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As regulatory requirements continue to move toward more stringent non-point source discharge requirements, effective sediment control becomes even more important. “Best Management Practices” are continually being adapted to account for new technology and methods of operation. This article reviews various sediment control measures that are currently in use on construction sites and have proven to be effective. It reviews their strengths and weaknesses and discusses how they have been used in some real world applications. If you have not yet seen or used any of these sediment control measures, you most likely will in the near future.
Not all sediment control measures are equal. There is no “Silver Bullet”. Sediment control measures need to fit site conditions. As with most tasks in life, the importance of planning cannot be overstated. A site manager must review the site’s characteristics before the water starts flowing. This includes taking a careful look at the project and the environment in which it is operating:
1. What types of soils are present
2. How much water is expected
3. How often is water expected
4. What are the anticipated peak flows
5. What is the required storage capacity
6. What is the terrain
7. Where does the water go when it leaves the site
8. What are the discharge goals
The above information allows a site manager or consultant to evaluate the strengths and weaknesses of the array of sediment control measures and equipment for each site. Sediment control measures differ dramatically in their effective range, cost, and maintenance requirements.
This does not mean that there will not be unforeseen conditions that require modifications or augmentations made to the planned sediment control measures, but it starts the project off on a strong footing—and one that is most often mandated for the Stormwater Permit.
Sediment control measures and equipment can be grouped into three basic categories.
1. Gravity based settling systems
2. Passive Filtration Systems
3. Polymer Treatment Systems
1. GRAVITY BASED SETTLING SYSTEMS
Gravity based settling systems are currently the most used sediment control measures across the country.
Sediment basins have a long history and are almost universally known. An important fact about sediment basins is not often discussed, however: sediment basins are often pumped down. If care is not taken when pumping a sediment basin, the sediment that was retained in the basin can be sucked downstream. A pump inlet should always be attached to a floating suction or a well-packed and placed well point. If using a floating suction, use one that has a plate on the bottom to guard against sediment being sucked off the bottom.
Strengths: Sediment basins hold large volumes of sediment. Heavy settling sediments such as sands settle out very effectively.
Weaknesses: Sediment basins are ineffective in removing fine sediments such as fine silts and clays. As sediments settle into the basin, the retention time decreases. This decreases efficiency. They require a relatively large surface area.
Tanks are another form of gravity based settling systems. There are two basic designs of tanks. A standard storage tank and a weir tank. Both types of tanks can be mobile up to 18,000 gallons. This allows them to be brought in and out as needed. A standard storage tank is just one large chamber. A weir tank has weirs built into the tank to enhance the sediment settling efficiency. Sediment laden water is pumped into one end of the tank from a collection point. The water travels over and under a series of weirs (baffles) before reaching the outlet at the other end of the tank. The weirs serve to maximize the distance the water must travel inside the tank. They also minimize water turbulence. Both of these factors greatly increase the settling efficiency of the weir tank over a standard tank. The minimized turbulence also allows weir tanks to be used for continuous flow operations much more effectively than standard tanks. For these reasons, weir tanks tend to make a better choice for sediment control applications.
It helps to ensure that the weir tank comes with a cleanout manway in each compartment. Otherwise, cleanout becomes problematic. Usually, a weir tank will not need to be cleaned out during the project. However, long term projects or flows with very high sediment loads may require cleanout during the project. Weir tanks come with open top and closed top designs. For safety reasons, it is preferable to have a closed top or lids that can cover the tank. Open top tanks have no protection to prevent workers or kids from falling into the tank. This is very important because these tanks are often placed in unsecured areas.
GET DRAWING OF WEIR TANK FROM MIKE.
Strengths: Mobile tanks eliminate the need for permanent dedication of space or construction of earthen dikes. Small sites rarely have much available land for sediment control. The tanks can hold a large volume of solids before requiring clean out. Tanks require very little operational maintenance. The flexibility of tanks gives the site manager the option of operating in either batch or continuous operating modes. Properly designed tanks are easier to drain down than sediment basins. This allows a more rapid return to full storage capacity.
Weaknesses: Like sediment basins, tanks are rather ineffective in removing fine to medium sized sediments. Mobile tanks have a limited storage capacity—typically 18,000 to 21,000 gallons. This means there are limitations to the flows that can be handled. A weir tank has a practical limit of 65 gpm per tank for adequate sediment settling. (It can be higher for larger sediments such as large sands.) The tanks must be cleaned out when the project is completed.
