By Professor Nicolas ROCHE
Professor Nicolas ROCHE
Coord. Pôle de Recherche Interdisciplinaire et Intersectoriel Environnement Lab. Mechanics, Modeling and Clean Processes (M2P2 – UMR 7340)
Dept. Chemical Engineering, Process Engineering, IUT of Aix-Marseille
AIX – MARSEILLE UNIVERSITY AMU
IUT of Aix-Marseille
SAS SOLABLE has developed an innovative purification system (LAVIE® process) for the water network.
According to SOLABLE:
- This system, based on UVA radiation, must allow to “break” the chlorine molecules while transforming the bottle into POA (Advanced Oxidation Process, proven by the abatement of methylene blue in 15mn). This POA must also allow in fine to treat deeply the taste and the odors, even other dissolved pollutants of the order of the pesticides or traces of drugs, without filter or passage of the water in a secondary circuit.
- This totally preserves bacterial pollution such as can be found in carafe filter systems, and allows the purified water to be stored in glass bottles, which fit perfectly into refrigerators.
- The absence of the use of different consumables (supports or chemical reagents) is also a plus which allows to dispose of it at any time, without any handling error. A safety feature (cutting off the UVA light when the box is opened) even allows the use to be extended to children.
These new uses, in particular in comparison with the use of filtering carafes, lead SOLABLE to ask for a scientific expertise on the quantitative performances of dechlorination of drinking water and on its qualitative capacities of decoloration of a water colored with methylene blue.
As this product is intended to purify drinking water, the tests were carried out on tap water drawn from Lambesc (13), doped with chlorine within the limits of chlorination authorized in France: between 0.2 and 0.4mg / liter.
Drinking water must have a certain number of characteristics so that it can be used for direct human consumption without risk. While the values of the different criteria monitored may differ from country to country, they can be classified into three main categories:
- physico-chemical parameters (pH, TOC, temperature, conductivity, nitrates, ammonium, chlorides, sulfates, …)
- toxic substances
- microbiological parameters (pathogens such as fecal coliforms).
For France, these criteria can be found in the decree of January 11, 2007 concerning the limits and quality references of raw water and water intended for human consumption mentioned in articles R. 1321-2, R. 1321-3, R. 1321-7 and R. 1321-38 of the public health code
In order to guarantee the quality of these waters, treatments are applied to the withdrawn waters, according to their origins (surface water, groundwater, desalination, rainwater, reuse) before being able to distribute them by ensuring that they remain clean for human consumption until their points of use.
The required physico-chemical parameters are achieved by the implementation of physico-chemical treatment and separation processes. Disinfection requires the use of advanced oxidation solutions such as ozonation, chlorination and UV light treatment. Whatever the process used, a chlorination step is very often carried out before distribution to ensure the bacteriological quality of the distributed water up to its point of use, due to the persistent power of chlorine.
Chlorination processes are very frequently used to disinfect water for consumption (Health Canada, 2009)*. Chlorine has a strong oxidizing power that allows it to kill or inactivate rapidly (contact time of about 30 minutes) the micro-organisms contained in water and it is generally used in two main forms, solid (tablet) or liquid (bleach or sodium hypochlorite), depending on its wide range of uses (collective or individual). Moreover, chlorine has a certain stability in water, which gives it a persistent power. It is therefore important to add sufficient doses of chlorine to ensure the disinfection of the water on the one hand and its persistent power on the other hand.
Chlorination disinfection processes have many real advantages in that they are effective, fast, inexpensive, multi-scale and relatively simple to implement. Nevertheless, they have a number of disadvantages that are important to take into account because they can have an effect on human health. Indeed, an overdose of chlorine can lead to reactions of chlorine with other compounds present in the water and in particular organic matter (OM) to form products commonly called “Chlorination By-Products” (CBP), some of which are considered harmful to human health. Numerous studies (Mouly et al., 2008†) have been able to highlight the conditions for setting up such reactions leading to the formation of TCS. Nearly 600 SPCs have been identified, including major families such as trihalomethanes (THMs) and haloacetic acids (HAAs), which together represent between 20 and 30% of the total mass of SPCs. In addition, too much residual chlorine can give an unpleasant taste to the drinking water.
