Total Solids and Total Dissolved Solids in Water Objective In this lesson we will answer the following questions: • • • •
What are Total Dissolved Solids? Where do dissolved solids come from? What is the effect of solids on drinking water? How are solids taken care of?
Reading Assignment Along with the online lecture, read chapter 25 in Simplified Procedures for Water Examination.
Lecture Introduction to Solids Water is a good solvent and picks up impurities easily. Pure water - tasteless, colorless, and odorless - is often called c alled the universal solvent. Dissolved solids refer to any minerals, salts, metals, cations or anions dissolved in water. Total dissolved solids (TDS) comprise inorganic salts (primarily calcium, magnesium, potassium, sodium, bicarbonates, chlorides and sulfates) and some small amounts of organic matter that are dissolved in water. TDS in drinking water originate from natural sources, sewage, urban runoff, r unoff, industrial wastewater, and chemicals used in the water treatment process, and the nature of the piping or hardware used to convey the water, i.e. the plumbing. In the U.S. elevated TDS has been due to natural environmental features such as mineral springs, carbonate deposits, salt deposits, and sea water intrusion, but other sources may include salts used for road de-icing, de- icing, drinking water treatment chemicals, stormwater and agricultural runoff, and point/non-point wastewater discharges.
Solids analyses are performed in a variety of applications in the fields of environmental engineering and science. For example, one of the most important parameters used in making control decisions in the activated sludge process of wastewater treatment systems is mixed liquor suspended solids (MLSS) and/or mixed liquor volatile suspended solids (MLVSS). Total solids can be subdivided into total suspended solids (TSS) and total dissolved solids (TDS). Each division can be further subdivided into fixed or volatile. Total solids is the term applied to the material residue left in a vessel after evaporation of a sample and its subsequent drying in an oven at a defined temperature (either 103°C or 180°C) Total solids includes total suspended solids, the portion of total solids retained by a filter, and total dissolved solids, the portion that passes through the filter. The type of filter holder, the pore size, porosity, area, and the thickness of the filter and the physical nature, particle size, and amount of material deposited on the filter are the principal factors affecting separation of suspended from dissolved solids. Dissolved solids is the portion of solids that passes through a 2.0 µm (micrometer) or smaller nominal pore size filter. Suspended solids is the portion retained on the filter. Fixed solids is the term applied to the residue of total, suspended, or dissolved solids remaining after combustion at 500°C. The weight lost during combustion is referred to as volatile solids. Fixed and volatile may not be the best measure of inorganic or organic material. For example, the loss of mass during combustion is not confined to organic material, and may include the decomposition or volatilization of some mineral salts. Most appropriate methods of characterizing organic material include total organic carbon (TOC), BOD and COD. More appropriate methods of characterizing inorganic material include alkalinity, hardness, and chromotography techniques for the analysis of specific constituents. The selection of the most appropriate method requires knowledge of both sample characteristics and the intended use of the data. Settleable solids is the term applied to the material settling out of suspension within a defined period of time. Settleable solids are not directly related to total solids, total suspended solids or total dissolved solids. Total Suspended Solids Total Suspended Solids (TSS) are solids in water that can be trapped by a filter. TSS can include a wide variety of material, such as silt, decaying plant and animal matter , industrial wastes, and sewage. High concentrations of suspended solids can cause many problems for stream health and aquatic life.
High TSS can block light from reaching submerged vegetation. As the amount of light passing through the water is reduced, photosynthesis slows down. Reduced rates of photosynthesis causes less dissolved oxygen to be released into the water by plants. If light is completely blocked from bottom dwelling plants, the plants will stop producing oxygen and will die. As the plants are decomposed, bacteria will use up even more oxygen from the water. Low dissolved oxygen can lead to fish kills. High TSS can also cause an increase in surface water temperature, because the suspended particles absorb heat from sunlight. This can cause dissolved oxygen levels to fall even further (because warmer waters can hold less DO), and can har m aquatic life in many other ways. High TSS in a water body can often mean higher concentrations of bacteria, nutrients, pesticides, and metals in the water. High TSS can cause problems for industrial use, because the solids may clog or scour pipes and machinery. The flow rate of the water body is a primary factor in TSS concentrations. Fast running water can carry more particles and larger-sized sediment. Heavy rains can pick up sand, silt, clay, and organic particles (such as leaves, soil, tire particles) from the land and carry it to surface water. A change in flow rate can also affect TSS; if the speed or direction of the water current increases, particulate matter from bottom sediments may be resuspended.
