5.3 Membrane Filtration & Reverse Osmosis (Florida RO)

5.3 Membrane Filtration & Reverse Osmosis (Florida RO)

Quick Answer: Membrane processes span microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) from larger pore / lower pressure to denser salt-rejecting membranes. Florida utilities use RO extensively on brackish groundwater and some coastal sources; operators manage recovery, salt rejection, scaling, fouling, antiscalants, cartridge prefilters, and concentrate disposal. Daily monitoring centers on pressures, flows, conductivity/TDS, and feed silt density index (SDI).

Membranes are no longer “specialty only” in Florida—they are mainstream for salinity, hardness, color/TOC, and pathogen barriers. Class C and B outlines expect spectrum literacy (MF/UF/NF/RO), RO process control vocabulary, and Florida-specific issues like concentrate management.

The MF / UF / NF / RO Spectrum

Think of the spectrum as pore size / molecular weight cutoff decreasing and operating pressure and salt rejection increasing from MF → RO. MF/UF are often called low-pressure membranes for particle removal; NF/RO are membrane desalting processes.

Integrity and Credits (Conceptual)

MF/UF systems used for pathogen removal require integrity testing (pressure decay, marker-based tests, etc.) because a broken fiber can short-circuit the barrier. RO/NF provide high dissolved-solids rejection; product conductivity is a continuous integrity-ish indicator for salt passage, but plants still follow manufacturer and regulatory integrity programs where applicable.

Why RO Is Big in Florida

Florida’s hydrogeology and coastal development create:

• Brackish Floridan and other aquifer zones with elevated chloride and TDS
• Saltwater intrusion and upconing near coasts and heavy wellfields
• Customer and regulatory pressure on chloride, sodium, TDS, and sometimes sulfate or hardness
• Interest in alternative water supplies under Water Management District constraints

Brackish groundwater RO is more common inland/coastal utility practice than open-ocean seawater RO for many public systems, though seawater and estuary plants exist in the broader Florida desalination landscape. Compared with seawater, brackish feed usually means:

• Lower feed TDS → lower required pressure
• Higher achievable recovery (more product per gallon of feed)
• Different scalant profile (often calcium carbonate, calcium sulfate, silica, barium/strontium sulfates depending on chemistry)

RO Process Train (Operator View)

A simplified brackish RO train:

1. Raw water (wells) → equalization as needed
2. Pretreatment — oxidation/filtration for Fe/Mn, acid or antiscalant feed, cartridge filters, sometimes UF
3. High-pressure pumps → RO skids (pressure vessels with spiral-wound elements in series/stages)
4. Permeate (product) → degasification (CO₂), corrosion control/stabilization, disinfection, storage
5. Concentrate (brine/reject) → disposal pathway

Post-treatment is critical: RO permeate is low in hardness and alkalinity and can be aggressive to concrete and metal. Operators blend, add alkalinity (e.g., caustic, calcite contactors), and adjust pH for stabilization and corrosion control (Lead and Copper Rule relevance).

Recovery, Salt Rejection, and Mass Balance

Recovery

Recovery = (permeate flow / feed flow) × 100%.

• Higher recovery → less concentrate volume, but higher salt concentration in reject and higher scaling risk.
• Brackish plants may target recoveries often in roughly the 65–85% band depending on design and chemistry; seawater plants run much lower recovery. Design and pilot data rule—exams test the definition and the tradeoff.

Salt Rejection and Passage

Salt rejection ≈ (1 − C_permeate/C_feed) × 100% for a given ion or conductivity proxy.
Salt passage is the complement (what gets through).

Operators trend normalized salt passage and differential pressure because temperature and flux affect raw readings. A sudden rejection loss can mean damaged O-rings, telescoped elements, oxidation damage to membranes (e.g., free chlorine attack on polyamide), or severe fouling changing transport.

