| Basic Terms in Water and Wastewater Treatment |
Drinking water:
Drinking water quality is regulated. In many jurisdictions, standards are set for approximately 90 contaminants in drinking water. For each of these contaminants a legal limit, called a maximum contaminant level, is set and if that limit is exceeded, it requires a certain treatment. Water suppliers will not provide water that doesn't meet the drinking water standards. Water that meets these standards is safe to drink and every water utility providing drinking water sees that this standard is maintained.
Water Treatment Plant:
A water treatment plant should produces drinking water that is free of bacteria, sparkling clean and without an objectionable taste or odor. Depending on the location of the well water or reservoir, the quality of the untreated water, and the local codes and standards for water quality, the plant may put the water through several treatments. Although treatment processes vary from plant to plant, there are three basic processes: coagulation and settling, filtration and disinfection. There are a number of ways to purify water. Chemical purification of above ground water is mandatory in all states. Since most areas lack alternate filtering mechanisms, such as ozone treatment, chlorine is used as the most common sterilizing agent.
Wastewater Treatment Plant:
The process carried out in treatment of wastewater is briefed below:
a. Screening:
Wastewater entering the wastewater treatment plant includes items like wood, rocks, even dead animals. If they are not removed, they could cause problems later in the treatment process. Most of these materials are sent to a landfill.
b. Pumping:
Wastewater treatment plants are located on low ground, often near a river into which treated water can be released. If the plant is built above ground level, the wastewater has to be pumped up to the aeration tanks. From here on, gravity takes over to move the wastewater through the treatment process.
c. Aerating:
One of the first steps that a wastewater treatment facility can do is to just shake up the sewage and expose it to air. This causes some of the dissolved gases (such as hydrogen sulfide, which smells like rotten eggs) that taste and smell bad to be released from the water. Wastewater enters a series of long, parallel concrete tanks. Each tank is divided into two sections. In the first section, air is pumped through the water. In the second section…(please finish the point).
As organic matter decays, it uses up oxygen. Aeration replenishes the oxygen. Bubbling oxygen through the water also keeps the organic material suspended while it forces 'grit' (sand and other small, dense particles) to settle out. Grit is pumped out of the tanks and taken to landfills.
d. Sludge removal:
Wastewater enters the second section or sedimentation tanks. Here, the sludge (the organic portion of the sewage) settles out of the wastewater and is pumped out of the tanks. Some of the water is removed in a step called thickening and then the sludge is processed in large tanks called digesters.
e. Scum removal:
As sludge settles at the bottom of the sedimentation tanks, lighter materials floats up to the surface. This ‘scum’ includes grease, oils, plastics, and soap. Slow-moving rakes skim the scum off the surface of the wastewater. Scum is thickened and pumped to the digesters along with the sludge. Filtration is also used in sewage treatment in many cities.
The liquid sewage is filtered through a substance, usually sand, by the action of gravity. This method gets rid of almost all bacteria, reduces turbidity and color, removes odors, reduces the amount of iron, and removes most other solid particles that remained in the water.
f. Killing bacteria:
Wastewater flows finally into a 'chlorine contact' tank, where the chemical chlorine is added to kill bacteria, which could pose a health risk. The chlorine is mostly eliminated as the bacteria are destroyed, but sometimes it must be neutralized by adding other chemicals. This protects fish and other marine organisms, which can be harmed by the smallest amounts of chlorine. The treated water (called effluent) is then discharged to a local river or the ocean.
Desalination
Desalination is the process of converting saline water into fresh, drinkable water. Seawater desalination is a process that removes salts and other impurities from seawater. This process is used in many areas of the world where fresh water is scarce. Seawater desalination produces very safe, high-quality water suitable for drinking and other potable water uses. The future seawater desalination facilities will use reverse osmosis technology, regarded as the most efficient and cost-effective desalination process. Reverse osmosis uses semi permeable membranes to separate fresh water from seawater.
Reverse Osmosis
Reverse osmosis is often used in commercial and residential water filtration. It is also one of the methods used to desalinate seawater. Sometimes reverse osmosis is used to purify liquids in which water is an undesirable impurity. Reverse osmosis occurs when the water is moved across the membrane against the concentration gradient, from lower concentration to higher concentration. To illustrate, imagine a semi permeable membrane with fresh water on one side and a concentrated aqueous solution on the other side. If normal osmosis takes place, the fresh water will cross the membrane to dilute the concentrated solution. In reverse osmosis, pressure is exerted on the side with the concentrated solution to force the water molecules across the membrane to the fresh water side.
Water Meter
A water meter measures the quantity of water passing through a particular outlet. The readings from your water meter determine the amount you are charged every month on your water bill. There are two basic types of water meters - straight reading and circular reading. The straight reading meter records the cubic feet of water used in much the same way that a car's odometer records miles. The dial with a single hand measures tenths of a cubic foot. The circular reading meter uses a series of circular dials to record cubic feet of water used. To read this type of meter, start with the 100,000 circle, then read the 10,000 circle, and so forth on down to the circle that reads in 1 cubic foot increments. If a hand is between two numbers, always read the lower number.
Major American and UK water utilities
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