The objective of aeration is to oxidize dissolved Fe²⁺ as it is one of the main pollutants in AMD. If the wastewater contains more than 50 mg/l Fe²⁺ then it must be aerated. Aeration increases the level of dissolved oxygen (DO), promoting the oxidation of iron and manganese, which increases the efficiency of chemical treatment and thus reduces costs. During aeration, dissolved carbon dioxide from underground mine water will be released, resulting in an increase in the pH and a reduction in the cost of reagents [1] .

AMD can be neutralized by chemicals such as sodium and calcium hydroxide and their carbonates in order to precipitate metals. Neutralization and precipitation are used quite often due to the feasibility of treating large volumes of contaminated water, the low cost and the simplicity of the process [10] .

Quicklime (CaO) and hydrated lime (Ca(OH)₂) are used for the neutralization of AMD due to their abundance and high reactivity. During neutralization metals such as Fe²⁺, Fe³⁺, Al, Cu, Zn and Pb are precipitated in the form of metal hydroxides. The sludge resulting from this process is a mixture of metal hydroxides and gypsum (CaSO₄). Equation (6) below gives the main neutralization reaction when using hydrated lime [10] .

Sludge containing Fe³⁺ is more stable than sludge containing Fe²⁺, and air is therefore used during neutralization to oxidize Fe²⁺ to Fe³⁺. Clarifiers or thickeners are used to settle the sludge produced, and if the solids content is less than 1 mg/l, sand filters can be used to polish the treated water. The solids content of sludge is strongly affected by the concentration of metals in the water and the type of treatment process applied, and can vary from 1% to 30%. The process is optimized by adjusting the process parameters (neutralization rate, oxidation rate, ratio of Fe²⁺/Fe³⁺, ion concentration, temperature, sludge age, crystal formation and sludge recycling) in order to obtain a denser sludge, thus reducing the volume [10] .

This process is called high sludge density (HSD) and it is a modification of conventional neutralization process and it aims to produce a higher sludge density [11] . Neutralization reactors are used to oxidize iron from Fe²⁺ to Fe³⁺ at certain pH. Treated water from the reactors is flocculated with a polymer, and the solids are separated from the liquid in a thickener or clarifier. The sludge produced in the thickener is routed back to the process [12] . The illustration of HSD process can be seen in Figure 2.

The configuration illustrated in Figure 2 is the standard commercial HDS process used for the treatment of AMD, and has the following advantages: the low cost of lime and its efficient use, only a small site is required for sludge disposal due to high density of the sludge, the water/solid separation is good, and it is a very robust process, with the ability to treat AMD with different properties (flow, metal loading and acidity) [1] .

Limestone has been used to treat AMD for many years in the coal mining industry because it is the cheapest material available, it is easy to handle, and is the safest chemical for treating AMD. The contaminants of greatest concern are iron and aluminum, and limestone is very effective in neutralizing these. However, the application of limestone is limited because it has a low solubility and has tendency to form an external coating of Fe(OH)₃ during the treatment of AMD [10] .

Under certain conditions, HDS can be achieved using limestone to neutralize AMD instead of lime [10] . Limestone reacts with acid water, leading to the dissociation and release of carbon dioxide, as in Equations (7) and (8) below.

The carbon dioxide released forms carbonate ions, which buffer the pH to an upper limit of 6.5. As a consequence of this, some metals cannot be removed as they require a pH above 6.5 for precipitation. To overcome this problem, a combination of limestone and lime can be used, as shown in Figure 3.

This process has three different steps: 1) pre-neutralization with limestone, which is a little cheaper than lime; 2) neutralization with lime in order to reach a certain pH that is determined by the metal to be removed; and 3) adjustment of the pH and re-carbonation using carbon dioxide produced in the limestone neutralization reactor [1] . When choosing the appropriate neutralization agent for the treatment of acid water from a particular mining site, the following parameters must be take into account: the type of material (including transportability, storability and dosing), the hazardous properties of the material, the reliability and availability of suppliers of reactants, the efficiency of neutralization, problems such as coating, clogging and scaling of the equipment, and the cost of the process [10] . Neutralization and hydrolysis are key aspects in the treatment of AMD. Table 1 lists different types of alkalis and materials used to treat AMD [1] .

To determine the amount of alkali required to treat a certain AMD, it is necessary to consider the cost of the alkali, the objective of the treatment (in this case, the removal of metals), and the effects of the residue produced (INAP, 2013). The data given in Table 1 can be used to determine the amount of alkali required to neutralize a certain amount of acid, and to estimate the cost of the alkali required to perform the task. The flowchart shown in Figure 4 and the data in Table 1 can be used to design an appropriate neutralization system for any coal mining wastewater, providing the flow rate and water chemistry of the AMD are known. In order to select the

^*Acidity is expressed as CaCO₃; ^**Market prices in January 2009.

appropriate neutralization agent, it is important to known the concentrations of iron and manganese. Manganese is very soluble in the pH interval 4.5 to 8 making its removal difficult. The best way to remove Mn is by raising the pH to a value above 9 in order to oxidize Mn²⁺ to Mn³⁺ or Mn⁴⁺, allowing the insoluble manganese carbonate or manganese oxide to be removed [13] .

