Fertilizer Manufacturing

SoundGRO Fertilizer Manufacturing

The Fertilizer Manufacturing Facility at Pierce County's Chambers Creek Regional Wastewater Treatment Plant utilizes a direct drying and pelletization process.
Fertilizer Building


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% dry solid (DS) and ready for the heat drying and pelletization process.


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


The mixture is then sent to the drying drum. The drying drum consists of three concentric cylinders which the pellets pass through consecutively. Heated, re-circulated air is blown into the drying drum from a furnace that burns both natural and methane gas. The inlet temperature of the drying drum is 850°-950° F and the outlet temperature is 210° F. After emerging from the drying process the pellets are 93% DS, nearly perfectly dry.
Manufacture Diagram

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 poly cyclone, 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 air treatment and recirculation stage.

Pellet Sorting

The solids from the pre-separator and the poly cyclone 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), oversize 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° F 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. The storage silos each have 10,000 cubic feet of storage capacity and together can hold 4 weeks of manufactured pellets at current production rates.

Air Treatment and Recirculation

The air is sent back to the furnace and re-circulated 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

  • Meets EPA standard for Class A 'Exceptional Quality' biosolid
  • 93% Dry Solid
  • 0.5-2.5 mm diameter pellet
  • N-P-K (Nitrogen-Phosphate-Potassium) ratio of 5-5-0
  • A quality fertilizer that improves landscaping and gardens
  • Registered with the Department of Agriculture as a commercial fertilizer