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Biosolids photo identifier

Event Recycling

Site Design

When designing a biosolids application site, extra consideration should be taken with:


Buffers are areas within a permitted biosolids application site where biosolids are not applied.

They function to:
  • Keep biosolids applications on the permitted site.
  • Control runoff from the treated area.
  • Protect surface water and other non-target areas.
  • Reduce off-site odors.
Buffers must be large enough to protect all receiving waters of the United States. Minimum buffers for biosolids that do not meet exceptional quality criteria are outlined in the statewide general permit for biosolids management.

Three categories of site buffers: Important things to remember about buffers:
  • A buffer is part of the permitted area.
  • During biosolids application, buffers should be clearly marked and monitored.
  • If biosolids applications are encroaching on the buffer area, determine the cause and correct any application problems.
  • In some cases it may be good practice to increase the size of the buffer beyond what is required by the site permit. For example, a five-foot property buffer in a crop field might be increased to the width of the farmer's fertilizer applicator. This larger buffer would allow fertilization of the buffer strip without overlapping the biosolids application area.

Surface Water Buffers

Surface waters include rivers, lakes, seasonal waters, and ditches. The table below lists surface water buffers recommended for agricultural biosolids application sites. In some cases the buffers may need to be adjusted based on site-specific circumstances.

Surface water buffers recommended for agricultural biosolids application sites.
Application Method Ground Surface Cover Slope Effect
Suitability Rating
Type of Water Body
(buffer width- feet)
River, Lake, or Stream Seasonal Ditch
Surface Bare soil Poor/Fair 200 100 50
Good/Excellent 100 50 33
Surface Permanent vegetative cover Poor/Fair 100 50 33
Good/Excellent 50 33 33
Injected or incorporated Bare soil Poor/Fair 100 50 33
Good/Excellent 50 33 33

The larger bodies of water need larger buffers because:
  • Larger waterways are more likely to be used for domestic purposes.
  • Larger streams have banks that are less well-defined and that are farther away from the normal waterline.
The largest buffers are for sites where:
  • Biosolids are applied to the surface and are not incorporated into soils.
  • The soil is bare.
  • The slope effect suitability rating is poor/fair (slope and soil types result in high amounts of runoff).
The smallest buffers are for sites where:
  • Surface runoff is slowed by permanent vegetation, or the biosolids are injected or incorporated into soils.
  • The slope effect suitability rating is good/excellent (slope and soil characteristics allow little runoff).
Surface water buffers must be increased for sites with:
  • High potential for runoff. Slope and soil characteristics determine runoff potential.
  • High potential for contaminant transport. Vegetation filters and slows runoff. A bare soil buffer provides little filtering of runoff, so these buffers should be larger than those with good vegetative cover.
The following factors are used to design buffer widths (see table below):
  • Biosolids application method (surface or injected/incorporated).
  • Ground surface cover.
  • Slope effect suitability rating (see table below). This rating estimates potential runoff based on percent slope and soil factors such as texture, structure, and organic matter.
  • Type of water body.

Groundwater Buffers

Applying biosolids at agronomic application rates protects the groundwater under the application site from becoming contaminated by nitrate, pathogens, and trace elements.
All wells provide a potential direct path to groundwater, and have similar buffer requirements regardless of how they are used (irrigation or domestic water supply). The suggested horizontal buffer from wells is 100 feet. In some cases buffers can be adjusted based on site-specific considerations. Buffer requirements in wellhead protection zones may override these recommendations.

A well that has been abandoned may provide a direct path to groundwater if it has a cracked or nonexistent well casing . If there are improperly abandoned wells on or near a biosolids application site, contact the Ecology regional office for guidance on well closure.

Depth to Groundwater
If the seasonal ground water is 3 feet (0.91 meters) or less below the surface, a management plan describing how you will protect ground water is required in the Site Specific Land Application Plan portion of your permit application.

Property Buffers

Property buffers include non-application areas near roads, dwellings, and fence lines. When designing property buffers, your objective will be to reduce any nuisance to neighbors and the public.

Property buffers must be designed to:
  • Make sure that no biosolids will be applied off the permitted site,
  • Reduce odors,
  • Restrict public access and exposure.
How biosolids are applied affects both application accuracy and the potential for odors. Accurate applications that control odors have the smallest property buffers. For example, required property buffers should be less for injected biosolids than for liquid biosolids applied with a high pressure big gun applicator.

