Treatment of wastewater by combination of ozonization and membrane separation

Zsuzsanna László, Cecilia Hodúr

Membrane filtration.

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A membrane is a selective barrier that permits the separation of certain species in a fluid by combination of sieving and diffusion mechanisms

Membrane filtration configurations: • Dead-end• Crossflow

Driving forces: • Pressure difference,• Concentration difference,• Voltage difference, etc.

Types of membrane filtration

•Membranes can separate particles and molecules over a wide particle size range and molecular weights

Lower pore size/molecular weight cut off (MWCO) means:

Higher transmembrane pressureLower flux (The rate of extraction of

permeate measured in litres per square meter of membrane surface per hour (L/m2/h))

Higher costs

Problems: Membrane fouling

Service periods

Membrane fouling – increasing filtration resistance

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(1) Surface (temporary) fouling•Foulant appears an evenly deposited layer on the membrane surface•Can be easily removed by cleaning solution•Permeation rate of membrane can be regenerated by cleaning

(2) Pore (permanent) fouling• Particulate matter diffuses into the membrane

•Could be caused by the poor quality of the cleaning water•Flux cannot be regenerated by cleaning•Determines the lifetime of the membrane

Methods for minimisig membrane fouling

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• Physical methods: – Stirring– Vibration– Ultrasonic treatment ect.

• Physical-chemical methods: – Coagulation-flocculation before treatment

• larger particles can be removed by higher pore-size membranes – lower pressure – lower cost

• Larger particles form gel layer on the surface of the membrane instead of pore fouling

- This can be achieved by combining waste water pretreatment with ozone and membrane separation

What is ozone? .

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• Ozone is triatomic oxygen with the chemical formula O3.

• Ozone is the very strong oxidant and disinfectant available for the treatment of aqueous solutions and gaseous mixtures.

• Ozone is generally used in wastewater treatment and drinking water plants (chemical oxidation, disinfection).

• It is generated generally from oxygen in the air by electrical discharges or by UV radiation <200 nm)

• Must be produced on-site (it is not stable)

Ozone’s properties provides multiple benefits in water treatment:

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– Oxidation

• COD/TOC• Fe/Mn• Hydrogen Sulfide • Taste & Odor• Color• Specific Chemicals

– Disinfection• Bacteria• Virus• Moulds

– Flocculation• Less Chemical Coagulant• Lower solids Handling• Lower Turbidity & Particles• Longer Run time• Less Backwashing

Organic colour reduction by ozonation

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• Surface waters are generally colored by natural organic materials such as humic, fulvic and tannic acids.

• Ozone is attracted to break organic double bonds. As more of these double bonds are eliminated, the color disappears.

• Surface water can usually be decolorized when treated with 2 to 4 ppm of ozone.

After Ozone Process

Ozone biocidal behaviour.

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Microflocculation:Improved coagulation and turbidity removal

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•Oxidation of dissolved organic materials by ozone results in polar and charged molecules

•pre-ozonation destabilizes the colloid with a resultant reduction of the amount of coagulant needed to produce a clear filtrate Improved Floc/Clarification & Filtration

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Microflocculation with ozone. .

Mechanisms of ozone-induced particle destabilization:

(1) Increases in carboxylic acid content caused by ozonation – lead to greater precipitation of metal salts/complexes or increased adsorption to alum flocs

(2) Ozone reduces the molecular weight of adsorbed organics, causing disruption of stabilized organic coatings – lead to reduced stearic hindrance or charge neutralization

(3) Polimerization of metastable organics – lead to direct precipitation or adsorption

(4) Ozone may liberate biopolimers from algae that could act as coagulating polymers

(5) Ozone may form amphipatic molecules from organic pollutants that form micelles

Combination of ozone treatment and membrane filtration

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•The aim of pre-ozonation should be:- increasing particle size by microflocculation- reducing biofilm building in membrane surface by disinfecting the solution

•Experimental set-up for combined process:

Effect of ozone pretreatment onpermeate flux:

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It strongly depends on the nature of the feed:

(1) Waste waters with high Fe, Ca, Mg-content, waste thermal waters – ozone pretreatment decrease the flux by precipitating metal salts

(2) the chemical nature of the wastes determine structure of the microfloccules: this affects the permeate flux, and e.g. in the case of meat industrial waters the stucture of the flocs are more loose, resulting increased permeate fluxes.

The effect of ozone pretreatments on relative permeate fluxes in thermal waste waters in the function of ozonation time

The effect of ozone pretreatments on permeate fluxes in meat industrial waste waters in the function of amount of absorbed ozone

Effect of ozone pretreatment on filtration resistances:

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Depends on the nature of the fouling compounds, and their reaction with ozone :

(1) Waste waters with high Fe, Ca, Mg-content, waste thermal waters – ozone pretreatment increase the membrane fouling by precipitating metal salts, or by formation of degradation by-products

(2) the chemical nature of the wastes determine structure of the microfloccules: this affects the filtration resistances, and e.g. in the case of meat industrial waters the loose structure of the flocs resulted decreased polarization layer and fouling resistances.

The effect of ozone pretreatments on filtration resistances in thermal waste waters in the function of ozonation time

The effect of ozone pretreatments on filtration resistances in meat industrial waste waters in the function of amount of absorbed ozone

Effect of ozone pretreatment on elimination efficiency:

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Comparing the elimination efficiency of membrane filtration and combined method, a

higher retention could be achieved with ozonated waters in consequence of the microflocculation effect of preozonation

Elimination efficiency of COD, phenolic index and ion content expressed by conductivity during

nanofiltration, ozone treatment and preozonation and nanofiltration of model thermal water

COD retention during ultrafiltration of meat industrial waste water in the function of

absorbed ozone

Summary . .

• The ozone treatment slightly degrades the organic content, but ion content remains high

• The remaining organic and inorganic wastes can be eliminated by nanofiltration, and larger molecules or particles by ultrafiltration

• the appearance and diminishing of degradation by-products determine the efficiency of the membrane filtration probably by changing membrane-solution interactions. The ozonation may produce

(a) microfloccules which may be more effectively eliminating from water than the original wastes. The structure of this floccules determine the structure of the polarization layer and resistance, and thus strongly affect the permeate flux.

(b) smaller by-products like oxalis acid, which may react with metallic ions of water (e.g. Ca2+ or Mg2+-ions) producing insoluble precipitates. These are easily removable by NF, but may cause higher pore fouling and decreased relative flux.

The combination of ozone treatment and membrane filtration enhance the efficiency of elimination of COD and other pollutants.