8.3 Pretreatment strategies for fouling prevention

Membrane fouling is a major challenge in water treatment. Pretreatment strategies help prevent fouling by removing contaminants before they reach the membrane. This improves membrane performance and extends its lifespan.

Physical, chemical, and adsorption methods are key pretreatment approaches. These include coagulation, filtration, pH adjustment, disinfection, and activated carbon adsorption. Each targets specific foulants to protect the membrane system.

Physical Pretreatment

Coagulation and Flocculation

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• Coagulation involves adding chemicals (coagulants) to destabilize suspended particles and colloids in the feed water
• Common coagulants include aluminum sulfate (alum), ferric chloride, and polyaluminum chloride (PAC)
• Coagulation neutralizes the electrical charges on particles, allowing them to stick together and form larger aggregates (flocs)
• Flocculation gently mixes the water to promote contact between the destabilized particles and encourage the formation of larger, more easily settled flocs
• Flocculation is typically achieved using mechanical mixers or hydraulic mixing in a flocculation basin

Sedimentation and Filtration

• Sedimentation allows the flocs formed during coagulation and flocculation to settle out of the water by gravity
• Sedimentation basins or clarifiers are designed to provide sufficient retention time for the flocs to settle, forming a sludge layer at the bottom
• Clarified water is then drawn off from the top of the sedimentation basin
• Filtration removes remaining suspended particles and flocs that did not settle out during sedimentation
• Common filtration methods include sand filtration, multimedia filtration (sand, anthracite, and garnet), and microfiltration (MF) membranes
• Filters are periodically backwashed to remove accumulated solids and maintain their effectiveness

Chemical Pretreatment

pH Adjustment and Antiscalant Addition

• pH adjustment involves adding acids or bases to the feed water to optimize the pH for membrane performance and minimize scaling potential
• Common acids used for pH adjustment include sulfuric acid, hydrochloric acid, and carbon dioxide
• Antiscalants are chemical compounds added to the feed water to inhibit the formation of mineral scale on the membrane surface
• Antiscalants work by disrupting the crystallization process of scale-forming minerals such as calcium carbonate, calcium sulfate, and silica
• Common antiscalants include polyacrylic acid, phosphonates, and polymaleic acid

Disinfection

• Disinfection is used to inactivate or remove microorganisms (bacteria, viruses, and protozoa) from the feed water to prevent biofouling of the membranes
• Common disinfection methods include chlorination, ultraviolet (UV) irradiation, and ozonation
• Chlorination involves adding chlorine compounds (chlorine gas, sodium hypochlorite, or calcium hypochlorite) to the water, which inactivate microorganisms through oxidation
• UV disinfection uses UV light to damage the DNA of microorganisms, preventing them from reproducing and causing infection
• Ozonation involves injecting ozone gas into the water, which is a powerful oxidant that inactivates microorganisms and breaks down organic compounds

Adsorption Pretreatment

Activated Carbon Adsorption

• Activated carbon adsorption is used to remove dissolved organic compounds, taste and odor compounds, and certain inorganic contaminants from the feed water
• Activated carbon is a highly porous material with a large surface area that attracts and adsorbs organic molecules from the water
• Common types of activated carbon include granular activated carbon (GAC) and powdered activated carbon (PAC)
• GAC is used in fixed-bed adsorbers, where the water flows through a bed of carbon granules
• PAC is added directly to the water, mixed, and then removed through sedimentation or filtration
• Activated carbon adsorption helps to reduce organic fouling of membranes and improve the overall water quality of the feed water