Objective
In this lesson we will learn about the following concepts:
- The hydrologic cycle and how it is like nature's way of treating water
- An introduction to water treatment
Reading AssignmentAlong with the online lesson, read Chapter 1: Water Treatment Plant Operator, in your textbook Operation of Water Treatment Plants Volume I.
Lecture
History of Drinking Water Treatment
When most people turn on the faucet, they probably never think about where that water came from and how it became safe to drink. If you get your water from a municipal water system, the process is pretty neat and involves processing, purification, storing and transporting through a network of pipes intracately designed and plotted beneath the city.
Civilizations have always formed around water supplies. Even though they knew they needed water, they were not aware that the quality may not be suitable for drinking. Water treatment originally focused on improving the aesthetic qualities of drinking water. Methods to improve the taste and odor of drinking water were recorded as early as 4000 B.C. During the mid to late 1800s, scientists gained a greater understanding of the sources and effects of drinking water contaminants, especially those that were not visible to the naked eye. In 1855, Dr. John Snow proved that cholera was a waterborne disease by linking an outbreak of illness in London to a public well that was contaminated by sewage. In the late 1880s, Louis Pasteur demonstrated the "germ theory" of disease, which explained how microscopic organisms (microbes) could transmit disease through media like water. Federal regulation of drinking water quality began in 1914, when the U.S. Public Health Service set standards for the bacteriological quality of drinking water. The Public Health Service revised and expanded these standards in 1925, 1946, and 1962. The 1962 standards were the most comprehensive federal drinking water standards in existence before the Safe Drinking Water Act of 1974.
Industrial and agricultural advances and the creation of new man-made chemicals had negative impacts on the environment and public health. Many of these new chemicals were finding their way into water supplies through factory discharges, street and farm field runoff, and leaking underground storage and disposal tanks. Many of the treatment techniques used today by drinking water plants includes methods that have been used for hundredds of years. However, newer treatment techniques, such as reverse osmosis and granular activated carbon, are also being used by some modern treatment plants.
Hydrologic Cycle
In water and wastewater treatment, 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. 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 soda from the refrigerator and set it in a room. Notice how the outside of the soda can "sweats"? 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).
Water Treatment Introduction
You probably don't think much about where the water in your tap comes from, but odds are that it has come through a municipal water treatment plant. There are two main types of treatment plants: drinkin gwater and wastewater. Both serve the purpose of cleaning the water, but in general, the output of wastewater plants are streams or rivers, and the output of drinking water plants are your city's pipe network distribution system. So, how does a treatment plant take dirty river water and turn it into clean water? Through processes involving chemicals and filters, water can be removed of most toxins and hazards and become potable again.
All drinking water will start off at the water source, which is usually a freshwater lake, river, well, or even a stream. The first step of treatment is to remove the settleable and dissolved solids suspended in the water. In order to speed the settling and removal process up, chemicals called coagulants are added to the water. The most common coagulant is aluminum sulfate, but this varies by water treatment plant. Essentially, this chemical has the opposite charge from the suspended solids, like clays or silts, which then neutralizes the charge and allows the particles to stick together. Now that the solids in the water can begin sticking together, the mixture is slowly mixed in a flocculation basin in order to continue to form floc particles. The floc particles then settle out of the mixture in a sedimentation basin and cleaner water flows overtop a weir.
This process is only the first step, and it has removed mainly larger particles in the water, but some smaller particles may still remain, as well as chemicals and bacteria. Following sedimentation, the next step is typically filtration through a sand filter. A sand filter is essentially what it sounds like, a basin of fine to coarse sand that filters water. After passing through the sand filter the water is clear, but bacteria are still present. The final step in the process is disinfection. In the U.S. the main method of disinfection is by adding chloramines, or chlorine based compounds. When these chemicals are added, they kill microorganisms, but they also react with any organic material left in the water. The reason you would add chlorine at the last step is because its reaction with organic matter can create disinfection byproducts which can result in carcinogens or other harmful chemicals being present in the final water product. Chlorine is used mainly because of how it kills pathogens. Chlorine concentrations are actively present in the final drinking water, keeping pathogens from entering the water from pipes or other contamination sources. 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.
