Those planning to take the Water Resources and Environmental Depth version of the PE Civil exam need to be familiar with the topics of Water Quality and Drinking Water Treatment. This blog will provide an overview of the relevant subtopics with which the exam-taker should be familiar. My intention is not to cover these topics in depth but to provide an introduction to the items which you should study further for the exam.
Aspects of water quality determination and analysis covered on the exam include: stream degradation; oxygen dynamics; total maximum daily load; biological contaminants; and chemical contaminants, which includes the topic of bioaccumulation.
1. Steam Degradation
Stream degradation refers to reduced water quality in a stream as a result of various types of pollution, erosion, or environmental degradation. The quality of the water may be considered to be degraded based on measurements of water temperature, turbidity (the clarity of the water), low dissolved oxygen levels (potentially due to microorganisms), pH balance, and/or the amount of solids content, which includes total solids (TS), total suspended solids (TSS), and total dissolved solids (TDS). Chemicals, such as from agricultural or industrial sources, which may flow into streams through surface runoff, are also contributors to stream degradation.
2. Oxygen Dynamics
Exam-takers should be familiar with concepts related to the determination of saturated content of dissolved oxygen, which is based on Henry's law. In terms of oxygen dynamics, it is also necessary to understand biochemical oxygen demand (BOD) and chemical oxygen demand (COD) and how they are determined.
3. Total Maximum Daily Load
Total maximum daily load (TMDL) is a concept in water quality which refers to the maximum quantity of pollutant that could potentially flow into a body of water on a daily basis without the water exceeding set maximum pollution levels. It is calculated as the sum of allocated waste loads from point sources, allocated loads from non-point sources, and a safety margin.
4. Biological Contaminants
Contaminants of concern in water that are biological in nature include bacteria such as E. coli (and other coliforms), Legionella, Cryptosporidium, Giardia lamblia, as well as enteric viruses. These specific microorganisms of concern (with the exception of Legionella) are largely associated with human and animal fecal waste contamination of water. The overall concentration of bacteria in a water source can be measured with a heterotrophic plate count (HPC). It should be noted that bacteria are naturally present in water sources, but lower concentrations of bacteria (as measured by an HPC test) can indicate a higher quality of water.
5. Chemical Contaminants
Chemical contaminants of concern for drinking water include inorganic contaminants (IOCs), volatile organic contaminants (VOCs), and synthetic organic contaminants (SOCs). Common IOCs of concern include arsenic, nitrate, nitrite, asbestos, lead, and copper. Bioaccumulation is the accumulation of contaminants within an organism as a result of ingestion of the contaminants.
6. Water Distribution and Treatment
Aspects of drinking water distribution and treatment that are covered on the exam include: drinking water distribution systems, drinking water treatment processes, demands, storage, sedimentation, taste and odor control, rapid mixing (such as with coagulation), flocculation, filtration, disinfection (including the byproducts of disinfection), and water hardness and softening.
7. Drinking Water Distribution Systems
The typical municipal systems for the distribution of public drinking water consist of a water source, treatment plant, storage tanks, and water mains and pipes which convey the water to the points at which the water can be consumed.
It should be noted that the enforced drinking water quality standards at the national level in the United States are set by the EPA in the National Primary Drinking Water Regulations (NPDWRs). These set maximum contaminant levels (MCLs). The water quality standards were first set in 1974 with the passage of the Safe Drinking Water Act (SDWA). The act has since been amended in 1986 and 1996 with updates to the list of contaminants which must be limited and the particular concentration levels which define those limits.
8. Drinking Water Treatment Processes
The treatment plants utilize processes of treatment that typically include a number of steps. These include rapid mixing (such as with coagulation), flocculation, sedimentation, filtration, and disinfection. The coagulation step aims to collect small and dissolved pollutants into larger particles resulting from their mixing and binding with chemicals, such as salts, which are introduced into the water. The flocculation step also involves the introduction of chemicals into the water, with the intention of forming "flocs," which are clusters of solids that can more easily be removed from the water than the smaller individual particles. These flocs settle to the bottom of a tank in the sedimentation step of the treatment process. Filtration involves the passage of the water (which has been separated from the flocs) through various filtering media, which may include charcoal, gravel, and sand. This step aims to remove any remaining sizable particulate matter from the water. Ultrafiltration is sometimes also done either as an additional step or as a substitution for the traditional filtration methods. This involves the use of a filter membrane with very small openings. The water is finally treated with disinfectants, such as chlorine, in order to prevent contamination by microorganisms both in the water leaving the treatment plant and in the water mains and pipes which carry the water from the plant to the points of use. Sometimes ultraviolet light is used for disinfection purposes either in addition to the chemical treatment or as a substitution.
9. Water Hardness
Water hardness refers to the total amount of dissolved minerals in water, typically calcium, magnesium, and iron. Though not typically a concern for health, it often leaves mineral buildup on pipes and fixtures. This buildup is also referred to as "scale." Because the particular elements contribute differently to the amount of hardness, they must be put into equivalent terms before adding them together to find the total hardness. The equivalent concentrations can be found by dividing the measured mineral concentrations by the equivalent weights of the elements. Water softness is, by contrast, a relatively low amount of dissolved minerals.
10. Unfavorable Taste and Smell
When it comes to unfavorable taste and smell of supplied drinking water, it can be caused by a number of factors. These often occur where secondary maximum contamination levels (SMCLs) are exceeded. These concentration levels are set in the National Secondary Drinking Water Regulations established by the EPA. These are guidelines are not required to be met by most jurisdictions but are recommendations for water quality that the agency sets. Where the concentrations of the contaminants exceed the recommended levels, though they are not understood to be risks to health, they can cause unpleasant taste, smell, or appearance, which can alarm consumers. These contaminants are also sometimes referred to as nuisance constituents. Contaminants negatively affecting taste or smell include chloride, iron, sulfates, and copper, among others. Those which may negatively affect appearance include aluminum, copper, and manganese, among others. The presence of silver in water can cause skin discoloration (though it is sometimes used for its antibacterial properties in home water treatment systems). Excess fluoride can cause tooth discoloration in children. The methods by which the odors, unpleasant tastes, and other negative effects can be controlled involve limiting the concentrations of these contaminants through processes at the water treatment plant.
Water quality and treatment is an important topic as it directly affects human health and well-being. For those taking the Water Resources and Environmental Depth version of the PE exam, it is especially important to have a thorough knowledge of the particular water quality contamination issues which water supply systems face and the processes by which the water is treated to resolve these issues and provide quality water to end users.
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