Question 5 of 5: Principles and Operation of a Trickling Filter

Question 5: Principles and Operation of a Trickling Filter (25 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

A trickling filter is an attached-growth biological reactor. Settled sewage is distributed over the top of a bed of coarse media — historically 50–100 mm rock, now more often a plastic cross-flow or vertical-flow module — and trickles down through it as a thin film while air passes through the voids. The name is doubly misleading: nothing is filtered, and the water is not the medium being treated so much as the carrier delivering substrate to a fixed biological film. It should be understood as one component of a secondary treatment system, since the secondary clarifier that follows is integral to the process rather than a polishing step.

Biofilm formation and sloughing

Within days of start-up, bacteria in the applied wastewater attach to the media and secrete extracellular polymeric substances that anchor them and each other, forming a gelatinous film 0.1–2 mm thick. Substrate and oxygen reach the organisms only by diffusion from the liquid film flowing over the surface, and both are consumed as they penetrate. This produces the characteristic stratification shown in the figure: an outer aerobic zone, perhaps 0.1–0.2 mm deep, where the useful oxidation happens; an anoxic intermediate zone where nitrate serves as the electron acceptor; and, once the film thickens past a few tenths of a millimetre, an anaerobic layer next to the media where fermentation and sulfate reduction proceed. That deepest layer is the key to the process’s self-regulation. Its organisms are starved of substrate, produce organic acids and gases, and progressively lose their grip on the media, so when the hydraulic shear of the passing liquid exceeds the weakened adhesion the film detaches in sheets — sloughing. A fresh thin film immediately begins to regrow. Sloughing is therefore not a malfunction but the mechanism by which the reactor wastes its excess biomass, exactly as sludge wasting does in an activated-sludge plant. Its consequence for design is decisive: the sloughed solids leave in the effluent, so a secondary clarifier is an integral part of the trickling filter process, without which the plant would discharge more suspended solids than it removed. Sloughing is seasonal in cold climates, producing a heavy spring slough that can overload the clarifier if the plant is not ready for it.

Aeration of the biomass

Oxygen reaches the biofilm from the air in the media voids, not from the wastewater, which is why void ratio and ventilation govern the design. In most installations airflow is by natural draft: the air inside the filter is at the temperature of the wastewater, the air outside is at ambient, and the density difference drives a chimney flow through the underdrain plenum and up (or down) through the bed. The direction reverses with the season — upward when the wastewater is warmer than the air, downward in summer — and, critically, the draft stalls when the two temperatures are equal, which is when odour complaints and loss of nitrification occur. Design provides generous underdrain vents, peripheral ventilation ports of at least 1 per cent of the plan area, and clear space above the media; heavily loaded plastic-media towers use forced-draft fans. Recirculation assists aeration because it re-oxygenates the liquid film as it falls through the distributor.

Effluent recirculation

Returning part of the filter or clarifier effluent to the filter inlet serves several purposes at once. It maintains a minimum wetting rate so the distributor keeps turning and no part of the bed dries out at night; it dilutes a strong or toxic influent; it returns partially degraded substrate and acclimated organisms for a second pass; it increases hydraulic shear, keeping the film thin and preventing ponding; and it dampens diurnal load variation. The cost is a larger clarifier, larger pumps and additional energy, and beyond a ratio of about two the returns diminish sharply. The classic NRC formulation quantifies the benefit through the recirculation factor \(F = (1+R)/(1+0.1R)^2\), whose denominator penalises the diminishing value of each additional pass.

Given. A single-stage rock filter treating \(Q = 5000\) m³/d of settled sewage at 150 mg/L BOD₅; diameter 20 m, media depth 2.0 m, recirculation ratio \(R = 1.0\).

Find. The loadings, the NRC removal efficiency with and without recirculation, the effluent quality, and the ventilation air requirement.

Approach. Compute the plan area and media volume, express the loadings in the units that classify the filter, then apply the NRC equation twice to isolate the effect of recirculation.

1. Geometry and loadings. \(A = \pi(20)^2/4 = 314\ \text{m}^2\) and \(V = 314(2.0) = 628\ \text{m}^3\). The organic load is \(W = 5000(150)/1000 = 750\) kg BOD/d, giving a volumetric loading of \(750/628 = 1.19\ \text{kg BOD/m}^3\!\cdot\!\text{d}\) and a hydraulic loading, including recirculation, of \(2(5000)/314 = 31.8\ \text{m}^3/\text{m}^2\!\cdot\!\text{d}\) — an intermediate-rate filter.
2. Recirculation factor. \(F = (1+R)/(1+0.1R)^2 = 2/1.21 = 1.65\).
3. NRC efficiency. In SI form with \(W\) in kg/d and \(V\) in m³, \[E = \frac{100}{1 + 0.4432\sqrt{W/(VF)}} = \frac{100}{1 + 0.4432\sqrt{750/(628\times1.65)}} \;\Longrightarrow\; \boxed{\ E = 72.6\ \%\ }\] so the effluent carries \(150(1-0.726) = 41\) mg/L BOD₅, requiring a second stage or a polishing process to reach a 25 mg/L consent.
4. Value of the recirculation. Repeating with \(F = 1\) gives \(E = 67.4\) per cent, so doubling the hydraulic load to the filter buys about five percentage points of removal — real, but modest, which is why recirculation is justified more by wetting, dilution and shear than by the NRC gain alone.
5. Ventilation air. Removing \(750(0.726) = 545\) kg BOD/d requires of the order of 545 kg O₂/d. Air at 1.2 kg/m³ is 23.2 per cent oxygen by mass, and only about 5 per cent of it is transferred in a single pass, so \(545/(1.2\times0.232\times0.05) = 3.9\times10^4\ \text{m}^3/\text{d}\), i.e. 125 m³ per m² of plan area per day. That is the flow the natural draft must sustain, and it is why ventilation area, not media volume, is often the binding constraint.

Nuisance organisms

An open, moist, nutrient-rich bed inevitably supports a macrofauna, and three nuisances are routine. Filter flies (Psychoda, the moth fly) breed in the film at the media surface where wetting is intermittent; they are harmless but emerge in swarms that travel to neighbouring properties and generate complaints. Control is by maintaining continuous wetting through recirculation, periodic flooding of the bed, keeping the walls and distributor clean, and dosing an approved larvicide. Ponding is the second: excessive film growth, media degradation or fine media plug the voids, water backs up on the surface, air cannot pass, and the bed goes anaerobic. It is prevented by using uniform, durable media, limiting the organic loading, and maintaining hydraulic shear through recirculation; it is cured by flooding, high-pressure hosing, or chlorinating the influent for a few hours. Odour is the third, arising from anaerobic conditions in a ponded or poorly ventilated bed and from septic influent, and is controlled by ventilation, pre-aeration, and in modern plants by covering the filter and treating the exhaust in a biofilter. Snails, worms and nuisance nitrifier-grazing organisms occur in warmer climates and can strip the nitrifying population from a tertiary filter.