Introduction to Groundwater
In this section we will answer the following questions:
- What is groundwater?
- How is groundwater removed from the ground?
What is Groundwater?
As you will remember from Lesson 1, when water falls to the earth as rain, it either runs across the surface of the ground as surface water or sinks down into the soil and becomes groundwater. Groundwater is the primary source of drinking water for most households in southwest Virginia. This lesson covers the methods used to treat groundwater for domestic uses - use in homes.
Groundwater is an economical source of water, especially for small communities. Groundwater has already been cleaned to some extent by filtering down through the soil particles. In addition, groundwater can be brought to the surface even in areas where there are no rivers nearby to supply surface water. But where is groundwater located in the earth?
If you dig a hole down through the earth, the soil initially has pockets of air between the soil particles. But as you dig deeper, soon water would fill in all of the gaps in the soil. The location where all of the holes first become filled with water is called the water table. This is the upper limit of the zone of saturation, also known as an aquifer, which is the part of the earth containing the groundwater.
The bottom of the zone of saturation is marked by an impermeable layer of rocks, clay or other material. Water cannot soak through this layer, so it instead slowly flows downhill.
Sources of Groundwater
Groundwater is considered a mineral in riparian rights, so it can be extracted by anyone who owns the land above it. Groundwater is extracted from the layer of saturation either through springs or wells. Springs are areas where the water table naturally rises above the surface of the earth, allowing groundwater to flow out of the soil.
When springs are not available, we use wells to reach the groundwater. A well is a hole dug down through the water table. Pumps are used to bring groundwater up to the surface.
The amount of water which a well will yield is determined by several factors. The size of the aquifer which feeds the well will determine how much water is available to the well. In addition, as water is pumped out of the well, a cone of depression, or area of low groundwater, surrounds the well. If the cone of depression descends past the bottom of the well, then the pumps will not longer be able to extract water from the well. Water will slowly fill the cone back up as gravity fuels more drainage of water into the aquifer, so the rate of drainage into the aquifer will also influence the yield of the well.
Although it can be costly to drill sample wells, a well will not yield water if it is built in impermeable rock, so several locations should be sampled before the well is built. The bottom of the well should have good drainage, which can be tested by pumping air into the bottom of the well. The amount of drainage depends on the permeability of the soil at the bottom of the well. The drainage will determine turbidity.
Drainage at the bottom of the well also means that two wells cannot be put side by side. Otherwise, the wells will tap into the same aquifer and will quickly go dry.
Treatment of Well and Spring Water
Groundwater is often more pure than surface water because it has filtered down through the soil. As a result of this filtration, groundwater contains less bacteria than surface waters do. But it still contains some bacteria and dissolved solids, so it has to be treated before it can be used for domestic purposes.
The types of treatment used depend upon the chemical and bacteriological quality of the water. Study and experimentation are required to tailor the process to the groundwater in each area.
Below, we have outlined the typical steps used in treating groundwater, but all steps are not used in every situation. The rest of the lesson will explain in more depth the typical processes used to treat groundwater.
There are certain minimum requirements which must be met in constructing and developing wells, springs, and treatment facilities. Before construction is begun, the Health Department must issue a permit.
Once the well is built, it must undergo an initial disinfection. Then water can be pumped from the well to be treated and then sent to the customers.
As water is pumped from a well or spring, it is screened to remove debris. Next, the water is aerated to remove carbon dioxide and some impurities from the water. Potassium permanganate to remove iron is added in the collection tray of the aerator.
The water then enters a flash mix chamber. Here, various chemicals are added and are mixed into the water. Coagulants cause fine particles to clump together into larger particles. Alkali are added to adjust the pH.
After flowing out of the flash mix chamber, the water goes through a chamber which causes coagulation and flocculation to occur. Here, the fine particles of contaminants gather together into large clumps called floc. When the water flows on into the sedimentation basin, some of the floc settles out of the water and is removed. Next, the water is passed through filters which remove particles too small to settle out in the sedimentation basin.
Finally, chlorine is added to the water. The water is left in the clear well for a period of time to allow the chlorine to kill bacteria in the water. The water is now treated and ready to be distributed.
In this section, we will answer the following questions:
- How is a new well disinfected?
- How does the process of continuous chlorination work?
Disinfection is the process of killing the microorganisms in water, usually through the use of chlorine (Cl2) or some similar substance. All public water supply wells are required to chlorinate the water being treated.
When disinfecting well water, first the new well must be disinfected. Then all of the water removed from the well must undergo continuous chlorination before being distributed. After chlorination, water must be tested for bacterial content and for adequate residual chlorine.
