counties in the state have a soil survey completed by the USDA Natural Resources Conservation Service.

A soil survey contains three parts: maps, text, and tables. Soil maps show the spatial distribution of the different soils in an area. The text describes the properties of each soil series mapped in the county.

Information for each soil series includes:
  • Soil series name, general location, and acreage
  • Landscape position and slope
  • Texture, color, and structure of each soil horizon
  • Soil depth
  • Soil-water relations
    • Drainage and permeability (water movement through soil)
    • Water-holding capacity (water retention after drainage)
  • Use and management of the soils
    • Average pH and native fertility
    • Average crop yields
    • Soil limitations and appropriate management practices

Map Units

On most soil maps, the smallest area that can be shown is three or four acres. This area on the map is called a map unit. Rather than representing a uniform soil series (a set of soils whose profiles are alike), it usually contains small areas of several soil series. The map unit descriptions detail the kind and extent of soil series found in that map unit.

Landscape Distribution

The soil survey also shows the landscape distribution of suitable and nonsuitable soils. Areas of uniform suitability are easier to manage than areas of non-uniform suitability. When areas of fair or poor soils are distributed throughout larger areas of good or excellent soils, management may be restricted to what is required for the least suitable soil. For example, access to the more suitable areas may be limited to times when the poorly suited soil can handle vehicle traffic.

Transportation and Accessibility

Transportation is an important part of a land application program. It is rare for a biosolids generator to have land where biosolids can be applied near the treatment plant.

The most common way of transporting biosolids to a land application site is by truck. There are several requirements that must be followed:
  • Haulers must always comply with state and federal transportation regulations.
  • Trucks, farm vehicles, and application equipment must be thoroughly cleaned before being used for other purposes.
When evaluating biosolids haul routes, important considerations are:

The farther the application site is from the treatment plant, the higher the hauling costs will be. In these cases, it is often more cost-effective to undertake volume and weight reduction processes (for example: dewatering) before transporting.

Transport Corridors
To determine how suitable the roads are, biosolids managers should check with local planning and transportation officials. Load restrictions, either permanent or seasonal, can eliminate access along certain routes. There may be public objections to trucks driving through some neighborhoods.

Access to the Site
Trucks also need safe access from a public all-weather road to and from the site. Again, seasonal conditions may restrict access. When biosolids will be delivered faster than they can be applied, storage areas must be identified and authorized in a site permit.


Transport routes that avoid waterways and rough terrain will reduce the risk of contamination if there is a spill. Biosolids haulers should have an emergency spill response plan in case of a vehicle accident or spill.


The temperature and precipitation at the site affects the number of days during the year when snow, freezing conditions, or rain might interrupt operations. Elevation will affect the number of days that snow or freezing conditions will make a site unsuitable for applications. Microclimate is an important factor because it influences how much sunlight an area receives. South and southwest facing sites get the most sunlight and have the warmest temperatures. Biosolids will dry faster and the ground won't be frozen or snow-covered as long.


The condition of vegetation at forest application sites influences how biosolids are applied and how successful the application will be.
Hybrid cottonwood and Douglas fir are better choices than mixed hardwoods due to high nitrate uptake rates. Red alder is not acceptable because it fixes its own nitrate from the atmosphere in excess of what it uses.

Stand Age and Condition
Young, vigorous forests that include both trees and understory can handle a higher application rate. Age also affects type and ease of application. When the trees are young, biosolids are applied over the top; when trees are older, biosolids are applied underneath the canopy. In between these ages there are times that interference from tree limbs and foliage may hamper application of biosolids. It is best to apply biosolids to well stocked or well thinned trees.

A well-established understory increases nutrient uptake in a forest, and decreases the potential for runoff compared with bare soil.

See our Site Design page for further information.