REFINEMENTS IN ANAEROBIC DIGESTION
FOR WASTE WATER TREATMENT

arial view of South Shore treatment plant

Located on Lake Michigan, the South Shore Wastewater Treatment Plant uses anaerobic digestion to generate methane gas. The plant uses the gas as fuel for its energy needs.

Anaerobic digestion is a biochemical process for converting organic wastes into a stable granular material and a usable methane gas. Taking place in the absence of oxygen, microorganisms break down the biosolids into usable by-products.The methane gas can produce process heat or electrical power; the solid residue is suitable for application on agricultural lands as an organic soil conditioner.The South Shore plant of the Milwaukee Metropolitan Sewarage District (MMSD) was an early large-scale application of the process.

Anaerobic digestion reportedly was first applied on a commercial basis at sewerage treatment plants in Birmingham, England and in Baltimore, Maryland in 1912.They were batch systems with open tanks, heated with steam during winter month.However, they fell into disfavor because of the odor and scum they produced.

In 1921 a pilot project demonstrated a continuous digestion process that was essentially free from odor. A production plant was installed and began operation at Brownsville, Texas in 1923.

Because of the temperature requirements for the digestion process, there still remained challenges for using the process in northern climates.In 1926, they were overcome at Antigo, Wisconsin, when an anaerobic digestion plant began operation, using a heated, covered, continuously fed, mechanically mixed digester with a methane collection system. It became the forerunner of all present day anaerobic digestion processes.

stationary engines for generators

These three spark-ignited Nordberg six cylinder turbocharged gas engines are fueled with digester gas. Each drives an Allis-Chalmers electric generator.

The South Shore Waste Water Treatment plant in Oak Creek illustrates a modern day anaerobic digestion plant.Waste solids are digested, stored in lagoons, then applied to agricultural lands.The plant uses methane produced in the digesters to generate much of the electric power needs of the plant, to produce the air needed for its secondary treatment processes, to provide the necessary heat to the digesters, and to incinerate grit, screenings and grease.

For greater flexibility, the Jones Island Treatment Plant can pump biosolids by pipeline the eleven miles to the South Shore facility where it can be anaerobically digested. Similarly, the South Shore plant can pump waste activated sludge and digested sludge from the to Jones Island where it becomes part of the raw material for Milorganite, a lawn and garden fertilizer produced by MMSD from sewage waste.

The by-product from the South Shore facility is a thickened sludge, marketed under the trade name Agri-Life, that can be spread onto farm fields as a liquid or as filter cake during spring or fall.The product is stored on site until it can be applied.

According to a June 1993 update to South Milwaukee’s Facilities Plan, the South Milwaukee facility is designed to treat an annual average flow of 6.0 million gallons per day (MGD) with a peak hydraulic capacity of 25.0 MGD.Currently, the average daily flow is 3.6 MGD.

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