All new wells must be adequately disinfected before being used to supply water. Below, we list a suggested procedure for disinfecting new wells using a hypochlorite (chlorine) solution. You may need to carry out this procedure more than once to thoroughly disinfect the well.
First, pump water from the well until the water coming from the well is clear. This water has not been disinfected and should be considered wastewater. Once the water coming from the well is clear, stop pumping.
Next, add hypochlorite (HTH) to the water in the well until the concentration of HTH in the water is 50 PPM (parts per million.) Allow the HTH to remain in the well for at least 6 hours, preferably for 12 hours. Then pump the water to waste (out of the well as wastewater) until all of the chlorine has been removed.
Finally, collect samples of the well water for bacteriological analysis. If the samples show that the levels of bacteria in the water are suitably low, then the well has been successfully disinfected. If not, repeat the entire process until the bacteriological analysis shows that the water is suitable. If the samples continue to be unsatisfactory after repeating the procedure twice, the Health Department should be contacted for further instructions.
For many wells, you can simply disinfect the well once. But when building a new well which will use a submersible pump, the well should be disinfected before and after installation of the pump.
After the initial disinfection of the well, all water taken from the well must be treated with chlorine before being sent to the distribution system. This process is called continuous chlorination.
The amount of chlorine used to treat well water depends upon the bacteriological quality of the water - what type and what amount of bacteria are present in the water. The type of bacteria which water treatment operators are primarily interested in are coliform bacteria. Coliform bacteria often grow in the guts of warm-blooded animals such as humans, but can also be found in plants, soil, water, or air. If coliform bacteria are present in the water, then other microorganisms which cause disease are also likely to be present.
The goal of chlorination is to remove all disease-causing microorganisms from the water. If chlorination removes all of the coliforms (coliform bacteria) from the water, then we can safely assume that the disease-causing microorganisms have also been removed. After chlorination, the water should have 0 coliforms per hundred millimeters of water sampled.
After chlorinating the water, it must be tested. If the tests suggest that the water is contaminated, the water must be chlorinated again. Turbidity above the maximum allowable level of 0.5 NTU (nephelometric turbidity units, a unit used to measure turbidity) can be an indicator of elevated levels of coliform bacteria.
In addition to testing for coliform bacteria, the chlorine residual test is also used to determine the correct amount of chlorine to be added to the water. When chlorine is added to water, some substances, such as hydrogen sulfide (H2S), react with the chlorine and use some of the chlorine up. The chlorine residual is how much chlorine is left in the water aftera certain amount of time. This leftover chlorine is what kills the bacteria in the water.
In a large system, chlorine must be sampled every two hours at the plant and at various points in the distribution system. It is important to test the chlorine residual at various points in the distribution system rather than just at the point where the chlorine is put in the water. The operator is concerned with the amount of chlorine in the water received by the customer rather than the amount near the pump. Since at least a trace amount of chlorine must be present at the end of all water lines, it is necessary to have a higher concentration of chlorine at the treatment facility.
The simplest method of continuous chlorination of wells less than 75 gpm (gallons per minute) is by the use of a motor driven pump called a hypochlorinator. The pump pulls chlorine solution out of a holding chamber and into the water to be treated. Where the pipe from the pump joins the pipe carrying the raw water, the Venturi effect creates a small vacuum and pulls the chlorine solution into the water.
The hypochlorinator's pump can be adjusted to feed various amounts of chlorine solution. Hypochlorinators can also be controlled to automatically work in sequence with the well or service pump.
It is often necessary to increase or decrease the amount of chlorine added to the water as conditions change. Hypochlorinators allow you to adjust the amount of chlorine in three ways. You can adjust the stroke length or machine speed by varying the pulley size. Both of these adjustments change the hypochlorinator feed rate - the speed at which the machine puts chlorine into the water. You can also adjust the amount of chlorine added by changing the strength of the chlorine solution. This solution is prepared by adding the proper amount of dry chlorine compound to water.
While hypochlorinators are usually used to perform continuous chlorination in smaller systems, chlorinators are more economical in larger systems. Chlorinators are machines which use liquid chlorine supplied in steel cylinders.
Chlorinators are used when the supply source is greater than 75 gpm and may sometimes be used in smaller wells also. Anticipated pumping periods and chlorine demand (based on the chlorine residual test) determine whether a hypochlorinator or chlorinator should be used in each situation.
In this section we will answer the following questions:
* What is aeration?
* What is the purpose of aeration in water treatment?
* What methods are used to aerate water?
The Purpose of Aeration
Aeration is the intimate exposure of water and air. It is a way of thoroughly mixing the air and water so that various reactions can occur between the components of the air and the components of the water.