2. PASSIVE FILTRATION SYSTEMS
The classification of Passive Filtration Systems includes sediment control measures that have barriers to remove the sediment from the water.
Portable Sand Filters remove heavy to medium sized sediment under a wide range of flows. Sand filters use a sand media bed as a barrier filter to sediments as water travels through the sand. There are two basic types of sand filters: Gravity Sand Filters and Pressurized Sand Filters. As the name implies, gravity sand filters rely on gravity to draw the sediment-laden water through the sand. Pressurized sand filters hold the sand media bed in two or more pressure rated vessels and the water is pumped under pressure through the sand media bed. Pressurized sand filters are capable of handling much higher flows per square foot of sand media bed (referred to as the flux rate) than gravity sand filters. Gravity sand filters are increasingly used as a post development sediment control measure. Due to the comparatively low flux rate and inability to easily remove retained sediment, they are not very practical as a construction site sediment control measure. In this article, we will discuss the pressurized sand filters.
Over time, sediments build up on the top and within the sand media bed. Removal of these retained sediments is accomplished by backwashing the sand media bed. Backwashing simply means reversing the flow of the water through the sand media bed. The water flows up through the bed dislodging the retained sediment. The backwash water carries the sediment out through a backwash line into a collection tank, sediment basin, or other temporary holding facility. A sand filter can be backwashed hundreds of times. Industrial sand filters often use a separate water source for backwash. On construction sites, this is often not practical. For this reason it is best to use a sand filter that is capable of using the discharge water from the sand filter for backwashing needs.
It is recommended to use a sand filter with automatic backwash capability. The backwash controller allows a backwash sequence to initiated on a regular time interval or based upon an increase in the pressure drop across the sand media bed. (Sediment build up on the media bed causes an increase in the pressure drop.)
Strengths: The ability to backwash makes a sand filter a very cost-effective choice in situations with medium to heavy sediments. Self-cleaning backwashing capability makes them effective in removing large amounts of sediments. An automatic backwash controller eliminates the need for constant operational supervision. Sand filters have a small footprint.
Pressurized sand filters have a high flux rate. This means that the footprint of a sand media filter is very small compared to sediment basins and tanks. A 200 gpm sand filter will typically have a footprint that is 3 feet wide by 8 feet long. A 100 gpm sand filter requires a footprint no greater than 5 feet wide by 20 feet long. Sand filters produce reliable results. By altering the grade of the sand used in the media bed, the micron rating (what size microns are removed) can be adjusted to meet site specific conditions. A portable sand filter using very fine sand can remove sediment down to the 50 micron range.
Weaknesses: Sand filters do not effectively remove fine silts or clays. A medium head pump is required to pressurize the system. The backwash generates a concentrated waste stream that must be addressed.
Bag Filters are another type of passive filtration system. There are gravity based bag filters and pressurized bag filters. Both of these types of filters have roles as sediment control measures on construction sites and dewatering operations.
The gravity based bag filters are not contained within any vessel or enclosure. They lay on the ground. Water is pumped into an opening in the bag filter. The water flows from the inside of the bag, through the filter cloth, and out onto the ground. The filter cloth acts as a barrier to the sediment. As a result, the sediment is retained inside the bag. The retained sediment tends to form a filter cake along the inside of the bag. This will lead to better filtration later in the life of the bag filter than early on—though the flow capacity will also tend to diminish. It is important not to disturb the bag filter. This will break up the filter cake and reduce its efficiency. Once the bag is filled to capacity, it can be cut open and the sediment removed or disposed in a landfill. Some areas will allow the bag filter to be buried in place.
Until a solid filter cake is built up, the bag filter is not effective in removing fine sediments such as silts and clays. The length of time required to build up an adequate filter cake is site dependent. It depends on the types of sediment in the water, the sediment load, and the flow.
Filter bags come in a variety of coarseness. The tighter the bag the more effective it will be in removing sediments, but the greater the resistance to flow.
Storm drain filters and inlet protectors operate on the same principal as gravity based filter bags except the water flows into the opening.
It is important to situate the bag filters in a location where the effluent water does not cause further erosion. Often times this is done by placing the bag filter on a bed of hay bales or gravel. Bio swales can be used, also.
These bags come in various different sizes and can accommodate a wide range of flows.
Strengths: Effective in removing heavy sediments. If used in a vegetated area they can be easily setup. Storm drain filters and inlet protectors can be reused numerous times.