Total Dissolved Solids Total Dissolved Solids (TDS) are solids in water that can pass through a filter. TDS in a measure of the amount of material dissolved in water. This material can include carbonate, bicarbonate, chloride, sulfate, phosphate, nitrate, calcium, magnesium, sodium organic ions, and other ions. A certain level of these ions in water is necessary for aquatic life. Changes in TDS concentrations can be harmful because the density of the water determines the flow of water into and out of the organism's cell. However, if TDS concentrations are too high or too low, the growth of many aquatic life can be limited, and death may occur. Similar to TSS, high concentrations of TDS may also reduce water clarity, contribute to a decrease in photosynthesis, combine with toxic compounds and heavy metals, and lead to an increase in water temperature. TDS is used to estimate the quality of drinking water, because it represents the amount of ions in thew ater. Water with high TDS often has a bad taste and/or high water hardness.
In general, the total dissolved solids concentration is the sum of the cations (positively charged) and anions (negatively charged) ions in the water. Therefore, the TDS test provides a qualitative measure of the amount of dissolved ions, but does not tell us the nature or ion relationships. In addition, the test does not provide us insight into the specific water quality issues, such as elevated hardness, salty taste or corrosiveness. Therefore, the TDS test is used as an indicator test to determine the general quality of the water. The sources of total dissolved solids can include all of the dissolved cations and anions, but the following table can be used as a generalization of the relationship of TDS to water quality problems. Cations combined with Carbonates (CaCO3, MgCO3, etc)
Associated with hardness, scale formation, bitter taste
Cations combined with Chloride (NaCl, KCl)
Salty or brackish taste, increase corrosivity
Some rock and soil release ions very easily when water flows over them; for exam ple, if acidic water flows over rocks containing calcite (CaCO 3), such as calcareous shales, calcium (Ca2+) and carbonate (CO 32-) ions will dissolve into the water. Therefore, TDS will increase. However, some rocks, such as quartz-rich granite, are very resistant to dissolution, and don't dissolve easily when water flows over them. TDS of waters draining areas where the geology only consists of granite or other resistant rocks will be low (unless other factors are involved.) Factors Affecting TSS and TDS Soil Erosion Soil erosion is caused by disturbance of a land surface. Soil erosion can be caused by building and road construction, forest fires, logging and mining. The eroded soil particles can be carried by stormwater to surface water. This will increase the TSS and TDS of the water body.
Urban Runoff During storm events, soil particles and debris from streets and industrial, commercial, and residential areas can be washed into streams. Because of the large a mount of pavement in urban areas, infiltration is decreased, velocity increases, and natural
settling areas have been removed. Sediment is carried through storm drains directly to creeks and rivers.
Decaying Plants and Animals As plants and animals decay, suspended organic particles are released and can contribute to the TSS and TDS concentration.
Wastewater and Septic System Effluent The effluent from wastewater treatment plants can add suspended solids to a stream. The wastewater from our houses contain food residue, human waste, and other solid material that we put down our drains. Most of the solids are removed from the water at the treatment plant before being discharged to the stream, but treatment plants only remove some of the TDS. Important components of the TDS load from the treatment plants include phosphorus, nitrogen, and organic matter.
Measurement of TSS To measure TSS, the water sample is filtered through a pre-weighed filter. The residue retained on the filter is dried in an oven at 103 to 105°C until the weight of the filter no longer changes. The increase in weight of the filter represents the total suspended solids. TSS can also be measured by analyzing for total solids and subtracting total dissolved solids. You will read more about this in the lab section of this lesson.
Measurement of TDS To measure TDS, the water sample is filtered, and then the filtrate (the water that passes through the filter) is evaporated in a pre-weighed dish and dried in an oven at 180°C, until the weight of the dish no longer changes. The increase in weight of the
dish represents the total dissolved solids, and is reported in milligrams per liter (mg/L). You will read more about this in the lab section of this lesson.
Interpreting Test Results The Environmental Protection Agency (EPA) establishes standards for drinking water which fall into two categories - Primary Standards and Secondary Standards. Pr imary Standards are based on health considerations and Secondary Standards are based on taste, odor, color, corrosivity, foaming, and staining properties of water. There is no Primary drinking water standard for total dissolved solids, but the Secondary standard for TDS is 500 mg/L. The treatment options for an elevated total dissolved solids really depends on the nature of the cations and anions. If the elevated total dissolved solids is due to cations like calcium, magnesium, and iron, it may be possible to remove these ions using a water softner. This process may not reduce the TDS concentration, but reduce the aesthetic problems with the water. If the problem is associated with an elevated concentration of sodium, potassium, etc, the primary recommendations would include a reverse osmosis system or distillation unit.