The process of aeration has a number of useful functions in water treatment. If water has a high concentration of free carbon dioxide, it will have a low pH which can cause corrosion of pipes when the water is distributed. Aeration allows some of the carbon dioxide to leave the water and enter the air, raising the pH to a more normal level.
Aeration can also remove iron and manganese from the water. By mixing the water with air, oxygen in the air comes in contact with these minerals. The minerals are oxidized and precipitate, or settle out of the water.
Hydrogen sulfide and other compounds can cause foul odors in water. Aeration will reduce the odors caused by these compounds. It will also strip organic contaminants from the water.
The goal of an aerator is to increase the surface area of water coming in contact with air so that more air can react with the water. As you break air or water up into small drops/bubbles or thin sheets, the same volume of either substance has a larger surface area. Let's consider three different sets of water drops, drawn as squares to make it easy to measure the volume and surface area.
The volume is the amount of water in each drop. The drop on the left has a volume of four, as does the elongated drop in the center. The small drops on the right each have a volume of one, so all four together have the same volume as each of the other two shapes.
The surface area is the length of a line drawn all the way around an object. The large drop has a surface area of 8, but the other two shapes have greater surface areas. As you can see, a thin sheet of water (like the elongated drop) has an intermediate surface area. When a drop of water is broken up into several smaller drops (like the four drops on the right), the surface area is greatly increased.
Methods of Aeration
There are several different methods used to aerate water, but all either involve passing water through air or air through water. Water can be exposed to air by spraying or by distributing it in such a way that small particles or thin sheets of water come in contact with the air. Water can also be aerated by pumping large volumes of air through the water.
The method of aeration to be used depends on which materials on the water are to be removed. The chemical characteristics of the water to be treated can also influence which treatment method is used. Finally, each method has a different efficiency. In general, pumping water through air is much more energy efficient than pumping air through water. Different types of aeration and other methods of treatment should all be compared to determine the most efficient and practical method of treatment in each case.
Chemical Characteristics of Water
The chemical characteristics of water which influence the efficiency of each aeration method are: pH, total alkalinity, carbon dioxide content, and the presence or absence of hydrogen sulfide. The first three factors are interrelated. A low pH (meaning that the water is acidic) may be a result of a low total alkalinity or of a high carbon dioxide concentration.
As mentioned earlier, aeration can raise pH if the low pH is caused by high carbon dioxide content in the water. But a low pH can also be due to a low total alkalinity. Total alkalinity is the capacity of water to neutralize acids, so it depends on the concentration of various buffers in the water. Even if the carbon dioxide content of water is high (>25 PPM), if it has a low total alkalinity (<25 PPM), then aeration is unlikely to be effective in raising the pH. Instead, the best treatment choice would probably be to feed an alkali which will raise the total alkalinity of the water.
In contrast, let s consider a situation in which water is treated in large quantities and has a high alkalinity and a high carbon dioxide content. Aeration would be the most efficient treatment method to raise pH in this case. Aeration would remove carbon dioxide from the water, which would cause the pH to rise.
If water has hydrogen sulfide present, then aeration may be a necessary treatment method to remove the hydrogen sulfide from the water. However, an aerator should not be installed based only on the need for reducing hydrogen sulfide. In most cases, chlorination would effectively correct this condition.
All four of these chemical characteristics must be considered before deciding whether aeration is a useful treatment method in each situation.
Types of Aerators
Air diffusion is a type of aerator in which air is blown through a trough of water. As water runs through the trough, compressed air is blown upward through porous plates on the bottom. This method is not very efficient due to limited air transfer.
Most of the other aeration methods work by passing raw water through air in small streams rather than by passing air through water. A few, such as spray nozzle aerators, pump water through nozzles breaking the water into a fine spray.
Cone tray aerators and cascade aerators both work by forming little waterfalls.
Cone tray aerators consist of several cones in which water flows through the cone and over the rim of the cone.
Cascade aerators allow water to flow in a thin layer down steps. In both cases, the waterfalls allow the water to come in contact with air.
Coke tray aerators also pass water through air in small streams. A coke tray aerator is comprised of a series of activated carbon trays, one above another, with a distributing pan above the top tray and a collecting pan below the bottom tray. The distributing pan breaks the water up into small streams or drops. The holes in the trays should be designed to develop some head loss to provide for equal distribution to the lower tray.
As the water moves through the coke tray aerator, small streams of water flow through the air from tray to tray. A great amount of water surface area is also exposed to air as the water passes over the coke beds. The water is collected in the bottom pan and given
further treatment if necessary.
In addition to aerating water, the activated carbon trays in a coke tray aerator filter organic contaminants out of the water. A similar method was once used to treat people who had swallowed poison. Bread was toasted in the oven until it blackened, turning into activated carbon. Then the patient ate the burnt toast. The carbon drew the poison into the carbon and out of the patient's system. Coke tray aerators work in a similar manner, drawing contaminants out of the water.
