Feds introduce new ammonia discharge regulation for metal and diamond mines

A mining operation near Sechelt Inlet, B.C.
A mining operation near Sechelt Inlet, B.C. Credit: edb3_16 / AdobeStock

By Kareena Gill

Mining is a foundational industry and contributes billions to the economy through taxes and spending. The sector is subject to strict environmental regulations and is gaining global recognition for ethical mining.

On June 1, 2021, amendments to the Metal Mining Effluent Regulations (MMER) under the title of Metal and Diamond Mining Effluent Regulations (MDMER) came into force. Of particular significance is the ammonia discharge limit.

Several activities in a mining operation introduce ammonia to its wastewaters. Two major sources are the cyanidation of gold ore and the use of ammonium nitrate-based explosives. Degradation of cyanide and the ammonium and nitrate in the explosive residue results in the release of ammonia. If it is discharged to the environment, untreated ammonia in mine-contacted water is harmful to aquatic life.

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The presence of ammonia in fish habitats has proven to be highly toxic. Fish do not possess the ability to eliminate ammonia from their body and therefore accumulate it even at low concentrations in water. Salmonids, especially trout, are extremely sensitive to ammonia. The low toxicity threshold for un-ionized ammonia for rainbow trout has been established to be 0.4 mg-N/L by the Canadian Council of Ministers of the Environment.

To limit the risks of the negative effects of mines on fish and fish habitat, the latest criteria under MDMER impose a limit on ammonia release for end-of-pipe discharges.

The discharge of un-ionized ammonia from mines will now be required to comply with a monthly mean concentration limit of 0.5 mg-N/L. This regulation will apply to new, existing, and re-opening mines from June 2021.

To treat elevated ammonia levels in mine-contacted water before discharge, existing mine water treatment systems will have to be upgraded and/or new systems designed per the new criteria for ammonia discharge. A brief overview of ammonia removal technologies follows:

Biological treatment – With the help of different types of bacteria, ammonia is oxidized to nitrate (nitrification) in an aerobic process and then the nitrate is reduced to nitrogen gas (denitrification) under anoxic conditions.

Oxidation – Oxidizing reagents such as chlorine, hydrogen peroxide and ozone are used to oxidize ammonia to nitrogen gas, or an electrochemical system is used to produce the oxidizing reagent.

Adsorption – Total ammonia in the form of ammonium and ammonia-metal complexes is removed by adsorption through the use of zeolites or ion exchange resins.

Membrane separation – Technologies like reverse osmosis (RO) and nanofiltration (NF) are available for ammonia removal from the mine effluents.

Air stripping – Various systems available include conventional packed towers and aerators with the ability to be installed in storage ponds. Ammonia is removed from mine-impacted water by mass transfer from water to air.

This new ammonia regulation is expected to affect over 100 metal and diamond mines across Canada. The best treatment approach should be selected based on a site-specific comprehensive assessment of the mine data and a thorough feasibility study.

Kareena Gill is with McCue Engineering Contractors.
Email: kareena@mccuecontracting.com

Read the full article in ES&E Magazine’s June 2021 issue:

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