Potential Health Effects An elevated total suspended solids (TSS) or total dissolved solids (TDS) concentration is not a health hazard. The TDS concentration is a secondary drinking water standard and therefore is regulated because it is more of an a esthetic rather than a health hazard. High total dissolved solids may affect the aesthetic quality of water, interfere with washing clothes and corroding plumbing fixtures. An elevated TDS indicates the following: •
The concentration of the dissolved ions may cause the water to be corrosive, salty or brackish tase, result in scale formation, and interfere and decrease efficiency of hot water heaters; and
•
Many contain elevated levels of ions that are above the Primary or Secondar y Drinking Water Standards, such as: an elevated level of nitrate, arsenic, aluminum, copper, lead, etc.
Calculations
where: A = weight of filter + aluminum dish, g B = weight of filter + aluminum dish + residue, g
Review Dissolved solids refer to any minerals, salts, metals, cations or anions dissolved in water. This includes anything present in water other than the pure water molecule and suspended solids. Suspended solids are any particles that are neither dissolved nor settled in the water, such as wood pulp. Some dissolved solids come from organic sources such as leaves, silt, plankton, and industrial waste and sewage. They can also come from inorganicmaterials such as rocks, calcium bicarbonate, nitrogen, iron and other minerals. Water may also pick up metals such aslead or copper as they travel through pipes used to distribute water to consumers. An elevated total dissolved solids conentration does not mean t hat the water is a health hazard, but it does mean the water may have aesthetic problems or cause nuisance problems. These problems may be associated with staining, taste, or precipitation. With respect to trace metals, an elevated total dissolved solids may suggest that toxic metals may be present at an elevated level. It is important to keep in mind that water with a very low TDS concentration may be corrosive and corrosive waters may leak toxic metals such as copper and lead from the household plumbing. This also means that trace metals could be present at levels that may pose a health risk.
Assignment Complete the questions for Assignment 20, which deals with the TSS, MLSS and MLVSS labs. When you have gotten all the answers correct, print the page and either mail or fax it to the instructor. You may also take the quiz online and directly submit it into the database for a grade.
Lab Read the Total Suspended Solids and Mixed Liquor Suspended Solids labs and do the assignment listed above, there are questions concerning the virtual labs included.
Quiz Answer the questions in the Lesson 20 quiz . When you have gotten all the answers correct, print the page and either mail or fax it to the instructor. You may also take the quiz online and directly submit it into the database for a grade.
Lab 17: Total Suspended Solids Reading Assignment Read Chapter 25 in Simplified Procedures for Water Examination .
Introduction The laboratory exercises you have performed up until this point have been primarily concerned with water treatment. This lab, and the rest of the labs in this course, are primarily used for testing wastewater and should be carried out in a wastewater treatment plant. The primary purpose of wastewater treatment is to remove solids from water, so this lab will be concerned with testing for one of the types of solids found in water. There are a variety of terms referring to solids in wastewater, each of which is defined below: •
Total solids - all solids in water. Total solids are measured by evaporating all of the water out of a sample and weighing the solids which remain. Dissolved solids - solids which are dissolved in the water and would pass through a filter. Dissolved solids are measured by passing the sample water through a filter, then drying the water which passes through. The solids remaining after the filtered water is dried are the dissolved solids. Suspended solids - solids which are suspended in the water and would be caught by a filter. Suspended solids are measured by passing sample water through a filter. The solids caught by the filter, once dried, are the suspended solids. Settleable solids - suspended solids which would settle out of the water if given enough time. Settleable solids are measured by allowing the sample water to settle for fifteen minutes, then by recording the volume of solids which have settled to the bottom of the sample. o
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Nonsettleable solids - suspended solids which are too small and light to settle out of the water, also known as colloidal solids. Nonsettleable solids are measured by subtracting the amount of settleable solids from the amount of suspended solids.
This lab focuses on the total suspended solids, which includes both settleable and nonsettleable solids. Total suspended solids should be tested at least five times per week using 24-hour, flow-proportioned composite samples. The test should be performed on both raw water (to determine the solids content of water entering the plant) and on finished water (to determine the efficiency of treatment at the plant.)
