11.1 Trickling Filters & Attached Growth Fundamentals

11.1 Trickling Filters & Attached Growth Fundamentals

A trickling filter is a bed of media over which settled wastewater is distributed, allowing a biological slime layer (biofilm) growing on the media to remove organic matter as the water trickles past. The name is misleading — nothing is strained. It is a fixed-film or attached-growth bioreactor, in contrast to activated sludge, which is a suspended-growth process.

Fixed-film processes are common at smaller Arizona plants because they use far less energy than aeration, tolerate shock and intermittent loading, need less operator attention, and recover quickly from upsets.

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The Biology

Wastewater flows over a biofilm attached to the media. Within that film:

• The outer aerobic layer contacts oxygen from air moving through the bed and does the organic removal.
• As the film thickens, oxygen cannot penetrate to the media surface, and the inner layer turns anaerobic.
• The anaerobic layer loses its grip on the media, and a patch of film detaches. This is sloughing.

Sloughing is normal and necessary. It removes excess biomass and is the process's equivalent of wasting sludge. It appears at the secondary clarifier as a temporary increase in effluent suspended solids, and heavy sloughing after a warm spell or a loading change can briefly degrade effluent quality. Excessive or wholesale sloughing, by contrast, signals a toxic load, a pH excursion, or a hydraulic surge.

Air movement through the bed is driven by natural draft from the temperature difference between the wastewater and the ambient air, and it reverses direction seasonally. Underdrains must stay clear and vents unobstructed; a plugged underdrain suffocates the bed. In cold weather natural draft weakens, which is one reason performance drops in winter — though in Arizona the more common problem is summer heat reducing oxygen solubility and driving odors.

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Media

Plastic media transformed the process: its high void ratio permits far higher organic loading without ponding, and its light weight allows tall towers with a small footprint.

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The Distributor

Most trickling filters use a rotary distributor: two or four arms with orifices, rotating on a center column.

• Rotation is normally driven by reaction force from the discharging jets (hydraulic drive); some are motor driven.
• Dosing rate, expressed in inches of liquid applied per pass of the distributor arm, controls how the bed is wetted. Higher instantaneous dosing with fewer passes flushes the media and controls flies and ponding.
• Routine maintenance: unplug orifices, check arm level, service the center column seal and bearing, and adjust the end gates that balance the arms.

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Loading Calculations

Hydraulic Loading

$\text{Hydraulic Loading (gpd/ft}^2\text{)} = \frac{\text{Total Flow Applied (gpd)}}{\text{Surface Area (ft}^2\text{)}}$

Total flow applied includes recirculation.

Organic Loading

$\text{Organic Loading (lb BOD/day/1,000 ft}^3\text{)} = \frac{\text{BOD (mg/L)} \times \text{Flow (MGD)} \times 8.34}{\text{Media Volume (ft}^3\text{)} \div 1,000}$

Worked example. A rock trickling filter is 80 ft in diameter and 6 ft deep. Primary effluent is 1.1 MGD at 140 mg/L BOD.

$\text{Area} = 0.785 \times 80^2 = 5,024\text{ ft}^2$ $\text{Volume} = 5,024 \times 6 = 30,144\text{ ft}^3$ $\text{BOD load} = 140 \times 1.1 \times 8.34 = 1,284\text{ lb/day}$ $\text{Organic loading} = \frac{1,284}{30,144 \div 1,000} = \frac{1,284}{30.14} = 42.6\text{ lb BOD/day/1,000 ft}^3$

That figure places the unit in the high-rate range for a rock filter.

Recirculation Ratio

$\text{Recirculation Ratio} = \frac{\text{Recirculated Flow}}{\text{Influent Flow}}$

If influent is 1.0 MGD and 1.5 MGD is recirculated, the ratio is 1.5:1 and total applied flow is 2.5 MGD.

Why recirculate:

• Dilutes strong or toxic influent, protecting the biofilm
• Maintains continuous wetting at night and during low flow, which prevents the film from drying and prevents filter fly breeding
• Increases hydraulic flushing, controlling ponding
• Improves treatment by giving wastewater additional passes
• Returns dissolved oxygen to the applied flow

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Classification by Loading

Low-rate filters nitrify well because the low organic loading lets slow-growing nitrifiers compete for space on the media. Heavily loaded filters are dominated by fast-growing heterotrophs and nitrify poorly — the same competition that governs sludge age in activated sludge.

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Operating Problems

[!IMPORTANT] Ponding and filter flies have the same primary fix from opposite directions: both are controlled by managing how the bed is wetted. Ponding needs flushing, and flies need the bed never to dry out. Increasing recirculation addresses both, which is why it is the first operational lever an operator reaches for.