Physicochemical treatment: phosphorus removal

Purpose

• Phosphorus removal, phosphate precipitation

• Supplementary or combined fluoride removal and sulphate removal

Principle

By adding the appropriate chemicals to the waste water non-soluble phosphorus compounds can be formed which are then easily removed from the waste water as a precipitate. This principle can be applied in both biological water treatment plants as well as in physicochemical treatment stations. Various chemicals are possible:

• Milk of lime (Ca(OH)₂) is a strong base and precipitates phosphorus as calcium phosphate at high pH values. Milk of lime is used mainly on physico-chemical water treatment plants and is simultaneously used for the correction of the pH, the removal of heavy metals, the removal of sulphate and the removal of fluorides.

• Iron trichloride (FeCl₃) and aluminum chloride (AlCl₃) are already functional at neutral pH and for this reason better suited for use on biological water treatment stations.

In a biological water purification station the formed phosphorous salts precipitate on the bio sludge after which it is removed together with the sludge through the sludge discharge. In a physicochemical wastewater treatment station the formed flocs are enlarged with the aid of a polymer and separated by sedimentation or flotation.

As already mentioned above other anions such as, among others, fluoride and sulfate can be removed in an analogous manner by forming a non-soluble compound. Typical examples are fluoride and sulphate.

Scheme

Operational costs

Besides the investment in the purification equipment there are also operational costs during the physicochemical removal of heavy metals from waste water. The main operational costs in this case are the consumption of chemicals and the removal of the physicochemical sludge.

Approach Trevi

Trevi always checks whether the costs for physicochemical wastewater treatment could be optimised by at-source measures or by side-stream treatment of certain waste waters. The most efficient choice and dose of chemicals is determined based on jar tests, keeping a minimal chemical consumption and waste sludge production in mind.