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Alkalinity is the capacity of water to neutralise acids. It results from the presence of hydroxides, carbonates, and bicarbonates of elements such as calcium, magnesium, sodium, potassium, or ammonia. Wastewater is normally alkaline, receiving its alkalinity from the water supply, the groundwater, and the materials added during domestic use. It is determined by titrating against a standard acid and the results are expressed in terms of calcium carbonate CaCO3, mg/l as CaCO3. For most practical purposes alkalinity can be defined in terms of molar quantities.

The corresponding expression in terms of equivalents is

In practice, alkalinity is expressed in terms of calcium carbonate. To convert from meq/l to mg/l as CaCO3, it is helpful to remember that

Milliequivalent mass of CaCO3 = (100 mg/mmole) / 2 meq/mmole = 50 mg/meq

Alkalinity plays an important role in the treatment of wastewater, as it indicates the buffer capacity of water. This affects the growth and activity of microbes present in activated sludge, which are responsible for the treatment of wastewater. It is also an essential parameter to be estimated to design and implement the corrosion and odour-control processes.

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