Dear  Hossien

UV units for water treatment                                                                                        Special low-pressure mercury vapor lamps produce ultraviolet radiation at 254 nm, the optimal wavelength for disinfection and ozone destruction. The UV lamp never contacts the water; it is either housed in a quartz glass sleeve inside the water chamber or mounted external to the water which flows through UV transparent Teflon tubes. Some ultrapure water systems use 185 nm UV units for reducing TOC (total organic carbon). (http://www.water-research.net/Waterlibrary/privatewell/UVradiation.pdf)

Good Luck

Dear Hamida

Thank you so much for your attention.

Design of such disinfection process requires following steps:

1. determination of the optimum circulation flow rate

2. selection of pump, UV light and piping equipment according to the determined flow rate.

However, I need information for the first step. How can I determine the optimum flow rate?  

Regards

400 J/m2 at 253,7 nm

References:

Chapter 0603 in

http://www.esv.info/978-3-503-14103-6

and these publications cited and described there:

Bernhardt et al (1994) gwf Wasser/Abwasser 135: 677-689

Kolch und Schoenen (1992) Zbl Hyg Umweltmed 192: 565-570

Sommer et al. (2000): J Food Prot 63: 1015-1020

UV fluence required for disinfection varies from 40-190 mJ/sq. cm. 

Accordingly you can set the liquid flow rate.

Thanks

Dear all

Thank you so much for your responses, but I need to know the optimum flow rate for reservoir water turnover. The optimum UV light should be selected according to this flow rate and supplier catalog.

There are guidance available for drinking water and water reuse applications. The average dosage calculation is complicated and you will need the UV supplier to do it for you using the software. The flow rate will be really dependent on the UV system you have and the dosage requirements.  

It is unclear if by "fresh drinking water" you mean "raw untreated water" or if you mean "treated water received by a distribution system". In either case, you will need to know something about the water quality to determine the amount of UV energy needed to deliver a  particular dose (400 J/m^2 or more as noted above). The two parameteres to know are the turbidity and the percent UV transmittance at 254 nm across 1 cm. The dose required will depend on the target organism and its sensitivity to UV inactivation. For raw water, this would involve selecting an organism that represents the greatest risk to human health, typically chlorine resistant parasites such as giardia and cryptosporidia. But if you are dealing with water that has already been treated and simply wish to inhibit bacterial regrowth then a different target organism should be selected. Once the organism, extent of inactivation (in log units typically), and dose are determined, the required flow for a given reactor can be determined from manufacturer supplied information or the manufacturer's own calculations. Recirculation of the water through the reactor back into the storage tank will need to take mixing within the storage tank into consideration, so as to avoid short circuiting, dead zones etc... 

However, if prevention of regrowth is what is desired for a tank of water over the period of one week, then a much simpler solution would be to add chlorine in the form of sodium hypochlorite (common household bleach) dosed to provide 1 to 2 mg/L of free chlorine and thoroughly mixed upon addition (i.e. filling of the tank). Measurement of the chlorine throughout the week can easily be monitored and is a proven method of reducing regrowth on the time scale of a few days. If removal of the chlorine is desired before use, it can be removed easily by allowing the water to be exposed to air over night, gentle heating, sodium bisulfite or equivalent, or activated carbon. This would be a substantially simpler and cheaper alternative. 

If the water is raw water, an alternative design would be to use UV as a single pass while filling the tank (primary disinfectant) and addition of chlorine to the tank for subsequent residual (secondary) disinfectant. This method of primary and secondary disinfection is increasingly common in North American drinking water systems. 

This system is not going to provide bacteria-free water at any flow rate.  This is not a "disinfection cycle" - only a site treatment.  UV only treats the water as it flows through and the poly tank as well as the rest of the system is not sterile and will suffer microbial growth.  You will have a biofilm develop in the tank and you can't effectively sanitize a poly tank.  

As Laith said, if you want bacteria-free system, you'll need to add an additional treatment and chlorine can serve that role.

Thank you Phil Gate. 

Phil and Laith are correct.  For this type of system, you need to disinfect the water before it enters the tank, not after.  UV will work if the turbidity is sufficiently low and the contact time in the UV reactor, itself, is sufficient to inactivate the microorganism that are of concern locally.  This would require that the tank is not open to further contamination and dark to avoid algae growth.  There are many guidance documents on the necessary contact times in UV reactors to inactivate organisms.  If you already have the UV system, reconfigure and place it before the tank, and to avoid additional cost (of an additional chlorinator), add NaOCl (bleach) to the tank once a week at a concentration that results in a residual chlorine concentration of 1-1.5 mg/L as Cl2.  Bleach is often 5.25% NaOCl, which translates to 5% as Cl2, so it is easy to dilute and not much is needed for such a large tank.  For example, at this concentration of bleach (5.25%), it only takes 0.6 mL added to 10 L to result in a concentration of 3 mg/L as Cl2.  Some of this will react with organics in the water, resulting in a lower residual concentration after about 30 minutes of reaction time.  If the concentration of ammonia is high in the water, more bleach will be required.  If the tank is filled once per week, the chlorine can be added immediately after filling.  The standard way of measuring chlorine as by the DPD method.  It is a colorometic method and very inexpensive.  There are even sticks with DPD on them, like measuring pH colorometrically.

