In the operation of reverse osmosis membrane systems, the combined use of reverse osmosis membrane biocides and scale inhibitors is a crucial element in ensuring system stability. However, the differences in their chemical properties can lead to compatibility issues, requiring comprehensive management from multiple dimensions, including chemical mechanisms, environmental conditions, and operating processes.
The interaction of chemical functional groups is the core contradiction in compatibility issues. The strong oxidizing properties of oxidizing reverse osmosis membrane biocides (such as chlorine dioxide and sodium hypochlorite) directly attack the chelating groups (such as phosphonic acid and carboxylic acid groups) in scale inhibitors, causing the scale inhibitor molecular chains to break and lose their dispersing and chelating abilities. For example, phosphonic acid scale inhibitors are easily oxidized to phosphate precipitates in oxidizing environments, not only losing their scale inhibition function but also potentially forming secondary deposits that clog membrane pores. While non-oxidizing reverse osmosis membrane biocides (such as isothiazolinones and DBNPA) do not directly oxidize scale inhibitors, their amine or thiol groups may form hydrogen bonds or complexes with polycarboxylic acid scale inhibitors, reducing the solubility of the scale inhibitor and inducing flocculation. Furthermore, metal ions (such as iron and manganese) catalyze the above reactions, significantly increasing the compatibility risk between oxidizing reverse osmosis membrane biocides and scale inhibitors in high-ferrous water.
Environmental conditions significantly impact compatibility. pH is a key parameter: acidic conditions (pH < 7) accelerate oxidation reactions; for example, sodium hypochlorite oxidizes HEDP (hydroxyethylidene diphosphonic acid) at pH 5 at three times the rate under neutral conditions. Alkaline conditions (pH > 8) may lead to the hydrolytic inactivation of non-oxidizing reverse osmosis membrane biocides; for example, isothiazolinone's half-life is shortened to 2 hours at pH 9. Increased temperature exacerbates the chemical reaction rate; at high temperatures (> 35°C), the efficiency of ClO⁻ in oxidizing scale inhibitors increases 3-5 times, but may also trigger thermal decomposition of the scale inhibitor. A high concentration ratio (the ratio of the concentration of the reverse osmosis concentrate to the feed water) (>3 times) can lead to the accumulation of reagent concentrations, increasing the risk of localized reactions. For example, excessively high concentrations of antiscalant and reverse osmosis membrane biocide on the concentrate side may form colloidal precipitates.
The order of dosing and the contact method directly affect compatibility. If the reverse osmosis membrane biocide is added before the antiscalant, the residual oxidant may directly decompose the antiscalant; conversely, the protective film formed by the antiscalant on the membrane surface may hinder the reverse osmosis membrane biocide from contacting microorganisms. In engineering practice, a point-by-point dosing strategy is often adopted: the antiscalant is added after the multi-media filter to remove suspended solids through its dispersing effect; the reverse osmosis membrane biocide is added before the security filter to ensure uniform dispersion through turbulence. For oxidizing reverse osmosis membrane biocides, their contact time with the membrane must be strictly controlled, for example, by neutralizing residual chlorine with a reducing agent (such as sodium bisulfite) or by using activated carbon filtration to remove residual oxidants.
Chemical selection is the first step in compatibility management. In oxidizing environments, oxidation-resistant scale inhibitors should be prioritized, such as PBTCA (2-phosphono-1,2,4-tricarboxylate butane), which exhibits superior oxidation resistance compared to HEDP and can operate stably in environments with ClO⁻ concentrations <0.5ppm. In non-oxidizing environments, the polycarboxylic acid amine content needs to be controlled; for example, selecting polyacrylic acid scale inhibitors with low amine content can reduce the risk of complexation with reverse osmosis membrane biocide. Combined bactericidal systems (such as alternating the use of ClO₂ and isothiazolinone) can reduce the contact concentration of a single agent, minimizing compatibility issues.
Dynamic monitoring and parameter adjustment are the technical support for ensuring compatibility. An online ORP meter is used to control the ClO⁻ balance (<0.1ppm) to avoid excessive oxidant; a pH meter is used to maintain the system pH between 7.5 and 8.0, balancing scale inhibitor stability and reverse osmosis membrane biocide activity. Regular monitoring of chemical concentrations (such as scale inhibitor residue and reverse osmosis membrane biocide activity) can promptly detect compatibility issues. For example, an abnormal decrease in scale inhibitor concentration may indicate oxidative decomposition, and reduced reverse osmosis membrane biocide activity may stem from reaction with the scale inhibitor.
The characteristics of membrane materials impose differentiated compatibility requirements. Polyamide composite membranes are sensitive to oxidants, requiring strict control of oxidizing reverse osmosis membrane biocide concentrations. Cellulose acetate membranes exhibit better oxidation resistance, but it is necessary to avoid organic solvents (such as alcohols) in non-oxidizing reverse osmosis membrane biocides that could cause membrane swelling. Membrane surface modification techniques (such as coating treatments) can improve membrane tolerance to chemicals; for example, coating polyvinyl alcohol onto the surface of a polyamide membrane can reduce oxidant permeation.
During long-term operation, compatibility issues may trigger a chain reaction. Scale inhibitor failure leads to inorganic scale deposition, providing attachment sites for microorganisms; reverse osmosis membrane biocide failure causes biofouling, where metabolites (such as polysaccharides and proteins) further adsorb organic matter, forming a complex fouling layer. This fouling reduces membrane flux, increases operating pressure, and can even cause irreversible membrane damage. Therefore, periodic chemical cleaning (such as using citric acid to clean inorganic scale and sodium hypochlorite to clean the biofilm) is necessary to restore membrane performance, while optimizing reagent formulations and dosing strategies to reduce compatibility risks at the source.
