Overview of Wastewater Disinfection Mark Gross University of Arkansas Kitt Farrell-Poe University of Arizona University Curriculum Development for Decentralized Wastewater Management
NDWRCDP Disclaimer This work was supported by the National Decentralized Water Resources Capacity Development Project (NDWRCDP) with funding provided by the U.S. Environmental Protection Agency through a Cooperative Agreement (EPA No. CR827881-01 01-0) 0) with Washington University in St. Louis. These materials have not been reviewed by the U.S. Environmental Protection Agency. These materials have been reviewed by representatives of the NDWRCDP. The contents of these materials do not necessarily reflect the views and policies of the NDWRCDP, Washington University, or the U.S. Environmental Protection Agency, nor does the mention of trade names or commercial products constitute their endorsement or recommendation for use.
CIDWT/University Disclaimer These materials are the collective effort of individuals from academic, regulatory, and private sectors of the onsite/decentralized wastewater industry. These materials have been peer-reviewed reviewed and represent the current state of knowledge/science in this field. They were developed through a series of writing and review meetings with the goal of formulating a consensus on the materials presented. These materials do not necessarily reflect the views and policies of University of Arkansas, and/or the Consortium of Institutes for Decentralized Wastewater Treatment (CIDWT). The mention of trade names or commercial products does not constitute an endorsement or recommendation for use from these individuals or entities, nor does it constitute criticism for similar ones not mentioned.
Citation Gross, M.A. and K. Farrell-Poe. 2005. Disinfection - PowerPoint Presentation. in (M.A. Gross and N.E. Deal, eds.) University Curriculum Development for Decentralized Wastewater Management. National Decentralized Water Resources Capacity Development Project. University of Arkansas, Fayetteville, AR.
Disinfection, NOT Sterilization The goal of disinfection is to rid the wastewater stream of organisms capable of causing infection Sterilization is freeing the wastewater stream of ALL LIFE.
Human Health Hazards (Diseases) Pathogenic Bacteria Typhoid fever Paratyphoid fever Gastroenteritis Cholera Bacillary dysentery Infectious Viruses Poliomyelitis Infectious hepatitis Aseptic meningitis Encephalitis Protozoa Giardiasis Gastroenteritis Amoebic dysentery Cryptosporidiosis Helminths Gastroenteritis Chronic anemia
How does Disinfection Work? Sedimentation Septic tank/soil absorption system Predation Biomat (soil absorption system) Dessication Soil Adsorption System Destroy Cell Wall Chlorine Ozone Affect biochemical processes UV
Commonly Used Disinfection Techniques: Chlorination Liquid Gas Solid (tablet erosion type) Ultraviolet Light Ozonation
Factors affecting Disinfection Contact Time ph Concentration or Intensity of the disinfectant Concentration of the organisms Concentration of interfering substances
Some Disinfection History Chlorine was introduced as a deodorizer and disinfectant in hospitals in 1835 Chlorine was used in water supplies in London during the cholera outbreak of 1854 Routine chlorination of public water supplies began in 1904 in London, and in 1908 in the U.S. In 1909, the courts upheld that chlorination was in the best interest of public health. (the needs of the many.)
Chlorination
Clean Water Chlorine Chemistry Cl 2 + H 2 O HOCl - + H + + Cl - HOCl - H + + OCl - The ph affects this reaction. Lower ph drives the reaction to make HOCl. HOCl (hypochlorous acid) is a more effective disinfectant than OCl - (hypochlorite ion). A ph below 7.4 is conducive to HOCl formation
Chlorine Destroys target organisms by oxidation of cellular material. Some organisms are resistant to low doses of chlorine Oocysts of Crytosporidium parvum Cysts of Entamoeba histolytica Cysts of Giardia lamblia Eggs of parasitic worms
Forms of Chlorine Free available chlorine is the concentration of chlorine existing in the form of hypochlorous acid and hypochlorite ions. Combined Chlorine is the concentration of chloramines formed when non-nitrified nitrified effluent is chlorinated
Some Wastewater Chlorination Considerations Chlorine is an oxidizer. In clean water (low BOD) it is more effective as a disinfectant since it is not stolen to oxidize organic matter.
Chlorination Considerations Chlorine reacts with ammonia to form chloramines. Chloramines are not as effective as hypochlorous acid and hypochlorite ion for disinfection.