Phil and Laith are correct.  For this type of system, you need to disinfect the water before it enters the tank, not after.  UV will work if the turbidity is sufficiently low and the contact time in the UV reactor, itself, is sufficient to inactivate the microorganism that are of concern locally.  This would require that the tank is not open to further contamination and dark to avoid algae growth.  There are many guidance documents on the necessary contact times in UV reactors to inactivate organisms.  If you already have the UV system, reconfigure and place it before the tank, and to avoid additional cost (of an additional chlorinator), add NaOCl (bleach) to the tank once a week at a concentration that results in a residual chlorine concentration of 1-1.5 mg/L as Cl2.  Bleach is often 5.25% NaOCl, which translates to 5% as Cl2, so it is easy to dilute and not much is needed for such a large tank.  For example, at this concentration of bleach (5.25%), it only takes 0.6 mL added to 10 L to result in a concentration of 3 mg/L as Cl2.  Some of this will react with organics in the water, resulting in a lower residual concentration after about 30 minutes of reaction time.  If the concentration of ammonia is high in the water, more bleach will be required.  If the tank is filled once per week, the chlorine can be added immediately after filling.  The standard way of measuring chlorine as by the DPD method.  It is a colorometic method and very inexpensive.  There are even sticks with DPD on them, like measuring pH colorometrically.

Phil and Laith are correct.  For this type of system, you need to disinfect the water before it enters the tank, not after.  UV will work if the turbidity is sufficiently low and the contact time in the UV reactor, itself, is sufficient to inactivate the microorganism that are of concern locally.  This would require that the tank is not open to further contamination and dark to avoid algae growth.  There are many guidance documents on the necessary contact times in UV reactors to inactivate organisms.  If you already have the UV system, reconfigure and place it before the tank, and to avoid additional cost (of an additional chlorinator), add NaOCl (bleach) to the tank once a week at a concentration that results in a residual chlorine concentration of 1-1.5 mg/L as Cl2.  Bleach is often 5.25% NaOCl, which translates to 5% as Cl2, so it is easy to dilute and not much is needed for such a large tank.  For example, at this concentration of bleach (5.25%), it only takes 0.6 mL added to 10 L to result in a concentration of 3 mg/L as Cl2.  Some of this will react with organics in the water, resulting in a lower residual concentration after about 30 minutes of reaction time.  If the concentration of ammonia is high in the water, more bleach will be required.  If the tank is filled once per week, the chlorine can be added immediately after filling.  The standard way of measuring chlorine as by the DPD method.  It is a colorometic method and very inexpensive.  There are even sticks with DPD on them, like measuring pH colorometrically.

Phil and Laith are correct.  For this type of system, you need to disinfect the water before it enters the tank, not after.  UV will work if the turbidity is sufficiently low and the contact time in the UV reactor, itself, is sufficient to inactivate the microorganism that are of concern locally.  This would require that the tank is not open to further contamination and dark to avoid algae growth.  There are many guidance documents on the necessary contact times in UV reactors to inactivate organisms.  If you already have the UV system, reconfigure and place it before the tank, and to avoid additional cost (of an additional chlorinator), add NaOCl (bleach) to the tank once a week at a concentration that results in a residual chlorine concentration of 1-1.5 mg/L as Cl2.  Bleach is often 5.25% NaOCl, which translates to 5% as Cl2, so it is easy to dilute and not much is needed for such a large tank.  For example, at this concentration of bleach (5.25%), it only takes 0.6 mL added to 10 L to result in a concentration of 3 mg/L as Cl2.  Some of this will react with organics in the water, resulting in a lower residual concentration after about 30 minutes of reaction time.  If the concentration of ammonia is high in the water, more bleach will be required.  If the tank is filled once per week, the chlorine can be added immediately after filling.  The standard way of measuring chlorine as by the DPD method.  It is a colorometic method and very inexpensive.  There are even sticks with DPD on them, like measuring pH colorometrically.

Phil and Laith are correct.  For this type of system, you need to disinfect the water before it enters the tank, not after.  UV will work if the turbidity is sufficiently low and the contact time in the UV reactor, itself, is sufficient to inactivate the microorganism that are of concern locally.  This would require that the tank is not open to further contamination and dark to avoid algae growth.  There are many guidance documents on the necessary contact times in UV reactors to inactivate organisms.  If you already have the UV system, reconfigure and place it before the tank, and to avoid additional cost (of an additional chlorinator), add NaOCl (bleach) to the tank once a week at a concentration that results in a residual chlorine concentration of 1-1.5 mg/L as Cl2.  Bleach is often 5.25% NaOCl, which translates to 5% as Cl2, so it is easy to dilute and not much is needed for such a large tank.  For example, at this concentration of bleach (5.25%), it only takes 0.6 mL added to 10 L to result in a concentration of 3 mg/L as Cl2.  Some of this will react with organics in the water, resulting in a lower residual concentration after about 30 minutes of reaction time.  If the concentration of ammonia is high in the water, more bleach will be required.  If the tank is filled once per week, the chlorine can be added immediately after filling.  The standard way of measuring chlorine as by the DPD method.  It is a colorometic method and very inexpensive.  There are even sticks with DPD on them, like measuring pH colorometrically.