Pathogen Removal Mechanisms and Pathogen Credits in MBR-Based Potable Reuse Trains

Transcription

1 Pathogen Removal Mechanisms and Pathogen Credits in MBR-Based Potable Reuse Trains Ufuk G. Erdal, PhD, PE 2017 NWRI Clarke Conference 10/20/2017 1

2 Outline Background Objectives Comparison of Pathogen Credits MBR Pathogen Removal Mechanisms Approaches to Improve Pathogen Removals in an MBR Based AWTF Discussion Questions and Comments 2

3 Background Currently full advanced treatment (FAT) consisting of RO and AOP is required to meet GWRR via subsurface injection MF for pretreatment of RO Very effective; meets all primary and secondary MCLs, notification level chemicals, and other requirements MICROFILTRATION REVERSE OSMOSIS UVAOP DECARBONATOR OCSD PLANT #1 SEC. EFF SODIUM HYPOCHLORITE BW Waste to WWTP Influent ANTISCALANT SULFURIC ACID RO CONC. OCSD OCEAN OUTFALL H2O2 LIME TO BARRIER INJECTION WELLS AND SPREADING BASINS 3

4 MBR AT FAT Comparison of Pathogen Credits Primary and Secondary MF/UF RO UVAOP Treatment 1 6-Month Retention in Ground Total Minimum LRV for GWR via Injection Crypto Giardia Virus MBR RO UVAOP 6-Month Retention in Ground Total Minimum LRV for GWR via Injection Crypto Giardia Virus Based on lower 10 th percentile values, Rose (2004) 4

5 Background and Objective Due to lack of DIT or other approved methods to assess membrane integrity, no pathogen credits have been given to MBR in many states. Several questions arise. Can we apply DIT to MBR systems for pathogen credits? Can MBR, in reality, achieve better pathogen removal? What surrogate we can use to assess membrane integrity? Can MBR deserve pathogen credit? How can we improve pathogen credits in an MBR based AWTF? Objective is to provide answers to above questions. 5

6 MBR Pathogen Removal Mechanisms 6

7 1. Direct Removal via Size Exclusion - Molecules larger than membrane pore sizes will be rejected regardless of their surface properties (i.e., charge, polarity) 4 µ 0.1 µ

8 2. Absorption into Biomass (MLSS) and Sequential Removal through Membrane Filtration 0.1 µ Floc

9 3. Pore Blocking - Large molecules block the pores of the membranes and restrict passage of small viruses CH 0.1 µ

10 4. Reduction of Effective Pore Size due to Biofilm Growth, Gel and Cake Layer Formation on Membrane Surface and Pores 0.1 µ

11 5. Self Healing of Damaged Membranes 0.1 µ

12 Can We Apply DIT to MBR Systems? DIT begins by pressurizing membrane fibers from inside to approximately psi about seconds. Once the pressure is stabilized the pressure source was isolated and the decay test started. The pressure was recorded over a 5-minute, or till the pressure decreased to the minimum permissible pressure as required by the test resolution, whichever occurred first. Daily pressure decay test Calculate pathogen removal 12

13 Concerns with DIT in MBR Systems DIT test pressure is relatively high and cannot be applied to flat sheet MBR membranes DIT test pressure also exceeds most of the MBR hollow fiber membrane suppliers pressure requirements (3-5 psi) 13

14 Concerns with DIT in MBR Systems Lack of correlation between PDT and LRV in MBR; due to the action of mechanisms other than pure size exclusion Pore blocking, cake and gel layer formation Presence of predator organisms that consume pathogenic organisms Absorption of pathogens to MLSS and their removal thru membrane filtration and periodic sludge wasting VCF? 14

15 Continuous Monitoring of Turbidity Limited data collected from NV tests indicate good correlation between MBR permeate turbidity and MS-2 LRV 15

