6.1 Membrane Treatment & Aeration

Why This Topic Matters for the Exam

Advanced treatment questions on the Texas water operator exams focus on what each process removes, how operators know a unit is fouling, and when aeration is the right tool for groundwater quality problems. Membrane plants and aeration towers are increasingly common in Texas systems that face hard, iron-bearing, or high-organics groundwater—or that must meet stricter turbidity and pathogen requirements. You will be expected to distinguish MF/UF from NF/RO, explain recovery and concentrate, and connect aeration chemistry to Fe/Mn, VOC, and CO2 control.

Membrane Process Basics

Membrane filtration separates contaminants by forcing water through a semi-permeable barrier. Pore size (or molecular weight cutoff) determines what passes into the permeate and what is rejected into the concentrate (also called reject or brine for RO).

MF and UF are primarily physical barriers for particles and microbes. They are widely used as filtration alternatives or as pretreatment ahead of NF/RO. NF softens and reduces disinfection byproduct precursors better than conventional softening in some plants. RO is the workhorse for desalting brackish groundwater and for removing specific dissolved contaminants when other processes are insufficient.

Key operating terms:

• Feed — raw or pretreated water entering the membrane skid
• Permeate — product water that passed through the membrane
• Concentrate / reject — water and rejected solids/salts that did not pass
• Transmembrane pressure (TMP) — pressure difference across the membrane; rising TMP at constant flux often signals fouling
• Flux — permeate flow per unit membrane area

Recovery, Concentrate, and System Balance

Recovery is the percentage of feed water converted to permeate:

$\text{Recovery (\%)} = \frac{\text{Permeate Flow}}{\text{Feed Flow}} \times 100$

Higher recovery means less wastewater (concentrate) but higher salt and foulant concentrations in the reject stream. Operators must balance recovery against scaling risk, pump energy, and disposal limits. For RO, antiscalants, pH adjustment, and staging are used so recovery can be raised without rapid mineral precipitation on the membrane surface.

Fouling, Pretreatment, and Cleaning

Membrane fouling is the accumulation of material that reduces permeability. Common categories include:

1. Particulate / colloidal fouling — silt, clay, coagulated solids that were not removed upstream
2. Organic fouling — natural organic matter (NOM) and biofilm
3. Biological fouling (biofouling) — microbial growth on the membrane
4. Inorganic scaling — calcium carbonate, calcium sulfate, silica, and similar precipitates (especially on NF/RO)

Pretreatment is the first line of defense: screening, coagulation/flocculation, multimedia or cartridge filtration, and—where required—oxidation or adsorption. Operators watch differential pressure, TMP, normalized permeate flow, and salt rejection (for NF/RO). When performance declines beyond manufacturer or plant setpoints, a clean-in-place (CIP) is performed using acid cleaners for mineral scale and alkaline/detergent or oxidant-compatible cleaners for organics and biofilm—always following membrane vendor limits so the polymer is not damaged.

Integrity testing (for example pressure decay or marker challenge tests on MF/UF) verifies that the barrier still provides the credited pathogen removal. A failed integrity test is a compliance and public-health issue, not just a maintenance inconvenience.

Aeration: Purpose and Chemistry

Aeration transfers gases between water and air. In drinking-water practice it is used to:

• Oxidize dissolved iron and manganese so they precipitate and can be filtered
• Strip volatile organic chemicals (VOCs) and gases such as hydrogen sulfide
• Remove carbon dioxide (CO2), which raises pH and reduces corrosivity when excess free CO2 is present
• Add dissolved oxygen when needed for subsequent oxidation or biological processes

Dissolved ferrous iron (Fe²⁺) oxidizes to ferric iron (Fe³⁺) when oxygen is introduced; manganese oxidation is slower and often needs higher pH, stronger oxidants, or catalytic media after aeration. VOC and CO2 removal follow Henry’s law: transferring water into fine droplets or thin films across a large air–water interface drives volatile compounds out of solution.

Packed Tower and Diffused Aeration

Two common configurations appear on exams and in Texas plants:

Packed towers excel when aggressive stripping of VOCs or CO2 is required. Diffused aeration is simpler for many groundwater iron/manganese applications when followed by detention and filtration. Cascade or tray aerators are also used where head is available and moderate gas exchange is enough.

Texas Applicability

Many Texas public water systems rely on groundwater with elevated iron, manganese, hardness, or dissolved gases. Aeration followed by filtration is a standard Fe/Mn approach when source chemistry cooperates. Membrane treatment (especially UF and RO) appears in plants addressing turbidity, pathogen credits, brackish sources, or specific MCL challenges. Under 30 TAC Chapter 290, membrane and aeration facilities must be designed, approved, operated, and monitored consistent with TCEQ drinking-water rules—operators are responsible for keeping units within approved operating ranges, documenting integrity/cleaning events, and ensuring finished water meets applicable standards before distribution.

Exam Focus Checklist

• Match MF/UF vs NF/RO to particle vs dissolved-salt removal
• Define recovery and explain why very high recovery increases fouling/scaling risk
• Link rising TMP or falling normalized flow to fouling and CIP need
• Choose aeration when the problem is Fe/Mn oxidation, VOC/H2S stripping, or CO2 removal
• Distinguish packed-tower stripping from diffused aeration oxidation