Question 4 of 5: Biological and Physical Treatment Unit Processes

Question 4: Biological and Physical Treatment Unit Processes (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. Anaerobic Sludge Digestion (6 marks)

Anaerobic sludge digestion biologically stabilizes primary and waste-activated sludge solids in an oxygen-free reactor through a consortium of anaerobic and facultative organisms working in four sequential stages: hydrolysis (extracellular enzymes break complex carbohydrates, proteins and lipids into soluble monomers), acidogenesis (fermentation of those monomers to volatile fatty acids, alcohols, CO₂ and H₂), acetogenesis (VFAs converted to acetate, H₂ and CO₂), and methanogenesis (strict anaerobes convert acetate and H₂/CO₂ to methane and CO₂ — the slowest, most sensitive step, and the one that governs digester design). Typical operation is single- or two-stage complete-mix, mesophilic (≈35°C, SRT 15–20 d) or thermophilic (≈55°C); the process destroys roughly 40–60% of volatile solids, reduces pathogens and odour, and produces biogas (60–65% methane) usable as an energy source. Stability is tracked via pH (near-neutral, 6.8–7.2, since methanogens are pH-sensitive) and the volatile-acids-to-alkalinity ratio, since VFA accumulation signals an overloaded or "souring" digester.

b. Filter Headloss, Schmutzdecke and Filter Backwash (6 marks)

Filter headloss is the pressure drop across a granular filter bed as water passes through it, rising over the length of a filter run as captured solids progressively narrow the pore spaces (clean-bed headloss follows the Kozeny–Carman/Rose relation; the rise above that baseline tracks accumulated solids). A filter is taken offline for cleaning once headloss reaches a set terminal value, or a turbidity/time criterion is met. Schmutzdecke ("dirty skin") is the thin, biologically active mat of trapped solids, algae and microorganisms that forms on the surface of a slow sand filter — not a rapid filter, which is cleaned too frequently for a mature biofilm to develop; it is the primary treatment mechanism in slow sand filtration (biological degradation plus straining) and must be allowed to mature ("ripen") after each cleaning before effluent quality is acceptable. Filter backwash is the cleaning process itself: flow is reversed and pumped upward through the bed at a rate sufficient to fluidize/expand the media (often paired with an air scour), shearing off and washing out the accumulated solids and restoring headloss and capacity; the spent backwash water is collected and typically returned to the head of the plant.

c. Biomass Sloughing in a Trickling Filter (7 marks)

In a trickling filter, wastewater trickles over a fixed rock or plastic media coated with an attached biofilm. As the biofilm thickens it draws substrate from the flowing liquid film faster than it can diffuse to the organisms nearest the media surface; the innermost layer eventually loses access to substrate and oxygen, dies, and loses adhesion to the media. Combined with the continuous hydraulic shear of the flowing water, chunks of the weakened biofilm periodically detach and are carried out in the effluent — this detachment is sloughing. Sloughing is a natural, essential part of trickling-filter operation, preventing indefinite biofilm thickening and eventual ponding/clogging of the media, but it discharges solids that must be captured downstream; a trickling filter is therefore always followed by a secondary (humus) clarifier. Sloughing rate increases with hydraulic and organic loading and, for rock-media filters, is often more pronounced or pulsed following periods of high flow (e.g., recirculation surges after storm events) or heavy grazing by filter-fly larvae.

d. HRT and SRT in Biological Treatment Systems (6 marks)

Hydraulic retention time (HRT, $\theta$) is tank volume divided by influent flow rate, $V/Q$ — the average time the water/wastewater itself spends in the reactor. Solids retention time (SRT, $\theta_c$, "sludge age") is the mass of solids in the system divided by the mass of solids wasted per day — the average time the biomass spends in the system. In a reactor with no solids recycle these are equal; the defining feature of activated sludge and most modern biological processes is that settled solids are recycled (RAS) from the secondary clarifier back to the aeration tank while a separately controlled stream (WAS) is removed, which decouples SRT from HRT — SRT can then be made far longer than HRT (typically 5–20+ days versus a few hours), letting slow-growing organisms, notably nitrifiers, be retained and reach steady state even though the liquid itself passes through quickly. SRT is the primary design and control lever for effluent quality (degree of stabilization, nitrification capability, sludge settleability) while HRT mainly sets the reactor volume required for a given flow.