Ammonia is a nitrogen compound that is found in nature in trace amounts, most notably as a waste by-product when microorganisms utilize nitrogen gas or from the decomposition of organic waste. It is also produced industrially for use in fermentation, fertilizers, cooling systems, explosives and cleaning supplies. Ammonia at concentrated levels is hazardous.
Whereas mammals possess mechanisms to incorporate ammonia into amino acids or dispose of it in urine, fish and most aquatic life do not. Without the ability to eliminate ammonia which can accumulate in blood or tissue, ammonia even at low concentrations is highly toxic to aquatic life. This has led Environment Canada and the US EPA to enact guidelines for ammonia concentrations in water. In Canada, the Environmental Protection Act limits ammonia levels in the receiving water body to 0.019 mg/L. The Canadian mining industry has also seen a recent revision to the Metal and Diamond Mining Effluent Regulations (MDMER) which added new ammonia limits for end-of-pipe discharges.
In mining, ammonia is introduced to waste streams through the use of the common blasting agent Ammonium Nitrate Fuel Oil (ANFO), cyanide destruction or degradation, and human activities. A portion of this ammonia reports to the aqueous phase, necessitating an ammonia treatment system to comply with water quality regulations. Common ammonia removal technologies applicable for mining include biological treatment, oxidation, adsorption, membrane separation, air stripping and combinations thereof. Here we provide an overview of each treatment and their application to mine impacted waters.
This process relies on different types of bacteria as a catalyst to remove ammonia from water in either one or two stage processes depending on the target nitrogen end product. In the one stage process, ammonia is oxidized to nitrate (nitrification) in an aerobic process. In the two stage process, nitrate reporting to the second stage is reduced to nitrogen gas (denitrification) under anoxic conditions.
Biological treatment is widely applied on domestic wastewaters and streams from agricultural and food industries. However, its application in mining raises concerns including:
- Inability to accommodate large variations in water flow or contaminant concentration
- Sensitivity of bacteria to trace metals
- Long start-up period including recovery after facility upsets
- Sensitivity of the kinetics of reactions catalyzed by bacteria to changes in temperature
- Need to add phosphorus and/or organic carbon to plant feed to ensure microbial population can be sustained
- Need to remove unutilized phosphorus and organics to avoid eutrophication in downstream environment
- Risk of effluent toxicity due to unwanted treatment by-products (NO2, NH4, P, BOD, H2S, organo-Se, methyl Hg)
- Increase in Total Suspended Solids (TSS) in the final effluent over the MDMER limit of 15 mg/L from fine biomass passing through conventional solid-liquid separation stages
- Inter-stage complexities where conditions upstream influence downstream stages
- Large footprint
Many mining projects experience large variations in wastewater flow and composition. And unlike municipal operations, mines are located in remote areas and discharge into pristine environments. Consequently, these concerns need to be carefully considered when assessing biological treatment for mining applications.
A number of oxidizing reagents such as chlorine, hydrogen peroxide and ozone can be used to oxidize ammonia to nitrogen gas. These reagents can be brought to site or they can be produced on-site with in-situ/in-stream electrochemical systems. If an electrochemical system is used to produce the oxidizing reagent, the process is referred to as indirect oxidation. An example of indirect electrochemical oxidation is using the chloride contained in wastewater to produce chlorine on the anode in an electrochemical cell where the chlorine subsequently reacts with ammonia.
Some of these electrochemical systems can also be used to directly oxidize ammonia. In this process, an oxidizing reagent is not required and ammonia is oxidized in an electrochemical cell directly on the anode. Direct oxidation is expensive due to its reliance on novel anode materials such as boron doped diamond.
Some oxidation processes have been applied widely on an industrial scale such as chlorination while others are in pilot and demonstration phase. Limitations that should be noted with oxidation technologies for mine waters include:
- Non-selective oxidation of ammonia as oxidants will react with other species contained in water.
- High dosages of oxidizing reagents or power consumption depending on water chemistry.
- Sensitivity to the presence of suspended solids.
- Treatment residuals and by-products can be toxic (e.g. chloramines).
- On-site generators may require ultra-pure water making the treatment impractical.
Adsorption involves adhering the targeted constituent to a surface. Ammonia removal utilizing this process can be divided into two main categories. The first is removal of cationic ammonium. In its cationic form NH4+, ammonium is like any other cation and can be removed from water by cation exchangers. Naturally occurring and synthetic zeolites are the most commonly used exchangers of ammonium.
Zeolites have been used for ammonium removal in mining as single-use and as regenerable media. Its relatively low cost and tolerance to flow and temperature fluctuations makes zeolites attractive but other cationic species in the water can compete with ammonium, reducing zeolite efficiency. Zeolites are also subject to physical degradation over time. When used as regenerable media, the regeneration process produces an ammonia laden brine requiring management.
The second category is complexation of unionized ammonia. In its non-ionic form NH3, ammonia complexes with metals such as copper and zinc. An ion exchange resin infused with such metals or activated carbon can remove ammonia from water.
Complexation of ammonia with metals is a function of the solution ionic strength. The efficiency of these adsorbers will be reduced with increasing total dissolved solids (TDS) levels in the mine water.
Membrane based technologies such as reverse osmosis (RO) and nanofiltration (NF) can be used to remove ammonia from mine impacted waters with RO achieving an ammonia removal efficiency as high as 99%. In order to ensure high removal efficiency, unionized NH3 must be converted to NH4+ by pH adjustment upstream of the membranes. One of the main advantages of membranes is their modular design reduces plant footprint and facilitates deployment.
However, prior to selecting membrane as the ammonia treatment method for any mine site, the following should be taken into consideration:
- Risk of membrane fouling caused by mine waters which typically have high concentrations of calcium, sulphate and alkalinity. Anti-scalants can be added to mitigate fouling but this may also create difficulties with downstream processes.
- Production of a liquid brine by-product with high ammonia concentration requiring handling, disposal or treatment. Brine management which varies from site to site can be a large portion of project costs.
This process relies on the mass transfer of unionized NH3 from water to air. To increase the rate of mass transfer, a large interfacial area between gas and liquid must be created. Various systems are available to achieve this including conventional packed towers and aerators that can be installed in storage ponds. The ammonia transferred from water to air either leaves the system in the air stream or is captured and transferred back into an aqueous form using sulphuric acid to produce an ammonium sulphate solution that has potential use as a fertilizer.
A relatively simple process, the use of air stripping to remove ammonia from mine water raises these concerns:
- Low efficiency removal due to the high solubility of ammonia in water.
- Air stripping is sensitive to temperature as ammonia solubility in water increases at lower temperatures. If heating is required it adds to the operating costs.
- High reagent cost associated with adjusting the pH up to enable NH3 removal and then back down to ensure the final effluent pH is within the acceptable range for discharge.
- Scale formation in air-stripping equipment caused by elevated hardness present in mine water.
- Air emissions for atmospheric ammonia.
Ammonia removal in mining can be a complex issue. Selecting the best treatment approach should be based on a holistic assessment of site specific factors that influence life cycle costs and implementation risks for the full life of the mine.
Farzad Mohamm, PhD and Chim Xiao, MEng