AREGGER CHEMIE AG
In the third wastewater treatment stage, the contaminated water is treated by precipitation with iron salts, hydrated lime and a flocculant. This stage primarily removes sediment and suspended solids, dissolved inorganic substances and those substances from the wastewater that are difficult or impossible to break down biologically. This also includes a number of colourants.
In precipitation, substances dissolved in water are converted into a completely or partially insoluble state by precipitants. Precipitation is used to separate metal ions, sulphides and phosphates as well as for softening. The precipitation product – in colloidal form – is converted into a separable form by flocculation.
Precipitation takes place in the wastewater treatment plant in so-called reaction tanks and is a chemical process that converts dissolved compounds into an undissolved state by adding suitable chemicals – such as iron and aluminium salts.
In the presence of complexing agents (inorganic or organic compounds that bind metal ions), sulphide precipitation may be necessary. Interfering anions are precipitated in the form of their salts (calcium, iron or aluminium salts).
The initial phase of flocculation is coagulation. In order for the finely dispersed colloids to be separated effectively, it is necessary to form the largest possible flocs from the small particles. Floc formation is favoured by the addition of mineral, organic, artificial or natural flocculants.
Most of the particles (colloids) remaining after precipitation are so small that they cannot be filtered. By adding a coagulant (e.g. ferric chloride, aluminium sulphate, calcium hydroxide) and then adding a flocculant (e.g. polyelectrolyte PE), flocs are formed under defined pH conditions. The large flocs enable solid-liquid separation by sedimentation (sedimentation tank, lamella separator) or filtration (pebble filter with backwash).
The flocs are removed from the water using suitable physical processes such as sedimentation, filtration or flotation.
Source:
Federal Office for the Environment, Fact sheets on water and exhaust air treatment processes, dated 28 March 2017, p. 10 (slightly abridged)