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The key to successful operation of membrane systems is ensuring that membrane fouling and scaling are minimized. To achieve that, efficient separation of particles and organic materials in the primary treatment step is not enough. Coagulants, flocculants and antiscalants have to be compatible with the membranes and one another. Residuals of any treatment chemicals should either be minimized or chosen so that they do not adsorb on membranes and cause irreversible fouling.
Questioning a classic approach
It is a widely accepted practice in primary water and wastewater treatment to neutralize particle surface charge with inorganic coagulants. Such reagents have been considered safe and well-described in chemical engineering literature.
Engineers from chemical companies and engineering and consulting organizations prefer to use such reagents in drinking, municipal and industrial wastewater applications; however, results of recent research studies illustrate that classical inorganic coagulants such as aluminum sulfate can be detrimental to membranes due to surface precipitation of aluminum silicates, hydroxides, phosphates or antiscalants, including phosphonates and polycarboxylates.
Ferric ions catalyze oxidation of polyamide reverse osmosis (RO) membranes and any exposed metal surfaces. Prepolymerized aluminum reagents such as polyaluminum chloride also interact with chloramines disinfectants and produce oxidizing species.
Detailed studies performed by Chris Gabelich and coworkers from the Metropolitan Water District in Los Angeles; Yoram Cohen and colleagues at the UCLA Water Technology Research Center; and Kenneth Ishida at the Orange County Water District in Fountain Valley, Calif., confirmed what has been shown at Clean Water Technology, Inc. (CWT) since 1996. In addition, inorganic and blend coagulants produce voluminous bulky sludge that is difficult to dewater. Such reagents also interfere with the performance of flocculants, which often results in carryover of suspended solids in treated effluent.
This resulted in evaluating cationic and anionic polymeric coagulants and flocculants as possible treatment alternatives. It was shown that contrary to theory, cationic polymeric coagulants do not adsorb significantly on polyamide RO membranes or polysulfone ultrafiltration membranes. The results at CWT were confirmed by work at the aforementioned Los Angeles institutions. Excessive amounts (more than 20 mg/L), however, will adsorb.
CWT developed a dual polymeric flocculant approach in which efficient high-energy mixing and coagulation with cationic coagulants is followed with low-energy flocculation with the high molecular weight, high-charge anionic polyacryalmide. When used properly, this strategy results in less than 0.05 mg/L of residual cationic coagulants, such as polydimethyldiallylamoniumchloride. As an additional precautionary step, cationic coagulants can be labeled with fluorescence dye and sensors can be used to measure the residual concentration of cationic reagents prior to the membrane separation step.
This approach was developed by Brian Johnson and coworkers at Nalco Co. Other benefits of using polymer-only chemistry include production of sludge with much higher solids loading that is easier to dewater and an increased rate of primary flotation or sedimentation treatment (smaller footprint).
Recently developed mixing, flocculation and flotation systems are designed based on principles of computational fluid dynamics. Mixing energy and the nature of mixing can be adjusted to achieve maximum uncoiling, activation and adsorption of coagulants and flocculants on water contaminants. Consequently, any free residual treatment chemicals can be minimized.
High-rate flotation systems can operate at flows as high as 20 gpm/sq ft. A high removal rate of contaminants, particularly algae, can be achieved. Fast response rate to any changes in water parameters is also suitable for sensor-based chemical dosage control.
Centrifugal flotation systems use centrifugal force to enhance mixing of particles and bubbles with treatment chemicals and accelerate solid/liquid separation. One such technique, the air-sparged hydrocyclone, was designed to enhance collisions of mineral particles with sparged gas bubbles. On this basis, specific capacities up to 100 times those of traditional dissolved air flotation equipment were achieved.
One improved version of the ASH is called bubble-accelerated flotation. This system has achieved exceptional removal of contaminants from industrial wastewater and process water while requiring smaller footprints and a lower operating cost. In addition to the air-sparged BAF, CWT has developed an induced-air BAF, a vacuum flotation BAF and an electro-flotation BAF. In the most recent design, centrifugal hydrocyclone mixing was combined with small dissolved-air flotation bubbles to develop the hybrid, dissolved-air centrifugal flotation, which was termed gas energy management flotation.
Commercial applications of the systems include treatment of petroleum, heavy metal, laundry, textile, food processing and animal livestock wastewaters. Other recently developed centrifugal and jet flotation systems such as modified Jameson jet flotation and flocculation flotation are also available. The new generation of dissolved air flotation systems, such as Degremont’s high rate flotation or ITT Leopold’s systems, has also shown great efficiency as a membrane pretreatment choice, particularly in drinking water treatment applications.
In spite of strong research results indicating that polymeric coagulants and flocculants not only outperform inorganic reagents but also present less danger to the downstream membrane or biological processes, most practicing engineers and chemical company sales representatives are still more comfortable with the aluminum- or iron-based coagulants.
But as the cost of membrane cleaning chemicals, replacement membranes, sludge drying and dewatering increases, polymeric reagents become more attractive to water and wastewater professionals. Only the future will show just how resilient the engineering community is and how fast we can replace inorganic reagents in most coagulation applications.