It is therefore important to be able to limit and control the production of these SPCs in the processing lines but also in the distribution circuits. This is essentially done by an appropriate dosage of chlorine in order to oxidize the organic matter and to limit the residual chlorine concentrations at the end of the treatment. This is a sensitive point for chlorination processes and is not always easy to implement in treatment systems, especially if the resource has variable characteristics and OM contents over time. From this point of view, the WHO recommends a free chlorine concentration of the treated water distributed between 0.2 and 0.5 mg/l.
In recent years, UV light disinfection processes have been developed and commercialized. They are based on the principle that UV rays have an important bactericidal effect.
Ultraviolet radiation (UV) is an electromagnetic radiation whose emission wavelength is shorter than that of visible light and longer than that of X-rays. They represent about 5% of the electromagnetic energy emitted by the sun and can be produced by lamps, called UV lamps. In UV rays, there are classically three categories according to their wavelength: UVA (400-315 nm), UVB (315-280 nm) and UVC (280-100 nm). It should be noted that due to the absorption of UV rays by the ozone layer in the atmosphere, almost all (95%) of the UV light coming from the sun and reaching the earth is of the UVA category.
UV rays modify the DNA of bacteria, which, depending on the time of exposure, can kill or inhibit them and thus prevent their reproduction. Finally, it is important to note that UV rays also destroy certain chemical compounds, known as photosensitive, in water or the atmosphere. They can thus participate in the photodegradation of certain chemical pollutants contained in water, even at low concentrations.
UV lamp disinfection processes are relatively simple in design as they consist of placing the UV lamp in a small reactor with the water to be treated in what is commonly called the irradiation chamber. These processes are developed on an industrial and collective scale as well as on an individual scale due to their simplicity of use. The literature and industrial developments of these processes are extensive and extensive (Oppenheimer et al., 1997)‡ but they rely almost exclusively on the use of UV lamps of the UVC class.
Concerning the use of UVA lamps (radiation category of the LAVIE® process of the SOLABLE company) if to my knowledge, there is currently no industrial development of this type of water treatment lamps, works have nevertheless proved the bactericidal efficiency of UVA lamps (Hamamoto et al., 2007)§.
In conclusion, UV lamp disinfection processes have a number of advantages. Indeed, they are easy to use on an industrial scale as well as on an individual scale. They do not require the use of chemical reagents or particular supports and do not modify the physical and chemical properties of the water to be treated. Finally, if the initial investment on this type of process may seem higher than other processes such as chlorination, its operating cost remains very low with long lifetimes of the lamps used and makes it competitive from this point of view. Nevertheless, there are still some disadvantages to the use of these processes. The bactericidal action of UV lamps is not persistent, unlike chlorination, so its use must be carried out as close as possible to the point of consumption and use. The use of this type of process also requires sufficiently clear water in order not to limit the transmission of UV radiation in the water. Finally, it is difficult to immediately assess the effectiveness of the treatment by measuring a residual, as in the case of the use of chlorine, and to optimize the treatment efficiency of these processes in the case of a resource characterized by a temporal variation in its quality (particularly bacteriological).
Materials and methods:
The experiments were carried out, on November 20, 2017 on the site of the company SOLABLE in LAMBESC (13) from a drinking water drawn from the premises of the company at the time of the experiments. It is a water of low hardness (12°F on average) distributed with a low level of free chlorine, less than 0.1 mg/l. This is why we have added chlorine to this drinking water for the dechlorination experiments using the LAVIE® process. This spiking was done on a volume of 15 liters of water from the network in which we added 0.44 ml of a new commercial bleach solution at 3.6% giving us in fine a water with a measured free chlorine concentration of 1.04 mg Cl-/l. This “mother” solution was then diluted to obtain two solutions respectively at 0.41 and 0.27 to remain within the limits recommended by the WHO. These solutions were each prepared just before the start of the experiments. The company SOLABLE proposes for the use of the LAVIE® process an exposure time to UV light of 20 minutes. Therefore, for each water studied, we made two tests respectively at 15 minutes and 30 minutes of exposure.
