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Meeting the Standards

There are three criteria for the determination of biosolids quality: pollutant limits, pathogen reduction, and vector attraction reduction. The regulations specify two levels of pollutant limits and pathogen reduction. If biosolids meet the higher standards, they are called exceptional quality (EQ) biosolids. EQ biosolids can be applied to land with few reporting and site management requirements. This page describes requirements for meeting pathogen, vector attraction and pollutant standards, as well as the standards for EQ, Class A, and Class B biosolids.

Pollutants

There are nine regulated pollutants. The regulations set two concentration limits, maximum allowable concentrations for land application (known as Table 1 limits) and lower concentration limits (known as Table 3 limits) (see Concentration Limits Table below). Nearly all biosolids tested in Washington State meet Table 3 standards for pollutants.

Pollutant Limits
Element Symbol Table 3
mg/kg
Table 1
(Ceiling Limit) mg/kg
Arsenic As 41 75
Cadmium Cd 39 85
Copper Cu 1500 4300
Lead Pb 300 840
Mercury Hg 17 57
Molybdenum Mo --- 75
Nickel Ni 420 420
Selenium Se 100 100
Zinc Zn 2800 7500


If any of the pollutant concentrations do not meet Table 3 limits, biosolids applications are limited by cumulative loading limits shown in the Cumulative Limits Table. Cumulative limits specify the highest amount of any pollutant that can be applied to a land application site during its lifetime. There are no restrictions on cumulative loading rates based on pollutants below the Table 3 standard. If any pollutant exceeds the Table 1 concentration limits, the material does not meet the criteria for biosolids and cannot be applied to the land.

Cumulative Pollutant Loading Rates
Element Cumulative limit (lb/acre) Cumulative limit (kg/ha)
Arsenic 37 41
Cadmium 35 39
Copper 1340 1500
Lead 268 300
Mercury 15 17
Molybdenum * *
Nickel 375 420
Selenium 89 100
Zinc 2500 2800


Pathogens

The regulations allow two levels of pathogen removal before land application:
  • Class B is a less stringent level for applications where final removal will occur in the field and include waiting periods before public access and harvest can occur. When proper site management and access restrictions are observed, the use of Class B biosolids is considered to be as safe as Class A biosolids.
  • Class A is a more stringent level where access restrictions and waiting periods are unnecessary.

Class A Biosolids

Biosolids can meet Class A requirements (WAC 173-308-170 Pathogen Reduction) through any of the four treatment alternatives listed in the table below. To demonstrate that biosolids meet Class A standards, you must collect representative samples, see our Sampling page for more information.

All Alternatives: Fecal coliform less than 1000 most probable number (MPN) per gram total solids, or salmonella less than 3 MPN per 4 grams total solids. See our Sampling page for more information.
Alternative 1: Thermal Treatment.
Meet specified time and temperature requirements according to 173-308-170 (1).
Alternative 2: High pH - High Temperature Process.
Maintain pH above 12 for 72 hours, with temperature during the 72-hour period greater than 52°C for 12 hours. After 72 hours at pH above 12, biosolids are air-dried to greater than 50% total solids.
Alternative 3: Process to Further Reduce Pathogens (PFRP).
Use a recognized PFRP, as described below.
Class A: Processes to Further Reduce Pathogens (PFRP)


Composting.
Using within-vessel, static aerated pile or windrow composting method. Within-vessel and static aerated pile the temperature of the biosolids are maintained at 55°C or higher for three days.

Using the windrow composting method, the temperature of the biosolids is maintained at 55°C or higher for 15 days or longer. During the period when the compost is maintained at 55°C or higher, there shall be at least five turnings of the windrow.

Heat Drying.
Biosolids are dried by direct or indirect contact with hot gases to reduce the moisture content to 10% or lower. Either the temperature of the biosolids particles exceeds 80°C or the wet bulb temperature of the gas in contact with the biosolids as it leaves the dryer exceeds 80°C.

Heat Treatment.
Liquid biosolids are heated to a temperature of 180°C or higher for 30 minutes.

Thermophilic Aerobic Digestion.
Liquid biosolids are agitated with air or oxygen to maintain aerobic conditions, and the mean cell residence time of the biosolids is 10 days at 55 to 60°C.

Beta-ray Irradiation.
Biosolids are irradiated with beta rays from an accelerator at dosages of at least 1.0 megarad at room temperature (approximately 20°C).