The last type of aerator which we will discuss here, the forced draft aerator, combines both methods: it blows air through water which has been broken into fine streams. The forced draft aerator consists of a series of trays over which raw water runs. As the water comes to the end of each tray, it cascades off and falls down to the collecting tray (also known as a drip pan). At the same time, a fan at the top of the aerator pulls air up through the water. So, as small streams of water fall from the trays, they comes in intimate contact with the strong updraft of air. This type of aerator is most effective in the reduction of hydrogen sulfide and carbon dioxide.
Iron and Manganese Removal
In this section we will answer the following questions:
* What problems are caused by iron and manganese in water?
* How are the minerals removed from water?
Iron (Fe) and manganese (Mn) can be considered the two most troublesome minerals to be found in water supplies. These elements cause stains on porcelain plumbing fixtures and laundry and cause coffee or tea to be cloudy and unpalatable. In addition, they can cause diarrhea.
Water containing iron and manganese will be clear when first discharged from a well. Upon exposure to air for several hours, the minerals oxidize (react with oxygen) and colored water results. The presence of oxidized iron causes water to be red and results in stains of the same color. Manganese is a dark brown mineral and the resulting stains are dark brown or black.
The recommended limit on concentrations of these minerals in water is 0.3 PPM for iron and 0.05 PPM for manganese. In addition, some industries cannot tolerate even this quantity of either mineral. It is strongly recommended that whenever the combined totals are more than 0.2 PPM, treatment facilities should be installed for removal.
Overview of Treatment
The usual treatment to remove iron and manganese from water is to oxidize the minerals as rapidly as possible and then to remove the oxidized material. Manganese oxidizes and discolors water at a slower rate than iron, which affects the treatment method used for each mineral. In addition, pH affects the rate of oxidation for both minerals, so it is often necessary to change the pH of the water during treatment.
In some cases the oxidation is accomplished entirely by the addition of chemicals. In other cases the water is first aerated, then an alkali is added to complete oxidation. The alkali optimizes the pH and uses the oxygen in the air to oxidize the iron and manganese. At the same time, the alkali reduces the carbon dioxide in the water.
The type of treatment for iron and manganese depends on several factors and each must be considered in the design of a facility. The factors to consider are: pH, alkalinity, concentration of iron and/or manganese, and whether or not the water is corrosive.The three procedures commonly used in iron and manganese removal in well water are as follows:
- The most common way of treating iron and manganese is aeration, followed by the addition of potassium permanganate (KMnO4), chlorination, addition of an alkali, flocculation and settling, and then filtration. The addition of potassium permanganate is the only aspect of this process which is unlikely to be part of a typical water treatment process when iron and manganese are absent.
Aeration is the first step of the process. Any of the methods for aeration may be used. However, the forced draft or induced draft aerator located over the settling basin is recommended. The second choice is the coke tray type.
The optimal application point of potassium permanganate in a water treatment facility depends on several factors. But in most cases the potassium permanganate is added to the collection tray of the aerator.
From the aerator, the water goes to the flash mix chamber. The pre-chlorine and alkali feeds are usually applied in the flash mix chamber and all of the chemicals are well mixed into the water. Although the chemical reactions are instantaneous, the water and added chemicals must be mixed for several minutes to secure a uniform reaction.
The addition of the pre-chlorine serves a variety of purposes, including disinfection, odor and taste control, and aid in coagulation and settling. The potassium permanganate, of course, oxidizes the iron and manganese. The alkali is used to produce non-corrosive and non-deposit forming water by raising the pH to 8.3. Adding an alkali may not be necessary when potassium permanganate is used, or if the total alkalinity exceeds 30 to 40 PPM. Often, a plant is operated at a pH of 8.3 due to the ease of checking with phenolphthalein.
After the chemicals are mixed into the water in the flash mix chamber, the water goes through the typical stages of flocculation and settling. In this process, the settling basin is usually designed for a detention period of two hours or more.
Then the settled water is filtered through either a pressure or gravity type filter unit. These filters are designed for a higher filter rate than those used for filtering water from surface waters. The usual filtration rate is three gallons per minute per square foot of sand, as compared with two gallons for surface water purification. Finally, the water is chlorinated and pumped through a standard pressure filter to the system.
This type of treatment would normally reduce the iron to an acceptable level if the original concentration was not too high (<0.6 PPM). Only water which has a high pH and alkalinity and is not corrosive can be treated in this method.
- Oxidation by chlorine followed by filtration is used for borderline situations when the iron concentration is 0.3 to 0.4 PPM. A contact time of about 45 minutes is required. A pressure filter separates out the iron oxidized by the chlorine.
