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Digesters at Arcata Marsh

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Figure 1. Arcata's Wastewater Treatment Plant Digester. (Dustin Poppendieck)


Anaerobic Digester Overview[edit]

The main purpose of an anaerobic digester (pictured above) is to kill pathogens and reduce the amount of solid waste, removed from the municipal water supply, that is sent to a landfill. [1] When water arrives at the Arcata Wastewater Treatment Center it passes through the headworks where the larger objects in the water are removed through a metal grating. The water then moves to the clarifiers where it is circulated allowing the solid matter to settle to the bottom. This sludge at the bottom of the clarifier is then pumped into the digester for further processing while the water is pumped to the oxidation ponds for further treatment.

Anaerobic Digester Stages[edit]

Hydrolysis[edit]

Most of the waste found in the waste water consists of long strings of complex carbon polymers or repeated links of simple molecules. In order for the bacteria to break down the sludge, the bacteria first needs to go through the process of hydrolysis. This process utilizes acid to break down the polymers into monomers or single simple molecules like simple sugars, amino acids, and fatty acids. [2]

Acidogenisis[edit]

In this stage there is a further breakdown of simple compounds by anaerobic bacteria or bacteria that must live in a oxygen free environment, to create volatile fatty acids. Volatile fatty acids are a short chains of molecules that can be easily dissolved in water. [3]

Acetogenisis[edit]

In this stage the fatty acids produced in the last stage are used by other anaerobic bacteria to produce acetic acid, carbon dioxide, and hydrogen. [2]

Methanogenisis[edit]

This is the final stage of the process. Here methanogens, another type of anaerobic bacteria, use products from the preceding stages, mainly from the acid forming bacteria, to produce methane, carbon dioxide, and water. This process is especially sensitive to pH fluctuations. Methanogenisis takes place in a pH range of 6.5 to 8. This causes a need for a specific balance of acid producing bacteria and methane producing bacteria to allow the process to continue. [2]

Products[edit]

Biogas[edit]

The methane produced in the digester is used in a boiler in combination with natural gas to heat the sludge and the slude is then pumped back into the digester. This allows the anaerobic reactions to happen faster. [4]

Figure 2. Remaining solids drying in outdoor beds. (Dustin Poppendieck)

Compost[edit]

Once the sludge has made it through the first digester it is moved to a second unheated digester. Little anaerobic activity still takes place, though the main point of this digester is to allow the left over organic matter in combination with any dead or decaying bacteria to settle out. All water left over will be pumped back to the clarifier for further processing. The left over organic material is then pumped into the open air drying beds shown to the right. This material is high in nutrient content and is a good base for a compost. Yard trimmings and Hydrocotyl, an aquatic plant, are mixed into the sludge for composting. The compost produced is then used by the city in parks. [5]

Related Sites[edit]

California Energy Commission on Anaerobic Digestion

The Arcata Wastewater Treatment Plant Webpage- Digesters

References[edit]

  1. Principles of Environmental Engineering and Science. Mackenzie L. Davis, Susan J. Masten. McGraw-Hill Copyright 2009.
  2. 2.0 2.1 2.2 Anaerobic Digestion. Accessed online 11/2/08. http://en.wikipedia.org/wiki/Anaerobic_digestion
  3. Anaerobic Digestion. Accessed online 11/2/08. http://en.wikipedia.org/wiki/Anaerobic_digestion
  4. Arcata, California - A Natural System for Wastewater Reclamation and Resource Enhancement. Accessed online 11/2/08. http://2168704.blog.hexun.com/6064890_d.html
  5. The EcoTipping Point Project. Accessed online 11/2/08. http://www.ecotippingpoints.org/ETP-Stories/indepth/usaarcata.html

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