Suggested property buffers are listed in the table below. Generally, a minimum buffer of 5 feet is recommended to protect against off-site application. Where biosolids are injected into the soil and there are no sensitive adjoining property uses, the buffer may be reduced to near zero. Remember that biosolids must not be applied or allowed to run onto non-permitted areas.

Typical property buffers recommended for biosolids application sites.
Feature Buffer (feet)
Property line 5 - 50
Dwelling 50 - 200
Major arterial or highway 50 - 100
Minor road (usually dirt or gravel) 5 - 50

Crop Selection

When designing a plan for applying biosolids, you will be working with the management practices already followed by the farmer or site operator.

Crops that can benefit from biosolids application are:
  • Food crops (for direct human consumption).
  • Pasture and feed crops (for animal consumption).
  • Non-food crops (ornamentals and fiber crops).
Whether a crop is suitable for biosolids depends on several factors:
  • Number of months when biosolids can be applied to the crop.
  • Potential for repeat applications at the same site.
  • Capacity of crop to use nitrogen.
  • Required waiting periods between Class B biosolids application and harvest.
  • Marketing restrictions imposed by food processors.
  • Whether liquid, dewatered or composted biosolids can be applied.


The estimated crop yield will determine how much plant nutrients are applied via biosolids. Generally, a site with a higher yield potential will make better use of biosolids.

Field records are the best source for crop yield estimates. You can find proven yields for most grain farms from the local Farm Service Agency office. For most other cropping systems, grower records are the only source available. Be sure to note whether the yield records are on an as-is or dry matter basis. Where field records are not available, you can make first-year estimates for a project using NRCS soil surveys, county production averages, or other local data sources.

Yield data is typically not available for grazed pastures because grazing animals consume the crop directly in the field. Estimate plant nitrogen needs from the appropriate pasture fertilizer guide recommendation, based on the level of pasture management.

Site Manure Application History

Applying manure for long periods reduces the need for nitrogen fertilization, which also reduces site suitability for biosolids. In areas where the land is primarily used for dairy and livestock, you can locate farms with non-manured acreage by consulting with the local NRCS office.

Food Crops

Food crops are consumed directly by humans. The feasibility of applying biosolids to a food crop site depends on:
  • The class of biosolids (A or B) applied.
  • Whether the harvested portion of the crop is in contact with biosolids.
For Class B biosolids, the harvest waiting periods for crops with harvested parts that are below the soil surface depends on how long the biosolids remain on the soil surface prior to incorporation. It may not be feasible to raise some food crops (for example root crops and low-growing fruits and vegetables) on sites that use Class B biosolids because the waiting period is more than one growing season. Use the table below as a guide to applicable crop harvest waiting periods.

Class B Biosolids Application Method
Harvested part contacts biosolids? Part of plant harvested Biosolids remain on soil surface Waiting period from application to harvest
No Leaf/Fruit/Grain No time specified 1 month
Yes Root > 4 months 20 months
Yes Root < 4 months 38 months
Yes Leaf/Fruit/Grain No time specified 14 months

Before using biosolids on a processed food crop (for example sweet corn or hops), discuss your plan with the food processor. Some food processors have refused to accept crops grown on land amended with biosolids. Other food processors have recognized biosolids recycling as a sound crop production practice. By working with the food processors, you may be able to share how biosolids can benefit their products.

Pasture and Perennial Animal Feed Crops

These crops offer flexibility for timing of biosolids application, because sites are accessible most of the year. The sod created by pasture and perennial animal feed crops promotes infiltration and controls erosion. A disadvantage of pasture and perennial animal feed crops is that it is difficult to work biosolids into the soil.

With surface applications:
  • Much of the biosolids inorganic nitrogen may be lost by ammonia volatilization. This loss must be considered when calculating the amount of biosolids needed to meet nutrient requirements.
  • For grazed pastures, animal access must be restricted for 30 days after Class B biosolids application.


Overgrazed pastures are not good sites for biosolids applications unless they are being used as part of a renovation program for overgrazed pastures. The nutrients from biosolids can help speed the establishment or recovery of desirable pasture grasses. In these cases, a clear plan for maintaining the renovated pasture in a healthy state will be needed.

Properly managed pastures will be more productive and have a greater need for biosolids nutrients. Good pasture management includes rest periods for regrowth. To reduce direct ingestion of biosolids, animals should be removed from the pasture when there are at least two to four inches of standing forage remaining.

