Abstract
A direct method for measuring adenosine-triphosphate (ATP) in seawater was developed recently, in which commercial reagents are added directly to seawater. However, calibration is required if seawater quality changes (such as changes in salinity, pH, Mg2+, Fe3+) as the seawater matrix interferes with ATP measurement. In this research, a 0.1 μm filtration process is introduced to eliminate such interferences. In addition, a filter rinsing step with sterilized artificial seawater is proposed to eliminate interference of free ATP.The ATP-filtration method is fast (<5 min), reproducible (VC = 7%), six times more sensitive than the direct ATP-method and correlates (R2 = 0.72, n = 100) with intact cell concentration. Microbial ATP concentration measured using the ATP-filtration method and the ATP-direct method were comparable. Microbial ATP measured along the treatment train of a full-scale seawater reverse osmosis (SWRO) plant decreased from 530 in the raw seawater to 10 ng-ATP/L after pre-treatment and to 0.5 ng-ATP/L in the SWRO permeate. The method was also applied to monitor bacterial growth potential (BGP) across the pre-treatment train of a (pilot) seawater desalination plant, where the removal of BGP through the media filtration and ultrafiltration was 44% and 7%, respectively.
Highlights
In reverse osmosis (RO) desalination, microbial quantification has been implemented: (i) to quantify biomass accumulation on RO membranes for biofouling diagnostics [1,2], (ii) to measure biomass in the feed water and across pre-treatment trains in RO plants [3,4], (iii) as a biomass parameter for the determination of nutrients [5,6,7], and (iv) to measure bacterial growth potential [6,8,9].The common methodologies to quantify microbes are heterotrophic plate counts (HPC's) and total direct counts (TDC's)
The comparison was made based on the microbial adenosine triphosphate (ATP) concentration in 2 different applications: (1) In a full-scale desalination plant, seawater samples are collected along the treatment train of an seawater reverse osmosis (SWRO) desalination plant
Bowman et al [40] reported that smaller organisms were not retained by a 0.45 μm filter [41], and Macdonell and Hood [37] observed that smaller marine bacteria (Bdellovibrio) in the Gulf of Mexico can even pass through 0.2 μm filters
Summary
In reverse osmosis (RO) desalination, microbial quantification has been implemented: (i) to quantify biomass accumulation on RO membranes for biofouling diagnostics [1,2], (ii) to measure biomass in the feed water and across pre-treatment trains in RO plants [3,4], (iii) as a biomass parameter for the determination of nutrients (carbon) [5,6,7], and (iv) to measure bacterial growth potential [6,8,9]. Marine microorganisms captured on the filter were flushed out with Milli-Q water to eliminate interference from salt, and the filter, including the retained microorganisms, was placed into a cuvette with 2 mL of Milli-Q water before analysing microbial ATP using freshwater reagents. This method is complicated, and the use of Milli-Q water to flush marine microorganisms can rapidly burst bacterial cells due to osmotic shock resulting in loss of microbial ATP. The method is simple, direct, allows ATP determination at a low concentration level (limit of detection (LOD) = 0.3 ng-ATP/L) and is promising for monitoring microbial growth potential in SWRO systems. Measuring microbial ATP concentration across the pre-treatment train of a full-scale SWRO desalination plant
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