The water treatment facilities will vary from town to town and state to state as well, as there are different regulations. Everything is also regulated by the EPa, which means there are federal standards for safe drinking water. The EPA monitors for hundreds of organisms, metals, compounds, chemicals and the like. For each contaminant, the EPA allows for a maximum number. When water is tested at the plants, it must meet or be lower than this specified number. If there is a spike, the customers must be notified. This is why you sometimes see a "boil alert" on television. This means that a contaminant at a potentially unsafe level has entered the water supply. There are also secondary contaminant guidelines, which are not enforced. These may cause cosmetic damage, such as skin and tooth discoloration or aesthetic effects such as taste or odor. The include aluminum, chloride, copper and many others. Many of these are also linked to health effects and for this reason, even water treated at a plant may need to be filtered at home also.
Most water systems use the multiple barrier method to remove these contaminants and to meet EPA guidelines. But also, things are sometimes added to the water. Not all, but some follow a process like this: At the treatment plant, chlorine is added to the water to kill bacteria; aluminum sulfate is added to coagulate organic particles; then the water is put into a settling basin to let the precipitate settle; the water is filtered through sand filters; the water is treated with lime to adjust the pH, and finally is put into those storage reservoirs for home delivery.
Along with the Safe Drinking Water Act, everyone is allowed to know what is found in the water. In fact, annual reports are usually provided to customers along with their account statement.
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.
Types of Water Contaminants
Adequate supplies of water are necessary for agriculture, human consumption, industry and recreation. With the advancement of technology and industrial growth, fresh water resources all over the world are threatened. One-sixth of the world population suffers from unavailable freshwater. It is seen that developed countries suffer most from chemical discharge problems to health and leads to waterborne diseases which can be prevented by taking measures even at the household level. Providing safe water for all is a challenging task. Basically the types of water contaminants are four types associated with water pollution, including inorganic, organic, biological and radiological.
The types and concentrations of natural contaminants depend on the nature of the geological materials through which the groundwater flows and quality of the recharge water. Groundwater moving through sedimentary rocks and soils may pick up a wide range of compounds, such as magnesium, calcium, and chloride, arsenate, fluoride, nitrate and iron; thus, the effect of these natural contaminants depends on their types and concentrations. Other contaminants are man-made by-products of industry and agriculture, including heavy metals like mercury, copper, chromium, lead and hazardous chemicals, dyes and compounds like insecticides and fertilizers. Improper storing or disposing of household chemicals such as paints, synthetic detergents, solvents, oils, medicines, disinfectants, pool chemicals, pesticides, batteries, gasoline and diesel fuel can lead to groundwater contamination. The microbial contaminants include pathogens like bacteria, viruses, and parasites such as microscopic protozoa and worms. These living organisms can be spread by human and animal wastes knowing or unknowingly.
Some contaminants can be easily identified by assessing color, odor, turbidity and the taste of the water. However, most cannot be easily detected and require testing to reveal whether water is contaminated or not. Thus, the contaminants may result in unappealing taste or odor and staining as well as health effects. The presence of clays, silts or sand, or organic, algae, and leaf particles results in turbidity. The turbidity may shield bacteria, preventing disinfection chemicals from attacking and destroying the cells. The presence of organic materials in conjunction with chlorine can form trihalomethanes and other potentially harmful chemicals. Generally, surface water sources have higher turbidity compared to groundwater sources.
Organic Contaminants
Organic contaminants are composed of two basic elements: carbon and hydrogen. They are often responsible for taste, odor, and color problems in groundwaters. They can occur naturally from things like decaying vegetation, or they can be man-made. These decay compounds are called lignins or tannins. Pollution of organic chemicals in water sources occur from natural products of aquatic microorganisms and artificial contaminants from industrial chemicals or human wastes. The effects of organic pollutants on water sources differ for each contaminant. Only a few of the thousands of toxic organic chemicals that occur in drinking water are regulated by drinking water standards. You can view a list of organic pollutants that are regulated at https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations#Organic. The upper acceptable concentration of regulated contaminants in drinking water is called the Maximum Contaminant Level (MCL).
Estimated organic contaminants are as follows:
- Organic chemicals produced by algae or Actinomycetes which grow in the natural aquatic environment;
- Organic pollutants of industrial chemicals, from industrial activity;
- Water contamination by pesticides from agricultural or other uses;
- Organic chemicals from unintentionally produced contaminants during combustion or burning of organic materials such as polynuclear aromatic hydrocarbons (PAHs);
- Pollutants in waste from the human environment such sa domestic effluent, and
- Newly produced materials during the water purification process of drinking water.