The LAVIE® process used for the tests is a prototype of commercial dimensions equipped with two ribbons of UVA LED lamps (6 lamps per ribbon, emission wavelength: 365 nm, power consumption: 25.9 W, voltage: 21.6 V, intensity: 1.2 A) able to accommodate a cylindrical bottle of one liter (diameter: 80 mm, height: 280 mm) in transparent glass (Borosilicate).
The determination of total chlorine was carried out on site at the beginning and end of each experiment using a HANNA H1 711 chlorimeter whose characteristics are given in Table 1. In order to verify the bacteriological quality of the water before and during the experiments and the final chlorine levels, samples from each experiment were taken and sent immediately to the SOSCA ANALYSES** laboratory (31120, l’Union), respecting the recommendations for sampling, packaging and sending given by the laboratory.
Table 1: Technical specifications of the HANNA HI 711 Chlorometer
From a general point of view, all the experiments carried out did not show any variations on the bacteriological and physico-chemical quality of the water after exposure times of 15 or 30 minutes in the LAVIE® process of the company SOLABLE. Indeed, we note, on the results provided by the SOCSA laboratory, no variation of the bacteriological quality of the water nor of its physico-chemical characteristics.
Monitoring of the dechlorination capacities of the LAVIE® process:
The results obtained from experiments carried out on water with initial chlorine concentrations of 0.41 and 0.27 mg chlorine/l respectively for exposure times of 13 and 30 minutes are presented in Figure 1. It clearly appears that the LAVIE® process is more than 90% effective in dechlorinating solutions doped with chlorine after 15 minutes of exposure to UVA radiation. The residual chlorine concentrations measured at 15 and 30 minutes are within the detection limit, within the measurement error, of the dosing method used. Analyses performed by the SOSCA laboratory confirm the absence of total and free chlorine at 15 and 30 minutes for all samples.
Figure 1: Evolution of the chlorine concentration for exposure times of 15 and 30 minutes for an initial solution with 0.41mg/l of Chlorine (blue) and an initial solution with 0.27 mg/l of Chlorine (orange).
Monitoring of LAVIE® process bleaching capabilities:
The LAVIE® water decolorization test was performed on the mother water (spiked with 1.04mg/l of Chlorine) prepared for the dechlorination tests after adding 6 drops of Methylene Blue solution per liter of water. This addition was carried out in two identical bottles and one bottle was then subjected to UVA radiation for 30 minutes, the observations made are presented in Figure 2
Figure 2: Monitoring of the decoloration of a water colored with Methylene Blue, in the initial state (a) and after 30 minutes of UVA exposure for bottle 2 (b)
This test clearly shows that the LAVIE® process presents, in addition to its dechlorination capacities studied below, clear decoloration capacities, thus contributing to the improvement of the quality of drinking water that may present problems of slight colorations.
The tests conducted, on November 20, 2017, on the LAVIE® process developed by the company SOLABLE on drinking water doped with Chlorine and colored by adding Methylene Blue, clearly show that, without any addition of reagents or use of ion exchange media or filtration :
- As far as free or residual chlorine is concerned, and for concentrations that can be considered as important with regard to the WHO recommendations, the LAVIE® process presents from a quantitative point of view dechlorination yields higher than 90% and this from 15 minutes of exposure to UVA radiation.
- For its effects on water coloring (important character of water quality, especially from the point of view of its societal perception), the LAVIE® process presents, from a qualitative point of view, decoloration capacities that can be considered as complete on samples colored with Methylene Blue. These observations show the potential for oxidation and degradation of chemical molecules contained in the LAVIE® process water.
Finally, the use of UVA rays provides an additional guarantee of maintaining or improving the bacteriological quality of the water.
Done in Aix en Provence on December 06, 2017
Prof. Nicolas ROCHE