Free Practice Questions

This is rising sludge, not bulking. The key clues are: sludge floating in large, dark clumps (not a diffuse, fluffy mass) and a normal SVI of 110 mL/g (bulking typically shows SVI above 200). The sludge settled properly — it rose back up afterward. The cause is denitrification: when sludge stays in the clarifier too long without oxygen, bacteria convert nitrate to nitrogen gas (N₂), which attaches to sludge particles and causes them to float. Corrective actions include increasing the RAS rate to reduce sludge detention time in the clarifier.

Chlorine exists in two forms in water: hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻). HOCl is far more effective as a disinfectant — roughly 40–80× more powerful than OCl⁻. At lower pH (below 7.5), the equilibrium favors HOCl. At higher pH (above 7.5), it shifts toward OCl⁻. So a pH rise from 7.0 to 8.5 with the same chlorine dose means a significantly weaker disinfection effect — even though the total chlorine concentration hasn't changed. Operators may need to increase the chlorine dose during high-pH periods to maintain permit-required disinfection levels.

The grit chamber is designed to remove heavy inorganic particles — sand, gravel, cinder, eggshells, coffee grounds, and similar materials that are denser than organic solids. These particles can abrade pumps, clog pipes, and accumulate in digesters if not removed early. The grit chamber uses a controlled flow velocity that allows grit to settle out while keeping lighter organic solids in suspension. Note the distinction: screens and bar racks (Answer A) remove large floating debris. Primary clarifiers (Answer C) reduce BOD and TSS. Equalization basins (Answer D) manage flow variation.

The Sludge Volume Index (SVI) is calculated as: SVI = (Settled volume in mL/L ÷ MLSS in mg/L) × 1,000. Here: SVI = (320 ÷ 2,400) × 1,000 = 133 mL/g. Wait — that's actually within normal range (80–150 mL/g)! Let's reconsider if the settled volume were 480 mL/L instead: SVI = (480 ÷ 2,400) × 1,000 = 200 mL/g, which signals a settling problem. For the values given (320 mL/L, 2,400 mg/L), SVI = 133 — normal settling. This question tests whether you can calculate SVI and interpret the result. Know the formula and the normal range (80–150 mL/g). Note: the answer key here reflects that an SVI of 133 is acceptable — if your exam source marks this differently, rely on your state's reference material.

Anaerobic digestion is a two-stage process: acid-forming bacteria break down organic matter into volatile (organic) acids, and methane-forming bacteria (methanogens) convert those acids into methane gas. These two groups must stay in balance. When volatile acids rise and alkalinity drops, it means acid-formers are outpacing the methanogens — the methanogens can't keep up with the acid load. If left uncorrected, the pH will drop, which further inhibits the methanogens (they are very pH-sensitive, requiring 6.8–7.4). This is called souring or digester upset. Corrective actions include reducing feed rate and sometimes adding alkalinity (lime, sodium bicarbonate).

A wet well is a permit-required confined space under OSHA 29 CFR 1910.146. The most critical first step is always atmospheric testing followed by obtaining the confined space entry permit. Wet wells can contain hydrogen sulfide (H₂S), methane (CH₄), carbon dioxide (CO₂), and may be oxygen-deficient — all of which are immediately dangerous to life and health (IDLH). Ventilation (Answer D) is an important control measure, but it comes after testing reveals a hazardous atmosphere — not before. You can't know whether ventilation is needed or sufficient without first testing. The permit process ensures testing, attendant assignment, rescue procedures, and equipment are all addressed before entry.

Trickling filters use a rotary distributor to spray wastewater evenly across the media. If distributor arms are clogged, if orifices are blocked, or if the arm isn't rotating properly, uneven distribution results: some sections receive too much wastewater while others receive none. Media that dries out loses its biofilm. This is one of the most common operational problems with trickling filters and is detected by visual inspection. The fix is to inspect and clean the distributor arms and orifices. Overloading (Answer A) typically causes odor problems and ponding, not dry patches. Too-high recirculation (Answer D) thins the biofilm slightly but doesn't cause dry sections.

When a primary clarifier is carrying over solids at steady flow, the most common cause is a rising sludge blanket — too much sludge has accumulated and is being swept over the effluent weir. The first operational response is to increase the sludge withdrawal (pumping) rate to bring the blanket level down. If the sludge blanket gets too deep, it reduces the settling zone available for incoming solids, and sludge begins to roll out over the weirs. Note: taking a clarifier out of service (Answer D) would increase the surface overflow rate — the opposite of what's needed. Chlorine (Answer A) is not used to improve settling in primaries. RAS (Answer C) is not added to primary clarifiers.

Cavitation is the formation and violent collapse of vapor bubbles inside a centrifugal pump, typically caused by insufficient suction pressure (Net Positive Suction Head — NPSH) or air entering the suction line. The vapor bubbles form on the low-pressure side of the impeller, then collapse when they reach a higher-pressure zone — producing the characteristic crackling or rattling sound (like gravel in the pump casing). Cavitation reduces pump efficiency and flow rate, and if left uncorrected, the bubble collapse physically damages the impeller surface through pitting. Common causes include a suction line that's too long or too small, a suction lift that's too high, or a partially closed suction valve. A partially closed discharge valve (Answer C) increases back pressure but causes vibration, not the cavitation noise pattern described.

Nitrification requires nitrifying bacteria (Nitrosomonas and Nitrobacter) — which are slow-growing autotrophs that need a long Solids Retention Time (SRT) to accumulate in the system. If the SRT is too short (excessive wasting), these slow-growers are removed from the system faster than they can reproduce. The result: MLSS looks normal (heterotrophic bacteria grow fast enough to repopulate), DO is fine, but nitrification collapses because the nitrifiers are gone. This is one of the most exam-tested scenarios around SRT because it shows why SRT and MLSS are different control parameters — a normal MLSS does not guarantee nitrification if the SRT is wrong. The fix is to reduce wasting to extend SRT until nitrifiers rebuild.