These manmade organic contaminants can be grouped into subcategories that are often more recognized than the more formal chemical name. These subcategories include:
- Industrial solvents
- Hydrocarbons
- Pesticides
- Herbicides
Disinfection by-products like trihalomethanes (THMs) and haloacetic acids (HAAs), which are formed when chlorine in treated drinking water combines with naturally occurring organic matter is one form of organic contaminant. These are known to be potent carcinogens, but the EPA didn't mandate limits until 2004. These naturally occurring carbon compounds are not hazardous by themselves, but when combined with chlorine, produce byproduct reactants that have a health concern.
Pesticides, herbicides, insecticides, and fungicides are man-made chemicals that get into the water supply from run-off, sewers, and landfills. Many are extremely dangerous. The numbers of these things are proliferating rapidly and the amounts that are showing up in water supplies is increasing as well. Volatile organic chemicals (VOCs), are man-made chemicals which include solvents, degreasers, adhesives, gasoline additives, and fuels additives. Possible chronic health effects include cancer, central nervous system disorders, liver and kidney damage, reproductive disorders, and birth defects. All of these contaminant subcategories, with the exception of trihalomethanes, are manmade and the result of land use or other human activity such as agriculture, manufacturing or improper waste disposal.
Sources of organic compounds (VOCs, taste and odor compounds, disinfection byproducts, and free chlorine) found in groundwaters may include:
- leaking underground gasoline/storage tanks
- agricultural runoff containing herbicides or pesticides
- solid waste or hazardous waste landfills
- and improperly disposed industrial/chemical waste
There are three treatment methods that have been shown to be effective in removing organics from drinking water. These include aeration, adsorption using activated carbon, and oxidtaion. If the concentration of the contaminant is high, two treatment units (using different methods) are typically installed. The first unit is used to remove the "heavy" contaminant load, while the second provides a "polishing step" to assure full removal of the contaminant(s) and to address "breakthrough". This sequential treatment configuration is called a series configuration. If appropriate for your contaminant, aeration is often the first method used, while activated carbon is often used as the polishing step.
Inorganic Contaminants
Inorganic contaminants include the entire spectrum of non-carbon based elements and chemicals. The presence of contaminants can also be measured by its chemical parameters. Hardness of the drinking water is a naturally occurring contaminant, which basically depends on the geographical status. It is caused by significant amounts of calcium or magnesium components; the hardness is classified into carbonate or non-carbonate hardness depending on what molecules are combined with calcium or magnesium. Apart from carbonate/noncarbonate hardness, there are several inorganic substances, such as fluoride, arsenic, lead, copper, chromium, mercury, antimony, and cyanide, that contaminate water resources. They can get into drinking water from natural sources, industrial processes, as well as from plumbing systems. Inorganic chemicals are metals, salts, and other compounds that typically do not contain carbon. Inorganic chemicals occur naturally and can also come from human activities. Lead and copper are inorganic compounds that are different because they are rarely found in the sources of our drinking water. Usually these contaminants enter the water as it passes through pipes and plumbing systems. You can view a list of regulated inorganic contaminants at https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations#Inorganic.
As a rule inorganic contaminants don't contain carbon and commonly occur in nature. Contaminants like arsenic, chromium, iron and manganese are widely present on the earth's surface and naturally end up in our water. Not all inorganic contaminants occur naturally, some are the result of pollution. Not all inorganic contaminants occur naturally, some are the result of polllution. Some are a consequence of the interplay of pollution and natural processes. For instance, nitrate is naturally formed by the oxidation of ammonia in fertilizer or leaks from septic systems.
Some inorganics are harmful to human health. Nitrate and lead present many problems for drinking water utilities. Both are harmful to vulnerable populations such as infants and children and are hard to manage. We can effectively remove nitrate from water, but the disposal of the treated waste stream is challenging and expensive. Lead doesn't present a treatment issue since most water sources contain trivial amounts of lead. Instead it is mainly tied to existing distribution system infrastructures. Exposure to lead, released from lead pipes and lead-containing fixtures used in the 20th century is a serious health risk. These pipes and fixtures, sometimes installed 50 to 100 years ago, may leach lead into water at the customer's tap.
In several cases, small amounts of some inorganics are beneficial to human health; while a large amount of the same inorganic is harmful. It is a delicate balancing act. For instance, within a narrow range, fluoride promotes dental health. On the other hand, exposure to too much fluoride can discolor teeth and weaken bones.
