Grit removal targets heavy, inorganic solids that would threaten equipment if left in the flow. Sand, gravel, and similar grit are dense enough to settle quickly and cause abrasion and wear on pumps, valves, and piping, and they can clog downstream channels. By pulling these solids out early—before biological treatment and pumps—the plant protects equipment, reduces maintenance, and helps keep downstream processes running smoothly. The other options describe goals that grit removal doesn’t directly address: reducing dissolved organic matter to lower BOD happens in biological treatment; capturing readily biodegradable suspended solids happens in the aeration basin; and disinfection is a separate, downstream step.

Hydrogen sulfide in wastewater plants is a hazardous gas that is both toxic and flammable. It can cause severe health effects if inhaled, ranging from irritation of the eyes and airways to dizziness, unconsciousness, or even death at higher exposures. It also poses an explosion risk in confined spaces and can be corrosive to equipment. Because people can lose their sense of smell at higher concentrations, relying on odor alone is not a safe way to judge danger. Controlling it requires a layered approach: continuous gas monitoring with alarms to detect rising concentrations; proper ventilation to dilute and remove the gas; odor control measures at sources or exhaust points to reduce emissions; and safe working practices, including confined-space procedures, proper PPE, training, and controls to prevent ignition. This combination directly protects workers and reduces both exposure and ignition risks, making it the best approach. Other options miss the mark because hydrogen sulfide is indeed hazardous, not nonhazardous; it affects more than just pipe corrosion, and simply flushing with water does not address the health and safety risks. Adding fragrance to mask odor does not mitigate toxicity and can give a false sense of safety.

In an activated sludge plant, the last stage that separates the treated liquid from the biomass is the final clarifier. After the mixed liquor is aerated and biologically treated, it flows into this settling basin where solids settle by gravity. The clarified liquid then proceeds to disinfection or out to the effluent, while the settled solids are routed to sludge handling (return and waste activated sludge). The aeration basin provides the oxygen and mixing for biological treatment, not settling, and a sludge digester handles stabilization and storage of sludge, not the final separation of solids from the treated water.

Waste Activated Sludge is removed from the mixed liquor to keep the solids concentration, MLSS, at the proper level. This control is essential because too many suspended solids in the aeration basin slows oxygen transfer, harms sludge settling, and reduces treatment efficiency. By removing excess sludge, the system maintains stable biomass conditions for effective organic matter removal and consistent effluent quality. The solids that are removed are typically thickened and sent for digestion or disposal; returning solids to the basin is done by the return activated sludge, not WAS.