Summary #
Active treatment uses chemical additions to neutralize AMD acidity and precipitate metals. This page describes the six most common active treatment agents — limestone, hydrated lime, quick lime, soda ash, caustic soda, and anhydrous ammonia — including their applications, costs, handling requirements, and safety considerations.
Active treatment involves the continuous addition of alkaline chemicals to AMD water to neutralize acidity and precipitate metals. Unlike passive treatment systems, active treatment requires ongoing chemical supply, equipment maintenance, and often electrical power. Active treatment is typically used for high-flow, high-acidity discharges where passive systems cannot provide sufficient alkalinity.
The following chemical agents are commonly used in active AMD treatment. Information adapted from the EPA’s Mid-Atlantic Integrated Assessment: http://www.epa.gov/maia/html/AMD_active.html
Limestone #
Limestone is the least expensive and safest active treatment method for AMD. Chunks or ground limestone are added directly to AMD water. It is effective for low to moderate acidity levels.
Hydrated Lime #
Hydrated lime is commonly used because it is cost effective for very high acidity and large water flow situations. It does not dissolve in water and requires extensive mechanical mixing via a mixer/aerator. It is purchased in bulk as a powder. Excess hydrated lime should not enter the settling pond stage. Safety goggles are required when handling this chemical as it can cause blindness.
Quick Lime #
Quick lime (calcium oxide) is highly reactive, relatively inexpensive, and can be purchased in 50-pound bags or in bulk. It is typically dispensed using an Aquafix water wheel placed above a channel of water. The quicklime dissolves over time as water flows past. Feeder bins or silos hold the quicklime above the machine and feed it by gravity, powered by the flow of water.
Soda Ash #
Soda ash (sodium carbonate) comes as solid briquettes and is gravity fed into AMD water from a hopper mounted over a basket. Soda ash is typically used to treat AMD in remote areas with low volume flows and low amounts of acidity and metals.
Caustic Soda #
Caustic soda (sodium hydroxide) is used in remote locations where electricity is not available and flows are low. It is gravity fed as a liquid into AMD, dissolves quickly, disperses, and raises pH very rapidly. Because it is denser than water, it should be applied to the surface of a pond where it will then sink. Caustic soda is contained in tanks that can hold up to 8,000 gallons. In winter, it can freeze, so the tank may require a heater or be buried underground (with strict EPA compliance). Adding potassium hydroxide makes it solid but freeze-proof, and it can then be dispensed as flakes or beads via large drums.
Anhydrous Ammonia #
Ammonia is stored as a liquid but turns to gas when it contacts water, dissolving and rapidly raising pH. Special training and experience is needed to handle the hazardous ammonia. Additional analyses must be performed downstream to monitor biological effects on the ecosystem. There are dangerous consequences associated with excessive ammonia application.
EPA Mid-Atlantic Integrated Assessment — Active Treatment Methods: http://www.epa.gov/maia/html/AMD_active.html
Related Pages #
- Treatment Strategies — When to choose active vs passive treatment
- Neutralizing Acid (Chemistry Concepts)
- Operations, Maintenance and Replacement — For passive alternatives
- Restoration Planning — Funding and design considerations
Source and Last Reviewed #
Source: AMR Clearinghouse (amrclearinghouse.org). Migrated to AML-Connect. Last Reviewed: 2026-03-13.
[Admin: This post consolidates the formerly separate Limestone, Hydrated Lime, and Quick Lime posts with Soda Ash, Caustic Soda, and Anhydrous Ammonia content from the Active Treatment hub page. The individual short posts were too thin to stand alone.]
[Admin note: Some external links in this article may be outdated. Verify before relying on them. Flag dead links for removal or replacement.]
Tags: active-treatment, alkalinity, practitioner, safety
