Wastewater Process Optimization for Efficient UV Disinfection of Secondary Effluent

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1 Wastewater Process Optimization for Efficient UV Disinfection of Secondary Effluent March 21, 2012 Kati Bell, PhD, P.E., BCEE

Background UV Disinfection of Wastewater Drivers for UV Disinfection Risk management requirements for chlorine gas Lifecycle cost can be lower than alternatives when system size, design, and

2 Background UV Disinfection of Wastewater Drivers for UV Disinfection Risk management requirements for chlorine gas Lifecycle cost can be lower than alternatives when system size, design, and operations are optimized Increasing treatment requirements for BNR and DBPs Considerations for Implementation Site conditions/physical constraints Effluent quality with respect to UV transmittance (UVT) Total Suspended Solids (TSS)

3 Challenges of Implementing UV Disinfection for Secondary Wastewater Effluent UVT is used as a rule of thumb TSS may impact disinfection efficacy and efficiency TSS may first cause a decrease in the disinfection rate, especially at lower UV doses Second, a tailing of the dose-response curve

4 log Inactivation (fecal coliform) WATERCON 2012 Challenges of Implementing UV Disinfection for Secondary Wastewater Effluent Example - collimated beam studies, weeks apart UVT = 53%; TSS = 5 mg/l UVT = 58%; TSS = 12 mg/l Dose (mj/cm 2 )

log Inactivation (total coliform) Decreased UV Disinfection Performance Due to TSS Tailing appears at low bacteria concentrations and is often considered

filtration (Darby et al., 1993; Johnson and Qualls, 1984; Jolis et al., 2001; Lazarova et al.

5 log Inactivation (total coliform) Decreased UV Disinfection Performance Due to TSS Tailing appears at low bacteria concentrations and is often considered the primary limiting factor to compliance Rate reductions are attributable to solids because increase in UV disinfection performance occurs with filtration (Darby et al., 1993; Johnson and Qualls, 1984; Jolis et al., 2001; Lazarova et al., 1998) High rate clarification systems 5 also improve UV performance Secondary Effluent Test 1 CoMag Effluent Test 1 Secondary Effluent Test 2 CoMag Effluent Test Dose (mj/cm 2 ) WATERCON 2012

UV Disinfection Performance Challenges as a Function of TSS TSS concentration is used to predict disinfection performance, but correlations are inconsistent Suspended solids size is important because

6 UV Disinfection Performance Challenges as a Function of TSS TSS concentration is used to predict disinfection performance, but correlations are inconsistent Suspended solids size is important because of bacterial association with particles Factors that affect bacterial association with particles particle size the nature of particles treatment type sludge age (SRT) Studies show UV protection increases with particle size microns is where particle shielding starts Particles larger than 20 microns are more important

Results of TSS Impacts and Strategies for Improving Performance UV doses required to inactivate free microorganisms are well documented; however, microorganisms attach to particulates and may require

7 Results of TSS Impacts and Strategies for Improving Performance UV doses required to inactivate free microorganisms are well documented; however, microorganisms attach to particulates and may require 2X, 3X or more to achieve the same log reduction as for free organisms Improvement of disinfection performance can be achieved using two possible control strategies Physically remove TSS to address particle associated bacteria Modify upstream (biological) process to reduce number and size of particles

8 Selecting a TSS Control Strategy Some particle shielding can be overcome by increasing dose with disinfection of secondary effluent has become common Capital costs associated with implementing tertiary filtration processes may justify higher UV doses Higher dose requires additional equipment and energy over the project life of the project Studies report the impact of various upstream processes on the concentrations of particle associated microorganisms (Emerick, 1999, Loge, 1999; Emerick, 2000) Modifications of biological treatment processes as TSS control strategy have been well-documented; these options are not widely implemented, although significant project lifecycle costs and operational simplicity can be acheived WATERCON 2012

9 Percent Distribution by Particle Count (%) What Does Experience Tell Us? Development of a UV Project Database Secondary wastewater UV disinfection projects without tertiary filtration were compiled into a database (with costs) Continuous SBR; TSS = 2 mg/l High Performing TF; TSS = 4.8 mg/l Conventional AS (nitrified); 4.7 mg/l Low Performing TF; TSS = 24.9 mg/l > 2 to 5 > 5 to 10 > 10 to 15 > 15 to 20 > 20 to 25 > 25 to 50 > 50 to 100 > 100 Effluent TSS Particle Size (µm) WATERCON 2012

Biological Process Improvements to Enhance TSS Removal from Treated Effluent Enhanced TSS removal is met by enhancing formation of biological flocs, attributed to several genera of bacteria

10 Biological Process Improvements to Enhance TSS Removal from Treated Effluent Enhanced TSS removal is met by enhancing formation of biological flocs, attributed to several genera of bacteria Achromobacter Alcaligenes Arthrobacter Citromonas Flavobacterium Pseudomonas Zoogloea Cells and other components are enveloped in bridges of extracellular microbial polymers (Jenkins et al., 1993) In practice, this translates to a contact basin with a targeted SRT to promote the growth of the appropriate organisms

