16-Civ-B5 Water Supply and Wastewater Treatment · May 2013
Question 5 of 5: Trickling filter — principle, working, and operating problems
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
Paper format. National Examination, May 2013 — 98-Civ-B5 Water Supply and Wastewater Treatment. Three hours, closed book, one aid sheet written on both sides, approved calculator permitted. Question 1 is compulsory and the candidate attempts any three of the remaining four; every question carries 25 marks, so the examinable total is 4 × 25 = 100 marks. Page-1 Note 2 invites the candidate to submit a clear statement of any assumption made where a question is open to interpretation, and Note 6 makes clarity and organisation part of the mark. All five questions are solved here, because this set is a study resource rather than a timed sitting.
Reference texts. Metcalf & Eddy | AECOM, Wastewater Engineering: Treatment and Resource Recovery, 5th ed. (wastewater characterisation, primary sedimentation, attached-growth processes); J. C. Crittenden et al., MWHʹs Water Treatment: Principles and Design, 3rd ed. (coagulation, flocculation, settling theory); M. L. Davis & D. A. Cornwell, Introduction to Environmental Engineering, 5th ed. (water-quality parameters, unit operations); J. R. Mihelcic & J. B. Zimmerman, Environmental Engineering: Fundamentals, Sustainability, Design, 3rd ed. (mass balances on receiving waters); Health Canada, Guidelines for Canadian Drinking Water Quality (GCDWQ) and CCME, Canadian Environmental Quality Guidelines (CEQG) for the Canadian regulatory frame; Wastewater Systems Effluent Regulations, SOR/2012-139 (WSER) for national effluent limits.
Check — conventions used throughout this paper. Concentrations in mg/L are treated as g/m3 throughout, which is exact for dilute aqueous solutions and is what makes the load arithmetic in Questions 3 and 4 one-line conversions. Wastewater flows quoted as m3/d are converted to m3/s with 86 400 s/d and are taken as steady average-day values, since the paper gives no peaking factor. Where a Canadian regulatory number is quoted (WSER, GCDWQ, CEQG) it is named at the point of use; the exam itself sets no jurisdiction, and none of the numerical answers depends on the citation.
Figure 5.1. Single-stage trickling filter with recirculation. Settled wastewater is spread by the rotary distributor over the media bed, oxygen arrives by natural draft through the underdrain, and the sloughed biofilm is captured in the secondary clarifier.
Principle. A trickling filter is an aerobic, attached-growth (fixed-film) biological treatment process, and both words in that description matter. It is attached growth because the micro-organisms live as a biofilm fixed to the surface of a stationary medium rather than in suspension, so the biomass is retained independently of the hydraulic flow and cannot be washed out. It is aerobic because the void space in the bed is never flooded: wastewater passes over the medium as a thin film, the voids stay air-filled, and oxygen reaches the biofilm by diffusion from that air. The name is a historical accident and a persistent source of confusion — the unit does not filter anything. Removal is biological oxidation, not straining.
Working. Settled wastewater from the primary clarifier is applied to the top of the bed through a rotary distributor, usually two or four arms carrying orifices, turning slowly about a centre column. On most units the drive is hydraulic: the reaction of the jets themselves turns the arms, so the distributor has no motor. The applied wastewater breaks into a thin film that trickles down over the medium under gravity. As it descends, soluble and colloidal organic matter and dissolved oxygen diffuse from the liquid film into the biofilm, where heterotrophic bacteria oxidise the organics to carbon dioxide, water and new cell mass; below the depth at which the carbonaceous load has been largely satisfied, slower-growing autotrophic nitrifiers colonise the medium and oxidise ammonia to nitrate. The treated liquid is collected by an underdrain of block or grating construction and carried to a secondary clarifier.
Two features of the working deserve to be understood rather than memorised. The first is ventilation. Oxygen supply is by natural draft, driven by the density difference between the air in the voids and the ambient air, which in turn follows the difference between wastewater and air temperature; the draft reverses direction between summer and winter and passes through zero when the two temperatures are equal. The underdrain and the peripheral vents are the air path, and blocking them — by ponding, by submerging the underdrain in a flooded effluent channel, or by sealing an enclosure without providing forced ventilation — starves the process of oxygen. Because no blowers are needed, a trickling filter uses far less energy than an activated-sludge plant of comparable capacity; that is its principal advantage.
The second is sloughing, which is what makes the secondary clarifier an integral part of the process rather than an optional polish. Oxygen can only penetrate about 0.1 to 0.2 mm into a biofilm. As the film thickens past that depth its base becomes anaerobic, the organisms there lose their capacity to hold on to the medium, and a patch of film detaches and is carried away with the flow. Sloughing is therefore continuous, unavoidable and necessary — it is how the process wastes its excess biomass — and the solids it releases are what the secondary clarifier removes. A trickling filter without a downstream clarifier discharges its own biomass.