Chlorination Interferences Ammonia Forms chloramines Less effective as disinfection agent More persistent or stable than free chlorine
Chlorine and Ammonia Monochloramine NH 3 + HOCl NH 2 Cl + H2O Dichloramine NH 2 Cl + HOCl NHCl 2 + H 2 O Trichloramine NHCl 2 + HOCl NCl 3 + H 2 O
Chlorination Interferences Biochemical Oxygen Demand (BOD) Can exert chlorine demand
Chlorination Interferences Total Suspended Solids (TSS) Shield organisms
Chlorination Interferences Humic Materials (possibly from peat filtration systems) Exert chlorine demand
Chlorination Interferences Nitrite Oxidized by chlorine
Chlorination Interferences ph Affects distribution between hypochlorous and hypochlorite ions Affects distribution among chloramine species
Chlorination Interferences Chlorine can react with iron, manganese and hydrogen sulfide. The oxidized compounds can precipitate downstream from the chlorination point
Mixer Mixing Tank Liquid Chlorine (bleach) Solution Metering Pump Wastewater Flow Typical Liquid Chlorination System
Typical Chlorine Tablets
Tablet Chlorinator
Dual Tablet Chlorinator
Tablet Chlorinator
Tablet Chlorinator
Tablet Chlorinator
Observation: Tablets in chlorinators can bridge and result in a low chlorine dose. Tablets need to be replaced regularly Homeowner management of disinfection units in general has proven to be unreliable
Contact Time and Concentration The DOSE of disinfectant can be considered the time that the water is in contact with the disinfectant multiplied by the concentration or intensity of the disinfectant DOSE = Concentration X (Intensity) Time 10-States Standards requires 15 minutes of contact time following the chlorine injection and mixing (at peak flow)
Ultraviolet Light UV
Disinfection using Ultraviolet Light UV light is in the wavelength range of about 40nm to 400 nm. The most effective wavelength is 254 nm.
UV Spectrum
Disinfection using Ultraviolet Light The water should be clear so that the light can pass through the water to the organisms
Disinfection using Ultraviolet Light The UV light is typically generated by lamps and shines through quartz glass tubes into the water
UV Lamp Cutaway
Disinfection with UV Light Dissolved organic compounds (BOD, TOC, COD) inhibit the passage of UV light to the organisms
Disinfection with UV Light Organic Iron compounds can absorb UV light, and iron precipitates can coat the quartz glass tubes, inhibiting the passage of UV light into the water to the organisms
UV effects on DNA
UV Dosage Dosage (C t ) = Power (intensity) x surface area x time Dosage often measured as microwatt-seconds/cm
UV Assembly
UV Assembly
UV Assemblies
UV Monitor
UV Lamp Monitor
Observation: UV systems can be expensive to maintain a new bulb can cost in the range of $40 - $100 and should be replaced annually at a minimum
Observation: UV and Ozone have less of a residual disinfection ability as chlorine. This is favored in wastewater treatment
OZONE
Disinfection with Ozone O 3 Ozone is an unstable (half life = 30 min.+/-) ) colorless gas produced by discharging electricity in dry air (corona discharge) Ozone is generated at the point of use (not shipped in in gas cylinders) Ozone is the strongest oxidant of the commonly used disinfectants
Disinfection with Ozone Ozone is produced by discharging electricity in dry air (corona discharge) Ozone is corrosive Ozone is soluble in water up to about 5 mg/l
Disinfection with Ozone Ozone disinfects by oxidizing the cell material of organisms..
Ozone Considerations Ozone is generally not recommended for small systems because of unfavorable economics and operation and maintenance requirements
Ozone Considerations Ozone is an oxidizer, and can be expected to react with organic compounds in the wastewater stream Ozone also oxidizes iron, manganese, and hydrogen sulfide and cause precipitates downstream of the ozone injection point Ozone is not expected to cause a toxic end product or form disinfection byproducts
Ozone generator
Ozone Pocket Detector
Ozone Sensor
What about viruses and protozoa? Disinfection effectiveness is generally measured in terms of Coliform bacteria (total or fecal). This is an indicator organism, indicating the possible presence of fecal material. Just because no coliform survived, it doesn t mean that nothing else survived
So What about viruses and protozoa??? Giardia and Cryptosporidium cysts are more resistant to chlorination than coliform Viruses apparently are more resistant to chlorination than coliform - particularly Hepatitis A virus. Ozonation is fairly effective for Giardia and Crytpo disinfection, but there is some evidence that with ozonation,, it s all or none
Conclusions: Get it clean before you disinfect
Conclusions: Provide some contact time
Conclusions: Use enough disinfectant
Conclusions: Experience has shown that tablet chlorinators are not a reliable choice if the operation and maintenance is left up to the homeowner.
Conclusions: Ultraviolet disinfection is gaining popularity as the disinfectant of choice if pretreatment processes beyond the septic tank are used.