Case Study Example Modification of Upstream Biological Process The Massard Wastewater Treatment Plant (MWWTP) was constructed in 1966 was designed for 8 mgd with 30/30 limits Bar screens Influent

11 Case Study Example Modification of Upstream Biological Process The Massard Wastewater Treatment Plant (MWWTP) was constructed in 1966 was designed for 8 mgd with 30/30 limits Bar screens Influent pump station Primary clarification Trickling filter/solids contact Secondary clarification Chlorine contact basin To meet new permit limits, including disinfection during wetweather events, MWWTP was upgraded to meet its permit limits at 10 mgd average flow and 24 mgd peak flow WATERCON 2012

Massard WWTP Disinfection Process Selection Disinfection study conducted based on economic and noneconomic criteria Gas Cl 2 /sulfur dioxide Gas Cl 2 /bisulfite Bulk NaOCl/bisulfite UV disinfection

12 Massard WWTP Disinfection Process Selection Disinfection study conducted based on economic and noneconomic criteria Gas Cl 2 /sulfur dioxide Gas Cl 2 /bisulfite Bulk NaOCl/bisulfite UV disinfection Lifecycle costs (construction and annual operating costs) showed that UV disinfection could be cost-effective and reliable under consistent effluent conditions Average effluent UVT > 55% Average TSS < 20 mg/l (maximum < 30 mg/l)

13 What Does Experience Tell Us? Using the UV Project Database to Evaluate MWWTP Particle size distributions in trickling filter (TF) plant effluent can be highly variable - UV disinfection is often a challenge TF plants can experience periodic sloughing of biosolids UV may work most of the time, sloughing can cause violations To meet new permit and wet water at MWWTP, trickling filter improvements were evaluated against solids contact process improvements which were more economical

14 Process Selection for Facility Improvements at MWWTP UV disinfection selected based on cost and non-cost factors Eliminating the use of gas chlorine Minimizing chemical handling Additional costs for improving biological process for implementing UV was incremental Facility improvements Small solids contact activated sludge system, including new contact and re-aeration basins, blowers, and building Improvements were also made to the secondary clarification system to address the peak wet-weather flows

15 Massard WWTP Process Improvements WATERCON 2012

16 Average Annual TSS Concentration (mg/l) WATERCON 2012 Performance Results at MWWTP Once in operation, significant improvements in TSS concentrations (and quality) were achieved Biological Contact Basin Improvements Implemented

17 Massard WWTP UV Disinfection System WATERCON 2012

18 Percent Distribution by Particle Count (%) WATERCON 2012 Case Study Example Evidence from the Kingsport, TN WWTP TF/SC Process SC SRT = 4d Q peak = 35 mgd UVT = 70% TSS Average = 6 mg/l High Performing TF; TSS = 4.8 mg/l TF with AS; TSS = 5.6 mg/l Low Performing TF; TSS = 24.9 mg/l > 2 to 5 > 5 to 10 > 10 to 15 > 15 to 20 > 20 to 25 > 25 to 50 > 50 to 100 > 100 Effluent TSS Particle Size (µm)

19 Gas Chlorine Disinfection Conversion Chlorine Contact Basin Retrofit WATERCON 2012

at a time Performance testing as specified

20 Gas Chlorine Disinfection Conversion Chlorine Contact Basin Retrofit Construction nearly complete - MOP included construction in one contact basin at a time Performance testing as specified will follow and will include both intensive and extensive testing

21 Other Process Considerations for TSS Removal for Improved UV Disinfection of Secondary Effluent If using chemical coagulation for P removal or chemically enhanced clarification, there are other considerations Chemically induced flocs are more disinfection resistant than biologically formed flocs because: Particles are coagulant-bound and shielding can occur Higher density particles can adsorb more UV light Decreased porosity reduces light penetration into particles Thus, if UV disinfection is used downstream of a metal (alum or ferric) coagulation process, it may be preferable to use a filtration process for removal of these particles

Summary Biological flocculation can enhance TSS removal

investigators report that very small increases in the mean cell

concentrations exponentially (Loge, 1999) HOWEVER, addressing TSS

22 Summary Biological flocculation can enhance TSS removal potentially eliminating expensive tertiary filtration Other investigators report that very small increases in the mean cell retention time of treatment facilities reduce effluent bacterial concentrations exponentially (Loge, 1999) HOWEVER, addressing TSS does not guarantee increased UVT and understanding water quality is key to UV disinfection

23 Questions? Kati Bell, Ph.D., P.E., BCEE, CDM

ACTIFLO Process For Wet Weather and Wastewater Treatment

ACTIFLO Process For Wet Weather and Wastewater Treatment ACTIFLO Microsand Ballasted Clarification Process ACTIFLO is a high rate, compact process developed by Veolia Solutions & Technologies. The process