Improved biostability assessment of drinking water with a suite of test methods at a water supply treating eutrophic lake water

Highlights

• ATP-based growth tests detect easily and slowly-biodegradable compounds.

• The continuous biofouling monitor facilitates detection of water quality changes.

• ClO₂ dosage increases the biomass and manganese accumulation rates of drinking water.

• Iron enhances the biomass accumulation rate in the distribution system.

• Aeromonas regrowth may be related to biomass-turnover processes.

Abstract

Assessment of drinking-water biostability is generally based on measuring bacterial growth in short-term batch tests. However, microbial growth in the distribution system is affected by multiple interactions between water, biofilms and sediments. Therefore a diversity of test methods was applied to characterize the biostability of drinking water distributed without disinfectant residual at a surface-water supply. This drinking water complied with the standards for the heterotrophic plate count and coliforms, but aeromonads periodically exceeded the regulatory limit (1000 CFU 100 mL⁻¹). Compounds promoting growth of the biopolymer-utilizing Flavobacterium johnsoniae strain A3 accounted for c. 21% of the easily assimilable organic carbon (AOC) concentration (17 ± 2 μg C L⁻¹) determined by growth of pure cultures in the water after granular activated-carbon filtration (GACF). Growth of the indigenous bacteria measured as adenosine tri-phosphate in water samples incubated at 25 °C confirmed the low AOC in the GACF but revealed the presence of compounds promoting growth after more than one week of incubation. Furthermore, the concentration of particulate organic carbon in the GACF (83 ± 42 μg C L⁻¹, including 65% carbohydrates) exceeded the AOC concentration. The increased biomass accumulation rate in the continuous biofouling monitor (CBM) at the distribution system reservoir demonstrated the presence of easily biodegradable by-products related to ClO₂ dosage to the GACF and in the CBM at 42 km from the treatment plant an iron-associated biomass accumulation was observed. The various methods applied thus distinguished between easily assimilable compounds, biopolymers, slowly biodegradable compounds and biomass-accumulation potential, providing an improved assessment of the biostability of the water. Regrowth of aeromonads may be related to biomass-turnover processes in the distribution system, but establishment of quantitative relationships is needed for confirmation.

Introduction

About one-third of the drinking water distributed by the water supply companies in the Netherlands is produced from surface-water sources and two-thirds from groundwater (Geudens, 2012). Treated water is distributed without a disinfectant residual, although a low concentration of chlorine dioxide (<0.2 mg ClO₂ L⁻¹) is added to the water directly after treatment at a few surface-water supplies. The ClO₂ residual in the water leaving the treatment plant is < 0.05 mg L⁻¹ and ClO₂ is not detectable in the distribution system. Microbial growth in the distribution system can lead to water quality deterioration, e.g. elevated heterotrophic plate counts, presence of coliforms, opportunistic pathogens or invertebrates (van der Kooij and van der Wielen, 2014). Therefore, the water supply companies in the Netherlands aim at distributing drinking water with a high level of biostability (van der Kooij et al., 1999). Assessment of drinking-water biostability is generally based on the concentration of easily assimilable organic carbon (AOC) derived from growth of either Pseudomonas fluorescens strain P17 and Spirillum sp. strain NOX (van der Kooij, 1992, Volk and LeChevallier, 2002) or a natural microbial consortium (Hammes et al., 2010) in short-term batch tests. However, microbial growth in distribution systems is affected by water quality variations and multiple interactions between water, biofilms and sediments. Therefore, additional methods have been developed recently to improve the assessment and monitoring of the biostability of drinking water. These methods include:

• the use of an additional bacterial culture in the AOC test, Flavobacterium johnsoniae strain A3, with the ability to utilize polysaccharides and proteins at the microgram-per-litre level (Sack et al., 2010, Sack et al., 2011),

• the biomass production potential (BPP) test for determining the effect of incubation of a water sample with the indigenous microbial community on the active biomass concentration measured as adenosine tri-phosphate (ATP) (van der Kooij and Veenendaal, 2014),

• determination of the concentration of particulate organic carbon (POC) and particulate carbohydrate carbon (PCHC),

• the continuous biofouling monitor (CBM) for determining the accumulation rates of biomass, iron and manganese on glass beads (van der Kooij and Veenendaal, 2014).

These methods were applied to the water at various treatment stages and in the distribution system of a surface-water supply using eutrophic lake water as the source and distributing drinking water without a disinfectant residual. The colony count of Aeromonas, which is used as an indicator for regrowth, periodically exceed the legislative limit value of 1000 CFU (100 mL)⁻¹ in the distribution system of this supply. Moreover, at a few locations in the distribution system, consumers complained about discoloured water. The objectives of our study were to (i) evaluate the usefulness of the recently developed methods for an improved assessment of the biostability of the water and (ii) elucidate the cause(s) of the regrowth in the distribution system.