Continuous Disinfection and Filtration of Microbiologically Contaminated Water ( Microbiological – bacteria, viruses, and protozoans)
Private Water systems, in Ohio, requiring continuous disinfection and protozoan cyst reduction are rainwater cisterns, springs, ponds, and some shallow wells. Private water system ponds require additional continuous filtration equipment that includes either a rapid sand filter with coagulation and cyst reduction filters, or a rapid sand filter with a step filters arrangement. Slow filters may be used on pond systems without the additional of cyst filters since the slow sand filter design is intended to reduce cysts.
Continuous Disinfection Rules for Private Water Systems
Ohio Administrative Code Rule 3701-28-15. Each type of private water system a continuous disinfection system is being installed on has specific additional requirements. Please consult with your registered private water systems contractor, your local health department, or Private Water Systems Program staff at 614-644-7558.
Types of Continuous Disinfection (all private water systems)
Chlorine is one of the most widely used and effective disinfectant used for treating drinking water. Chlorination is the process of adding chlorine to drinking water to kill germs. Chlorine was first used, as a major disinfectant, in the United States in the early 1900’s. Its use has become more common through the following decades.
Drinking water using chlorine in small amounts does not cause harmful health effects and provide protection against waterborne disease outbreaks. Chlorine levels up to 4 milligrams per liter (mg/l) or parts per million (ppm) are considered safe in drinking water and no harmful health effects are likely to occur. Although some people do not mind the smell and taste of chlorine in the water, others do.
Private water systems such as ponds, springs, cisterns, wells with casing lengths of less than 25 feet are required to use a continuous disinfection system. One of the more effective continuous disinfection systems is a Chlorination Disinfection System. The requirements for the installation of a chlorine disinfection system is addressed in 3701-28-15 of the Administrative Code. A chlorination disinfection system shall include at a minimum a chlorine solution tank, a retention tank, a method to inject the chlorine into the retention tank for contact, and may be required to include cyst reduction filters for ponds, springs, cisterns, and wells with casing less than 25 feet.
Iodine systems are still being used in Ohio for continuous disinfection of private water system but are gradually being replaced by chorine or UV disinfection systems. An iodine system configuration can be similar to a liquid chlorine injection system or can use granular or pelletized iodine.
The sale of iodine had been more restricted due to it being an ingredient in some illegal drug manufacturing.
Iodine disinfection is not used on public water systems because some individuals with thyroid problems may have increased sensitivity to iodine.
Ozone is one of the best chemical oxidizers and maintains only a short-lived chemical residual. Ozone systems are used as oxidizing systems to treat sulfur and iron as well as for continuous disinfection.
Ozone can be generated by corona arc discharge or ultraviolet light. Only corona arc is allowed for generating ozone for private water system disinfection in Ohio. Air is passed through a high-voltage electric field called a corona. Corona arc discharge creates ozone in a similar same way that lightning creates ozone naturally. An air dryer should be added to help in the production of ozone.
It should be noted that many commercial ozone system use UV for creating ozone generation for oxidation. The UV light used to generate ozone is designed to work at a different frequency than UV light used for continuous disinfection only and has a more limited capability to disinfect water. Ultraviolet light does not produce a sufficient quantity of ozone for adequate continuous disinfection of private water systems. UV generates from 10 to 1000 times less ozone by weight than corona arc.
Ozone used for continuous disinfection in Ohio shall have sufficient ozone added to the water to satisfy the demand, and the CT value shall be no less than 0.6 at pH seven and 5 degrees Celsius (41 degrees F). Ozone must have a minimum detectable residual of 0.1 milligram per liter after six minutes of contact.
Mixing prior to contact for ozone can be achieved using a bubble diffuser and venturi nozzles. A venturi basically squeezes that water and ozone together at the point of injection. Bubble diffusers work by emitting ozone through hundreds of bubbles beneath the waters surface.
Due to the strong oxidizing properties of ozone, components used in ozone systems must be capable of resisting corrosive. Ozone is considered a potential indoor air hazard and needs to be destroyed or vented to the outside.
Ultraviolet light (UV) is an effective means of disinfecting water. Ultraviolet light disinfects water by striking a microorganism with a 254Å (nm) wavelength of electromagnetic radiation at a specified intensity. This action disrupts the DNA, preventing the microorganisms from reproducing and thus effectively killing it. Sunlight can disinfect water the same way.
NSF has UV devices divided into Class "A" and Class "B" UV systems that differ based on their ability to treat water for different levels of micro organisms. Pathogens are affected differently by varying dosage intensities of UV light.
The National Sanitation Foundation describes Standard 55 for "Class A" UV light disinfection systems as follows:
"Class A point-of-entry and point-of-use systems covered by this Standard are designed to inactivate and/or remove microorganisms, including bacteria, viruses, Cryptosporidium oocyst and Giardia cysts, from contaminated water. Systems covered by this standard are not intended for the treatment of water that has obvious contamination or intentional source such as raw sewage, nor are systems intended to convert wastewater to drinking water. The systems are intended to be installed on visually clear water."
NSF Standard 55 "Class B" UV systems are designed to operate at a minimum dosage and are intended to "reduce normally occurring non-pathogenic or nuisance microorganisms only." The "Class B" or similar non-rated UV systems are not intended for the disinfection of "microbiologically unsafe water."
Transmitted UV light dosage is affected by water clarity. Water treatment devices are dependent on the quality of the raw water. When turbidity is 5 NTU or greater and/or total suspended solids are greater than 10 ppm, pre-filtration of the water is highly recommended.