1.2: Wastewater Quality
Learning Outcomes
- Understand the purpose of NPDES permits
- Explain how TMDLs are obtained and how they improve water quality
- Assess the difference between chemical and physical water quality contaminants
- Describe how different contaminants can affect water bodies
National Pollutant Discharge Elimination System (NPDES) Permits
Wastewater treatment plants typically discharge their treated water into a water body that is nearby. Since 1972, with the passage of the Clean Water Act, it is illegal to discharge water from a point source into waters of the United States unless an NPDES permit is obtained. Since wastewater treatment facilities have all of their discharge leaving the plant from a single discharge pipe, it is considered a point source. This differs from nonpoint sources that have large areas of discharge like runoff from agricultural fields. An NPDES permit allows discharging the treated wastewater into a water body as long as it meets the requirements of the permit. These requirements can include limits on what can be discharged, how frequently samples must be taken, and other reporting requirements. There may also be other requirements specific to the discharger to assure that the water being discharged does not harm public health or the environment.
Total Maximum Daily Loads (TMDL)
The total maximum daily load (TMDL) is a calculated value of the maximum amount of a specific pollutant that a waterbody can receive without negatively impacting that water body. For example, 10,000lbs of pollutant X naturally occurs within a stream that has a TMDL of 50,000lbs for pollutant X and contains three point source discharges. Therefore, an additional 40,000lbs of pollutant X may be discharged into the stream without exceeding the TMDL. It’s the job of permit writers to use the TMDLs of a water body and the total number of point source discharges to determine the waste load allocation (WLA) for each discharge and incorporate that into the NPDES permit as a limit. To continue with the example, a permit writer could determine that with three point source discharges each one would be allowed to discharge 10,000lbs of pollutant X. This would leave a margin of safety of 10,000lbs. So in theory, the discharge of pollutant X would not negatively impact the water body because the sum of all the discharges and naturally occurring contamination is below the TMDL of the water body.
Wastewater Solids
The solids found in wastewater are comprised of many different constituents including organic and inorganic material. A variety of laboratory tests can be performed to determine the total amount of solids. The total suspended solids (TSS) test will determine the total amount of solids that are suspended and that are easily settleable. This test is done by filtering a specific volume of the wastewater through a filter pad. The water and dissolved solids will pass through the filter; and the settleable solids and suspended solids will remain on top of the filter. The difference in weight of the filter before and after filtration is used to calculate the amount of TSS.
While all the solids remaining on the filter after the TSS test are suspended, not all of the solids are settleable. Settleable solids can be determined by using an Imhoff cone. Again, a specific volume, typically 1,000 mL, is poured into the Imhoff cone and allowed to settle. The settleable solids will collect on the bottom of the Imhoff cone. After an hour, the amount of solids that settled can be determined by the graduations on the Imhoff cone as seen in the figure below.
The water that filtered through the filter pad during the TSS test still contains solids of contaminants that are dissolved into the water. Collectively this can be determined by completing a Total Dissolved Solids (TDS) test. This test takes the water left over from the TSS test and pours it into a pre-weighed dish. The water is then evaporated from the dish and weighed again. The difference in weight is then used to calculate the TDS.
Whether the solids are settleable, suspended, or dissolved, wastewater is generally made up of less than 1% solids. The other 99% is just water. The focus of the remainder of this text will be on treating the 1% of solids in wastewater. The difference in the type of solids will determine what type of treatment process is used. Settleable solids will easily settle during the primary sedimentation process. Suspended solids will require a secondary treatment process. TDS can only be removed by advanced treatment methods like membrane filtration or reverse osmosis. TDS will not be removed by conventional wastewater treatment methods.
Biochemical Oxygen Demand
Biochemical oxygen demand (BOD) is a measure of how much organic material is in the wastewater. When wastewater with high amounts of BOD is released to a water body, the organic material will consume the dissolved oxygen in the water body. If the BOD exceeds the TMDL of the water body, then the oxygen levels can be completely depleted. Without the dissolved oxygen, fish and other aquatic life will not be able to survive.
