Today, newer purification technologies have proven able to compete with long-established distillation. Following a brief review of these purification technologies are data for performances of a single and double distillation system, and the Millipore Elix UV system.
During distillation, water is heated in a boiler. Then water vapor is condensed and purified water collected in a receiving flask. Though it is expected contaminants initially present will not distill and will remain in the boiler, in addition to some specific ions, some organic molecules may be carried away with the water vapor as these molecules distill easier than water or may co-distill with water.
The Elix system combines reverse osmosis (RO), electrodeionization (EDI) and a germicidal UV lamp.
Organic Contaminants – TOC
Levels of organic contaminants, grouped under “Total Organic Carbon” (TOC), were monitored. TOC in tap water can vary widely day-to-day. Graph 1 shows TOC values for Elix, single distilled and double distilled water. RO-EDI is more ef?cient than single or even double distillation in removing organics. This is partly because filtration by reverse osmosis rejects most organic molecules due to the small pore size (10-8m) of RO membranes. Also, EDI eliminates many of the charged organics. Distillation is less ef?cient due to the volatility of small organic molecules that can co-distill with water.
Inorganic Contaminants – Ion Chromatography
Ion removal efficiency was compared between the Elix and single still and double still systems. Levels of the commonest cations and anions present in tap water were quantified using ion chromatography (IC). Tables 1 and 2 show results for cations and anions respectively. Considering the total number of inorganic ions remaining after each purification step, the Elix provides slightly better global results than single and double distillation. But, if we also take the charged organics into account, the difference is significant.
Table 3 reports microbiology data. The distillation process produces sterile water, which was checked by collecting water samples directly at the condenser outlet. No micro-organisms were found on the plates after incubation. However, samples collected at the entire system’s outlet appeared to be contaminated. The two boilers are connected by tubing 15 cm long, and analysis performed at the tubing outlet revealed micro-organisms here also, showing the drawbacks of tubing necessary to distillation systems.
But, bacterial content in water delivered by the Elix, including outlet tubing, was consistently between 0 and 1.3 cfu/ml.
Energy & Maintenance
Energy consumption: Elix required approximately 35 Wh and the double still system, 6000 Wh.
Water consumption: Elix required up to ten times less water than the double still system to produce the same amount of puri?ed water.
Water recovery: Double still system—2.1 % to 3.3 %.Elix—20.5 % to 22.1 %.
The Elix needed no maintenance during the three months when the experiments were conducted. The double still system required a weekly descaling treatment with strong acid.
Equipment used in experiment:
Double still: Aquatron® 4000D from Bibby Sterilin Ltd, (Staffordshire, U.K.) For single distillation step, water was sampled at entrance of still’s second boiler.
RO-EDI system: Millipore Elix 10 UV system, (Massachusetts USA).
Systems were fed with tap water from Saint-Quentin-en-Yvelines, France. Samples were taken when systems had been running for at least one hour. For the IC experiments, 100 ml samples were collected in thoroughly rinsed polyethylene bottles.
IC system: Dionex® DX-500 (Dionex Corporation, Sunnyvale, CA, USA) equipped with pre-concentration columns. Cations were separated on a CS12A column in isocratic conditions, using 20 mM methanesulfonic acid as an eluent. Anions were separated on an AS17 column in isocratic conditions. An EG40 eluent generator was used.
TOC measurements were made using a Sievers® 2244 AP analyzer (Ionics Instrument Business Group, Boulder, CO, USA), combining acid treatment, persulfate oxidation and UV oxidation. To eliminate risk of sample contamination through air exposure, sealed containers were used. Bacteria-free bottles were used for microbiology. Bacteria were grown for 72 h at 30°C on R2A plates. All tubing was changed before experiments.
Results showed the Elix was significantly more efficient than the single and double stills in reducing TOC levels and eliminating bacterial contaminants, supporting what is generally admitted in the scientific world: that distillation has been superseded by the RO-combined-with-EDI technology.
This is an edited version of a longer application note