Q.The primary treatment of waste water involves the removal of:

Primary treatment of waste water is a purely physical process designed to remove materials that can be separated by simple mechanical means. It tackles the coarse and suspended solids—things that float or settle easily—not the dissolved or biological contaminants.

The process uses screening to filter out large debris (rags, sticks, cans) and sedimentation in settling tanks where grit and suspended particles settle to the bottom as primary sludge, while oils and grease float to the top as scum. Both are removed, leaving the water clearer but still rich in dissolved organic matter and microbes.

• (A) Dissolved impurities remain untouched; they need biological treatment (secondary stage) to be broken down.
• (B) Stable particles—the settleable and floatable solids—are exactly what primary treatment removes. …

Wastewater treatment is designed as a multi-stage process, each targeting different categories of pollutants. Understanding what primary treatment does requires seeing where it fits in the overall scheme: it is the first major step after preliminary screening, and it relies entirely on physical processes rather than biological or chemical intervention.

When wastewater enters a treatment plant, it carries a mixture of contaminants — large debris, suspended solids, dissolved organic and inorganic matter, pathogens, and sometimes toxic chemicals. The preliminary stage removes coarse materials (rags, sticks, cans) through bar screens and grit chambers. Primary treatment then takes over to deal with the next layer of pollutants: the stable particles that will settle or float given enough time and calm conditions.

The heart of primary treatment is the primary settling tank or sedimentation tank. Here, wastewater flows slowly through large basins. Heavier suspended solids — soil particles, food waste, faecal matter — settle to the bottom as primary sludge. Lighter materials like oils and grease float to the surface as scum, which is skimmed off. Both are removed mechanically. This process is purely physical: gravity does the work, separating particles based on density. No chemicals are added, no microbes are cultivated — just time, stillness, and the natural tendency of solids to settle.

Ask 'would this survive if the water just sat still for a while?' -- solids that settle or float under gravity alone (stable particles) are exactly what a purely physical, no-chemicals-no-microbes stage can remove. Dissolved substances, toxin …