If the iron concentration increases above 0.4 PPM, this type of treatment will not be satisfactory. The water must be non-corrosive or other treatment will be required.
- The Zeolite Process method of removing iron and manganese is a specialized process and the manufacturer's recommendations must be followed. This is an iron exchange process, in that the iron and manganese are exchanged for the cation substance added to the
water. The process is expensive and the finished water may be corrosive.
The combination of iron and manganese is responsible for most of the discolored water from wells. In this area, seldom do iron and manganese problems occur separately in wells. However, manganese contamination alone occurs rather frequently in surface water supplies. So, surface water often must be treated to remove only manganese.
Several of the smaller water supplies in the State also have iron removal plants. These plants have performed well when operated properly, delivering a satisfactory quality of water with minimum maintenance and operation. Unsatisfactory results are almost always due to improper operation and maintenance.
In these plants, the water is aerated, alkali and chlorine are added, oxidized iron is settled, and then the water is pressure-filtered and pumped to the system. This type of plant should work as well for manganese removal if the pH is raised and potassium permanganate
is added to oxidize the manganese.
In this section we will answer the following questions:
* What are the similarities and differences between corrective treatment and other methods used to prevent red water ?
* When should corrective treatment include aeration? Addition of an alkali? Hexametaphosphate?
The previous section outlines the methods of removing iron from water if the iron entered the water by leaching from rocks in the ground. However, iron may also enter water due to corrosion of the metal pipes it runs through after being treated. The results, as far as
the consumer is concerned, are the same regardless of the origin of the iron. Consumers are likely to complain about red water. But the treatment methods for removing the iron are different and depend on the source of the iron.
The term "corrective treatment" is defined as the treatment of water to prevent corrosion in pipes. Corrosion is usually caused by acidic water, which in turn is caused by a high carbon dioxide content. So, corrosion is prevented by removing and/or neutralizing the free carbon dioxide in the water. In well waters with high free carbon dioxide content, treatment is most often accomplished by aeration followed by the addition of an alkali.
Carbon Dioxide Removal
Carbon dioxide is a gas which dissolves easily in water forming an acidic liquid and giving a pleasant sparkling quality to soda and mineral waters. The gas may enter groundwater when the water encounters decaying vegetation. Unless this gas is removed or neutralized, the resulting acidic water will attack metal pipes and cause iron rust in water.
An operator usually encounters carbon dioxide gas concentrations of less than 100 PPM. When the carbon dioxide concentration is greater than 10 PPM, the free carbon dioxide in the water is loosely bound or held. If the water with a high free carbon dioxide content is exposed to the air, the loosely bound portions of the gas will be expelled from the water. Therefore, high levels of carbon dioxide in water can be lowered by aeration.
But aeration does not remove all of the carbon dioxide from water. If water containing 75 PPM of carbon dioxide is aerated, 10 to 12 PPM of carbon dioxide will remain in the water. The remaining carbon dioxide must be neutralized by the addition of an alkali. The two alkalis most commonly used for removal of carbon dioxide are lime and soda ash. One part per million of lime or two parts per million of soda ash will neutralize about one part per million of carbon dioxide.
Choosing a Treatment Method
As with most other aspects of water treatment, there are many factors to be considered when deciding on a treatment method. The concentration of carbon dioxide in the water is one important consideration, since aeration will only remove high concentrations of carbon dioxide. When considering all factors, water low in carbon dioxide content can probably be treated most economically by treatment only with alkali.
Total alkalinity, as discussed in the section on aeration, is another factor. If the water is high in carbon dioxide and low in alkalinity, it will be necessary to add large quantities of alkali for water stability as well as for carbon dioxide removal.
Aeration is usually a more costly treatment method than addition of an alkali. Aeration requires pumping the water twice, in addition to other cost factors, so it is often more economical to use only the addition of alkali to remove carbon dioxide and prevent
In many small plants concerned with iron or carbon dioxide removal, the equipment used to add lime or soda ash for iron or carbon dioxide removal consists of a 55-gallon solution barrel with a continuous mixer and a hypochlorinator. Up to a 50-pound bag of lime can be mixed with water in the solution barrel if approximately one ounce of sodium hexametaphosphate (also known as glassy phosphate, Calgon, Sodium Polyphos, etc.) is
added to the lime solution to form a protective film on the pipes once the water is distributed. Then the hypochlorinator pumps the lime solution directly into the well pump discharge line (the water being pumped out of the well.)
Unlike soda ash, lime does not readily dissolve in water, so it is more difficult to add. Soda ash can simply be dissolved in a hypochlorinator attached to the well pump discharge line, while lime requires a solution barrel and continuous mixer.