Weaknesses: They will not remove fine sediments such as silts and clays until a filter cake builds up. The length of time it will take for a filter cake to develop is unpredictable. When the filter cake is built up, it is difficult to predict the removal efficiency for fine sediments, and the flow rate diminishes. If you cannot bury the used bag filter in place, they can be difficult to move and dispose. They are not readily portable once they have been used. Care must be taken to ensure that the discharge does not cause further erosion. In some cases this will require re-channeling the effluent flow.
Pressurized bag filters operate on the same principle as a gravity bag filter, but they are placed in a pressurized bag chamber. The water is pumped into the inside of the bag and passes through the bag filter element. The effluent water can then be piped to the desired discharge point. The filtration efficiency of a pressurized bag filter is similar to a gravity bag filter. The surface area of the bag filters tends to be rather small compared to gravity bag filters. A standard 7” diameter by 30” long bag filter can handle up to 100 gpm. Higher flow rates are achieved by using multiple bag filters—up to 1,000 gpm.
Strengths: The containment of the bag filters in vessels makes the unit very portable. It can be moved from location to location rather easily. They are most effective in removing medium to heavy sediments.
Weaknesses: They do not efficiently remove fine sediments such as silts and clays. The smaller bag surface area and volume means that the sediment holding capacity is much smaller than gravity bag filters.
Wound Cartridge Filter Units are the most efficient pressurized filter systems for removing fine sediments such as silts and clays. The cartridges are constructed by winding a polypropylene yarn around a core using microprocessor-controlled technology. The winding process creates an increasingly tighter barrier as you move towards the center. Wound cartridges work in the opposite direction as bag filters: the water flows from the outside of the cartridge to the inside—were the physical barrier is smallest. The water moves down the interior of the core to the outlet of the vessel. A pressurized vessel can hold from one to over 100 cartridges. Typically, each cartridge is 40 inches long with a 2 ˝” diameter. The greater the number of cartridges, the higher the flow capacity and sediment holding capacity.
The flow capacity of a wound cartridge filter unit depends on the number of cartridges in the vessel. For projects that can not tolerate maintenance shutdowns such as pipeline dewatering multiple chambers are placed in parallel. This allows the flow to continue through the other chambers while the cartridges are being changed out in one chamber.
Wound filter cartridge filtration units have a very small footprint. A 50 gpm unit requires less than a square foot. A 1,000 gpm system requires less than four square feet. The small footprint makes them ideal for mobile trailer mounted systems.
The microprocessor-controlled winding technology not only creates the most efficient sediment filter; it produces a very reliable removal curve. Unlike bag filters, wound cartridges do not require a filter cake to build up for fine sediment removal. This means that these units can be brought online with a high predictability of operational effectiveness.
The wound cartridges we are referring to have a nominal rating of 0.5 microns. It is important to note that wound cartridges can be made using a mechanical winding process. Though, they may have a 0.5 micron rating, they are not nearly as efficient at removing fine sediments and clays. For projects that do not have problems with fine sediments, a lower quality filter might be sufficient.
Strengths: Wound cartridge systems provide the best sediment removal efficiency without utilizing chemical treatment. These cartridge systems are effective in removing fine sediments not removed by sediment basins, sand filters, or bag filters. They are highly portable with a very small footprint. Operational effectiveness is very consistent.
Weaknesses: Wound cartridges will not remove colloidal clays. They have a low sediment holding capacity.
3. POLYMER TREATMENT SYSTEMS
Polymer treatment systems are highly effective in removing colloidal clays. Polymer treatment systems are different than other sediment control measures because they actually fall under the category of a water treatment system. As such, most areas require permits for their use.
There are two basic types of polymer treatment systems in use on construction sites: Cationic and Anionic polymer based treatment systems. Cationic polymers carry a positive charge. Anionic polymers carry a negative charge. Each polymer has its uses and benefits. The polymer attracts a large number of particles (sediments) with an opposite charge to create larger particles. The larger particle will now settle out at a faster rate. This is called flocculation. (Technically, this process incorporates both coagulation and flocculation. The two actions are closely related. However, it is easier and more common in the sediment control industry to call it flocculation.)
The amount of polymer needed to flocculate the sediments from the water depends on types and levels of sediment in the water. The appropriate amount is determined using jar tests. A jar test is conducted by adding different amounts of polymers to a representative sample of the sediment laden water in a series of jars. This process shows the optimum polymer concentration. Currently, batch treatment operation is the favored format.
The polymer is injected into the water flow just prior to a series of settling tanks or ponds in a manner that maximizes contact with the sediments. The sediments then bind with the polymer and rapidly settle into a floc on the bottom.
