Pocket colorimeter 2

Duplicate samples of primary drinking water and secondary water were collected. FCR was measured in one sample using a Hach® Pocket Colorimeter II (Loveland, CO, USA), according to manufacturer’s instructions. The second sample was aseptically collected in a 100 mL sterile bottle containing sodium thiosulfate (IDEXX, Westbrook, ME, USA) for microbiological analysis. Bottles were stored in a cooler containing ice during transport to a laboratory (average 8.5 h, range 3–14.5 h). Samples were tested for the presence of total coliforms and E. coli using the IDEXX Colilert®-18 methodology, according to manufacturer’s instructions. A field blank (distilled water) and positive control (distilled water seeded with fecal waste) were analyzed each day.

Total chlorine was measured during water sample collection using a Pocket Colorimeter II with DPD powder pillows (Hach Company, Loveland, CO, USA). During a subsequent campaign, assimilable organic carbon (AOC) was measured throughout the two systems (August 2015 and May 2017 for chloraminated and no-residual systems, respectively) using the Pseudomonas fluorescens P-17/Spirillum sp. NOX method [17 ]. The chloraminated utility provided raw water temperatures and treated water pH, total chlorine, hardness, free ammonia (NH ${}_3+{\mathit{\text{NH}}}_4^{+}$ ), nitrate (NO ${}_3^{-}$ ), and orthophosphate (P ${\mathrm{O}}_4^{3-}$ ) for 2014. Water temperatures, total chlorine, free ammonia, and nitrate were also provided for 13 monitoring sites in the distribution system for 2014. The no-residual utility provided raw water temperatures and treated water pH, total chlorine, hardness, ammonium (NH ${}_4^{+}$ ), and nitrate for 2014 to 2015. Free ammonia in the no-residual DWDS was estimated from ammonium using pH and temperature, as previously described [18 ]. In addition to water temperature, orthophosphate and total phosphorus were measured at three sites in the no-residual DWDS in May 2017 using Hach assay number LCK 349. The utilities analyze water quality using methods compliant with either the United States Environmental Protection Agency (chloraminated DWDS) or the Standards Norway (no-residual DWDS).

Water temperature was measured in accordance with the APHA St. Method 2550 B (20 ) using alcohol thermometer with graduation intervals of 0.1 °C. Residual free chlorine was measured according to ISO 7393-2:2018 (21 ) (quantification limit of Cl₂ 0.02 mg/L) using portable colorimeter Pocket Colorimeter™ II (Hach, Loveland, CO, USA). Electrical conductivity and pH values of the water were measured using multi-channel, modular instrument SevenExcellence (S47; Mettler Toledo, Giessen, Germany) according to ISO 7888:1985 (22 ) (quantification limit 9 µS/cm) and ISO 10523:2008 (23 ), respectively.
Turbidity was measured according to ISO 7027-1:2016 (24 ) (quantification limit 0.10 NTU) using laboratory turbidimeter 2100N IS (Hach). Permanganate index (consumption of KMnO₄) was determined according to ISO 8467:1993 (25 ) (quantification limit of O₂ 0.25 mg/L). Quality control was performed with resorcinol (Merck, Darmstadt, Germany) with recovery of 90-105%.