Larger plants often use large gravimetric lime feeders. Gravimetric feeders measure a specific weight of the dry chemical to add to the water during a specific time period. The hexametaphosphate is then added to the water using a hypochlorinator which pumps the chemical into the make up water line. The make up water line provides water to the lime feeder solution chamber.
Soda Ash or Lime?
Each alkali has its advantages and disadvantages. As mentioned above, soda ash is much more readily soluble in water than is lime. But too much soda ash will cause the water to feel slick, so it may not be possible to add enough soda ash to neutralize all of the carbon dioxide without causing other complaints.
Lime has the advantage of being much cheaper than soda ash. Lime costs about half as much as soda ash. In addition, it takes two pounds of soda ash to do the same job as one pound of lime when neutralizing carbon dioxide. So, using soda ash costs four times as
much to do the same job.
But lime has the ability to make the water be acidic or basic rather than acting as a buffer and neutralizing the water. If the pH is too low, the pipes will corrode, but if the pH is too high they will suffer from buildup of limestone.
Whichever alkali is chosen, the proper amount to be added to the water should be determined by tests.
Sodium hexametaphosphate is another chemical used in corrective treatment. At some well supplies, proper treatment facilities were not originally provided and would be expensive to construct, so glassy phosphate is used instead. At other well supplies, conventional treatment methods were not successful for red water control. Sodium hexametaphosphate may be dissolved and added with a hypochlorinator, so it is a relatively simple addition to the water treatment facility.
When using hexametaphosphate, the quantity of chemicals added should be carefully controlled. Hexametaphosphate is comparatively expensive and a large enough quantity must be used to yield the desired results.
If the pH of the water is too low (less than 6.5), then complaints may be increased rather than stopped when hexametaphosphate is added to the water. At a low pH, the glassy phosphate causes corrosion of copper pipes, resulting in complaints of "green" or "blue" water. In fact, unless the pH is raised to at least 7.0 and preferably to around 7.2, hexametaphosphate should not be used alone. It is important to maintain a pH in the range of 6.8 to 7.5 when using hexametaphosphate. But hexametaphosphate can be used in combination with an alkali at a pH of less than 6.5.
Usually when this chemical is first added, "red water" complaints are already common. In such cases, the mains are usually coated with corrosion products and large quantities of loose rust or scale may have settled in low places and at dead-ends. In order to clean the line thoroughly, the chemical must be fed at the rate of 10 PPM for the first week to 10 days to form a protective coating on the pipe walls. During this period, it is necessary to flush the lines regularly to draw in the treated water and to flush out the resulting loosened or dissolved materials. Flushing should be practiced at least once every other day. After the first week or ten days, the dose should be reduced each time a new batch of chemicals is
dissolved until the recommended minimum dose is reached. The final feed rate depends upon several factors and may vary from slightly less than 1 PPM to 4 PPM.
In this section we will answer the following questions:
* What problems are caused by hydrogen sulfide?
* How is hydrogen sulfide in water treated?
Hydrogen sulfide gas occurs in well water rather frequently. As the water passes through the ground, it comes in contact with sulfates. If the water is highly mineralized or contains products of decomposition, these minerals and other substances will react with the sulfates and change them to hydrogen sulfide (H2S).
Hydrogen sulfide gas turns into hydrosulfuric acid when it dissolves in water. The acid is weak but highly corrosive, eating up electrical contacts, causing a slight odor, and resulting in black water complaints. Water containing hydosulfuric acid will become very dark after remaining in the water lines for a few hours. The black water is most often noticed when flushing a fire hydrant.
The presence of larger quantities of hydrogen sulfide can be readily noted by odor. The disagreeable "rotten egg" odor is very characteristic of this gas and unless it is removed or reduced, the smell results in many complaints.
So, even though hydrogen sulfide gas in water is not injurious to people, it is usually removed when present.
Treatment There are three methods used for the removal of hydrogen sulfide. If there is a heavy concentration of the gas, the water should be aerated. This treatment breaks the water into droplets and most of the gas escapes to the air.
The gas remaining in the water after aeration can be oxidized by chlorine. Sufficient chlorine must be added in order to completely oxidize the material and still maintain a free chlorine residual.
Ozone converts hydrogen sulfide to sulfurous acid (SO2), but ozone is also corrosive so it may cause as many problems as it solves.
The test for hydrogen sulfide is not run when the usual chemical analysis of water is made. Its presence is disclosed by odor, which is described as strong, weak, faint, or no hydrogen sulfide odor to convey the idea of the quantity of hydrogen sulfide present. Under
conditions with a pH of 8.0, a hydrogen sulfide content of about O.015 PPM will give a weak odor, and 0.005 to 0.010 PPM will give a faint odor. Under other conditions with a lower pH, a concentration of 0.005 to 0.010 PPM may create a much stronger odor.