The recirculation line shown returning from the clarifier to the distributor performs several duties at once: it keeps the medium continuously wetted when the diurnal low flow would otherwise leave parts of the bed dry, it dilutes a strong or shock influent so that the surface layer is not overloaded, it maintains the distributor rotation speed and the flushing intensity, and it returns some dissolved oxygen and active organisms to the top of the bed. Recirculation ratios of about 0.5 to 3 are usual. The medium itself is either rock of 40 to 80 mm nominal size, giving roughly 40 to 60 m2 of surface per cubic metre in beds of 1 to 2.5 m depth, or moulded plastic cross-flow or vertical-flow modules, giving 90 to 150 m2/m3 at a fraction of the weight, which permits towers up to about 12 m tall on a small footprint. Filters are classified by their loading — low-rate, intermediate, high-rate, roughing and nitrifying — and performance is estimated with empirical formulations such as the NRC, Velz, Schulze and Eckenfelder equations rather than from first principles, because the biofilm kinetics are not tractable analytically.
Common operating issues. The following are the problems that recur across operating plants, with their usual causes and remedies:
Ponding of the bed surface. Excessive biofilm growth, accumulated debris and undersized or degraded medium block the voids, water stands on the surface, and flow short-circuits through the remaining channels. Ponding destroys the ventilation path and turns the affected zone anaerobic. Remedies: increase the flushing intensity by dosing at a higher instantaneous rate, chlorinate the influent for a short period, rake or fork the surface, flood the filter for 24 hours, and in the worst case replace the medium with a larger-graded material.
Odours. Caused by a septic influent arriving from a long collection system, by organic overloading, or by ponding and poor ventilation producing anaerobic conditions with hydrogen sulphide release. Remedies: pre-aerate the influent, increase recirculation, restore ventilation, and if the plant is close to a receiving neighbourhood, cover the unit and treat the exhaust.
Filter flies. The moth fly Psychoda breeds in the drier peripheral and outer-surface zones of the medium, and swarms of the adults are a serious nuisance around the plant. Remedies: maintain continuous wetting of the entire surface, increase the dosing rate, flood the filter for 24 hours to interrupt the life cycle, and keep the walls and surrounding area clear.
Cold-weather performance. The governing issue for Canadian installations. Biological rates fall by roughly 30 % for each 10 °C drop in wastewater temperature (a temperature-correction coefficient of about 1.035), and nitrification suffers far more than carbon removal; ice can form on the distributor arms, on the medium surface and in the effluent channels. A counter-intuitive consequence is that recirculation should be reduced in winter, because a recirculated stream is repeatedly exposed to cold air and cools the wastewater. Remedies: enclose the filter, operate with fewer units at higher hydraulic loading to maintain wetting, use windbreaks, and design for the winter kinetics rather than the summer.
Snail infestation. Common in nitrifying towers, where snails graze the biofilm and remove the slow-growing nitrifiers that took weeks to establish, then pass downstream to abrade pumps and to accumulate in digesters and pipework. Remedies: periodic flooding, controlled dewatering cycles and short chlorination or ammonia-shock treatments.
Distributor problems. Plugged orifices produce dry stripes down the bed; worn seals leak; unbalanced arms and worn bearings stall the rotation at low flow. The distributor requires routine inspection, orifice cleaning and seal maintenance, and hydraulically driven units need a minimum flow — which is another reason recirculation is run at night.
Solids carryover after heavy sloughing. A seasonal, often springtime, mass sloughing event can overload the secondary clarifier and push suspended solids into the effluent. Remedies: adequate clarifier surface area and sludge-withdrawal capacity, and management of the sloughing rate through the recirculation and flushing regime.
Limited process control and sensitivity to load. Unlike activated sludge, where the operator can adjust solids retention time, wasting rate and aeration to reshape the biomass, a trickling filter offers only two levers, recirculation and dosing rate. Its effluent quality is correspondingly harder to fine-tune, and meeting a stringent ammonia or BOD limit often requires a second stage or a downstream suspended-growth polishing step. On the other side of the ledger, the fixed biomass makes the process notably robust against toxic and hydraulic shocks and it recovers quickly after an upset.
Structural and hydraulic deterioration. Rock beds degrade under freeze–thaw and settle over time, plastic media can deform and collapse under the weight of a heavy biofilm if the modules are under-specified, and underdrains silt up. All three are slow failures that show first as ponding.
