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Des Moines Metropolitan
Wastewater Reclamation Authority
3000 Vandalia Road
Des Moines, IA 50317

Solids Stabilization

Back to Plant Processes

The sludge pumped to the anaerobic digesters is retained for 30 days. There are six large digesters, each approximately 115 feet in diameter with a side wall depth of approximately 30 feet. Each digester can hold approximately 2.7 million gallons of sludge. The digesters are heated to 90 to 100 °F. In the anaerobic digesters, stabilization occurs in three phases. In the first phase, extracellular enzymes convert solid complex organics, cellulose, proteins, lignins, and lipids into soluble organic fatty acids, alcohols, carbon dioxide and ammonia. In the second stage acid forming bacteria (Saprophytic, Clostridium aceticum) converts organic matter to organic acids. These organic acids then become the food source for the methane forming bacteria (the third phase). Methane forming bacteria are strict anaerobes and die quickly in the presence of oxygen. 

Digester gas (methane) is collected and stored in the high-pressure gas sphere (45 psig). The gas is used in the boilers to heat the buildings and the digesters. This gas will also be used to run the engine generators to produce electricity for peak shaving. In an effort to keep operating costs down, the treatment plant has three large generators driven by engines powered by our own digester gas. These generators will produce electricity for peak energy curtailment and may well save the City $500,000 a year in electrical costs. The main treatment plant uses approximately $100,000 worth of electricity a month. After 14 days, approximately 40% of the solids are destroyed. The remaining solids are pumped out, mixed with polymer and de-watered using belt filter presses. Here, 3 to 5% solids are de-watered to 20% solids. Most pathogenic bacteria will not live through the digestion process. This stabilized sludge is now ready for disposal.