Since many water works operators also help maintain the sewage system, it may be interesting to note that hydrogen sulfide gas is a rapid poison in extremely high concentrations of 700 PPM or more and an irritant gas in concentrations around 70 PPM. It is a common constituent of sewer gas and can be dangerous in manholes, sewage pump stations and in other places where it may collect. Inhaling high concentrations of hydrogen sulfide gas causes death due to respiratory failure. Overexposure to small amounts may cause damage to the eyes, known as "gas eyes". For these reasons, working around an aerator exhaust without proper ventilation should be discouraged.
In this section we will answer the following questions:
* What is turbidity?
* How is turbid water treated?
Turbidity is the cloudy appearance of water caused by small particles suspended in the water. Water with little or no turbidity will be clear. A maximum level of 0.5 NTU is allowable in groundwater.
Excess turbidity levels can occur in groundwater found in limestone and unconsolidated rock formations. In addition, many springs experience fluctuating levels of turbidity and extensive monitoring of the water is required. All springs must be equipped
with a constant monitoring turbidimeter to shut off spring pump operation if turbidity levels are excessive.
Filtration alone will not remove the turbidity and bacteria in groundwater. In addition, a coagulation step is necessary to concentrate the fine particles into floc, which can be removed by filters.
Coagulation involves the addition of chemicals called coagulant aids. Alum or polymers are often used for coagulation. Then the water is allowed to settle for one to two hours, which gives the coagulant aids time to concentrate the water's suspended particles into floc. The settling period also gives the chlorine contact time. The final step is filtration to remove the floc. Rapid sand filters provide the best treatment.
A treatability study is suggested for any groundwater reading treatment for turbidity removal. This study will ensure the facilities will perform properly once constructed. The Health Department will not approve this type of source and treatment as a single water source because of the unpredictability of groundwater quality.
In this section we will answer the following question:
* How do the filters used in groundwater systems work?
Filtration removes impurities and floc from the water being treated. In general, filtration consists of passing the water through sand and gravel or some other filter. The floc and impurities get stuck in the sand while the water passes through. Filtration is usually one of the last steps in the water treatment process.
There are three types of filters associated with water treatment: rapid sand, pressure, and slow sand.
Rapid sand filters are mainly used in connection with surface water treatment.
Pressure filters are commonly used when iron and manganese must be removed from well water, especially in smaller water systems.
Slow sand filters have been used for the treatment of relatively clear water when there is a possibility of the water becoming turbid. This filtration method can be used to treat spring or well water that is relatively safe from contamination, but should not be used to treat surface water (lakes, ponds, etc.)
Pressure filters and rapid sand filters have the same requirements for sand and gravel for filtration. Both use the same types of manifolds and laterals (types of pipes) and have the same velocities in the pipes. Here, we will consider pressure filters, since they are used in groundwater treatment. But many aspects of the two systems are similar.
A pressure filter is an upright, closed cylinder containing a filter bed of layered sand and gravel on top of a collection system. Water under pressure passes through the filter and then continues on through the water treatment system.
As the water passes through the filter, oxidized minerals (such as iron and manganese) and foreign matter collect in the top portion of the sand. Continued build-up of these particles tends to clog the filter. The clogged filter requires more force or pressure to pass water through and filter at the same rate as an unclogged filter.
A head loss gauge measures the pressure of the water leaving the filter. By comparing the pressure of the water leaving the filter to the pressure of the water entering the filter, you can determine how much pressure was lost due to the water passing through a clogged filter - a measurement also known as head loss.
When the head loss gauge shows that there is excessive build-up on the filter, then the filter must be washed. Excessive build-up can cause clogging, restricted flow, pressure build-ups, and possible breakthroughs. A breakthrough is a crack or break in a filter bed which allows the water to pass through without contacting the filter and being cleaned. In addition to damaging the filter bed and piping, a clogged filter can allow poor quality water to go into the system, causing problems and complaints.
Filters are cleaned by backwashing. The influent valve is closed and a waste line is opened. Treated water from the system is pumped upward through the filter bed. The water pumped upward has the velocity and volume to agitate the sand and carry away the foreign matter that has collected there.
Backwashing normally takes about 10 minutes, though the time varies depending on the length of the filter run and the quantity of material to be removed. Filters should be backwashed until the backwash water is clean.
The filter rate can be increased to 3 gpm (gallons per minute) per square foot of filter and the backwash rate may be reduced to 12 gpm per square foot of filter if this provides proper washing.
