OZONE WATER TREATMENT SYSTEMS LARGE SCALE

Ozone Water Treatment Systems in large Industrial or Municipal applications typically comprise a Pre-treatment step for organic and COD removal with a mid treatment step for disinfection. Final disinfection is normally catered for with dedicated chlorine dosage due to chlorine’s residual capability.

Oxidant demand

Ozone water treatment relies on its reaction with a number of organic and inorganic substances as follows:

Metals

Ozone will oxidise metals (except gold, platinum, and iridium) to oxides of the metals in their highest oxidation state,

e.g.:2Cu+(aq) + 2H3O+(aq) + O3(g) → 2Cu2+(aq) + 3H2O(l) + O2(g)

Iron

To oxidise the iron, 0.43 mg ozone is needed for every 1 mg of iron in solution.

Manganese

Ozone can be used to remove manganese from water, forming a precipitate which can be filtered: Mn2+ +O3 +H2O→MnO2 (s)+O2 +2H+

For manganese oxidation, 0.88 mg ozone is needed for 1.0 mg manganese.

Sulphides

In an aqueous solution, two competing simultaneous reactions occur, one to produce elemental sulphur, and one to produce sulphuric acid:

H2S+O3 →S+O2 +H2O

3H2S + 4 O3 → 3H2SO4

Organic material

Ozone can mineralise organic material to CO2:

C+2O3 →CO2 +2O2

The demand for ozone exerted by organics in the water has been proven by numerous studies to be between 0.5 to 1 mg O3/mg DOC.

Ammonia

Ozone does not react with ammonium salts but it reacts with ammonia to form ammonium nitrate: 2NH3 +4O3 →NH4NO3 +4O2 +H2O

Cyanides

Ozone will oxidise cyanides to one thousand times less toxic cyanates:

CN+O3 →CNO+O2

Urea

Ozone will completely decompose urea.

(NH2)2CO + O3 → N2 + CO2 + 2H2O

For example the Vaalkop WTW process in Gauteng taking water out of the Vaal River consists of the following:

 

VAALKOP WATER TREATMENT WORKS TREATMENT PROCESS
Process Treatment objective
Pre-chlorination
Pre-ozonation Oxidation of iron and manganese and organic compounds, colour removal, aid flocculation
PAC adsorption Taste and odour removal, algal toxin adsorption
Coagulation Destabilisation of suspended matter
DAF Removal of floating flocs and algae
Sedimentation Removal of heavier flocculated particles
Cocodaff Simultaneous floatation and filtration – removal of floatable flocs and algae at the top and gravity filtration of micro particles and micro organisms
Intermediate ozonation Oxidation of taste and odour compounds, inactivation of viruses and parasites
GAC filtration Adsorption of oxidised organic compounds
Rapid sand filtration Removal of micro particles and micro organisms
Chlorination Disinfection of bacteria and micro organisms
Chloramination Providing residual disinfectant in long distribution system

And then for the Roodeplaat plant….

 

ROODEPLAAT WATER TREATMENT WORKS TREATMENT PROCESS
Process Treatment objective
Pre-treatment Screening of debris
Pre-oxidation (KMnO4) Oxidation of iron and manganese, taste and odour control
Pre-ozonation* Taste and odour control, iron and manganese oxidation, DOC fractionation, colour reduction, chlorophyll-a reduction
Aeration Removal of volatile organic substance, iron oxidation
Coagulation pH correction and coagulation of suspended solids
Flocculation Flocculation of suspended solids
DAF Removal of algae and floatable suspended solids
Upflow clarification Removal of flocculated particles (capturing of PAC dosed at inlet if KMnO4 is not dosed), taste and odour control and DOC removal
Rapid Sand filtration Barrier for micro-organisms and removal of remaining fine particles
Ozonation* Taste and odour removal and inactivation of Cryptosporidium and Giardia
GAC filtration* Removal of DOC and taste and odour compounds
UV irradiation Inactivation of harmful micro-organisms such as Cryptosporidium and Giardia
Chlorination Inactivation of harmful micro-organisms and establishment of disinfectant residual
Chloramination* Establish disinfectant with longer ‘shelf life’

EFFICACY OF OZONE WATER TREATMENT

  • i)  High colour: Ozone has a very high oxidation potential and has the ability to break the double bonds of colour molecules and is effective in the removal of colour.
  • ii)  Taste and odour: Ozone is effective in the removal of both taste and odour. It is however qualified in the sense that the environmental factors and specifically the pH of the water will affect the mode of oxidation. The removal of particularly geosmin and MIB is more susceptible to removal by means of the indirect mode of ozonation via the highly active hydroxyl radical.
  • iii)  High manganese: Ozone will effectively oxidise the soluble manganese ions to an insoluble manganese form.
  • iv)  High iron: Ozone will effectively oxidise the soluble iron ions to an insoluble iron form.
  • v)  High turbidity: Ozone has limited impact on turbidity, although some positive impacts have been found on turbidity reduction.
  • vi)  High algae: Ozone can rupture the cell wall of algae and effectively kill these compounds in the water
  • vii)  High chlorophyll: Ozone is an effective bleaching agent and is effective in removing chlorophyll-a from the water
  • viii)  HighDOC:The ozone demand required by organic material is about 0.5-1mg ozone per mg DOC. The ozone does not completely mineralise the organics but does change the composition of the organic molecule to a state which is more biodegradable. Ozone is effectively used to treat high organic composition waters.
  • ix)  High coliforms: Ozone is very effective in inactivating coliforms in water treatment.
  • x)  High Cryptosporidium: Ozone is effective in killing this pathogen.
  • xi)  High Giardia: Ozone can effectively inactivate this pathogen.
  • xii)  High cyanobacteria toxins: Ozone is effective in oxidising these toxins.
  • xiii)  High ammonia:Ozone is not effective in oxidising ammonia to nitrates and nitrites,although oxidation to ammonium nitrate does occur.
  • xiv)  High pathogen: Ozone is effective against pathogenic organisms like bacteria, protozoa and viruses. 
  • High COD: Ozone should not be used when the objective is to oxidise high levels of organics to carbon dioxide.