**2. Targets for remineralization**

When designing remineralization systems and determining the appropriate water quality targets, a handful of key parameters are taken into consideration: alkalinity, calcium hardness and pH. The importance of these parameters are discussed further in detail in the following sections.

#### **2.1 Importance of alkalinity**

Alkalinity is one of the most important water quality parameter due to its ability to maintain a stable and buffered system, as well as its effectiveness in protecting against various mechanisms of corrosion. In general terms, alkalinity is defined as the capacity of an aqueous solution to accept a proton, or rather to neutralize an acid. Although other systems can contribute to alkalinity, for drinking water processes, it is only the carbonate system that is taken into account to define alkalinity. This is due to its predominance over other buffering systems in the pH ranges usually associated with water chemistry. Alkalinity for drinking water is therefore expressed as the following:

$$\text{Alkalinity} = 2\left[\text{CO}\_3\text{}^{2-}\right] + \left[\text{HCO}\_3^-\right] + \left[\text{OH}^-\right] \text{-} \left[\text{H}^+\right] \tag{1}$$

As seen in Eq. (1), alkalinity is determined primarily by the concentration of carbonate and bicarbonate ions, and to a lesser extent by the concentration of hydroxide and hydrogen ions, which also define the pH. Bicarbonate ions are particularly important, due to their ability to consume both hydrogen and hydroxide ions, and therefore provide a buffer to pH shifts in both directions.

In addition to providing a buffer to protect against shifts in pH, alkalinity is one of the most important parameters in corrosion control. The World Health Organization recommends high alkalinity levels as a suitable technique for preventing many mechanisms of corrosion in their Guidelines to Drinking Water Quality [7]. This is backed up by a multitude of studies into corrosion control and experts responsible for setting water quality targets.

More particularly, alkalinity is essential in controlling the corrosion of many metal materials of construction. One of the best methods of controlling iron, copper, zinc or galvanized iron corrosion, is the precipitation of the respective carbonates, such as siderite (FeCO3), basic copper carbonate and basic zinc carbonate, for the formation of a passivation layer on the surface of the material [8]. A higher alkalinity content is therefore imperative to ensure sufficient carbonate species are available for the formation of these compounds. These protective layers are also an effective strategy against microbiologically induced pitting [9]. Lead on the other

hand, is released into the water either directly from the pipe, or from leadcontaining corrosion products that are formed on the pipe surface. In terms of lead corrosion products, the most common of these is lead carbonate whose propensity of lead (II) carbonates to dissolve into the water stream is directly related to the concentration of carbonate species already within the water. Finally, alkalinity is essential to prevent the degradation of concrete and cement-based systems that in many cases are used as a lining to protect large bore conduits constructed from mild steel or ductile iron.
