Gravity-Based Settling Systems

Gravity-based settling systems are currently the most used sediment control measures. .

Sediment Basins. Sediment basins have a long history and are almost universally known. However, an important fact about sediment basins is not often discussed: they are frequently 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 well-packed and -placed well point or a floating suction 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: The basins are ineffective in removing fine sediments. As sediments settle into the basin, the retention time decreases, which decreases efficiency. They require a relatively large surface area.

Tanks. There are two basic designs of tanks: a standard storage tank and a weir tank. Both types can be mobile up to 18,000 gal., which 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 and to 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 a project unless the project is long-term or has flows with very high sediment loads. 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 children from falling into the tank. This is very important because these tanks are often placed in unsecured areas.

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 cleanout, and they require very little operational maintenance. The flexibility of tanks gives the site manager the option of operating in either batch or continuous modes. Properly designed tanks are easier to drain down than sediment basins, allowing a more rapid return to full storage capacity.

Weaknesses: As with sediment basins, tanks are rather ineffective in removing fine to medium-size sediments. Mobile tanks have a limited storage capacity-typically 18,000-21,000 gal. This means there are limitations to the flows that can be handled. A weir tank has a practical limit of 65 gal. per minute (gpm) per tank for adequate sediment settling (and can be higher for larger sediments). The tanks must be cleaned out when the project is completed.

Passive Filtration Systems

This classification includes sediment control measures that have bafflers to remove the sediment from the water. Portable sand filters remove heavy to medium-size sediment under a wide range of flows.

Sand Filters. Sand filters use a sand media bed as a barrier filter to sediments as water travels through the sand. 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 are gravity sand filters. Gravity sand filters are increasingly used as a postdevelopment sediment control measure. As a result of the comparatively low flux rate and inability to easily remove retained sediment, they are not very practical as a construction-site sediment control measure. This article 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, a 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. This is often not practical on construction sites, so it is best to use a sand filter capable of using the discharge water from the sand filter for backwashing needs.

Experts recommend using a sand filter with automatic backwash capability. The backwash controller allows a backwash sequence to be initiated on a regular time interval or based on an increase in the pressure drop across the sand media bed. (Sediment buildup 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 sediment. 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, which means that the footprint of a sand media filter is very small compared to that of sediment basins and tanks. A 200-gpm sand filter will typically have a 3-ft.-wide by 8-ft.-long footprint. A 100-gpm sand filter requires a footprint no greater than 5 ft. wide by 20 ft. 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 wastestream that must be addressed.

Bag Filters. There are gravity-based bag filters and pressurized bag filters. Both 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 lie 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 baffler to the sediment. As a result, the sediment is retained inside the bag and tends to form a filter cake. This will lead to better filtration later in the life of the bag filter compared to 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 of 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. The length of time required to build up an adequate filter cake depends on the types of sediment in the water, the sediment load, and the flow. Filter bags come in various levels 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. These operate on the same principle as gravity-based filter bags except that 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. Oftentimes this is done by placing the bag filter on a bed of hay bales or gravel. Bioswales can also be used. These bags come in various sizes and can accommodate a wide range of flows.

Strengths: They effectively remove heavy sediments. If used in a vegetated area, they can be easily set up. Storm-drain filters and inlet protectors can be reused numerous times.

Weaknesses: They will not remove fine sediments until a filter cake builds up. The length of time it will take for a filter cake to develop is unpredictable. Once it does, 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, it can be difficult to move and dispose of. It is not readily portable once it's been used. Care must be taken to ensure that the discharge does not cause further erosion. In some cases this will require rechanneling the effluent flow.

Pressurized Bag Filters. These operate on the same principle as gravity-based bag filters, 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 that of a gravity-based bag filter. The surface area of the bag filters tends to be rather small compared to gravity-based bag filters. A standard 7-in.-diameter by 30-in.-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. They are most effective in removing medium to heavy sediments.

Weaknesses: They do not efficiently remove fine sediments. The smaller surface area and volume means that the sediment holding capacity is much smaller than in gravity-based bag filters.

Wound Cartridge Filter Units. These pressurized filter systems are the most efficient for removing fine sediments. 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 toward the center. Wound cartridges work in the opposite direction as bag filters: The water flows from the outside of the cartridge to the inside---where the physical baffler 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 more than 100 cartridges. Typically, each cartridge is 40 in. long with a 2.5-in. 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 cannot tolerate maintenance shutdowns, 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 4 ft.2 The small footprint makes them ideal for mobile trailer-mounted systems.

Not only does the microprocessor-controlled winding technology create the most efficient sediment filter, it also produces a very reliable removal curve. Wound cartridges do not require a filter cake to build up for fine-sediment removal. This means that these units can be brought on-line with a high predictability of operational effectiveness.

The wound cartridges we are referring to have a nominal rating of 0.5 micron. It is important to note that wound cartridges can be made using a mechanical winding process. Though they might have a 0.5-micron rating, they are not nearly the most efficient in 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.

Polymer Treatment Systems

Polymer treatment systems are highly effective in removing colloidal clays. They are different from other sediment control measures because they actually fall under the category of water treatment systems. 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. 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.

The amount of polymer needed to flocculate the sediments from the water depends on the 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 floe 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 nephelometric turbidity units (NTUs). It is important to monitor the operation to determine system efficiency. Changes in influent water quality or flow rates might require changes in the polymer concentration.

The above is a very basic description of the process for informational purposes only. It is important not to think that any cationic or anionic polymer can be used. 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 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 and 10 ppm. Toxicity for unattached anionic polymers does not become a concern until over 150 ppm. If sediments are present in the water, these polymers become attached to the sediments. Neither cationic nor anionic polymers are toxic to aquatic life once they are attached to sediments. The polymers in use on construction sites have been used for years in agriculture and to clarify potable water. They have a long track record of safe and effective use.

When used as soil binders/tackifiers, anionic polymers cost much less than water-based polymer treatment systems. The polymer is mixed at a ratio of approximately 1 lb. per 1,000 gal. of water and sprayed over the ground. Each 1,000-gal. solution covers approximately 1 ac. The polymer binds the top of 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 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.

Strengths: Water-based polymer treatment systems provide consistent removal of fine sediments. Effluent water quality typically is below 20 NTUs. 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 easy and much cheaper, but the effluent water quality is much less dependable than when introduced directly into the water.

On most construction sites, contractors implement both erosion control and sediment control measures as part of their Best Management Practices 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. Experience has shown that it is cheaper to keep the sediments in their original place than it is to remove 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 fail-safe. Invariably, some sediment is dislodged from the soil and carried along with the stormwater runoff. Sediment control measures seek to prevent the sediment from migrating off-site into surface waters, storm drains, or sewer systems.

The "whole systems approach" is 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 complement one 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, the large volumes of sediment could inundate it.

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 above sediment control measures can be rented or purchased. While it is virtually impossible to provide pricing in this article, in general the more complex the equipment, the more it will cost. This holds true for both rental and purchase. However, equipment cost comparisons alone are not always adequate. Available land, project time line, regulatory requirements, and operational needs all factor into sediment control-equipment decisions.

Sediment levels in stormwater runoff are coming under increased scrutiny all across the country. Once-acceptable practices are now seen as insufficient. The aforementioned sediment control measures provide any contractor with a wide array of options to stay in compliance.