Question 5 of 5: Brief Descriptions — Water Treatment Concepts

Question 5: Brief Descriptions — Water Treatment Concepts (25 marks: a–e 5 each)

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) Jar Tests for Optimum Coagulant Dose

The jar test is a bench-scale simulation of the plant's rapid-mix/flocculation/sedimentation sequence, run in a multi-station gang stirrer holding a set of identical raw-water samples (typically 1 L jars). Each jar receives a different coagulant dose (and, in a companion series, a different pH); the stirrer runs a short high-speed phase (simulating rapid mix, seconds to about a minute) followed by a longer low-speed phase (simulating flocculation, roughly 15–30 minutes), then the paddles stop and the flocs are allowed to settle for a fixed period (simulating the sedimentation basin). The operator then compares the jars for floc size and settling characteristics, measures residual turbidity (and often residual coagulant, pH, colour, or UV254/TOC as a natural-organic-matter surrogate) in each settled supernatant, and plots these responses against applied dose; the optimum coagulant dose is the lowest dose that achieves the required residual turbidity (or the dose at the "knee" of the turbidity-versus-dose curve, beyond which further coagulant gives little additional improvement, or in some cases re-stabilizes the colloids through charge reversal and turbidity rises again). Because raw-water quality (turbidity, alkalinity, temperature, natural organic matter) drifts seasonally, the jar test is run routinely, not just once, so the full-scale coagulant dose can be adjusted to track it.

(b) Filter Headloss, Schmutzdecke and Filter Backwash

Filter headloss is the pressure (hydraulic-grade-line) drop across a granular filter bed, which rises steadily over a filter run as captured particles accumulate in the pore spaces and progressively narrow the flow channels through the media; a filter run is normally terminated (and backwashed) either when headloss reaches the maximum the plant's hydraulic profile allows (terminal headloss) or when effluent turbidity begins to break through, whichever occurs first. Schmutzdecke ("dirty skin," German) is the thin biologically active mat of trapped floc, algae, and microorganisms that forms on the top surface of a slow sand filter; it is the layer that actually performs most of the filtration and even some biological treatment (organic matter/ammonia degradation) in a slow sand filter, and is deliberately scraped off (rather than backwashed away) when it becomes too thick, since slow sand filters are not designed for hydraulic backwashing. Filter backwash is the cleaning process used on rapid (granular) filters: flow is reversed and pumped upward through the bed at a rate sufficient to fluidize and expand the media (often combined with an air scour step), scrubbing the captured floc off the grains and carrying it out to waste, restoring the bed to a low-headloss, clean-media condition before the next filter run begins.

(c) Discrete Settling and Flocculent Settling

Discrete (Type I) settling describes particles that settle as individual, unchanging entities — they do not coalesce, flocculate, or change size, shape, or density as they fall, so each particle settles at its own constant terminal (Stokes'-law) velocity independent of its neighbours and independent of depth; this is the idealization used for grit-chamber design (dense, discrete sand and inert particles). Flocculent (Type II) settling describes particles that DO collide and coalesce as they settle — typically dilute chemical floc (post-coagulation) or biological floc — so particle size, and therefore settling velocity, increases continuously with depth and detention time as smaller flocs merge into larger ones; because velocity is not constant, flocculent settling cannot be sized from a single terminal velocity the way discrete settling can, and instead requires settling-column test data (percent removal at various depth/time combinations) to size the tank. This is the dominant settling mode in primary clarifiers treating coagulated water and in the upper zone of most conventional sedimentation basins.

(d) Organic Compounds, Ammonia and Chlorination-Based Disinfection

Organic compounds (natural organic matter, humic and fulvic acids, algal metabolites) matter to chlorination in two distinct ways: they exert an immediate and ongoing chlorine demand (Zone I/continuing oxidation of the chlorination curve), diverting applied chlorine away from disinfection and forcing a higher dose to reach a target free residual; and they react with chlorine to form regulated disinfection by-products (trihalomethanes, haloacetic acids), so a water with a high organic-matter content faces a direct trade-off between dosing enough chlorine to disinfect adequately and limiting DBP formation. Ammonia matters because it reacts preferentially with chlorine to form chloramines (combined residual) rather than leaving free chlorine available — a fixed applied dose in an ammonia-bearing water yields far less disinfecting power than the same dose in an ammonia-free water, because combined chlorine is roughly 1/25 to 1/100 as strong a disinfectant as free chlorine per unit residual. Together, organics and ammonia are why the required chlorine dose for a given target free residual and CT (concentration × time) disinfection credit is set from the specific raw-water demand curve, never from a generic textbook dose.

(e) UV Disinfection vs. Chlorination — Advantages and Disadvantages

Two advantages of UV over chlorination: (1) UV is highly effective against chlorine-resistant protozoan cysts/oocysts — Cryptosporidium and Giardia — which are far more resistant to chlorine at practical CT values than they are to UV inactivation, making UV the preferred barrier for these organisms in many modern treatment trains. (2) UV disinfection produces no regulated disinfection by-products (no trihalomethanes/haloacetic acids), because it inactivates organisms physically (UV-induced DNA/RNA damage) rather than through an oxidizing chemical reaction with organic matter, so it does not carry chlorine's DBP-formation trade-off. Two disadvantages of UV relative to chlorination: (1) UV provides no persistent residual in the distribution system — once water leaves the UV reactor there is no ongoing disinfecting capability to guard against regrowth or a distribution-system intrusion, so a utility using UV as its primary disinfectant must still add a secondary chemical (typically a small chlorine or chloramine dose) purely for residual maintenance. (2) UV effectiveness depends strongly on water clarity/UV transmittance — particulate matter and elevated turbidity or colour can shield organisms from the UV dose (and can foul the quartz sleeves around the lamps), so UV performance is far more sensitive to upstream treatment quality (and to routine sleeve cleaning/lamp maintenance) than chlorine dosing is.