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Biosolids photo identifier

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Site Management

Major considerations in site management are:

Soil Conservation

To manage land application of biosolids properly, you must control runoff and erosion. Erosion decreases soil productivity, increases sediment loads in streams, and could carry biosolids into surface waters. A well-managed permanent cover, such as pasture and hay crops, has the lowest potential runoff and erosion. To reduce soil erosion in row crops, tillage operations must be managed carefully. Improving infiltration and slowing water that flows over the soil surface controls erosion. Organic matter in the biosolids helps form stable soil aggregates which increases the porosity and infiltration rate of the surface soil.

Effective erosion control may require additional conservation practices. Appropriate conservation practices vary widely depending on crop, soil, landscape, climate, etc. Consult with your local Natural Resources Conservation Service (NRCS) office to discuss appropriate conservation practices for your site.

Highly Erodible Land

Certain parts of Washington State are classified as highly erodible. Biosolids applications could conflict with required soil conservation plans if biosolids incorporation requires more tillage operations than normal. Before applying biosolids, growers should contact their local NRCS office to review their plan and avoid reducing surface crop residue levels below the requirement. Conflicts between biosolids application and conservation compliance are most likely in the drier areas (below 16 inches annual precipitation) that use a winter wheat-summer fallow system.

The following management practices help maintain surface soils on highly erodible sites:
  • In dryland grain rotations, apply biosolids to standing stubble after grain harvest and before the first tillage operation. This is the time during the fallow season when fields can best support vehicle traffic.
  • Do not apply biosolids to powder-dry soils after the soils have been tilled. Wait for rainfall to moisten the surface soil and the straw. When moist, the straw will bend rather than break when vehicles drive over it. Moist soil will greatly reduce dust problems during application.
  • Incorporate biosolids only when absolutely necessary to control odor.
  • Use tillage practices that maintain surface soils throughout the fallow cycle.

Vehicle Traffic Pattern

One option is to spread vehicle traffic as evenly as possible over the whole field. The other option is to use the same lane for all traffic, which reduces the area compacted, but requires remediation (such as subsoiling) on the lane itself. Where wind erosion is a problem, vehicle traffic should be kept away from the edge of the field to avoid dust problems.

Water Management

Many agricultural areas face problems because of too much or too little water in the soil. To manage these problems, farmers drain or irrigate their farmlands. Both of these practices must be considered when planning biosolids applications.

Ground Water

Groundwater should be at least two feet below the soil surface, and static or receding before biosolids are applied.


Crop yields are higher on irrigated land than on dryland, so the demand for nutrients is greater, allowing for higher application rates. Irrigation increases the potential for both leaching and runoff. Farmers can reduce leaching and runoff with an irrigation system that is properly designed and managed.

Practices that can reduce leaching and runoff include:
  • Scheduling irrigations based on crop need.
  • Diverting irrigation runoff water into re-use systems.
  • Monitoring soil salinity and sodium levels to maintain water infiltration and crop yields.
You can control the rate of surface runoff with sprinkler irrigation. Sprinkler irrigation immediately after a biosolids application can help reduce ammonia loss and wash biosolids off forage plants. To prevent runoff, don't over-irrigate. For information on irrigation management, you can contact WSU Cooperative Extension agents, NRCS personnel and qualified agronomists.

Application Equipment & Operation

Transfer Equipment

Transferring biosolids between vehicles and storage is by gravity or pumping, depending on the solids content of the biosolids and the position and configuration of the storage vessel.

The easiest way to unload a trailer from the treatment plant is by gravity. For gravity transfer:
  • The storage facility must be either in-ground or have a ramp the truck can be driven onto.
  • The biosolids must be dilute enough to flow from the trailer (<8% solids) or pressurized tank (<15% solids), or the trailer bed must be tilted.
  • Dewatered biosolids containing up to 24% solids can be emptied via gravity from a tilting bed or bottom-unloading dump, provided there is enough clearance for operation.
Biosolids can be pumped if they are liquid enough - below 10 to 15% solids. With a higher percentage of solids, the flow of biosolids to the pump will be restricted. Centrifugal pumps can be used on dilute biosolids (<10-13% solids). Chopper-type centrifugal pumps may be able to pump up to 15% solids. Dewatered biosolids may need to be diluted with water before pumping.


The suitable application methods for winter applications are:
  • Sprinkler application.
  • Application from spray trucks operating on all-weather, access roads.
  • Application via spray trucks or manure spreaders on well-drained soils.
During heavy rainfall, it is best not to apply biosolids.

Forest Application Systems

There are three general methods for applying biosolids to forests:
  • Spray irrigation with either a set system or a traveling gun,
  • Spray application by an application vehicle with a spray cannon, and
  • Application by a throw-spreader or manure-type spreader.
The table below lists these application methods, their range, relative costs, advantages and disadvantages, and their suitability for biosolids of different solids contents.

Forest Application Vehicles

Many application vehicles have been designed for agricultural applications, but can be modified for forest use by mounting a spray nozzle and pump on the tank. Application vehicles can also be custom-made. Depending on site needs, options include a specially designed all-terrain vehicle or a heavy-duty truck chassis with rear-mounted tank. Liquid application vehicles can be filled by a traveling tanker, directly from on-site storage, or may itself be an over-the-road multi-purpose vehicle that is filled at the wastewater treatment plant. Throw-spreaders or manure spreaders can be filled by loaders, or may be custom-fit with self-loading clamshells.

System & Range Relative Costs Advantages Disadvantage
Set Irrigation System
Range: 30' - 200'
High Capital.
Low Operations.
Low Maintenance.
Simple to operate.
2% - 8% Solids
Use only low percent solids.
Brush interference.
Traveling Big Gun
Range: 200'
Moderate Capital.
Low Operations.
Low Maintenance.
Simple to operate on appropriate sites.
2% - 8% solids
Use only low percent solids.
Brush interference.
Need even terrain.
Application Vehicle with Mounted Cannon
Range: 125'
Low to Moderate Capitol.
High Operations.
High Maintenance.
Any terrain.
Up to 18% solids
May need special trials.
Throw-spreader on ATV Chassis Moderate to High Capital.
Low Operations.
Low Maintenance.
Can apply dewatered biosolids with >20% total solids. Fast and good throw distance. Limited to >15% solids.
Manure Spreader
Range: 10' - 50'
Low Capital.
Low Operations.
Low Maintenance.
Can apply dewatered biosolids with >20% total solids. Limited to high percent solids.
Trails may need to be close together.

Calibrating Application Equipment

Calibrating application equipment is an essential part of biosolids application and record keeping. Because of the nature of the material and limitations of the application equipment, it is difficult to deliver the exact application rate planned. In most circumstances it is reasonable to expect actual application rates to be within15% of the planned rate.

Calibrate biosolids spreaders by determining the area applied per load. You can adjust either the ground speed of the spreader vehicle or the biosolids discharge rate. You will need truck scales to determine the weight of biosolids in the spreader, and a measuring wheel to measure the actual length and width of the application area in the field.

If you are applying liquid biosolids that vary in percent total solids, you will need to recalibrate the spreader as the solids content changes. Material with 4% total solids should be applied at half the rate of material with 2% total solids to maintain the same application rate in dry tons per acre. This recalibration is very important at low solids content, because a small error in calibration can result in a large error in the application rate.

At the end of each day you should verify the calibration by measuring the total area spread, and comparing the actual number of loads applied with the projected number based on area covered.

See our Spreader Calibration Worksheet.

Application Rate

To apply biosolids properly, application rates need to match crop needs with the nutrients provided by the biosolids.

For more detailed information see our Application Rate page.

Biosolids Application Process