Gamma-ray Irradiation.
Biosolids are irradiated with gamma rays from certain isotopes, such as Cobalt 60 and Cesium 137, at room temperature (approximately 20°C).

Pasteurization.
The temperature of the biosolids are maintained at 70°C or higher for 30 minutes or longer.

Alternative 4: Process Equivalent to a PFRP.
Use a process approved by the state or EPA as equivalent to an approved PFRP.


Class B Biosolids

The table below lists the three alternatives (WAC 173-308-170 Pathogen Reduction) for meeting Class B requirements.

Alternative 1: Fecal coliform are less than 2,000,000 Most Probable Number or 2,000,000 Colony-Forming Units per gram of total solids, based on a geometric mean of seven samples. See our Sampling page for more information.
Alternative 2: Use a Process to Significantly Reduce Pathogens (PSRP). Listed below.
Class B: Processes to Significantly Reduce Pathogens (PSRP)


Aerobic Digestion.
Biosolids are agitated with air or oxygen to maintain aerobic conditions for a specific time and at a specific temperature, ranging from 40 days at 20°C to 60 days at 15°C.

Air Drying.
Biosolids are dried on sand beds or on paved or unpaved basins. The biosolids must be dried for at least three months. During two of the three months, the ambient average daily temperature is above 0°C.

Anaerobic Digestion.
Biosolids are treated in the absence of air for a specific time and at a specific temperature, ranging between 15 days at 35 to 55°C and 60 days at 20°C.

Composting.
Using either the within-vessel, static aerated pile or windrow composting methods, the temperature of the biosolids is raised to 40°C or higher and remains at 40°C or higher for five days. For four hours during the five days, the temperature in the compost pile exceeds 55°C.

Lime Stabilization.
Enough lime is added to the biosolids to raise the pH to 12 after two hours of contact.
Alternative 3: Use a process determined by the state or U.S. EPA to be equivalent to a Process to Significantly Reduce Pathogens.


Vector Attraction Reduction

The biosolids rule (WAC 173-308-180 Vector Attraction Reduction) allows for eight alternative methods of meeting vector attraction reduction standards.
  • Alternatives 1 through 6 reduce vector attraction through treatment. These processes produce biosolids that are acceptable for application on any site, including lawns and home gardens. Exceptional Quality biosolids, which can be sold or given away in bags or other containers, must meet at least one of these six alternatives.
  • Alternatives 7 and 8 reduce vector attraction by tilling or injecting biosolids into the soil at the land application site. For more information see WAC 173-308-210. They can be used for biosolids that do not meet Alternatives 1 through 6. They do not meet vector attraction reduction requirements for Exceptional Quality.
Alternative 1: Volatile Solids Reduction.
Biosolids digestion processes with greater than 38% volatile solids reduction.
Alternative 1a: Bench-scale Test for Anaerobically-digested Solids.
Test end-product of anaerobic digestion process. Forty day anaerobic test at 30-37°C. Acceptable stabilization if less than 17% volatile solids reduction occurs during the test.

Alternative 1b: Bench-scale Test for Aerobically-digested Solids.
Test end-product of aerobic digestion process having less than 2% solids. Thirty day aerobic test at 20°C. Acceptable stabilization if less than 15% volatile solids reduction occurs during the test.
Alternative 2: Specific Oxygen Uptake Rate (SOUR).
Specific oxygen uptake rate (SOUR) test using end-product of digestion process. Acceptable stabilization if uptake is less than 1.5 mg oxygen per g total solids per hour at 20°C.
Alternative 3: Aerobic Process.
Fourteen days residence time at temperatures greater than 40°C, with average temperature greater than 45°C
Alternative 4: pH Adjustment.
Biosolids pH above 12 for 2 hours and greater than 11.5 for 24 hours.
Alternative 5: Percent Solids for Stabilized Solids.
Not to include unstabilized primary wastewater solids. Total solids content greater than 75% before mixing with other material.
Alternative 6: Percent Solids for Unstabilized Solids.
Can include unstabilized primary wastewater solids. Total solids greater than 90% before mixing with other materials.
Alternative 7: Injection into Soil.
No biosolids on soil surface 1 hour after application. For Class A biosolids, injection must occur within 8 hours of discharge from the pathogen reducing process.
Alternative 8: Soil Incorporation by Tillage.
Soil incorporation by tillage within 6 hours of application. For Class A biosolids, application must occur within 8 hours of discharge from the pathogen reducing process.