Equipment • • • • • • • • •
Dessicator Drying oven, for operation at 103 to 105°C Analytical balance, capable of weighing to 0.1 mg Magnetic stirrer with TFE stirring bar Wide-bore pipets Graduated cylinder Low-form beaker Glass-fiber filter disks with organic binder Filtration apparatus, which can be any one of the following: Membrane filter funnel Gooch crucible, 25 mL to 40 mL capacity, with Gooch crucible adapter Filtration apparatus with reservoir and coarse fritted disk (40 to 60 um) as filter support Filter flasks, of sufficient capacity for sample size selected Vacuum pump Tubing Stop watch Aluminum weighing dishes o o
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Reagents
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Reagent-grade water
Laboratory Procedure 1. Prepare the glass-fiber filter disks (unless they are pre-prepared.) Insert the filter disk with wrinkled side up in filtration apparatus. Apply vacuum and wash the disk with three successive 20 mL portions of reagent-grade water. Continue suction to remove all traces of water, turn vacuum off, and discard washings. Remove the filter from the filtration apparatus and transfer to an inert aluminum weighing dish. If a Gooch crucible is used, remove the crucible and filter combination. Dry in an oven at 103 to 105°C for 1 hour. Cool the filter in a desiccator to balance the temperature. Then weigh the filter and record the weight. Repeat the above cycle of drying, cooling, desiccating, and weighing until a constant weight is obtained or until weight change is less than 4% of the previous weighing or 0.5 mg, whichever is less. Store the filter in the desiccator until it is needed. You will need to prepare a filter disk for each sample you plan to test.
2. Select the filter and sample sizes. Choose a sample volume which will yield between 2.5 and 200 mg of dried residue. If the volume filtered fails to meet the minimum yield, you will have to increase the sample size up to 1 L. If the complete filtration takes more than 10 minutes, you will have to increase the filter diameter or decrease the sample volume.
3. Analyze the sample.
Assemble the filtering apparatus, as shown above, and begin suction. Wet the filter with a small volume of reagent-grade water to seat it. Stir the sample with a magnetic stirrer at a speed to shear larger particles, if practical, to obtain a more uniform particle size. While stirring, pipet a measured volume onto the seated glass-fiber filter. For homogeneous samples, pipet from the approximate midpoint of the container but not in the vortex. Choose a point both middepth and midway between the wall and the vortex. Use the stopwatch to measure the amount of time it takes for the sample water to flow through the filter. Remember that filtration should take no more than 10 minutes. If filtration takes too long, choose a smaller sample size or a larger filter and repeat the procedure. Record the filtration time. Wash the filter with three successive 10 mL volumes of reagent-grade water, allowing complete drainage between washings. (This washes down solids which may have stuck to the glass on the upper filter holder and removes dissolved solids from the suspended solids captured by the filter. Samples with high dissolved solids may require additional washings.) Continue suction for about 3 minutes after filtration is complete. Carefully remove the filter from the filtration apparatus and transfer it to an aluminum weighing dish as a support. Or remove the crucible and filter combination from the
crucible adapter if a Gooch crucible is used. Dry the filter for at least 1 hour at 103 to 105°C in an oven, cool in a dessicator to balance the temperature, and weigh. Repeat the cycle of drying, cooling, desiccating, and weighing until a constant weight is obtained or until the weight change is less than 4% of the previous weight or 0.5 mg, whichever is less. At least 10% of all samples should be analyzed in duplicate. Duplicate determinations should agree within 5% of their average weight.
4. Calculate the concentration of total suspended solids in the sample using the following formula:
Where: A = Sample and filter weight, mg B = Filter weight, mg
5. If two samples were measured, then the average total suspended solids can be calculated as follows:
Where: C = Total suspended solids of sample 1, mg/L D = Total suspended solids of sample 2, mg/L
6. Calculate the total suspended solids in kilograms per day (KGD) at the plant, as follows: Total suspended solids per day, KGD = (Average total suspended solids, mg/L) × (Flow, MGD) × 3.785
Data
Filter and sample preparation Weight - Trial 1 Weight - Trial 2 Weight - Trial 3 Final Weight
Filter Sample 1 Sample 2
Total suspended solids Sample Sample Source
Filter weight (mg)
Sample volume (mL)
Filtration time (min)
Sample and filter weight (mg)
----
----
----
----
Total suspended solids (mg/L)
1 2 Average ----
Total suspended solids (KGD) = _____________________________________________
Virtual Lab For more information on testing for total suspended solids, view the virtual lab.
Sources American Public Health Association, American Water Works Association, and Water Environment Federation. 1998. Standard Methods for the Examination of Water and Wastewater . American Public Health Association, Washington, D.C. Kerri, K.D. 1998. Operation of Wastewater Treatment Plants. California State University: Sacramento.
