Figure 1. Arcata's Wastewater Treatment Plant Compost Piles, photo courtesy of Dustin Poppendieck

Introduction[edit | edit source]

The Arcata Wastewater Treatment Plant processes the biosolids from the digester into compost at its onsite composting facility. The finished compost is used as a soil amendment by the City of Arcata Parks and Recreation department throughout Arcata city parks. [1]

Compost Ingredients[edit | edit source]

Biosolids[edit | edit source]

Figure 2. Biosolids in the outdoor drying beds, photo courtesy of Dustin Poppendieck

Biosolids are the solid organic matter produced from wastewater treatment facilities. Although historically called “sludge,” biosolids differ in respect to the fact that sludge refers to untreated, raw sewage, whereas biosolid refers to the beneficial by-product of sewage treatment that can be used in composting. [2] The Arcata Wastewater Treatment Plant processes its sludge into Class A- Exceptional Quality biosolid.[1] The biosolid is pumped from the digester onto covered outdoor drying beds (Figure 2), and left to air dry for up to six months. The two drying beds are rectangular in shape, four feet deep, and use one center drain. Dewatering is necessary in order to reduce the liquid content and increase the solids concentration to greater than 20 percent.[1] By converting the liquid biosolid into a damp “cake” it can easily be moved into the composting facility.

Figure 3. Mixing Wagon at the Composting Facility, photo courtesy of Dustin Poppendieck

Bulking Agent[edit | edit source]

In order for composting of the biosolid to take place, a bulking agent is added to allow for aeration, porosity, and proper carbon to nitrogen nutrient balance. [3] The composting facility uses wood chips, grass clippings, and hydrocotyl from the oxidation ponds to provide this function. Operators mix a 3:1 volume ratio of woodchips to biosolids in a double auger vertical mixing wagon (Figure 3) and dump the mixture into long, six to eight feet high compost piles.[1]

Composting Process[edit | edit source]

Active Process[edit | edit source]

The Arcata Wastewater Treatment Plant utilizes an Aerated Static Pile composting process (Figure 4). The "static", or unturned compost piles utilize an air supply hooked up to a pipe that is located at the bottom of the pile. The pipe runs through the entire length of the pile and maintains the constant oxygen supply necessary for aerobic activity. [4] During active composting, microorganisms break down the organic material in the biosolids. As a result, carbon dioxide, water, compost, and heat are produced. The composting facility follows the EPA requirements for biosolid compost, performing daily checks to ensure the pile reaches 150 degrees Fahrenheit for three consecutive days, followed by ten days at 135 degrees Fahrenheit.[1] The high temperatures allow for pathogen and fecal coliform elimination or reduction. A pH level of 6 to 8, and a moisture content of 50 to 60% of total weight are also considered ideal conditions. [3] The active process takes about one month to for microorganisms to turn the biosolid and wood chips mix into a stabilized product.

Figure 4. Aerated Static Pile Schematic Diagram

Curing[edit | edit source]

After the month long active composting process, the Arcata Wastewater Treatment Plant cures its compost piles for an additional 30 days.[1] During curing, the compost mix is stabilized as microbial activity decreases and the pile cools. Operators take eight samples from different locations of the pile and bring them to a lab to test for fecal coliform and trace metal contents. If the compost mixture has passes inspections for EPA standards, it is taken by the City of Arcata Parks and Recreation Department. The City of Arcata brings the compost directly to site locations, or store it by Health Sport, an Arcata community center.[1]

Public Use[edit | edit source]

The compost is used exclusively by the City of Arcata Parks and Recreation department and is not available for sale. Presently, the composting facility does not produce enough compost to supply the City Parks department and the general public.[1] The final compost product, however, satisfies the EPA requirements for a Class A biosolid, and can therefore be used in household gardens without any regulations.[5]

References[edit | edit source]

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Arcata Wastewater Treatment Plant. Multiple correspondences (03/08/08-04/30/08).
  2. EPA Report: Biosolids Use, Generation,and Disposal in the United States. Accessed online 4/20/08.
  3. 3.0 3.1 Decision Makers guide to Integrated Waste: Chapter 7, compost. Accessed online 4/20/08.
  4. Davis, Mackenzie, and Susan Masten. Principles of Environmental Engineering and Science. McGraw Hill: New York, NY 2004.
  5. EPA:Biosolids Technology Fact Sheet. Accessed online 4/20/08.