Membrane Desalination Systems

Transcription

1 Membrane Desalination Systems The State of the Science with Emphasis on Needed Research 10/17/05 R. Rhodes Trussell R. Shane Trussell trusselltech.com

2 Outline Introduction Today s Successs & Challenges Considerations for future research

3 Introduction Principle processes: RO, NF, ED and Membrane Filtration All are basically post-wwii developments First commercial ED in mid-50s First Commercial RO in mid-60s First Commercial NF in late-70s Large-scale membrane Filtration came into its own around 1990

4 Introduction Perhaps the most fundamental divide between membrane processes is in the driving force In the case of ED the electric field» Charged species move through the membrane and the water is retained With MF, UF, NF, and RO pressure» Water passes through the membrane & the other constituents are retained

5 Introduction Membrane processes fit into the Desalination & Reuse discussion in two places: As a means of removing particulates As a means of removing dissolved species

6 Characteristic Rejections of Pressure Driven Membranes Size exclusion Charge Steric Effects Microfiltration Ultrafiltration Nanofiltration Particles Sediment Algae Protozoa Bacteria Small colloids Viruses Dissolved organic matter Divalent ions (Ca 2+, Mg 2+ ) Reverse osmosis Monovalent species (Na +, Cl - ) Decreasing pore size Increasing pressure Water

7 A brief look at each of these Successes & Challenges

8 Membrane Filtration: Successes The most rapidly growing of membrane technologies Will displace granular media filtration technologies during the next decade Is also revolutionizing other processes: Solids separation for activated sludge (MBR) Pretreatment for RO (NF and ED?)

9 Membrane Filtration: Challenges Inconsistent virus removal Inconsistent regulation Fouling inadequately understood

10 A little aside on MBR

11 Flow Schemes for the MBR and Conventional Activated Sludge Process Conventional Microfiltration Tertiary Secondary Treated Clarifier Wastewater Aeration Basin Backwash Water WASTE Aeration Basin Primary Treated Wastewater (Equivalent to a 1-3 mm screen) MBR Tertiary Quality Wastewater WASTE

12 Why is MBR important to Reuse? Benefits: Excellent effluent quality A much smaller footprint Easily automated Sludge need not settle Effluent quality is not dependent on operations A MUCH better barrier to pathogens

13 Return to successes & challenges

14 MBR: Successes Consistently superior effluent quality Fouling and foaming are better understood Projects at increasingly larger scale Costs increasingly competitive Energy costs decreasing Long-term success demonstrated in numerous locations

15 MBR: Challenges Our thinking must shift to decentralized reuse Strategies for dealing with peak flows must mature MBR-specific strategies for operation must be developed (filter, not settle) Membranes must be made to the same QC standards as drinking water membranes

16 RO: Successes Cost has dropped significantly Membranes have improved Energy efficiency has improved

17 RO: Challenges Particulate Fouling The area where we ve longest felt we knew something (SDI, MFI, etc,) Yet:» A high SDI has always guaranteed fouling» But» A low SDI has never guaranteed good performance We need a better understanding of the role low levels of particulates play in fouling behavior

18 RO: Challenges Inorganic Fouling (scaling) The Good News:» We have good models to predict it» We know how to control CaCO 3 and CaSO 4 The Bad News:» For silica our principle strategy is to reduce recovery» Scale inhibitors are available, but their selection and use is more of an art than a science

19 RO: Challenges Organic Fouling The least-understood area of fouling Biological Fouling Pretreatment is helpful, but The key will be the use of disinfectants

20 RO: Challenges Water Quality and Corrosion We are stuck with the Langelier Index The LSI is an idea from nearly 100 years ago that the way to prevent corrosion is keep CaCO 3 supersaturated This strategy cannot be reconciled with US communities who successfully use naturally soft water: San Francisco, Oakland, Portland, Tacoma, Seattle, New York, and Boston We are wasting a lot of money hardening good quality water

21 RO: Challenges (reuse) Removing Microbiological Contaminants Little doubt that the capability is there, but SD work showed can t just rely on MW cut-ff Uniform standards, specifications and regulations are required Emerging Organics New compounds will continue to show up RO will not remove them all It is important that we understand those that RO does not remove and» Either establish there is no risk, or» Find other ways to remove them

22 Pretreatment RO: Challenges (seawater) It s unlikely that one universal treatment will work in every application The appropriate pretreatment depends on the site In some cases we ve been lucky Others have been less fortunate We need to invest more in pretreatment studies before we build projects With an emerging technology, large-scale failure sets back the entire industry

23 RO: Challenges (seawater) Boron Essential for plants, perhaps for humans Low levels have toxic effects on both What we know about plants Information is old Is limited to agricultural crops Reported toxic levels range from 0.3 to 2 mg/l Little information on horticulture (Grandma s roses) Boron is difficult to remove Doesn t ionize Current technologies all increase cost Need better information on safe levels Need better removal technologies

24 Considerations for further Research

25 Future Research Recommendation 1 - Improve Feedwater Recovery 1. Understand scale inhibitors 2. Study silica scale control (esp.) 3. Study chemical pretreatment & sidestream interventions 4. Optimize element design Recommendation 2 - Improve our understanding of pretreatment for SWRO

26 Future Research Recommendation 3 - Water Quality and Horticulture 1. Desktop study and field studies on common plants Recommendation 4 - Biofilm Control/Microbiofouling 1. Understanding it better Reuse, seawater, brackish water Which organisms? Important parameters, TOC, Temp, Stagnation, micronutrients, etc. Effectiveness of pretreatment 2. Use of chloramines Long-term exposure tests (>20,000 hrs) Better documentation on the redox chemistry of chloramines Seawater/bromamines

27 Future Research Recommendation 5 - Study Organic Fouling We should continue to fund strong studies in this area. We don t understand it Recommendation 6 - Removal of Microbials Here we have the understanding We need standard specifications, confirmation tests, & a design Manual

28 Future Research Recommendation 7 - MBR A huge new area We should keep an active research program in this area, funding only the best proposals Recommendation 8 - Documenting the rejection of emerging contaminants They will keep coming We will need to maintain our database

29 Future Research Recommendation 9 - Boron control SWRO - support studies to demo approaches Reuse - scoping study Recommendation 10 - Energy Recovery Studies/demonstrations of new techniques

30 Future Research Recommendation 11 - Corrosion Desktop study of natural low TDS supplies Testing of alternate strategies (besides Langelier) Recommendation 12 - Pretreatment for EDR & Reuse Strategies to prevent anion membrane fouling Research to understand the same

31 Finis