16 Australian Experience In Australia, Amos Branch and Pierre Le-Clech (2015) documented LRVs for MBR systems. Based on the data collected, they proposed the following LRV credits: For MBR systems, with 95 th percentile permeate turbidity 0.4 NTU, and 95 th percentile flux 16.9 gfd Virus LRV: 1.5, Protozoa LRV: 2.0 and Bacteria LRV:4.0 With membrane nominal pore size <0.1 µ, with 95 th percentile turbidity 0.3 NTU and flux never exceeding 17.7 gfd. Virus LRV: 1.5, Protozoa LRV: 4.0 and Bacteria LRV:4.0 16

17 Now DDW is Providing Conditional Approval for MBR Projects in CA If 95 th percentile turbidity <0.2 NTU and some other conditions meet: Virus LRV: 1.5, Crypto LRV:2.0 and Giardia LRV: 2.0 Is it enough to meet pathogen LRVs? MBR RO UVAOP 6-Month Retention in Ground Total Minimum LRV for GWR via Injection Crypto Giardia Virus

18 Approaches to Improve Pathogen Credits in an MBR Based AWTF 1. Get better pathogen credits for RO systems 2. Explore options to get better credit for UVAOP 3. Incorporate additional treatment processes into AWTF 18

19 1. Get Better Pathogen Credits for RO 19

20 Why Are We Getting ~2-log LRV for RO? Conductivity monitoring is widely used method to assess RO performance (i.e. salt passage) Easy to implement with a relatively low cost However, resolution of this method is low (up to 2-log) CF CP LRV RO=log(C F /C P ) 20

21 Cond. DLRV=2.3 DLRV=2.4 TRASAR MS-2 Observed TRASAR TRASAR dye dosage in RO feed= ppb TRASAR dye in permeate= ppb LRV can be reliably demonstrated with TRASAR (Fues, 2017) 21

22 2. Explore Better Pathogen Credits for UVAOP 22

23 Can We Get Better Pathogen Credits for UVAOP? Given that UVAOP gets 6-log credit for V/G/C 23

24 UVAOP Design UVAOP is designed to meet <10 ng/l NDMA Minimum 0.5-log 1,4-dioxane removal UV doses >850 mj/cm2 are needed for 0.5-log 1,4-diaxone removal Multiple reactors/banks are operated in series to deliver the target UV dose Each reactor/bank is operated and monitored independently 24

25 UVAOP Validation at Oxnard UVAOP was designed to meet Minimum 1.2-log NDMA removal Minimum 4-log MS-2 Inactivation Three chambers in series each has two reactors, (total of 6 reactors), each reactor has 72 lamps At MS-2 RED Dose of 115 mj/cm2, each reactor can achieve >>4-log Crypto and Giardia inactivation At an assumed validation factor of 3 for Crypto, five reactors in series could achieve >20-log Crypto and Giardia Inactivation 25

26 Disinfection Processes in Series vs. UVAOP Disinfection processes in series each may get up to 6 log pathogen credit (up to 12-log total) For systems with multiple UVAOP reactors in series, may deserve credit for each reactor if all the monitoring requirements are met? UVAOP + Free Chlorine Ozone + UV Ozone + UVAOP UVAOP+ Low Dose UV Chlorine + UVAOP Are these disinfection processes in series different than UVAOP reactors in series? 26

27 3. Add a UV Disinfection System to MBR Based Train 27

28 UV Dose Requirements A validation factor must be applied to these UV doses for full-scale operation 28

29 Where Can We Put UV Disinfection? After MF/UF (Before RO) Lower UVT (typically 70-78%) Without residual chlorine does not provide effective biological fouling control for RO After RO Higher UVT (typically 95-98%) Reduces both CAPEX and O&M Costs MBR UV RO AOP MBR RO UV AOP MBR RO AOP UV 29

30 Discussion and Conclusions MBR is a robust secondary treatment technology that produces solids free effluent and effectively removes all particulate material and most of the pathogens Using TRASAR, Rhodamine Dye, TOC, Sulfate may provide 2-log additional LRV for RO over conductivity monitoring UVAOP may deserve a better pathogen credits than currently given UV disinfection with <50 mj/cm 2 dose can provide 4-log Crypto and Giardia inactivation 30

31 Questions and Comments