Hello, I'm Sper3099. I would like to introduce you to a subject called water and wastewater treatment. In water and wastewater, there are many concepts to learn so I thought I would start out with the way nature treats 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.
Evaporation and Rainfall
Water evaporates into the air due to the sun's warming the water and changing it into a gas. The water escapes from plants as they breathe (a process called transpiration) or evaporates from surface water. After evaporating the moisture disperses into the atmosphere and forms particles which are collected in clouds.
Fog is an example of a cloud next to the earth's surface. As you drive through fog, you can tell that there is water in the air because droplets collect on the windshield as a result of the water's attraction (affinity) to the glass.
Water must collect together before it can fall out of the air. If clouds are in a dusty region, water will collect on the dust in the air and cause precipitation. For example, in times of war there is often a greater than usual amount of rainfall because explosions on the ground throw dust up into the atmosphere.
Dust particles form the centers of raindrops, but these drops usually fall only when the clouds are cooled. As air cools, it becomes supersaturated, meaning that the air has more water in it than it can hold. Clouds cool and become supersaturated when they gain in elevation or come in contact with a cold front. When clouds are forced over mountains they become extremely supersaturated and enormous amounts of precipitation can occur. On one of the Hawaiian Islands, approximately 500 inches of rain may fall every year.
Purification in the Sky
Evaporating water can carry contaminants such as bacteria into the air with it. However, the bacteria are killed by two substances: ultraviolet light and ozone. As a result, the rain falling out of the clouds is usually pure until it nears the earth's surface.
Although we cannot see ultraviolet light, also known as UV light, it causes sunburn in humans. Greater than usual amounts of UV light can kill both plants and animals. Luckily, most UV light penetrates into clouds to kill bacteria.
The same ozone which prevents most UV light from reaching the earth's surface is also a potent killer of bacteria. Ozone kills microorganisms in the atmosphere about 30,000 times faster than chlorine can kill microorganisms in water.
Whereas UV light shines down on the atmosphere in an endless stream from the sun, ozone must be produced. Energy from cloud movement causes ions (charged particles) to separate and the ionic charge released produces lightning. When lightning occurs, ozone and a soluble form of nitrogen are made.
Ozone production occurs primarily during warm weather. In cold weather, there is no need for the ions to separate as much. Thus, there are very few static charges in the clouds to create ozone. But the UV light and the cold temperatures still kill microorganisms.
Bacterial Contamination
In temperate conditions ozone and UV light cleanse the cloud of bacterial contamination. But once rain begins to fall, the rain drops start picking up contaminants, such as bacteria and dust, from the air. The bacterial contamination is all around us. For proof, expose some agar to air and you will see that it grows microorganisms. Agar is a gelatin-like food designed to grow only certain groups of organisms.
Some of the falling rainwater evaporates back into the air due to the saturation of the soil and the warmth of the earth. Other water is retained by the vegetation or trickles down through the soil. This groundwater is cleaned by filtering slowly through the earth's crust. Still other water runs across the earth's surface and ends up in rivers and streams.
As water runs across the earth's surface, it picks up any contaminants it comes in contact with, including bacteria and organic matter such as ammonia. Surface areas have high bacterial counts and bacteria are easily washed into rivers and streams. Some of these contaminants are harmful to humans. For example, TB is a disease which can be transmitted through food, animals, water, and soil.
But many of the bacteria which enter surface waters are not harmful. Instead, they eat the organic matter that is suspended in the water and start using up oxygen dissolved in the water. This action raises the biological oxygen demand (BOD) of surface waters. This is also how the natural world starts the process of water treatment.
Natural Water Treatment
By the time rain water reaches streams, it has picked up microorganisms, organic matter (such as ammonia), and materials of various sizes (such as dirt and rocks). Natural processes remove each type of contaminant from the water.
The materials suspended in the water come in sizes ranging from tiny particles of mud to large rocks. Over time, the materials and sediments within the water tend to settle selectively by velocity, as shown in the illustration below. Quickly moving water can carry large objects with it, but the largest rocks will soon fall to the bottom of the stream and stay there. As a stream leaves rapids and begins to move more slowly, smaller objects, such as pebbles, settle to the bottom. Even slower water drops sand. Finally, when the stream reaches a basin or pond where the water moves very slowly, sediments settle to the bottom and form mud.
The removal of organic matter begins in these impoundments of still water. Here, microorganisms begin to eat the organic matter. In the process, they use up oxygen in the water.
As water leaves these impoundments, it often flows over rocks, increasing the amount of water surface coming in contact with the air. The increased surface area enables the water to pick up oxygen to replace the oxygen used by the microorganisms. Carbon dioxide, a waste product from the microorganisms, is also released here. As the carbon dioxide is driven off, pH readings rise in the water.
Contaminants are also removed as the water flows over rocks in streams. In a living stream, rocks develop a slick coating because of the microorganisms that attach to them. So a living stream can be determined by the presence of slick rocks and plant growth.
In a healthy stream, the microorganisms are able to convert ammonia into nitrates. Plants in the stream absorb the nitrates which they in turn use for food.
So ammonia and sediments are both removed from the water during the natural flow of a stream. But microorganisms remain in the water until it flows to the ocean. There, salt kills most of the freshwater microorganisms.
Buffers
As I mentioned above, whenever carbon dioxide is released into water or removed from water, the pH of the water changes. Most of the living things in the stream, including the microorganisms which digest organic matter, are sensitive to the stream's pH. Changing pH can kill many of these organisms.
But in a natural system, the pH of the water does not usually change dramatically. Buffers in the water prevent it from becoming very acidic or basic. A buffer can best be described as an acid and a base mixed together. As the pH of the water begins to change, the buffer neutralizes the change. As a result, the microorganisms survive.
Buffer: A solution or liquid whose chemical makeup neutralizes acids or bases without a great change in pH. Any of certain combinations of chemicals used to stabilize the pH values or alkalinities of solutions.
Buffer Capacity: A measure of the capacity of a solution or liquid to neutralize acids or bases. This is a measure of the capacity of water or wastewater for offering a resistance to changes in pH.
Taken From: College Chemistry, 1980 LC 79-88562 page 427:
Mixtures of weak acids and their salts or mixtures of weak bases and their salts are called buffer solutions. They resist a change in hydrogen-ion concentration upon the addition of small amounts of acids or bases.
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