The BOD5 test is completed in a laboratory by taking a sample of wastewater and measuring the dissolved oxygen concentration. The sample is kept in an airtight container in a refrigerator set to a temperature of 25C. After 5 days, yes, that’s why there is a subscript 5 in BOD5, the dissolved oxygen concentration is measured again. The difference in oxygen concentrations is how much oxygen was consumed by the organic material in the wastewater. The more organic material there is, the higher the BOD5 will be.
The BOD5 test is the standard test that is used to measure the organic contamination in wastewater. It is one of the main regulatory components in an NPDES. However, it has one big drawback which is that it takes five days to get results. Another test that gives similar results is the chemical oxygen demand (COD). COD will measure the amount of oxygen needed to decompose organic material as well as inorganic chemicals. The difference between COD and BOD5 is that COD will also measure other chemicals that can be oxidized besides ones that are biological. COD laboratory results can be determined in approximately 6 hours. If a wastewater treatment facility is only treating wastes from a community that is mainly residential and does not have many industrial discharges, then the COD and BOD5 can be correlated after a significant number of samples are analyzed side by side. BOD5 is routinely done to meet regulatory requirements while COD results are used for process control to make sure the treatment plant is operating efficiently.
Microbiological Contaminants and Pathogens
There are many different types of microorganisms that can be found in wastewater. Some of these microbes are pathogenic. A pathogen is a microorganism that is capable of causing disease. Some common pathogens that can be found in untreated wastewater are Cholera, Giardia, Streptococcus, E. Coli, Cryptosporidium, and Salmonella. However, not all microorganisms are pathogenic and some non-pathogenic bacteria will actually be utilized as a treatment aide to reduce the amount of BOD5 in the wastewater.
One classification of microorganisms is a protozoa. A protozoa is a single-celled organism and is essential to the biological treatment processes. Amoebae, flagellates, and ciliates are some examples of protozoa commonly found in wastewater samples. Metazoa are multi-celled organisms that can also be found in wastewater treatment facilities. Some examples of metazoa are rotifers, nematodes, tardigrade (water bears). When we discuss the various biological treatment methods later in this textbook, we will learn that a microscopic examination of wastewater samples can show the health of the treatment system by comparing the quantities of protozoa and metazoa organisms.
Viruses can also be detected in wastewater samples. Viruses must have a host organism in order to live and reproduce. Unfortunately, they can remain dormant in water until they find such a host. Hepatitis A and polio are examples of viruses that can be found in wastewater samples. Viruses can be difficult to identify in the laboratory and are not routinely tested for in wastewater samples.
Coliform bacteria is the most common laboratory test done to determine the potential of the presence of pathogenic organisms. Coliform bacteria are abundant in the natural environment and can be found in water and soils. Coliform bacteria are not pathogenic but their presence can determine if pathogenic organisms may also be present. For example, if a tested wastewater sample has large amounts of coliform bacteria, then we can say that there is a high chance that pathogenic organisms are also in that sample. If we find that there are very low amounts of coliform bacteria, then we can say that there is a very small chance that pathogenic bacteria are also present. Because of this, coliform bacteria are often called an indicator organism. The laboratory test results can be used to indicate whether the sample has the potential for containing pathogens. Since there are so many pathogenic organisms, it would be extremely cumbersome to test for each one individually. It is much more economical to complete the coliform test instead. Now if the coliform test does indicate that there are coliforms present, further testing can be done to determine if pathogenic bacteria are also there. Typically, the test to determine if E. coli is present.
Eutrophication Effects of Nitrogen and Phosphorus
Algae typically is not present in wastewater but wastewater can contain nutrients such as nitrogen and phosphorus which will promote algae growth in waterbodies. In pond treatment systems, algae will actually be used to aide the treatment process by supplying oxygen to beneficial bacteria that will reduce BOD5.
