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.
Ozone water treatment relies on its reaction with a number of organic and inorganic substances as follows:
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)
To oxidise the iron, 0.43 mg ozone is needed for every 1 mg of iron in solution.
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.
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
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.
Ozone does not react with ammonium salts but it reacts with ammonia to form ammonium nitrate: 2NH3 +4O3 →NH4NO3 +4O2 +H2O
Ozone will oxidise cyanides to one thousand times less toxic cyanates:
CN– +O3 →CNO– +O2
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|
|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|
|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.