Low Pressure Membrane Basics

Microfiltration and Ultrafiltration Membrane Processes

Scott McClelland, P.E., MBA, BCEE, T-5 Director of Water Quality Sweetwater Authority

CAL‐NEV AWWA Water Education Seminar August 10, 2016

1 Microfiltration and Ultrafiltration

Submerged system 2 Microfiltration and Ultrafiltration

Pressurized System

3 MF/UF

• Water is forced through the membrane pores by pressure differential • Since UF has smaller pores, it will require higher pressures than MF • To make MF/UF practical, it may be necessary to remove the bulk of suspended solids by conventional means first

4 Low Pressure : -15 to 65 psi

Use of a material with distinct pores a. UF (.1 micron or less) b. MF (.05 to .5 micron)

Designed to separate undissolved materials (solids) from the water

5 Low Pressure : -15 to 65 psi

• Removes particles, parasites, bacteria • Reduces turbidity to < 0.1 NTU • Reduces SDI to <2 • Removes viruses • Removes organics and inorganics with proper chemistry • Can be pressure driven or vacuum driven

6 Membrane Processes – Low Pressure • Low Pressure applications remove solids from the water and do require regular scheduled backwashing to free up the module surfaces • Salts are not removed in low pressure applications • The chemicals allowed in low pressure applications are greatly different than those used in high pressure applications

7 Differences of UF/MF from RO/NF

• RO/Nanofiltration removes dissolved salts/solids • UF/MF removes suspended particles and some organics • MF/UF utilizes a backwash to clean the surface • MF/UF barrier layer not as susceptible to oxidation Pore size is 50-100 times smaller

0.1 micron pore size 4 to 6 micron

Giardia Cryptosporidium (4-15 micron) (4 - 6 microns) Flow Across the Membrane • Direct or Dead-end Flow – 100% of the feed water passes through the membrane surface – Direct flow allows systems to operate at higher recovery rates (95-98%) – Most of the MF, UF applications utilize this flow pattern • Crossflow – Similar to RO – Some of the MF/UF utilize this flow pattern Flow Across the Membrane

Filtered Water

Cross – Flow Filtration Dead - End Filtration Flow Across the Membrane

• Most MF/UF utilize direct or dead end flow • Water is passed through the membrane while contaminants build up on the surface • Pressure or vacuum requirement increases due to deposition • When maximum vacuum, pressure or flow rate reaches a set point, system must be backwashed and possibly cleaned Filtration and Backwash

FILTRATION BACKWASH Feedwater flows on the Backwash water flows from outside of the fibers the inside of the fibers

13 Applications

• Treatment of surface water • Groundwater treatment especially in high iron and manganese situations • Tertiary filtration of waste water for reuse • Wastewater treatment in MBR systems • Food and dairy applications • Industrial applications Operations • Modular design • Variety of layouts • Installation in small spaces (40% of conventional space used) • Retrofits into existing settling and media tanks Operations

• Variety of layouts • Expansions are possible with little additional investment • Additions can share cleaning systems, backwashing equipment and air blowers, etc. Operations

Four basic operating steps: • Filtration • Backwash • Cleaning • Integrity Testing Normal Filtration

In hollow fiber membrane filtration applications, flow path is outside-in or inside- out Flow In Filtered water out

Flow In Filtered water out

Inside-Out Outside-In Normal Filtration

• Particles retained on surface • Clean water, filtrate, collected • Most are “dead-end” or direct filtration • Vacuum systems use only outside-in • Pressurized systems use either inside-out or outside-in MF / UF Configuration & Devices

• Hollow Fiber (Straight Through) • Spiral Wound (Sheet) • Hollow Fine Fiber (U-Tube) • Tubular • Plate & Frame (Sheet)

20 Element Construction MF/UF material

• Thin skin with small pores on surface of a much more porous material

• Some products have a skin on the outside and the inside of the hollow fiber

22 Modules Containing Hollow Fibers

23 Hollow Fiber Cassette

24 Normal Filtration Normal Operation Forward Flow: • “Normal” operational mode • Feed water flows through the membrane to produce filtrate

Reverse Flow or Backwash: • Brief flow of filtrate back through membrane in the opposite direction to dislodge solids • Done periodically during normal operation Normal Filtration

• MF/UF systems operate at 95- 98% recovery due to less backwash water volume than conventional filters • Most systems operate without chemical pretreatment – When pretreatment is required, dose rates are typically much lower than conventional media filters Normal Filtration

