Surfactants belong to a group of chemicals that are well known for their cleaning properties. Their excessive use as ingredients in care products (e.g., shampoos, body wash) and in household cleaning products (e.g., dishwashing detergents, laundry detergents, hard-surface cleaners) has led to the discharge of highly contaminated wastewaters in aquatic and terrestrial environment. Once reached in the different environmental compartments (rivers, lakes, soils, and sediments), surfactants can undergo aerobic or anaerobic degradation. The most studied surfactants so far are linear alkylbenzene sulfonate (LAS), quaternary ammonium compounds (QACs), alkylphenol ethoxylate (APEOs), and alcohol ethoxylate (AEOs). Concentrations of surfactants in wastewaters can range between few micrograms to hundreds of milligrams in some cases, while it reaches several grams in sludge used for soil amendments in agricultural areas. Above the legislation standards, surfactants can be toxic to aquatic and terrestrial organisms which make treatment processes necessary before their discharge into the environment. Given this fact, biological and chemical processes should be considered for better surfactants removal. In this review, we investigate several issues with regard to: (1) the toxicity of surfactants in the environment, (2) their behavior in different ecological systems, (3) and the different treatment processes used in wastewater treatment plants in order to reduce the effects of surfactants on living organisms.
Combination of high performance membrane bioreactor (MBR) equipped with ultrafiltration and electro-oxidation process (EOP) by boron-doped diamond electrode (BDD) was used to effectively treat highly contaminated old landfill leachate. MBR and EOP were optimized for raw and pretreated landfill leachate. Seasonal changes dramatically affected the both processes' performance, as the landfill leachate was ¾ more concentrated in winter. For MBR, organic load rate of 1.2 gCOD/L/day and sludge retention time of 80 days was considered as the optimum operating condition in which COD, TOC, NH and phosphorous removal efficiencies reached the average of 63, 35, 98 and 52%, respectively. The best performance of EOP was in current intensity of 3 A with treatment of time of 120 min. Effluent of electro-oxidation was more toxic due to the presence of radicals and organochlorinated compounds. These compounds were removed by stripping or assimilation of sludge if EOP was used as a pretreatment method. Furthermore, the energy consumption of EOP was decreased from 22 to 16 KWh/m for biologically treated and raw landfill leachate, respectively.
Large amounts of ethylene glycol (EG) based antifreeze liquids are daily discharge into the receiving waters. The chemical oxygen demand (COD) detected in most of the lakes and rivers close to the discharge points exceeded 100 000 mg O 2 /L. With such an amount, biological and chemical processes are inefficient to completely remove EG. In order to efficiently remove EG, electrochemical oxidation (ECO) of EG was carried out. Niobium Boron Doped Diamond (Nb/BDD) was used as anode whereas carbon felt was used as cathode. Different operating parameters including current intensity, treatment time, electrolyte concentration and pollutant concentration were tested. The application of 0.077A/cm 2 of current density during 120 min in the presence of 7.0 g/L of Na 2 SO 4 allowed 89.6 ± 0.2 % of COD removal with 490 mg O 2 /L of residual COD. This result was far below the legislation standard set by the province of Quebec that allows the discharge of 800 mg O 2 /L of COD. The efficiency of ECO process was attributed to both direct and indirect oxidation that generate strong oxidizing species (OH°, S 2 O 8 2-, H 2 O 2 , etc.) capable of oxidizing organic matters on the surface of the anode and in the bulk of the solution.
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