Methods of removing phosphorus from wastewater

Methods of removing phosphorus from sewage and industrial water theoretically include:

  • chemical precipitation
  • ion exchange
  • adsorption
  • membrane methods

Chemical precipitation is the most commonly used method. It is effective and enables phosphate removal in a reasonable amount of time. The process is widely recognised and used on technical scale. Lime or iron and aluminium salts, with a possible addition of flocculants, are used in precipitation process. The addition of polymers (flocculants) enables the formation of bigger, more permanent structures. The resulting sludge is then removed in sedimentation processes. Orthophosphate ions are usually precipitated with aluminium sulphate, ion chloride (III) or ion sulphate (II). They can also be removed with lime; however nowadays this approach is becoming less popular. All of the aforementioned substances cause the precipitation of orthophosphate salts from the solution.

 

The following chemical reactions contribute to the process:

 

1. Al3+ + PO43- → AlPO4 ↓ (in this case, the required ratio of Al to P is 0.87:1)

2. Fe3+ + PO43- → FePO4 ↓ (, in this case, the required ratio of Fe to P is 1.8:1)

3. If lime is used in the process, hydroxyapatite is formed: [Ca9(OH)(PO4)3]. The following reaction occurs between phosphate and lime ions added to the solution:

 

5Ca2+ + 4OH- + 3HPO4- → Ca5OH(PO4)3 + 3H2O

 

Large amounts of sludge are generated during coagulation processes. If sewage alkalinity is below the required standard, alkalizing products, such as lime or NaOH, must be added to it. Lime addition increases the amount of accumulating sludge. However, lime has low solubility in water (approximately 1.65 g/ dm3 at 20°C) and thus it is difficult to maintain the correct concentration. Lime is over 200 times less soluble than aluminium salts and over 500 times less soluble than iron chloride (III). Moreover, lime increases pH to over 10. Its advantages include low cost and high effectiveness (lime dose and other parameters must be determined experimentally). Sulphates and chlorides, on the other hand, might, depending on the coagulant used, increase sewage corrosivity.

 

There is no economic justification of using adsorption and ion exchange in industrial wastewater treatment.

 

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