Fertilizer Manufacturing

SoundGRO®  Fertilizer Manufacturing

Fertilizer BuildingThe SoundGRO SoundGRO® Fertilizer Manufacturing Facility at Pierce County's Chambers Creek Regional Wastewater Treatment Plant employs a direct drying and pelletization process.

The process can be discussed in seven phases: dewatering, mixing, drying, air and solids separation, pellet sorting, pellet cooling and storage, and air treatment and recirculation.


After being anaerobically digested, the biosolids are conveyed to one of two centrifuges for dewatering. The centrifuges operate at 3200 RPM and use a liquid polymer to separate the solids and the liquids. Upon emerging from the centrifuge, the biosolids are 20% Total Solids (TS), which means that it is still 80% water at this point. This is the first step of the heat drying and pelletization process.


The biosolids continue to the mixer, where they are mixed with recycled pellets to form a consistency of 70% TS.


The mixture is then sent to the drying drum. The drying drum consists of three concentric cylinders, which the pellets pass through consecutively. Heated recirculated air is blown into the drying drum from a furnace that burns both natural and digester gas. The inlet temperature of the drying drum is approximately 1,000 degrees Fahrenheit and the outlet temperature is 200 degrees Fahrenheit. The high temperature kills harmful pathogens. After emerging from the drying process, the pellets are 92% TS, nearly perfectly dry. undefined Opens in new window

Air and Solids Separation

The pellets and the heated air pass through a pre-separator. The pre-separator has a baffle that slows the velocity of the air, causing the heaviest solids to drop out. The air and the lighter solids then pass through a polycyclone consisting of tunnels, which use centrifugal motion to allow the rest of the solids to drop out. The pellets are then sorted, and the air is transferred to the air treatment and recirculation stage.

Pellet Sorting

The solids from the pre-separator and the polycyclone are then sent to a vibrating screen. The vibrating screen consists of three tiers of screens that sort the solids into four sizes: product-sized pellets (0.5-2.5 mm), oversized pellets (2.5-10 mm), fines (<0.5 mm), and trash (>10mm). The oversized pellets are then crushed and combined with the fines before being sent to the recycle bin. These recycled pellets are used in the mixing stage. The product-sized pellets are sent to the pellet cooler. 

Pellet Cooling and Storage

Pellets that are the proper size are cooled to 90 degrees Fahrenheit in the pellet cooler and sent to one of three storage silos using a pneumatic conveyor. Trucks can be loaded directly from the silos and weighed with the on-site scale. The storage silos each have 10,000 cubic feet of storage capacity and together can hold 10 to 14 days of manufactured pellets at current production rates.

Air Treatment and Recirculation

The air is sent back to the furnace and recirculated through the system. Air that needs to be treated is scrubbed with water and chemicals to eliminate odors and then released to the atmosphere. Volatile organic compounds (VOCs) are a main source of air pollution. The regenerative thermal oxidizer (RTO) efficiently destroys the VOCs prior to discharge of the treated air to the atmosphere.

The Final Product - SoundGRO®

The final product of the SoundGRO® fertilizer manufacturing process:

  • Meets EPA standards for Class A 'Exceptional Quality' biosolids.
  • Contains 92% Total Solids.
  • Is made up of 0.5-2.5 mm diameter pellets.
  • Contains an N-P-K (nitrogen-phosphate-potassium) ratio of 5-5-0.
  • Is a quality fertilizer product that improves the health of lawns and gardens.

Nutrient Analysis

Pierce County tests our biosolids by a commercial lab every month to ensure that it continually meets the standards for quality and safety. One of the benefits of an organic fertilizer is that it contains micronutrients that are beneficial and essential to plant growth. 

SoundGRO contains the following nutrient concentrations:

Total Nitrogen*6.0%
Water insoluble nitrogen
Water soluble nitrogen
Available Phosphate (P2O5)*4.0%
Calcium (Ca)2.3%
Magnesium (Mg)1.1%
Iron (Fe)0.42%
Molybdenum (Mo)*0.0005%
Zinc (Zn)0.07%

*Minimum Guaranteed Values denoted by asterisk, remaining values are averages lab results from 2020-2022.

One concern associated with using biosolids as fertilizer is with possible heavy metal content. The U.S. Environmental Protection Agency sets standards for safe heavy metal concentrations in biosolids in the 40 CFR Part 503 ("Biosolids Rule").

SoundGRO's heavy metal concentrations are consistently well below the EPA's standards.

Heavy MetalSoundGRO Heavy Metal Concentration Averages in 2022 (ppm)EPA's Standards for Class A 'Exceptional Quality' Bio-solids (ppm)
Contact Us


(253) 798-3099 (information line only)
(253) 798-4050

Pick-up by appointment only

Chambers Creek Regional Wastewater Treatment Plant
10311 Chambers Creek Rd. W
University Place, WA 98467