Float intakes then draw the water off the top of the settled water. Depending on the situation, the water may then be pumped through a wound cartridge filter unit to catch residual suspended solids. Polymer treatment systems consistently discharge water that is less than 30 NTU’s. It is important to monitor the operation to determine system efficiency. Changes in influent water quality or flow rates may require changes in the polymer concentration.
The above is a very basic description of the process for informational purposes only. A comprehensive description is beyond the scope of this article. Some important factors such as pH, pond size, and average settling time must be taken into consideration. Also, it is important not to think that any cationic or anionic polymer can be used. Polymer is a very generic term. If polymer treatment systems are used in your area, your local agency should have a list of the polymers that work and are approved for use. Under no circumstances should untrained personnel attempt to design, implement, or use a polymer treatment system.
Cationic polymers tend to create a more complete flocculation when removing colloidal clays from water. The concern with cationic polymers is that unattached cationic polymers can be toxic to aquatic life as low as 150 ppm to 250 ppm, which is in the usage range. The toxic effects occur because the polymer attaches to the aquatic life’s gills. Anionic polymer usage range is between 1 ppm and 10 ppm. Toxicity for unattached anionic polymers does not become a concern until over 150 ppm. It is critical to understand that these numbers are for unattached polymer. If sediments are present in the water, these polymers become attached to the sediments. Neither anionic nor cationic polymers are toxic to aquatic life once attached to sediments. The polymers in use on construction sites have been used for years to clarify potable water and in agriculture. They have a long track record of safe and effective use.
There is another method of polymer utilization that is effective in sediment control. Using anionic polymer as a soil binder/tackifier costs much less than water based polymer treatment system. The polymer is mixed at a ratio of approximately 1 pound per 1,000 gallons of water and sprayed over the ground. Each 1,000 gallon solution covers approximately 1 acre. The polymer binds the top the soil together to protect it from erosion. The sediments that do break free tend to be larger. This means that they can be settled or filtered out more effectively than with untreated soil. Though this method is cheaper, effluent water quality appears to be less consistent than with water based polymer treatment systems. Because the polymer is not being used for water treatment purposes, most states will not require a water treatment permit for this activity. (However, it is best to check with your local agency.)
Strengths: Water based polymer treatment systems provide consistent removal of fine sediments—including colloidal clays. CHECK TO SEE ABOUT REDUCED NUTRIENT LEVELS. Effluent water quality typically is below 20 NTU’s. The settling tanks or ponds can be designed to hold large amounts of sediments. Anionic polymer ground application enhances erosion control and sediment control simultaneously, and has a low relative cost.
Weaknesses: Water based polymer treatment systems are water treatment systems. As such, they are more complex and costly than other sediment control measures. Trained personnel are required to design and monitor the system. They require continual monitoring during operation. Anionic polymer ground based application is much cheaper and easy, but the effluent water quality is much less dependable than when introduced directly into the water.
Though this article has focused on sediment control measures and equipment, it is worth mentioning the importance of a “Whole Systems Approach” to construction stormwater runoff and dewatering operations. On most construction sites, contractors implement both erosion control and sediment control measures as part of their Best Management Practices (BMP’s). This is because erosion control and sediment control each play vital roles in minimizing the amount of sediment that leaves the site. Effective erosion control measures also minimize the cost of overall erosion and sediment control activities. The reason for this is simple. Experience has shown that it is cheaper to keep the sediments in their original place than removing them from the stormwater runoff. Effective erosion control helps keep the levels of sediment in stormwater runoff at manageable levels. However, no system is perfect or failsafe. This is especially true of erosion control. Invariably, some sediment is dislodged from the soil and is carried along with the stormwater runoff. Sediment control measures seek to prevent the sediment from migrating offsite into surface waters, storm drains, or sewer systems.
The “Whole Systems Approach” is also a vital tool when focusing on your sediment control needs. The proper combination of sediment control measures might not only be required for acceptable effluent water quality; it will often mean the difference between cost-effectiveness and a busted budget. A good sediment control plan will utilize measures in such a manner that their individual strengths and weaknesses compliment on another. For example, a sediment basin is not effective at removing fine sediments, but it can hold large volumes of sediments. If a site has both heavy and fine sediments, a wound cartridge filter unit can be used following the sediment basin. In this manner, the sediment basin will adsorb the larger volume of the sediments and the wound cartridge filter unit will remove the remaining fine sediments. The sediment basin alone most likely will not produce acceptable effluent water quality in this case. If the wound cartridge filter unit was used alone, it could be inundated by the large volumes of sediment.