The best time to apply biosolids to perennial animal feed crops is immediately after harvest has been completed (ideally within seven days). This reduces the amount of biosolids adhering to the leaves and eaten by cattle. Also, if biosolids applications are delayed too long, the pasture will be past peak grazing or harvest quality by the end of the 30 day waiting period. This defeats the purpose of using biosolids to improve pasture quality. See the table below for harvest restrictions.

Site/Crop Grazing or Harvest Public Access
Animal feed & other crops (not for human consumption) 30 days until harvest --
Pasture 30 days until harvest --
Turf (includes sod production) 1 year 1 year
Public contact site -- 30 days (low potential for public exposure)
1 year (high potential for public exposure)

Grain and Grass Seed Crops

Grain (a food crop) and grass seed (a non-food crop) are similar in that both crops have harvested portions (seeds) that do not contact biosolids.

Grain and grass seed crops are well suited for biosolids applications, because the harvested portion of the crop does not contact biosolids. Biosolids are applied before seeding for all grain and grass seed crops. For fall-seeded crops (for example winter wheat and winter barley), biosolids can be applied any time before seed bed preparation, when the soil is dry enough to apply biosolids without a great risk of runoff, leaching, or soil compaction.

For spring-seeded grains (for example corn, spring wheat, barley or oats), the amount of time for applying biosolids can be short. In many areas the soil does not dry out enough to support traffic until planting time. In these cases, biosolids may need to be applied and incorporated into the soil, immediately followed by planting. In dryland cropping areas, the time for applications is longer - biosolids can be applied in the fall when soils are drier. Some spring-seeded crops may need supplemental (starter) fertilizers. The nitrogen in the starter fertilizer (usually 10 to 30 lb nitrogen per acre) is more immediately available to the crop and decreases the total amount of nitrogen needed from the biosolids. See the table above for harvest restrictions.

Non-Food Crops

These crops include turf, ornamental plants, and fiber crops. Some of these crops may be used in a situation that has a high potential for public exposure. The sale of products such as sod is restricted for one year after applying Class B biosolids. See the table above for harvest restrictions.


Applications to forested sites can be made either annually or once every several years. The best age and size for trees is more than five years old or more than about four feet high. This cuts down on the required maintenance (disking and herbicide application) because the trees are well established. For older stands, application takes place under the canopy. Since foliage is not affected, biosolids can be applied year-round. If possible, application to young plantations should take place during the rainy, nongrowing season. This helps wash the biosolids from the foliage. It also keeps the biosolids off new foliage, which could slow the current year's growth. Avoid extreme weather, especially with saturated soil conditions during rainy-season biosolids applications because the risk of runoff increases.

Biosolids Application in Western Washington

Winter biosolids applications in western Washington require careful selection, design and management. sites with perennial grasses on well drained soils are the best option for winter application. Winter applications must be at a low rate to match the limited uptake capacity of the perennial grass or cover crop. Biosolids application equipment must not compact wet soils, and must be able to apply the biosolids at low rates.
Factors that influence site suitability and management include:

At low-lying sites in western Washington, flooding can take place from November through April. Flooding is a problem at biosolids application sites because nutrients and pathogens may contaminate surface waters. The flooding hazard is listed by month in NRCS soil surveys as: frequent, occasional, or none.

Sites that occasionally flood may be acceptable provided that:
  • Biosolids are applied at least 60 days before the onset of flood season, which allows enough time for nutrient utilization and pathogen die-off. This will usually limit biosolids applications to May through August.
  • Perennial grass cover is maintained on the site.
  • Biosolids are not applied to draws that carry water at high velocity.
  • There are no conflicts with wetlands regulations.
Runoff and Leaching
The application schedule is designed so biosolids application will take place well before seasonal high water tables. Lower application rates are used during winter months to match the slow growth of the grass. Progressively lower rates are applied in July, August, and September on all soils to match the nitrate required by the crop for the remainder of the growing season. To design an application schedule to fit the soils, management, and precipitation at your site, contact your local NRCS office or a qualified soil scientist or agronomist.

Type of Crop
Crops at winter application sites must be able to:
  • Use the winter-applied nitrogen
  • Support vehicle traffic
  • Filter runoff from the site
The ability of different crops to use winter-applied nitrogen in western Washington is variable due to weather, crop species, soils, and crop management. Fallow ground and legume cover crops do not use nitrogen and are not suitable for winter biosolids application. Grass or cereal cover crops (for example rye planted in September after corn harvest) do not need fertilizer nitrate in order to become established in the fall. They can, however, use nitrogen applied in late winter (February). Established perennial grasses provide the most options for winter application, they can use conservative amounts of nitrogen applied throughout the winter period.