Slow Sand Filters
The slow sand filter is similar in design to a rapid sand filter and a pressure filter, except that the sand is usually 36 inches in depth and the filter cannot be backwashed. Instead, the filter is cleaned by removing the top two inches of sand from the filter. Once 6 to 12 inches of sand have been removed, additional sand must be added to bring the filter back to the original depth.
Unlike pressure filtration in which the water is forced through the filter medium, water is drawn through a slow sand filter by gravity. The raw water is pumped onto the filter bed and is filtered at a rate of 3 to 6 mgad (million gallons per acre per day) or 3 to 6 gallons per hour per square foot of sand area. The filter rate is controlled by a set valve on the effluent line (the pipe containing the water flowing out of the filter system.)
Water is usually chlorinated before slow sand filtration, but may also be chlorinated as the filtered water goes to the sump pump for pumping to the system. The slow sand filters are usually covered as well to prevent the growth of algae which will clog the filters.
Although the operation and maintenance cost of slow sand filters is low, they cover large areas of land. So the initial cost of land and construction makes this type of unit uneconomical in most instances.
Groundwater is water removed from the zone of saturation in the earth. Wells and springs are used to bring this water to the surface.
This lesson has explained the treatment methods used to prepare groundwater for distribution to customers. Many of the types of water treatment used for groundwater are also used for surface water, as you will learn in later lessons.
When choosing a treatment method for any water problem, many factors should be taken into account. Some treatment methods can correct various problems at the same time. So all of the treatment methods mentioned in the lesson will not be found in all water treatment facilities.
The treatment methods explained in this lesson include:
- kills microorganisms
- controls odors and taste
- aids in coagulation and settling
- removes carbon dioxide (CO2) and raises pH
- oxidizes iron (Fe) and manganese (Mn)
- removes hydrogen sulfide (H2S)
- removes organic contaminants
Addition of potassium permanganate (KmnO4) -
- oxidizes Fe and Mn
Addition of ozone -
- neutralizes H2S
Addition of coagulants -
- concentrates particles into floc, removing turbidity
Addition of alkali -
- oxidizes Fe and Mn
- neutralizes CO2 and optimizes pH
Addition of hexametaphosphate -
- prevents corrosion of pipes
- removes floc
- kills microorganisms
- oxidizes H2S
An overview of the treatment processes used on well and spring waters is given in the following charts, taken from Alabama Department of Environmental Management's Water Works Operator Manual (1989.)
Scenario pH T.Alk. CO2 Fe Mn H2S 1 7.8 75 10 0.3 0.05 No 2 5.5 10 30 0.0 0.0 0.0 3 5.5 20 25 0.0 0.0 0.0 4 5.5 20 25 0.6 0.0 0.0 5 6.5 50 30 0.0 0.0 Yes 6 7.0 50 10 <1.5 0.0 Yes 7 7.0 50 30 <1.5 0.0 Yes 8 7.0 50 15 >1.5 0.05 Yes 9 7.0 50 15 >1.5 0.3
Scenario Cl2 Aerate Alkali Filter 1 Yes No No No 2 Yes Yes Yes No 3 Yes No Yes No 4 Yes Yes Yes Yes 5 Yes Yes Maybe No 6 Yes Yes No Yes 7 Yes Yes Yes Yes 8 Yes Yes Yes Yes 9 Yes Yes Yes Yes
Scenario Notes 1 Acceptable under EPA Standards. Low levels of H2S could be treated by heavy chlorination. 2 30-minute contact basin. 3 Lime injected under pressure. 4 30-minute contact basin double pump. 5 30-minute contact basin double pump. 6 30-minute contact basin double pump. 7 30-minute contact basin double pump. 8 Add KMnO4. One minute rapid mix, 30-minute flocculation, settling, recycle sludge. 9 Add KMnO4. One minute rapid mix, 30-minute flocculation, settling, recycle sludge.
- You are digging a hole at the beach where the waves cannot reach you. But after digging down about one foot, the bottom of your hole begins to fill with water. Why?
- Go back and review Lesson 1. Which aspects of water treatment explained in this lesson have comparable aspects in the natural world? List the similarities and differences between the natural and man-made processes.
- Water in your water treatment plant has a pH of 6.5, a carbon dioxide concentration of 30 PPM, a total alkalinity of 50 PPM, and a high hydrogen sulfide concentration. What problems is the water likely to cause if distributed in this condition? What treatment methods would you recommend to correct these problems?
- You are new to a water treatment plant and don't know the treatment history. Customers tell you that they originally complained about red water staining their sinks. Water soon began to run clear, but then some customers complained of green and blue water. What caused the different water colors? What treatment method had been used to correct the red water problem? How will you correct the blue and green water problem?