In water and wastewater , there are many concepts to learn. Let’s start with the Nature’s way of treating water – the hydrologic cycle.
The hydrologic cycle follows water as it evaporates from the earth's surface, forms clouds, and then falls back to the earth's surface as rain. The diagram below shows an overview of the process.

Since the cycle is continuous, there is no actual beginning or end, so you can begin anywhere. However, there are five basic processes that make up the hydrologic cycle: Condensation, precipitation, infiltration, runoff, and evapotranspiration. Water vapor condenses to form clouds, which result in precipitation when the conditions are suitable. Precipitation falls to the surface and infiltrates the soil or flows to the ocean as runoff. Surface water evaporates, returning moisture to the atmosphere, while plants return water to the atmosphere by transpiration.
Condensation is the process of water changing from a vapor to a liquid. Water vapor in the air rises mostly by convection. This means that warm, humid air will rise, while cooler air will flow downward. As the warmer air rises, the water vapor will lose energy, causing its temperature to drop. The water vapor then has a change of state into liquid or ice. You can see condensation in action whenever you take a cold softdrinks from the refrigerator and set it in a room. Notice how the outside of the bottle can "sweat"? The water doesn't come from inside the can, it comes from the water vapor in the air. As the air cools around the can water droplets form.
Precipitation is water being released from clouds as rain, sleet, snow, or hail. Precipitation begins after water vapor, which has condensed in the atmosphere, becomes too heavy to remain in atmospheric air currents and falls. Under some circumstances precipitation actually evaporates before it reaches the surface. More often, though, precipitation reaches the Earth's surface, adding to the surface water in streams and lakes, or infiltrating the soil to become groundwater.
A portion of the precipitation that reaches the Earth's surface seeps into the ground through the process called infiltration. The amount of water that infiltrates the soil varies with the degree of land slope, the amount and type of vegetation, soil type and rock type, and whether the soil is already saturated by water. The more openings in the surface (cracks, pores, joints) the more infiltration occurs. Water that doesn't infiltrate the soil flows on the surface as runoff.
Precipitation that reaches the surface of the Earth but does not infiltrate the soil is called runoff. Runoff can also come from melted snow and ice. When there is a lot of precipitation, soils become saturated with water. Additional rainfall can no longer enter it. Runoff will eventually drain into creeks, streams, and rivers, adding a large amount of water to the flow. Surface water always travels towards the lowest point possible, usually the oceans. Along the way some water evaporates, percolates into the ground, or is used for agricultural, residential, or industrial purposes.
Evapotranspiration is water evaporating from the ground and transpiration by plants. Evapotranspiration is also the way water vapor re-enters the atmosphere. Evaporation occurs when radiant energy from the sun heats water causing the water molecules to become so active that some of them rise into the atmosphere as vapor. Transpiration occurs when plants take in water through the roots and release it through the leaves, a process that can clean water by removing contaminants and pollution.
As you can see, many processes are at work to give you the water you need. And these processes are always at work. Just because Antarctica is frozen doesn't mean that evaporation is not taking place (ice can turn directly to water vapor by a process called sublimation). And because the Sahara Desert is so dry doesn't mean that precipitation is not happening (it evaporates before it makes it to the ground).

Introduction to Water Treatment
Now that you know how nature treats water, let's see how it's done in the water treatment plant before we consume it in a glass of water.
Water treatment in a typical water treatment plant is shown in the picture below. Based on the characteristics of the raw water and on other factors, this treatment process may vary considerably from place to place.

As water is pumped from the source (a well, spring, river, or lake) it is screened to remove debris. Then, at the water plant, various characteristics of the raw water are tested.
The water may be prechlorinated to kill microorganisms, control odors and taste, and aid in coagulation and settling. The water may also be aerated, which removes carbon dioxide (CO2) and raises pH, oxidizes iron (Fe) and manganese (Mn), removes hydrogen sulfide (H2S), and removes organic contaminants. Potassium permanganate (KMnO4) may be added to the water in the collection tray of the aerator in order to remove iron and manganese from the water. Ozone may be added to the water to treat iron and manganese, remove algae, neutralize hydrogen sulfide (H2S), and aid in flocculation. Many of these are dependent upon the plant and the amount of water being distributed daily.
In most cases, the water then enters a flash mix chamber. Here, various chemicals are added and are mixed into the water. Coagulants cause fine particles to clump together into larger particles. Alkali are added to adjust the pH as well as to oxidize iron and manganese. Hexametaphosphate may be added to prevent corrosion of pipes.
After flowing out of the flash mix chamber, the water goes through a chamber which causes coagulation and flocculation to occur. Here, the fine particles of contaminants gather together into large clumps called floc. When the water flows into the sedimentation basin, some of the floc settles out of the water and is removed. Next, the water is passed through filters which remove particles too small to settle out in the sedimentation basin.
Finally, chlorine is added to the water. The water may also be fluoridated to reduce tooth decay in the consumers. The water is left in the clear well for a period of time to allow the chlorine to kill bacteria in the water and to oxidize hydrogen sulfide. The water is now treated and ready to be distributed.
That was a quick overview of the water treatment process and what happens to water as it goes through the plant step by step. We will get into greater detail in later lessons so that you will understand each step of the process, why it is done and how to make sure each step is being done efficiently.

Souce: EnvET 120 Lecture
(Introduction to Wastewater Treatment & Disposal) Lesson 1