When treated wastewater is discharged into a waterway, the amount of nitrogen and phosphorus must be closely examined. If there is too much nitrogen and phosphorus in the waterbody, it can lead to an overabundance of algae. When there is too much algae growth, the algae will consume more oxygen from the waterbody. With large amounts of algae consuming the oxygen, there is not enough oxygen for other aquatic organisms to sustain life. This excess growth of algae resulting from the increased nutrient loading on a waterbody is called eutrophication .
Other Chemical Contaminants
In addition to nitrogen and phosphorus, there are other chemical contaminants that can be found in wastewater. Almost every element that can be found on the periodic table will molecularly combine with water under the right conditions. Collectively these will be determined when completing the TDS analysis. There are some elements that we are particularly concerned about and these concentrations are measured independently.
Chlorides can be introduced into wastewater from road salts, food waste, water softeners, and naturally occurring surface water contamination. Chlorides can be found in water as sodium chloride, potassium chloride, calcium chloride, and magnesium chloride. When there are excess levels of chlorides in a waterbody, it can have adverse effects on aquatic organisms. Excessive chlorides can interfere with the biological processes going on inside the bodies of freshwater aquatic organisms.
Heavy metals found in wastewater typically come from industrial discharges. Metal finishing, electroplating processing, mineral extraction operations, and textile industries can contribute to heavy metals in wastewater. Lead, copper, zinc, mercury, arsenic, nickel, and silver are some common heavy metals that can be found in wastewater samples. Although, any metal on the periodic table can find their way into the sewer system. Heavy metals are a concern due to their toxicity in aquatic systems and adverse effects on plants and animals.
The pH of wastewater is very important and can influence the treatability of wastewater. If the pH is not between 6.5 and 8.5, chemicals may need to be added to bring the pH into that range. Alkalinity is another important characteristic of wastewater. Alkalinity is the ability of the wastewater to buffer against changes in pH. Alkalinity is measured by determining the amount of acid-neutralizing basics. It is commonly measured in mg of CaCO3 equivalents per liter. Alkalinity is critical to the physical, chemical, and biological treatment processes we will discuss throughout this book. These processes do not operate well under acidic conditions. Sufficient alkalinity is needed to ensure the pH stays within the 6.5 to 8.5 range.
Other Physical Contaminants
Odor, color, turbidity, and temperature are all physical characteristics of wastewater. Unlike the biological and chemical characteristics discussed previously, physical contaminants can be observed by the human senses. We can see what the color is, we can smell if the odor is foul, and we can feel if the sample is warm or cold. Although laboratory equipment can be used to obtain more quantitative data.
As you can imagine, untreated wastewater can give off a foul odor. However, odor can give an operator great insight. If the wastewater smells like rotten eggs, this is a sign that there can be high amounts of hydrogen sulfide in the wastewater. In densely populated areas where the wastewater treatment plant is in close proximity to residents, odors may be captured and sent to an air treatment unit to reduce the objectionable odors from the facility.
Color can also provide insight into what’s happening in the wastewater or where it’s coming from. An unusual color in the wastewater can indicate an industrial discharge such as dye from a manufacturing process. If the wastewater is dark or black, then it has most likely undergone septic or anaerobic conditions. Fresh wastewater typically has a murky yellowish/brown color.
Turbidity can be physically seen in wastewater samples and is often described as the cloudiness in the water. In the laboratory, a nephelometer is used to obtain a quantitative result measured in Nephelometric Turbidity Units (NTU). A nephelometer works by shining a light through the sample of wastewater. If the water has high amounts of turbidity the light will scatter and not make it to the detector on the other side of the sample. High amounts of turbidity can impact the effectiveness of disinfection. The small particles that cause turbidity can shield microorganisms from coming in contact with the disinfectant.