Scaling, Fouling, and Antiscalants

Scaling

Scalants precipitate when concentration polarization and recovery push sparingly soluble salts past solubility:

• Calcium carbonate (pH and LSI/CCPP dependent)
• Calcium sulfate, barium/strontium sulfates
• Silica
• Other site-specific minerals

Controls:

• Limit recovery to design
• Acid feed to lower pH (carbonate scale control) where used
• Antiscalant dosing per manufacturer projection software
• Softening or ion exchange pretreatment in some designs
• Proper flush/shutdown procedures so stagnant concentrate does not precipitate in elements

Fouling

Fouling is deposition of particulates, colloids, organic matter, or biofilm that is not necessarily a crystalline scale:

• Particulate fouling — inadequate cartridge/UF protection
• Colloidal / organic fouling — color, TOC, tannins in some Florida feeds
• Biofouling — microbial growth, especially with nutrients and warm temperatures
• Iron/manganese carryover if oxidation/filtration pretreatment fails

Clean-in-place (CIP) uses low/high pH cleaners and other specialty chemicals on a normalized performance trigger—not only on a fixed calendar if monitoring shows earlier need.

Antiscalants

Antiscalants are specialty chemicals that delay precipitation and crystal growth, allowing higher safe recovery. Operator duties:

• Dose accurately (underfeed → scale; gross overfeed can contribute to fouling in some cases)
• Maintain day tanks, dilution water quality, and injection points upstream of cartridges as designed
• Re-evaluate dose when feed chemistry changes (new wells, seasonal blend, intrusion events)

Concentrate Disposal Issues in Florida

RO does not destroy salt—it partitions it. Concentrate is higher TDS than feed and may contain antiscalant residuals and whatever was rejected.

Florida disposal pathways (availability is highly site- and permit-specific):

Exam theme: concentrate management can control project feasibility as much as membrane price. Coastal discharge and environmental sensitivity make planning and permit compliance a real operating constraint—not an afterthought.

Cartridge Prefilters and Upstream Protection

Cartridge filters (often 5-micron nominal, plant-specific) protect RO elements from particles that escaped multimedia/UF pretreatment. Operator practices:

• Monitor differential pressure across cartridge housings; replace at setpoint
• Never bypass cartridges to “keep production up” during fouling events
• Use SDI / turbidity / particle counts on feed to judge pretreatment health
• Coordinate Fe/Mn removal so oxidized metals do not load cartridges and membranes

UF as RO pretreatment is increasingly common: stable low-turbidity, low-SDI feed improves RO run length.

Operator Monitoring: Pressure, Conductivity, SDI

Pressures and Flows

• Feed, interstage, and concentrate pressures
• Permeate and concentrate flows (stage mass balance)
• Normalized permeate flow and normalized ΔP for apples-to-apples trends

Conductivity / TDS

• Feed and permeate conductivity continuously
• Spot checks on individual vessels/elements when salt passage rises
• Temperature compensation awareness

Silt Density Index (SDI)

SDI is a standardized fouling index using a 0.45-µm filter at fixed pressure; it estimates how quickly particulates plug a membrane-like surface. RO manufacturers often want feed SDI₁₅ below a threshold such as < 3 (sometimes < 5 depending on warranty—know the concept: low SDI = acceptable particulate pretreatment). Rising SDI means fix pretreatment before you sacrifice RO elements.

Other Florida-Relevant Checks

• Free chlorine before polyamide RO must be removed (dechlorination with bisulfite, etc.); polyamide thin-film composites are chlorine-sensitive.
• ORP/chlorine residual monitoring on feed
• Antiscalant pump verification
• Well blend changes and chloride trends that shift osmotic pressure requirements

Exam Scenarios to Mentally Rehearse

Scenario A — Coastal wellfield chloride rising
Conventional coagulation will not fix TDS/chloride. RO (or blending/source management) is the process class that addresses dissolved salts.

Scenario B — Rising RO feed pressure and ΔP, stable rejection
Think fouling/scaling on the feed-concentrate path; check cartridges, SDI, recovery, antiscalant, CIP need.

Scenario C — Stable ΔP but permeate conductivity climbing
Think salt passage / integrity (O-rings, damaged membranes, oxidation) as well as instrument error.

Scenario D — After hurricane, colored high-TOC surface blend enters RO plant
Organic fouling risk; pretreatment and maybe temporary recovery reduction; do not assume brackish well chemistry still applies.

Integration with Granular Filtration

Many Florida trains combine both worlds: granular media or UF removes particles; RO removes dissolved ions. Operators who only understand sand filters will miss salinity problems; operators who only understand RO will destroy membranes without particle control. Chapter 5’s through-line is match the barrier to the contaminant size and type—and monitor the KPIs that prove the barrier is working.