• Upsets in feed water quality do not affect filtrate quality – This is a key feature over conventional treatment options – For example, Lake turnover, disturbance of intake area, runoff, etc. can result in a quick change in feed conditions and yet the MF/UF provides a consistent filtrate Normal Filtration

• MF/UF systems provide physical barrier

• Efficiency measured using microbial log removal, NTU, and SDI – Up to 6 log microbial removal – Turbidity from 0.02 – 0.1 NTU – SDI 0.5 – 2.5 Modular Construction

• Provides flexibility • Staging provides capacity

Air blower

Permeate pump Membrane cassettes Backwashing

• Systems are designed to be backwashed automatically and on a frequent basis

• Procedures similar to conventional filter backwashing – Clean water pumped under low pressure in reverse direction – Cleans surface of filter when solids are removed from the membrane surface – Backwashing takes minutes to complete Fouling • Reversible or Irreversible – Is it removed easily? • Type of foulant – Biological – Organic – Particulate – Dissolved • Means of Fouling – Cake formation – Pore blockage Fouling

• Fouling causes: – Decreased flow at constant pressure – Increased pressure or vacuum requirements • Backwashing removes most solids, however a chemically enhanced backwash (CEB) may also be needed for certain types of fouling Backwashing

Air can accompany backwash or air can be sparged through unit without water Chemical Cleaning • Modules may be cleaned in place (CIP) or submerged cassettes may require removal to a separate cleaning tank • Rows/cassettes may be isolated for cleaning without loss of production • Cleaners might be heated, recirculated, and used for a soak of the membrane • Cleaning cycles can be short durations (minutes) or long cycles (5-6 hours) that tend to be a manually completed operation similar to cleaning RO/NF membranes Chemical Cleanings

Effective vs. Ineffective Cleanings

110

100

90

80 Normalized Flow , % Normalized

70 0 20406080 Time, Weeks

Effective Clean Ineffective Clean

Slide courtesy of Avista Technologies Integrity Testing

• Detects leak in individual fibers or modules

• Verifies physical barrier is intact – Visible air bubbles on the feed side indicate breach

• Fibers can be easily located and repaired by a variety of techniques Finding a Compromised Fiber Example of how to spot the location a broken fiber

Photograph courtesy Pall Corporation Pinning a broken fiber MF/UF Membrane Construction

• Common polymer materials: PVDF, PAN, PS, PES, PP

• Can be composite (different support material) or asymmetric (single material and support material) Element Construction

• MF/UF membranes are bi-directional – Membrane surface allows flow both ways, which allows for a reverse flow backwash and cleaning • Most materials are oxidant resistant, allowing chlorine, ozone, etc. – Reduces biological fouling and removal of Iron and Manganese – Hollow fiber is most common Pressurized Element Construction

• Hollow fibers are packed into a cylindrical housing (6000 – 10,000 fibers)

• Fibers are typically 1.2 – 1.5 mm OD (0.6 – 0.75 mm ID) – Small diameter allows high packing density

• Fibers are free to move or vibrate – Helps dislodge solids Element Construction

Feed Membrane element

Filtrate

Membrane housing Feed Cassette Construction

Picture courtesy of GE Water and Process Technology 44 Hollow Fiber Cassette

Courtesy South Valley Sewer District/Jordan Basin Water Reclamation, UT 45 New Seawater system with hollow fibers

Photo courtesy of GE Water and Process Technology 46 Pressurized Systems Housing modules are mounted in racks with piping manifolds Submerged Systems • Membranes are immersed in open tanks, exposed directly to feed water • Hollow fibers arranged in modules and cassettes, submerged in open tanks Submerged Systems

• Vacuum pumps (permeate pumps) pull filtrate through top of modules • Provision to remove individual modules/cassettes with overhead crane Submerged Systems Pretreatment • Strainers are required for wastewater treatment • UF and MF feeds may need some equipment to remove grit and large particulates • The use of flocculants and coagulants may be necessary to remove organics • Oxidation of Iron and Manganese may aid in their removal • More on UF/MF Pretreatment in Module 3

51 Summary

• UF and MF separate solids from the liquid treated • Dissolved species are not removed • Pressurized and vacuum systems are utilized • Backwashing and cleaning are part of the regular process

52 Knowledge Questions

How do MF and UF systems make separations occur?

MF/UF membrane is susceptible to oxidation? Yes or no.

Name the four items in a MF/UF operation.

53 Special Thanks to SEDA (Southeast Desalting Association) and SWMOA (Southwest Membrane Operators Association) for use of their material

http://SWMOA.ORG For more information on becoming a member of SWMOA

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