3.3 Advanced Disinfection (Ozone, UV) & Byproducts

3.3 Advanced Disinfection (Ozone, UV) & Byproducts

Why This Topic Matters for the Exam

While chlorine is the most common disinfectant, its reaction with natural organic matter (NOM) forms carcinogenic disinfection byproducts. To meet strict EPA regulations, many modern treatment plants have transitioned to advanced disinfection technologies. On the operator certification exam, you must understand the operational requirements, advantages, and disadvantages of ozone, UV light, and chlorine dioxide. You must also know the maximum contaminant levels (MCLs) for regulated byproducts and how to modify treatment processes to minimize their formation.

Ozone Disinfection

Ozone ($O_3$) is a powerful gas produced on-site by passing dry air or oxygen through a high-voltage electrical discharge (corona discharge). It is an exceptionally strong oxidant and disinfectant, highly effective against Cryptosporidium, Giardia, and viruses.

Key characteristics of ozone include:

• No secondary residual: Ozone decays rapidly back to oxygen, leaving no disinfectant residual to protect the distribution system. A secondary disinfectant, such as chlorine or chloramines, must be added post-treatment.
• Byproduct formation: Ozone does not form chlorinated byproducts like trihalomethanes. However, if bromide is present in the raw water, ozone oxidizes it to form bromate, a regulated carcinogen.
• Safety hazards: Ozone is a highly toxic gas that requires ambient ozone monitors and ozone destruct units to safely convert off-gas back to oxygen.

Ultraviolet (UV) Light Disinfection

Ultraviolet light disinfection is a physical, non-chemical process. Water flows past low-pressure or medium-pressure mercury arc lamps that emit UV radiation (specifically in the germicidal range of 254 nm). The UV light penetrates the cell walls of microorganisms and permanently damages their DNA or RNA, preventing replication.

Key characteristics of UV disinfection include:

• Pathogen inactivation: UV is extremely effective at inactivating Cryptosporidium and *Giardia at very low doses, which are highly resistant to chlorine. However, it is less effective against viruses unless high doses are applied.
• No chemical byproducts: UV does not add chemicals to the water, meaning it forms no trihalomethanes, haloacetic acids, or bromate.
• Water quality sensitivity: UV performance depends on UV transmittance (UVT), which is the percentage of light that passes through the water. High turbidity, dissolved iron, or hardness can cause scaling on the protective quartz sleeves or scatter the light, reducing effectiveness.
• No residual: Like ozone, UV provides no residual disinfection, requiring a secondary chemical disinfectant.
• Quartz Sleeve Maintenance: UV lamps are housed inside protective quartz sleeves to prevent contact with water. Over time, calcium, iron, and manganese deposit on these sleeves, causing fouling that blocks light. Operators clean sleeves using mechanical wipers or mild acids. Operators also track lamp runtime (typically replaced after 8,000 to 12,000 hours) and monitor sensor readings to ensure the design dose is delivered.

Chlorine Dioxide

Chlorine dioxide ($ClO_2$) is a synthetic gas generated on-site by reacting sodium chlorite ($NaClO_2$) with chlorine gas or hydrochloric acid. It is a strong disinfectant that does not form chlorinated organic byproducts (THMs or HAA5) and remains active in the distribution system longer than ozone. However, it forms chlorite, a regulated byproduct that must be monitored closely, and has a strict maximum residual disinfectant level of 0.8 mg/L to prevent taste and odor complaints.

Regulated Disinfection Byproducts (DBPs)

When chlorine reacts with natural organic matter (such as humic and fulvic acids), it forms DBPs. The EPA regulates these under the Disinfectants and Disinfection Byproducts Rule (DBPR):

Under the Stage 2 DBPR, water utilities must identify monitoring locations with the highest DBP concentrations, known as Locational Running Annual Average (LRAA) sites. These sites are typically at the far ends of the distribution system where water age is highest. To reduce DBP formation, the rule establishes Total Organic Carbon (TOC) removal percentages during the coagulation process, based on raw water TOC and alkalinity. Higher raw water TOC and lower alkalinity require higher TOC removal percentages (up to 50%). If a plant cannot achieve these percentages, they must perform bench-scale tests to demonstrate enhanced coagulation compliance, where coagulant dosages are increased to achieve maximum organic removal regardless of turbidity. Strategies to control DBPs include:

• Removing organic precursors prior to chlorination using enhanced coagulation, activated carbon, or membranes.
• Switching to chloramines for secondary disinfection.
• Moving the point of primary chlorination further downstream in the treatment process.

Realistic Exam Scenarios

1. Scenario 1: A water plant utilizing ozone disinfection notices a sudden increase in bromate levels in the finished water, exceeding the 0.010 mg/L MCL. The operator must either lower the ozone dose, shift the pH of the water downward (which reduces bromate formation), or add ammonia to bind with bromine precursors before ozonation.
2. Scenario 2: A plant using UV disinfection experiences a drop in UV transmittance from 95% to 75% due to a seasonal turbidity spike. The operator must immediately initiate backwashing of upstream filters, adjust coagulant dosages to lower turbidity, or inspect the quartz sleeves for scaling and activate the mechanical wiper system.