How UV Disinfection Can Reduce Disinfection Byproducts While - - PowerPoint PPT Presentation

How UV Disinfection Can Reduce Disinfection Byproducts While Enhancing Microbial Inactivation

Glenn Palen, P.E. / CH2M HILL

Project Background - Current Plant Configuration

City of Norfolk VA Department of Utilities

Raw Water Source ─ Reservoir System consisting of three lakes in Suffolk and additional lakes in Norfolk and Virginia Beach ─ Fed from Various Sources

• Runoff to Drainage Area
• Lake Gaston
• Blackwater River
• Nottoway River

─ Average TOC in Raw Water = 3-7 mg/l

City of Norfolk VA Department of Utilities

City Operates Two Water Treatment Plants 37th Street (28 mgd) Moores Bridges (108 mgd)

37th Street WTP

Plant Originally Constructed in 1920 Expanded in 1940, with Numerous Subsequent Upgrades

37th Street WTP

4.0 MG FW Storage Tank Floc/Sed (above) Clearwells (below) Filter Bldg Residuals PS High Pressure PS Backwash Equalization Basin Intermediate Pump Station Chemical/ Operations Bldg

37th Street WTP

Treatment Process:

Ammonia Rapid Mix Floc/ Sed Filters Disinfection In Clearwells Pump/ Storage Storage FW Pumps Chlorine

Enhanced Coagulation with Ferric Sulfate at Depressed pH Chlorinate Upstream of Filters for Mn Control DBP Control Strategy: Enhanced Coagulation + Chloramines in Distribution System

37th Street WTP

Clearwells are Located Beneath Floc/Sed Basins (Double Deck Arrangement) Limited Baffling in Existing Clearwell

Floc/Sed (Upper Level) Clearwell/Disinfection (Lower Level)

Raw Water Raw Water Settled Water to Filters Filtered Water Disinfected Water

37th Street WTP

Potential for Contamination from Sed Basins Above Favors Chlorinating the Entire Clearwell Large Clearwell Volume results in a High Degree of Log Inactivation with Free Chlorine

Sed Basins Sed Basins Clearwells Clearwells

37th Street WTP Phase III Upgrade – Disinfection Design

37th Street WTP Site Layout

• Exist. 4.0

MG FW Storage Floc/Sed (above) Settled Water CCB (below)

• Exist. Filter Bldg

New Settled Water PS

• Exist. Residuals PS
• Exist. High

Service PS New Filters New UV Disinfection Exst. Backwash Equalization Basin IPS Exst. Chemical/ Operations Bldg

Add Serpentine Baffles to existing clearwells and operate as parallel Settled Water Chlorine Contact Basins (CCB) Replace existing 1920/1940 vintage filters with new Deep Bed Filters CCBs limit settled water contamination risk from above

Initial Phase III Upgrade Design Approach

Rapid Mix Floc/ Sed New Filters CCB Storage FW Pumps New SW Pumps Low Lift Pumps Ammonia Chlorine

Initial Design Approach

Disinfection Contact Time Effectively Increased with Improved Baffling (less short circuiting)

Current Clearwell Baffling Proposed CCB Baffling

Summer 2012

First Application of New Stage II DBP Distribution System Sampling Points Tropical Storm Irene Flushed Natural Organic Material into Reservoirs in fall of 2011 and Stirred Things Up ─ Raw Water TOC Levels and Finished Water DBP’s Increased in 2012 ─ Enhanced Coagulation Treated the Water and Regulatory Compliance Maintained but DBP Levels were Higher City Reviewed Finished Water DBP Goals and Lowered their THM Target This Series of Events Revealed the Vulnerability of Proposed Phase III Disinfection Approach

Treatment Approach Reviewed to Achieve Revised DBP Goals

Further Enhancement of Coagulation Process seemed Impractical Considered Shifting Point of Chlorine Addition Further Downstream in Clearwells ─ Risk of Contamination from Sed Basins Above Still Present ─ Existing Disinfection Benchmark Difficult to Meet Considered Lowering Operating Level in Clearwell to Reduce Chlorine Contact Time ─ Reduction in Settled Water Storage Volume Created New Risks to Plant Operations /Reduced Operational Flexibility ─ Additional Chloraminated Finished Water Storage Could be Added to Maintain Flexibility at a Higher Project Cost

DBP Formation Rates Reviewed to Confirm Optimum Free Chlorine Contact Time

Significant reductions in free chlorine contact time are required to achieve city’s finished water DBP goals ─ Contact Times <30 minutes are preferred for optimal DBP control ─ 10-12 minute Contact Time required for 0.5-log Giardia inactivation @ 3.0 mg/L free chlorine concentration ─ 2-log Virus inactivation also required, but Giardia criteria governed Conclusion: Based on DBP formation rates; the disinfection benchmark requirements and the city’s DBP goals are difficult to achieve without adding a new disinfection process that does not produce chlorinated DBP’s

New Multiple Barrier Disinfection Approach Selected to Meet DBP and Disinfection Goals

Rapid Mix Floc/ Sed New Filters SW Storage Basin Storage FW Pumps New SW Pumps Low Lift Pumps Ammonia Chlorine UV

Chlorine fed upstream of filters to maintain Mn control strategy Free chlorine Contact Time limited to 10-12 minutes at design flows (0.5-log Giardia inactivation) ─ Achievable through deep bed filters and downstream PS wet well UV sized to provide a minimum of 1-log giardia inactivation with one reactor out of service ─ Conservative design dose = 40 mJ/cm2 Ammonia added immediately downstream of UV

New Multiple Barrier Disinfection Approach Selected to Meet DBP and Disinfection Goals

Advantages ─ Safeguards drinking water against most microorganisms treated by chlorine ─ Inactivation of chlorine-resistant protozoa, including Cryptosporidium and Giardia.

Trojan

New Multiple Barrier Disinfection Approach Selected to Meet DBP and Disinfection Goals

Advantages: ─ Disinfection process enhanced by adding a second disinfection barrier ─ Design free-chlorine Contact Time achieves 0.5-log Giardia inactivation ─ City’s DBP goals are achieved ─ Current Settled and Finished Storage Volumes Maintained

New UV Disinfection Building Constructed on Existing Backwash Equalization Basin

Plan View

New UV Building Existing South Backwash Equalization Basin (Below)

UV Reactor UV Reactor

UV Reactor Building - Section View Feed and Discharge Piping Installed Overhead to Reduce Footprint to Match Existing Basin Flow Meter and Control Valve Provided on Each Discharge Pipe

New UV Disinfection Building Constructed on Existing Backwash Equalization Basin

UV Reactor UV Reactor

UV Disinfection Reactors Designed Around Trojan and Calgon Equipment

Long Term Manufacturer’s of UV Disinfection Equipment Medium Pressure UV Design Reactors Validated in Accordance with EPA UVDGM Maximum Flow per Reactor = 17 mgd Reactor Size: 24-inch

Conclusions

UV Disinfection Can Provide a Relatively Low Cost Option to Reduce Finished Water DBP’s and Enhance Disinfection in Plants that Already Practice Enhanced Coagulation and Chloramine Secondary Disinfection UV Disinfection Coupled with Free Chlorine Disinfection Can be Combined to Meet Disinfection Benchmark Requirements

Questions?