3.9 Tertiary Filtration, Effluent Polishing & Water Reuse

Tertiary Filtration, Effluent Polishing & Water Reuse

Table 1 of 9VAC25-790-290 lists filtration among the advanced waste treatment processes that push a facility into a higher classification, and the ABC Wastewater outline names "tertiary treatment processes: filtration processes, media filtration (e.g., sand, anthracite, disc filters)" and "reclaim/non-potable reuse" as distinct tasks.

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1. Why Tertiary Filtration Exists

Secondary clarification typically leaves 10 to 30 mg/L of TSS in the effluent. Filtration is required when:

• The VPDES permit sets TSS below about 10 mg/L, or CBOD below about 10 mg/L (because much of the remaining BOD is particulate);
• Phosphorus limits are below roughly 0.5 mg/L, since chemically precipitated phosphorus must be physically removed;
• Disinfection must reach low bacterial limits, because particles shield organisms from chlorine and absorb ultraviolet light; or
• Effluent is being reclaimed for reuse.

Tertiary filters are sized on hydraulic loading rate, typically 2 to 6 gpm/sq ft for granular media and higher for cloth media, and they receive a light, variable solids load rather than the heavy consistent load of a water treatment filter.

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2. Filter Types

Backwash return is a real process issue: filter backwash from a tertiary filter is returned to the head of the plant, where it adds solids and, if the filter is a denitrification filter, adds nitrate and residual carbon.

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3. Denitrification Filters

A denitrification filter is a deep-bed anoxic filter that simultaneously removes nitrate and TSS. It is how plants meet total nitrogen limits below about 5 mg/L.

• The bed is anoxic. Nitrate is the electron acceptor.
• Carbon must be added. Secondary effluent has almost no remaining BOD, so an external carbon source is fed at the filter inlet.

Carbon dosing is the central control problem. Underdose and nitrate passes through; overdose and residual BOD appears in the effluent, sometimes accompanied by sulfide generation once nitrate is exhausted. Dose is flow-paced against measured influent nitrate, and effluent nitrate is trimmed by feedback.

Nitrogen gas binding is the characteristic operational nuisance: N2 produced within the bed accumulates and increases headloss without solids. Denitrification filters therefore run periodic short "bump" backwashes to release gas between full backwashes.

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4. Chemical Phosphorus Polishing

Biological phosphorus removal reliably reaches about 0.5 to 1.0 mg/L. Virginia's enhanced nutrient removal targets of 0.18 to 0.30 mg/L total phosphorus require chemical polishing plus filtration.

Chemistry. Metal salts precipitate orthophosphate:

Al3+ + PO4^3- -> AlPO4 (down-arrow) Fe3+ + PO4^3- -> FePO4 (down-arrow)

Stoichiometry versus reality. The theoretical molar ratio is 1:1, which works out to about 9.7 lb of alum per lb of phosphorus removed. In practice, at low residual targets the required dose ratio climbs steeply - metal-to-phosphorus molar ratios of 2:1 to 6:1 are common below 0.3 mg/L - because the reaction becomes adsorption-limited and because at very low concentrations the remaining phosphorus is largely particulate and colloidal.

Two-point dosing is the standard approach: a bulk dose upstream (primary or secondary) to remove most of the phosphorus economically, and a small polishing dose ahead of the tertiary filter to capture the remainder. Overdosing has consequences - depressed alkalinity and pH, increased sludge production, and elevated aluminum or iron in the effluent and the biosolids.

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5. Membrane Bioreactors

An MBR submerges microfiltration or ultrafiltration membranes directly in the mixed liquor (or in a separate membrane tank), replacing the secondary clarifier entirely.

Operating constraints unique to MBRs:

• Fine screening is mandatory - typically 1 to 3 mm perforated plate screens - because hair and fiber braid onto the membrane fibers and are extremely difficult to remove.
• Mixed liquor viscosity rises sharply above about 12,000 mg/L, cutting oxygen transfer efficiency and increasing the energy needed for membrane scour.
• Fouling control is by continuous coarse-bubble air scour, periodic relaxation or backpulse, maintenance cleans with hypochlorite or citric acid, and periodic recovery cleans.
• FOG and surfactants foul membranes rapidly, so the pretreatment program matters more at an MBR than at a conventional plant.

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6. Water Reuse in Virginia

Virginia regulates reclaimed water under the Water Reclamation and Reuse Regulation, 9VAC25-740, administered by DEQ. The regulation establishes two quality levels tied to the degree of human exposure.

Reclaimed water systems must meet the regulation's standards for BOD5 or CBOD5, turbidity, and bacteria, with continuous turbidity monitoring where filtration is required. Additional requirements that operators encounter directly:

• Reclamation system operations plan and a reuse management plan approved by DEQ.
• Cross-connection control: reclaimed water piping must be physically separated from potable piping, marked in purple (Pantone 522), and labeled "CAUTION: RECLAIMED WATER - DO NOT DRINK." No physical connection to a potable system is permitted; makeup water must be delivered through an approved air gap or an RPZ assembly.
• Storage sized for periods when reuse demand is below production.
• Notification and signage at reuse sites.
• Operator licensing - a reclamation system is a treatment works, so it requires an appropriately classified Virginia wastewater works operator.

Indirect potable reuse is a distinct, more heavily regulated category. Virginia's best-known project, the Hampton Roads Sanitation District's SWIFT program, treats secondary effluent through advanced processes and recharges the Potomac Aquifer - an aquifer replenishment application governed by additional groundwater and drinking water requirements beyond 9VAC25-740.