Hybrid Process of Adsorption/Coagulation/Ceramic MF for Removing Pesticides in Drinking Water Treatment—Inline vs. Contact Tank PAC Dosing

Abstract

Two pilot trials of powdered activated carbon (PAC)/(coagulation)/ceramic microfiltration were conducted to compare continuous 10–12 mg/L PAC inline dosing with 8–10 mg/L dosing to a 2 h-contact tank. Two low turbidity/low natural organic matter (NOM, total organic carbon <2 mg C/L) surface waters spiked with 7.2–10.3 µg/L total-pesticides were tested and the dosing options were compared towards operational performance, average removal of pesticides and NOM and costs. Removal differences between the two PAC dosing options depended on pesticides’ amenability to adsorption and NOM characteristics (254 nm absorbance, A254). Waters containing low A254-absorbing NOM and only pesticides amenable to adsorption showed very high removals (all pesticides ≥93%) and no significant differences between the two PAC dosing options. Waters containing higher A254-absorbing NOM and high loads of pesticides less amenable to adsorption (dimethoate, bentazone) required higher inline PAC dose. Those or more severe conditions may require PAC doses higher than tested to comply with the Drinking Water Directive limits for pesticides. Cost analysis showed PAC inline dosing is more cost-effective than PAC dosing to the contact tank when identical PAC dose is sufficient or when the doses are low, even if 50% higher for inline dosing, and the plant is small.

Adsorption/Coagulation/Ceramic Microfiltration for Treating Challenging Waters for Drinking Water Production

“…time for PAC/Alum/MF depends on microcontaminants' characteristics and NOM-microcontaminants competition, some compounds benefiting with contact time increase, others not being much affected, as demonstrated in other studies including ours comparing inline vs. tank PAC dosing [28].…”

“…The 11 spiking trials presented in this paper were conducted throughout the 1.5-year period and a conservative flux of 133 L/(m 2 •h), possible to be used for all waters tested, was selected, corresponding to 2.4 h average contact time (2 h minimum), which was not further subjected to optimisation. The adequate contact time for PAC/Alum/MF depends on microcontaminants' characteristics and NOM-microcontaminants competition, some compounds benefiting with contact time increase, others not being much affected, as demonstrated in other studies including ours comparing inline vs. tank PAC dosing [28].…”

“…Pesticides such as atrazine, tebuconazole, diuron and dimethoate have shown critical occurrence in several water abstraction areas Pilot studies with pressurized PAC/ultrafiltration reported removals of sulfamethoxazole, carbamazepine and diclofenac from secondary effluent of 60-95% [43] and <30% to >80% [44] with 20 mg/L PAC, while in our previous studies with a pressurized PAC/(Alum)/ceramic MF pilot [28] we reached individual pesticides' removals of 53% to >97% from surface water using 8-10 mg/L PAC. Overall, PAC seems to be more efficient for the adsorption of neutral hydrophobic or positively charged compounds [28,45]. For low-hydrophobicity compounds, positively charged functional groups and low surface polar area and/or high number of aromatic rings seem to act as adsorption enhancers [28,46].…”

“…Diverse PhCs with potential deleterious acute and synergistic chronic health effects have been reported in surface, groundwater and even in drinking water at ng/L to low µg/L range [2,5,6], with carbamazepine, diclofenac, propranolol, and atenolol amongst the major compounds detected [7,8]. Pesticides such as atrazine, tebuconazole, diuron and dimethoate have shown critical occurrence in several water abstraction areas Pilot studies with pressurized PAC/ultrafiltration reported removals of sulfamethoxazole, carbamazepine and diclofenac from secondary effluent of 60-95% [43] and <30% to >80% [44] with 20 mg/L PAC, while in our previous studies with a pressurized PAC/(Alum)/ceramic MF pilot [28] we reached individual pesticides' removals of 53% to >97% from surface water using 8-10 mg/L PAC. Overall, PAC seems to be more efficient for the adsorption of neutral hydrophobic or positively charged compounds [28,45].…”

“…For low-hydrophobicity compounds, positively charged functional groups and low surface polar area and/or high number of aromatic rings seem to act as adsorption enhancers [28,46]. As adsorption onto PAC depends on contaminant properties, water matrix, PAC properties, dose and dosing conditions (e.g., contact time, mixing conditions) [28,43,45,46], the prevalent contaminant-PAC-water interactions are site-specific and a wider range of studies is important to further support generic predictions.…”