Question 5 of 5: Brief Descriptions — Water and Wastewater Treatment Concepts

Question 5: Brief Descriptions — Water and Wastewater Treatment Concepts (25 marks: a 6, b 7, c 6, d 6)

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. Design and operating principle of aerated grit removal tanks (6 marks)

An aerated grit chamber is a rectangular tank fitted with diffused-air piping along one side, arranged to induce a helical (spiral) roll pattern across the tank's cross-section rather than a simple straight-through flow. The air roll velocity, not the forward (hydraulic) flow, controls the scour/settling velocity acting on particles, so the design is independent of the plant's flow rate over a fairly wide range — a key advantage over velocity-controlled channels, which must be re-sized or split into multiple velocity-controlled units to track flow variation. Air rate is set (typically about 0.2–0.5 m³/min per metre of tank length) so that dense grit (specific gravity ∼2.65) settles to a hopper at the tank base while the lighter organic solids stay in suspension and carry through to the next unit; a screw or bucket-elevator mechanism periodically removes settled grit from the hopper for washing and disposal. Because the roll velocity (not throughput) governs separation, aerated units also provide some pre-aeration/odour-control benefit and tolerate flow surges better than a simple channel.

b. Ion exchange process in water treatment (7 marks)

Ion exchange removes dissolved ionic species by passing water through a bed of resin beads carrying a fixed, exchangeable counter-ion; as water percolates through, target ions in solution swap places with the resin's counter-ion on a stoichiometric (equivalent-for-equivalent) basis. In the classic water-softening application, a cation-exchange resin in the sodium form exchanges hardness cations for sodium: $\text{Resin-Na}_2+Ca^{2+}\rightarrow\text{Resin-Ca}+2Na^+$ (and similarly for $Mg^{2+}$), removing hardness without any chemical sludge. The resin's exchange capacity is finite and is progressively exhausted as the bed loads with hardness ions, at which point hardness begins to break through the effluent and the bed must be regenerated — typically backwashed to remove trapped solids and re-expand the bed, then rinsed with a concentrated brine (NaCl) solution that drives the reverse reaction, restoring the resin to the sodium form (the displaced $Ca^{2+}/Mg^{2+}$ leave in the spent regenerant, the process's main waste-disposal concern). Ion exchange is also used for nitrate removal, arsenic removal (anion resins) and specialty demineralization, but is generally uneconomical at high total dissolved solids because of the correspondingly frequent regeneration it would require.

c. Fluoridation and defluoridation (6 marks)

Fluoridation is the controlled addition of a fluoride compound (commonly fluorosilicic acid, sodium fluorosilicate, or sodium fluoride) to drinking water to raise its fluoride concentration to an optimal level (roughly 0.7 mg/L in most current Canadian guidance) for reducing dental caries, since fluoride incorporated into developing tooth enamel makes it more resistant to acid demineralization; dosing is carefully controlled because excess intake causes dental (and at much higher levels, skeletal) fluorosis. Defluoridation is the reverse problem: removing naturally-occurring excess fluoride (common in some groundwater sources, sometimes several mg/L or more, well above the Canadian aesthetic/health guideline) down to a safe level, typically by adsorption onto activated alumina or bone char, by precipitation with lime/alum (co-precipitation as calcium fluoride or aluminum fluoride complexes), or by ion exchange/reverse osmosis for smaller or higher-strength sources; the appropriate method is chosen based on the source's fluoride concentration, competing-ion chemistry and the scale of the supply.

d. Sludge volume index and sludge bulking (6 marks)

The sludge volume index (SVI) is an operational settleability metric for activated-sludge mixed liquor: a sample is allowed to settle for 30 minutes in a 1-litre graduated cylinder, and $$SVI=\frac{\text{settled sludge volume (mL/L)}\times1000}{\text{MLSS (mg/L)}},$$ giving the volume (mL) occupied by one gram of dry solids after settling. A healthy, well-settling floc typically has SVI in the range of roughly 80–150 mL/g; values much higher than that indicate poor settleability. Sludge bulking is the operational problem this high-SVI condition describes — the sludge settles slowly and compacts poorly in the secondary clarifier, risking solids carryover in the effluent and loss of return-activated-sludge concentration. The most common cause is filamentous bulking: filamentous organisms (e.g. Sphaerotilus natans, type 1701, Nocardia) grow out from the floc surface and physically interfere with compaction and interlock adjacent floc particles, often triggered by low dissolved oxygen, low food-to-microorganism (F/M) ratio, septic/low-nutrient influent, or low pH; a less common cause is viscous (non-filamentous, zoogloeal) bulking, a slimy overproduction of extracellular polymer under nutrient-deficient or overloaded conditions. Control is typically by correcting the underlying cause (raising DO, adjusting F/M via wasting rate, adding a limiting nutrient) or, as a short-term fix, selective chlorination of the return sludge to suppress filaments.