By Correspondent
Before the extensive pollution of Lake Chivero, Harare only needed 3 chemicals to purify the water.
The three were liquid aluminium sulphate (alum), lime and chlorine.
However the state of water has become so bad that 9 chemicals are required to make the water safe.
Below is the list of chemicals needed and their purpose.
Ammonia
It is used for residual chlorine retention.
Residual chlorine is the low level amount of chlorine remaining in the water after a certain period or contact time after its initial application.
It constitutes an important safeguard against the risk of subsequent microbial contamination after treatment—a unique and significant benefit for public health.
Sulphuric Acid
PH reduction to improve efficacy of aluminium sulphate (also consumes sulphuric acid to prepare)
Calcium Hypochlorite (HTH)
Substitute of gas chlorine (granular form)
Sodium Silicate
Coagulant aid to aluminium sulphate (also consumes sulphuric acid to prepare).
Sodium silicate is used directly in water treatment as a component of activated silica coagulant aids, for corrosion control, and for iron and manganese stabilization.
Sodium silicate is not used as a precursor in the commercial manufacture of other water treatment chemicals.
Gas Chlorine
Oxidation (control of algae) Disinfection (control of bacteria)
Granular Aluminium Sulphate
Used as a substitute and/or to supplement liquid aluminium sulphate
Liquid Aluminium Sulphate (Alum)
Main coagulant for removal of suspended solids and colloidal matter.
Powdered Activated Carbon
Removal of odours
Hydrated Lime
PH correction.
The simplest method of elevating pH is to pass the water through a filter bed of alkaline granular material.
Such filters are referred to as pH correction filters or neutralizing filters.
The alkaline media is usually calcium carbonate or magnesium oxide, or a combination of both.