The simplest type of passive treatment is the aerobic wetland, but it cannot be used to treat all types of acid water efficiently. Its capacity to neutralize acidity is limited, but it can be used to treat net alkaline water that has a high content of iron. Mine water is aerated while it flows slowly through the vegetation, and dissolved iron will thus be oxidized, and the oxidation product precipitated. The pH will fall as a result of the precipitation of iron due to the generation of H⁺ ions, and the treated water will thus have a lower pH than the influent water, despite the fact that the iron concentration is higher in the influent water. Aerobic wetlands can also be used to remove Mn, but the oxidation of Mn only starts when the oxidation of Fe is completed. To remove Mn using aerobic wetlands it is necessary to have large areas to allow the complete oxidation of Fe and thus the oxidation of Mn. Alternatively, another wetland cell can be added. Figure 5 shows a typical aerobic wetland where aquatic plants transport oxygen through the roots to the subsurface to help the oxidation process. Composted organic matter or natural soil can be used as substrate, and water levels between 10 - 30 cm are used to maintain aerobic conditions, and to allow the cattails to grow in order to improve wetland performance [14] .

Anaerobic wetlands are a modification kind of aerobic wetlands, where a bed of limestone and a layer of biodegradable organic matter are added in order to allow the treatment of acid water.

The limestone is located below the substrate to enhance the generation of alkalinity in the form of. Under anoxic conditions (low oxygen levels) sulfate can be reduced in the presence of bio­degrad­able organic matter. Sulfate-reducing bacteria use the oxygen in that enters the system under anoxic conditions to reduce sulfate to H₂S gas or to a solid sulfide by the biodegradation of organic matter in a metabolic process [14] .

This process is illustrated by Equation (9).

If metals (M) are present in the solution, the reduction process leads to metal sulfides, as can be seen from Equation (10). These metal sulfides are deposited in the substrate.

Alkalinity can also be generated by the reaction between acid water and the limestone below the substrate, as in Equation (11).

These three equations illustrate the production of bicarbonate ions, which are the source of alkalinity, and they can raise the pH by the neutralization of H⁺ (Equation (12)), thus contributing to the precipitation of soluble metals present in acid water.

Figure 6 illustrates anaerobic wetlands also known as composted wetlands and the curved arrows in Figure 6 indicate diffusion of water.

Anoxic limestone drains (ALDs) (Figure 7) is an engineered method where limestone is used to intercept AMD in anoxic conditions. Limestone in contact with AMD dissolves and generates alkalinity. To avoid contact between oxygen and AMD, limestone is crushed and buried with compacted soil or clay and the effluent water is led to a settling pond where the pH is adjusted to bring about the precipitation of metals [15] . When ALDs are working properly, they are more cost-effective than wetlands, but they cannot be used to treat AMD with significant amounts of Fe³⁺, Al and DO, due to clogging resulting from the precipitation of metal hydroxides when the pH reaches or exceeds 4.5. To avoid clogging, the influent concentrations of Fe³⁺, Al, and dissolved oxygen must all be below 1 mg/l. Under anoxic condition armoring by iron hydroxide cannot take place because Fe²⁺ cannot precipitate as Fe(OH)₂ at a pH below 6 [14] .

Most of AMD can be treated by the passive methods described above, but if mine water contains DO, Fe³⁺ and Al in great quantity, successive alkalinity-producing systems (SAPS) or vertical flow system which is a combination of ALDs and anaerobic wetlands with the objective of compensating for the limitations of each method can be used [16] . When AMD enters the system it flows vertically downwards through the organic layer where dissolved oxygen is removed by aerobic bacteria using biodegradable organic matter as their energy source, while other bacteria generate alkalinity by reducing sulfate to sulfide (Figure 8). The organic matter layer must

be able to reduce the level of dissolved oxygen to less than 1 mg/l to avoid limestone armoring and to allow the reduction of sulfate. The limestone layer allows the dissolution of CaCO₃ by acid water, and under anoxic conditions more alkalinity will be produced. Finally, the water is discharged to a settling pond where the acid is neutralized and the metals precipitated. When the influent AMD contains a significant amount of Fe³⁺ and sediments, pretreatment in an aerobic wetland or a settling pond is necessary to avoid the accumulation of solids. When the influent is highly acidic, it is necessary to divide the system into several vertical flows that can be separated by different settling ponds [14] .

The flowchart in Figure 9 can be of use when choosing the appropriate kind of passive treatment of AMD. As with active methods, it is important to know the flow rate and the chemical composition of the AMD. Samples should be collected from tailing seepage or mine discharge, and the levels of Fe, Mn, alkalinity, pH and acidity measured (Hedin et al., 1994). The composition of AMD can change considerable with the seasons, and it is thus important to collect and analyze samples at different times of the year [17] .