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Compost or Anaerobic Wetlands
Compost wetlands, or anaerobic wetlands as they are sometimes called, consist of a large pond with a lower layer of organic substrate. The flow is horizontal within the substrate layer of the basin. Piling the compost a little higher than the free water surface can encourage the flow within the substrate. Typically, the compost layer is made from spent mushroom compost that contains about 10 percent calcium carbonate. Other compost materials include peat moss, wood chips, sawdust or hay. A typical compost wetland will have 12 to 24 inches of organic substrate and be planted with cattails or other emergent vegetation. The vegetation helps stabilize the substrate and provides additional organic materials to perpetuate the sulfate reduction reactions.

Anaerobic wetlands are used to treat AMD from active mine discharges to meet established effluent requirements. Generally, the design of these wetlands is conservative and can treat discharges that contain dissolved oxygen, Fe3+, Al3+ or acidity less than 300 mg/l. When treating discharges from abandoned mines the goal is to reduce the pollution to levels that will restore the receiving stream. In these cases, wetlands can accept discharges with an acidity in the 500 mg/l range. The compost wetland acts as a reducing wetland where the organic substrate promotes chemical and microbial processes that generate alkalinity and increase the pH. The compost removes any oxygen in the system. This allows sulfate to be reduced and also keeps the metals from oxidizing and armoring or coating the limestone present in the compost, thereby preventing its dissolution. Microbial respiration within the organic substrate reduces sulfates to water and hydrogen sulfide. The anoxic environment within the substrate also increases the dissolution of limestone.

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