What natural source of contamination is most commonly removed through pre-treatment processes?

The most commonly removed natural source of contamination through pre-treatment processes is gravel and sand. During water treatment, pre-treatment processes are designed to protect downstream processes and remove larger particles that can interfere with subsequent treatment stages. Gravel and sand are typical constituents of sediment that can enter a water supply from natural sources, making their removal critical to ensure that the water is clear and that the equipment used in further treatment processes is not damaged.

Pre-treatment typically involves methods such as screening, sedimentation, or filtration, which are specifically aimed at removing these larger particulates before the water undergoes more intensive treatment. This ensures that the quality of the water is improved and protects subsequent treatment methods from potential clogging or abrasion caused by these larger materials.

The other sources of contamination listed—microorganisms, nutrients, and chemicals—are usually addressed in later stages of water treatment rather than during pre-treatment. Microorganisms require disinfection processes, nutrients often necessitate biological treatment, and chemicals might involve more complex chemical treatment techniques. Thus, gravel and sand as natural particulate contaminants are primarily handled during pre-treatment.