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Biosolids photo identifier

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Compost Facility Operator Training Photo Courtesy of Dan Corum


Composting is a good alternative for processing biosolids from small or large wastewater treatment plants, whether it's onsite or sent to another permitted facility. The compost site must meet design standards, and the compost process must meet requirements for pathogen reduction and vector attraction reduction. Composting is more expensive than some alternatives, but the product has a higher market value and is more versatile than Class B biosolids liquid or cake.

This page outlines the basic methods and requirements that must be met for biosolids to be composted. For further information see our Regulatory Links & Resources page. For further information on requirements and permitting, please contact your Department of Ecology Regional Coordinator or local health jurisdiction.


The Washington Organic Recycling Council (WORC) offers Compost Facility Operator Training (CFOT) each fall. Upon successful completion of the training, continuing education credits (CEU's) are awarded to be put towards Ecology's Wastewater Operator Certification Program. For more information about eligible CEU's, contact Poppy Carre.


Any of the following methods can produce compost that meets Class A pathogen reduction standards as outlined in WAC 173-308-170 (Pathogen Reduction).

This is the simplest method with the lowest start-up costs. Raw materials are blended and formed into long, linear mounds (windrows) that are up to nine feet high and eighteen feet wide at the base. To meet pathogen reduction standards, the compost must be turned five times. The size of the mounds depends on the raw materials and the type of turning equipment used - for example, front-end loaders, tractor-pulled turners or large self-propelled turners. Material on the outer layer is eventually mixed into the center where it will be exposed to high temperatures.

Some windrows are aerated using a blower system with aeration ports embedded in the pad. This keeps the windrow supplied with oxygen and can reduce odors. Time, temperature, and turning requirements for pathogen reduction are the same for simple windrows and aerated windrows.

Aerated Static Pile
This method also uses piles but does not require turning. Instead, the pile is built over a base of porous materials such as wood chips. A perforated pipe in the base is attached to a blower, which pushes or pulls air through the pile for aeration. Some facilities have aeration pipes and ports built into the underlying concrete pad. The piles are typically five to eight feet high, and ten to sixteen feet wide. The length of the pile depends on blower capacity. The blower should maintain even air flow through the pile.

Since the pile is not turned, you must be sure the initial mixing of the materials is thorough. Covering piles with finished compost or another insulating material is essential to achieve the required minimum temperatures throughout the pile. When air is pulled through the pile, odors can be controlled by venting the system through a pile of cured compost, woodchips or "overs", which acts as a biofilter. Operating the blower intermittently leads to more uniform composting because air and moisture will equilibrate in the pile when the blower is off. You should monitor temperature and oxygen throughout the composting process and adjust the aeration schedules accordingly. The same principles apply for aerating a windrow. Although pathogen reduction can occur within a few days, it takes longer to produce finished compost.

These are enclosed, aerated containers that allow composting under controlled conditions of temperature and aeration.


Pathogen Reduction
Composting can be used to meet the Class A or B pathogen reduction requirements outlined in WAC 173-308-170 (Pathogen Reduction). The cost of the composting process, however, will generally not justify the production of a Class B product. Class B compost should be viewed as a failed batch and be reprocessed to achieve Class A standards. Following are Class A requirements for biosolids compost produced by the standard composting methods (aerated static pile (ASP), windrow, or in-vessel). Using either the in-vessel composting method or the aerated static pile composting method, maintain the temperature of the biosolids at 55°C (131°F) or higher for three days. Using the windrow composting method, maintain the temperature of the biosolids at 55°C or higher for 15 or more days. During the period when the temperature is maintained at 55°C or higher, turn the windrow at least five times.

As part of obtaining permit coverage, you must develop a sampling program to demonstrate that your facility meets the required standards. For more information about testing and sampling see Sampling.

Vector Attraction Reduction
Vector attraction reduction must be achieved during the composting process. Pathogen reduction must occur at the same time as or before vector attraction reduction is complete. The requirement for vector attraction reduction during composting involves maintaining the pile for at least fourteen consecutive days at temperatures greater than 40°C, with average temperature greater than 45°C.