A separate article could be written on each of the above sediment control measures. If you are not knowledgeable about any of the above measures, make sure you talk with someone who has experience with them before attempting to put them into use.
Case Study #1
A contractor on a public works project in the San Francisco Area had two sediment control problems: stormwater runoff, and low flow dewatering operations. Space for sediment basins was minimal. The site managers wanted to consolidate both stormwater runoff and dewatering sediment control measures. It was estimated that the maximum flow during a storm would be 200 gpm. A Sediment basin followed by a weir tank proved inadequate in removing the fine sediments. The next step was to bring in a trailer mounted bag filter and wound filter cartridge unit. The effluent from the weir tank was pumped through the filter unit. This solved the problem with the fine sediments.
Case Study #2
A contractor in the Northwest had very little land with which to work. The soil contained very high levels of clay. Stormwater runoff averaged about 40 gpm. The discharge went to a small creek with sensitive habitat. The site managers first tried using weir tanks. While these retained the large solids, the effluent water was still over 125 NTU’s. The next step involved bringing a small sand filter, a bag filter, and a wound cartridge filter unit online. The effluent water went down to less than 70 NTU’s. This was acceptable to the local regulatory agency. If it had not worked, the contractor was going to have to install a polymer treatment system.
Case Study #3
A site manger in the Northwest faced the difficult task of working on a site with almost no extra space, but the soil was mostly clay. It was obvious that the runoff was going to contain colloidal clays. Add to this, high projected flow rates—between 200 and 500 gpm. He chose to apply anionic polymer as a tackifier to the exposed ground. This greatly reduced the amount of sediment coming off the site. As a follow on measure, he installed two weir tanks, a sand filter, and a particulate filter. Though this was not as reliable as a treatment system that injecting polymer into the stormwater runoff, it was a much simpler system. The system worked well enough that the manager did not have to use polymer in the stormwater runoff.
Case Study #4
A large scale construction project in the Seattle area faced the danger of being shutdown during the rainy season, if the contractor could not remove high levels of colloidal clays from the stormwater runoff. The site had enough space to put a cationic polymer treatment system in place. Flow rates were anticipated to be as high as 500 gpm. The cationic polymer treatment system worked throughout the entire rainy season. The effluent water was less than 20 NTU’s, and the contractor never had to shut down because of effluent stormwater quality.
Case Study #5
A sub-division overlooking the Pacific Ocean had its main sediment basin fill up with sediment before the rainy season ended. As a result, the stormwater runoff contained unacceptable levels of sediment. The contractor brought in a trailer mounted wound cartridge unit to draw water from the sediment basin via a floating suction. This prevented sediments from being discharged into the adjacent stream. By drawing the water level down after each rain, the sediment basin was able to hold the next rain without overflowing. After each rain, the water was pumped through the wound cartridge filter unit for about eight to twelve hours at 250 gpm before reaching the bottom.
Case Study #6
A contractor in the New York/New Jersey area had a project that would last for up to six months. The site was small enough that the contractor did not want to build a sediment basin. Instead he rented a weir tank, a bag filter, and a wound cartridge filter. During each rain event, the water was pumped into the weir tank. The water was then pumped out of the tank and through a bag filter and wound cartridge filter unit at about 20 gpm.
Case Study #7
A new sewer pipe was being installed in the Tahoe Basin. The process involved moving in and out of saturated soils. It was impractical to construct one single sediment control measure for the entire length of the installation. Colloidal clays did not appear to be a problem. A weir tank and mobile wound filter cartridge unit were brought to areas where groundwater created a problem along the length of the installation.
Except for material that is used up in the process such as filter bags and polymer, all of the equipment needed to implement any of the sediment control measures above can be rented or purchased. Because of an almost limitless number of variables, it is virtually impossible to provide pricing in this article. In general, however, the more complex the equipment the more it will cost. This holds true for both rental and purchase. However, equipment cost comparisons alone is not always adequate. Available land, project timeline, regulatory requirements, and operational needs all factor into sediment control equipment decisions.
Sediment levels in stormwater runoff are coming under increasing scrutiny all across the country. Once acceptable practices are now seen as insufficient. The above sediment control measures provide any contractor with a wide array of options to stay in compliance.
 For detailed information of Sediment Basin design please refer to your local agencies Field Manual. The San Francisco Regional Water Quality Control Board’s Erosion and Sediment Control Field Manual is also a good source.