12.1 Tertiary Filtration & Advanced Effluent Polishing

12.1 Tertiary Filtration & Advanced Effluent Polishing

Tertiary treatment is whatever follows secondary treatment. Its most common form is filtration, and its purpose is to remove the residual suspended solids that a secondary clarifier cannot capture. In Arizona this step is not optional at most plants, because reclaimed water classes are defined partly by filtration and because effective ultraviolet disinfection depends on it.

---

Why Filter Secondary Effluent

[!IMPORTANT] A tertiary filter is a polishing device, not a primary solids removal device. It is designed for a secondary effluent of roughly 10 to 30 mg/L suspended solids. When the secondary process upsets and solids carry over, the tertiary filter blinds almost immediately — it cannot absorb an activated sludge failure. Filter performance problems very often trace back upstream, and chasing them at the filter wastes effort.

---

Filter Technologies

Granular Media Filters

The same deep-bed sand and dual-media configurations used in drinking water, adapted for the higher and more variable solids load of wastewater.

• Deep bed monomedia (sand or anthracite, 4 to 6 ft deep) tolerates higher solids loading than shallow beds.
• Dual media (anthracite over sand) provides coarse-to-fine filtration in the direction of flow.
• Continuous backwash upflow sand filters (moving bed) clean themselves continuously with an airlift that moves sand from the bottom to a washer at the top, so there is no backwash cycle and no downtime.
• Typical filtration rates in wastewater service run about 2 to 6 gpm/ft², lower than drinking water because the solids are more difficult.

Backwash may be water only, or air scour followed by water. Because wastewater solids are sticky and biological, air scour is often necessary to break the accumulated mat.

Cloth Media (Disk) Filters

Pile cloth media mounted on rotating disks submerged in a tank. Solids accumulate on the cloth's outer surface; permeate passes inward and out through the center.

Cloth filters have become the standard retrofit choice for small and medium plants adding filtration to an existing hydraulic profile. Operator tasks: monitor the level in the filter tank (a rising level means the cloth is blinding), verify backwash pump and shoe operation, and periodically chemically clean the cloth to remove grease and biological growth. Cloth media is eventually replaced; life is typically several years.

Membrane Filtration

Microfiltration or ultrafiltration applied to secondary effluent, distinct from an MBR where membranes sit in the mixed liquor. Produces the highest quality effluent, essentially free of suspended solids and with substantial pathogen removal credit. Used where advanced reuse or reverse osmosis pretreatment is required.

---

Operating Parameters

$\text{Filtration Rate (gpm/ft}^2\text{)} = \frac{\text{Flow (gpm)}}{\text{Filter Surface Area (ft}^2\text{)}}$

$\text{Percent Removal} = \frac{\text{In} - \text{Out}}{\text{In}} \times 100$

Worked example. A tertiary filter bed is 12 ft by 20 ft and receives 1,150 gpm. Influent TSS is 18 mg/L and effluent is 3 mg/L.

$\text{Area} = 12 \times 20 = 240\text{ ft}^2$ $\text{Rate} = \frac{1,150}{240} = 4.8\text{ gpm/ft}^2$ $\text{Removal} = \frac{18 - 3}{18} \times 100 = 83.3\%$

Head loss across the bed is the primary indication of filter condition, and a filter is normally backwashed on terminal head loss, elapsed run time, or effluent turbidity breakthrough — whichever comes first.

---

Filtration and UV Disinfection

The link between the two is direct and heavily tested.

UV transmittance (UVT) measures the fraction of ultraviolet light at 254 nm passing through a 1 cm path of the water, expressed as a percentage. Higher is better; wastewater plants commonly target 55 to 65 percent or greater after filtration.

Two distinct effects degrade UV performance:

1. Dissolved constituents absorb UV, lowering UVT. Iron, humic material, and some industrial discharges do this.
2. Particles shield organisms. An organism embedded in or behind a particle receives less than the delivered dose regardless of what the UVT reading says.

[!WARNING] Low turbidity does not guarantee good UV performance, and good UVT does not guarantee adequate disinfection. Particle-associated organisms are the reason UV systems are validated on the combination of dose, UVT, and particle content rather than on turbidity alone. This is why tertiary filtration precedes UV in essentially every modern design.

---

Chemical Addition for Phosphorus

Biological phosphorus removal alone typically reaches about 1 mg/L total phosphorus. Permits requiring 0.1 to 0.5 mg/L need chemical precipitation followed by filtration.

• Alum, ferric chloride, or polyaluminum chloride is dosed ahead of the filter.
• The metal precipitates phosphate as an insoluble solid.
• The filter removes the precipitate — the chemistry alone accomplishes nothing without the physical removal step.
• Doses well above stoichiometric are needed at very low targets, because driving the last fraction of phosphate out of solution requires excess metal.
• The resulting chemical sludge adds to the plant's residuals load, and the added metal salt consumes alkalinity and depresses pH.