6.7 Tertiary Filtration, Effluent Polishing & Wastewater Disinfection with Chlorine, Dechlorination and UV

Polishing what the clarifier could not remove

WPI's outline lists tertiary treatment equipment: filtration, media filtration (sand, anthracite, disc filter) and tertiary treatment processes: filtration processes, media filtration. Tertiary filtration is added when a permit requires effluent TSS below what secondary clarification reliably achieves, when phosphorus limits require capture of chemical precipitate, when UV disinfection needs low turbidity to work, or when the effluent will be reused.

What tertiary filtration achieves

A well-operated secondary clarifier delivers roughly 10 to 30 mg/L TSS. Tertiary filtration takes that to 5 mg/L or below, and often to 2 to 3 mg/L. Because much of the residual BOD in a secondary effluent is particulate — it is the solids themselves — removing solids also removes BOD, typically bringing 15 to 25 mg/L down to under 5 mg/L. Filtration also removes the particulate fraction of phosphorus, which is why plants with total phosphorus limits below about 0.5 mg/L almost always need it.

Critically, filtration improves UV transmittance by removing the particles that shield organisms and scatter light. A UV system downstream of an unfiltered effluent must be sized for far more dose than one downstream of a filter.

Technologies

Cloth media disc filters have become the default retrofit for existing plants because they need very little head, occupy a small footprint, backwash with only 1 to 3 percent of throughput, and have no media to lose. The pile cloth is inspected for wear and cleaned chemically on a schedule; solids that mat into the cloth cause rising differential and reduced capacity.

Continuous backwash upflow sand filters never come offline. Water enters at the bottom and flows upward through a downward-moving sand bed; an airlift in the center draws dirty sand up, washes it in a reject weir, and drops it back on top. There is no backwash cycle to sequence, but the reject stream runs continuously at about 5 percent of feed and must be returned to the head of the plant.

Deep bed media filters in tertiary service work exactly like drinking water filters and share the same failure modes: mudballs, gravel upset, air binding, and breakthrough. The difference is loading, since secondary effluent carries a lighter, more biological floc.

Operating essentials

• Backwash return. Every filter backwash stream goes back to the head of the plant, and it is concentrated. Returning it as a slug upsets primary clarification and aeration. Backwash is equalized and returned at a controlled, steady rate, ideally during low-flow hours.
• Chemical addition. A small dose of coagulant or filter-aid polymer ahead of the filter dramatically improves removal of fine, non-settleable particles and is essential when chemical phosphorus precipitate must be captured.
• Biological growth. Secondary effluent still contains nutrients and organics, so filters grow biofilm. Periodic chlorination of the filter or a hypochlorite soak controls it, and unaddressed growth blinds the media and creates odor.
• Solids loading limits. Tertiary filters are polishing devices, not clarifiers. If secondary effluent TSS rises above roughly 30 mg/L, the filter blinds rapidly and backwash frequency becomes unmanageable. A tertiary filter cannot rescue a failing secondary process, which is the most important operating fact about it.

Wastewater disinfection

Both the equipment and the process outlines list chlorination (chlorine, hypochlorite, chloramine), dechlorination (sodium bisulfite, sodium thiosulfate, sulfur dioxide), and ultraviolet (UV).

Why wastewater disinfection is different

Drinking water disinfection starts from a clean, filtered water with low demand. Wastewater disinfection starts from an effluent with residual organics, ammonia, and particles. Three consequences follow:

1. Ammonia converts free chlorine to chloramines almost immediately. In a non-nitrifying effluent, essentially all the chlorine residual is combined. Chloramines are far weaker disinfectants, so wastewater chlorine doses and contact times are much larger than drinking water equivalents.
2. Particles shield organisms. Bacteria embedded in floc survive; this is why effluent TSS and disinfection performance are directly linked.
3. The residual usually must be removed before discharge, because chlorine is acutely toxic to aquatic life. Typical permit limits for total residual chlorine are extremely low, and many Colorado permits express them in the tens of micrograms per liter.

Chlorination and contact

• Dose is set to achieve the permit's bacteriological limit, commonly expressed as E. coli or fecal coliform as a geometric mean with a daily maximum.
• Contact time in the chlorine contact chamber is typically 15 to 30 minutes at peak flow, and the chamber is baffled to force plug flow. A poorly baffled chamber short-circuits, and the measured detention time overstates the actual contact time. Dye or tracer testing determines the true T10.
• CT applies here too, but with combined chlorine the required CT is roughly two orders of magnitude greater than with free chlorine.
• Solids deposits accumulate in contact chambers and must be cleaned out; they exert demand and harbor organisms.

Dechlorination

Reducing agents destroy the residual before discharge:

• Sulfur dioxide gas (SO2) — equipment is nearly identical to gas chlorination, including all-vacuum design, and the same room and safety requirements apply. SO2 is also heavier than air and is a severe respiratory irritant.
• Sodium bisulfite and sodium metabisulfite — liquid, most common on small and mid-size plants.
• Sodium thiosulfate — used mostly for sample dechlorination and small applications.

Stoichiometric demand is roughly 1 part sulfur dioxide per part of total residual chlorine, with about 10 percent excess in practice. Overdosing dechlorination consumes dissolved oxygen in the effluent, which can itself violate a permit's minimum DO requirement, so the operator trims to just enough. Mixing at the point of addition must be rapid, since the reaction is fast but requires contact.

Ultraviolet disinfection

UV inactivates organisms by damaging nucleic acids, forming thymine dimers that prevent replication. It adds no chemical, creates no disinfection byproducts, leaves no toxic residual to remove, and is effective against chlorine-resistant protozoa.

Operating variables:

• Dose equals intensity multiplied by exposure time, expressed in millijoules per square centimeter. Typical wastewater design doses are 30 mJ/cm² and upward.
• UV transmittance (UVT) at 254 nanometers is the single most important water quality parameter. A secondary effluent might have 55 to 65 percent UVT; a filtered effluent 65 to 75 percent. Falling UVT means the lamps must work harder for the same dose.
• Lamp fouling. Quartz sleeves scale with hardness and iron and film with organics. Cleaning is mechanical (wipers), chemical, or both. Fouled sleeves are the most common cause of UV underperformance.
• Lamp aging. Output declines over lamp life, typically 9,000 to 15,000 hours, and the control system must account for it.
• Ballast and lamp failure alarms, and intensity sensors calibrated against a reference.
• No residual. UV provides no lasting protection, so regrowth downstream is possible.
• Photoreactivation. Some bacteria can repair UV damage when exposed to visible light, which is one reason design doses carry safety factors.