What is dewatering of sediments

The cleanup plan, which has been under study for 25 years, calls for dredging 2. Once the contaminated sediment is removed, a protective cap of 2 ft of sand and 1 ft of habitat support would be placed over the dredged area. The dredged material then would be taken by barge to a local facility for dewatering.

The water would be treated and returned to the river; the sediment would be transported by rail for thermal treatment or landfilling. Finalizing the cleanup plan and conducting the engineering and design work will be carried out over the next few years, and EPA estimates that it will take another five years to dredge the 8 miles of river bed. This article is intended to describe and compare various types of mechanical dewatering techniques and will examine the role of polymers in enhancing their performance.

Polymers long chain-like molecules are used in dredging projects to improve solids capture and increase cake dryness. Polymers aggregate suspended solids by flocculation, which improves solids capture in downstream dewatering devices.

Flocculation occurs when polymers act like bridges between particles, resulting in larger particles called flocs. In the case of contaminated sediments, polymers also are able to enhance the attachment of contaminants to the large flocculated sediment particles, thus leaving cleaner sediment behind. This reduces the amount of contaminated sediment that needs to be treated, disposed of or contained.

The variables that influence equipment choices are many and must be considered in conjunction with the overall objectives of a given sediment dewatering project. The driest cakes are generated by plate and frame presses and geotextiles.

This is due to the length of time the solids are exposed to hydraulic pressures and the relatively low shear forces exerted during their use. Larger amounts of chemical additives are required to build stronger bonds, which can withstand stronger shear forces in certain types of equipment.

Accordingly, chemical dosing is lowest for plate and frame presses and geotextiles with low shear forces. Ease of operation depends on the operating complexity of the equipment. Geotextiles are the easiest to operate because there are no moving parts. On the other hand, they also require large amounts of space. Plate and frame presses also have relatively large footprints because they operate in batch mode, and multiple units are required to ensure a continuous operation.

The overall cost takes into account the initial capital cost, as well as labor, maintenance and energy expenses, and the cost of chemical additives. Likewise, properly selected sediment filters used in conjunction with sediment traps or basins or similar BMPs removes fine sediment that contributes to turbidity. For example, some cationic polymers may increase toxicity risks to fish and other aquatic organisms if not managed and used properly. Anionic polymers are much less toxic, and are widely used for construction stormwater applications.

See here for more information and consult product material safety data sheets and product specifications. The use of stormwater filtration does not require prior approval. Dewatering practices are applicable to all construction sites where excess water needs to be removed and discharged, particularly excavated areas where water collects and does not otherwise drain, such as trenches, pits, sediment basins, and traps.

Accumulated water and sediment must be removed from these practices during construction to ensure their effectiveness throughout the construction process. Water pumped from low areas of construction sites can contain high concentrations of suspended soils, resulting in high turbidity. These fine particles may already be suspended in water or may have become suspended due to the pumping process.

Dewatering discharge to surface waters is not permitted until adequate treatment is achieved such that the discharge will not negatively impact the receiving water and aquatic ecosystems.

Chemical treatment and sediment filtration should be considered where turbid discharges to sensitive waters cannot be prevented using sediment basins and traps and other traditional BMPs. Various practices can be used for sediment removal from dewatering discharge. Sedimentation is primarily effective at removing larger sized particles, while filtration and chemical treatment can also remove the fine particles.

These approaches are less effective for dissolved nutrients and metals that are non-adsorbed. Effectiveness of chemical treatment depends greatly on the pH and temperature of the water being treated. Expected performance benefits for various dewatering practices. Link to this table. All appropriate safety precautions must be followed when handling chemicals, and designs must consider and minimize all hazards related to the particular chemicals used.

Consult material safety data sheets for chemical application, mixing, and use information, and keep them with the SWPPP for further reference.

Chemical safety equipment should be stored on site in a lockable shed and should include the chemical injector, secondary containment for acid, caustic, buffering compound, and treatment chemical , emergency shower and eyewash, and monitoring equipment including a pH meter and turbidimeter.

Any tanks or ponds should have ladders, steps, or similar features extended from top to bottom. Per Section The use of chemicals for treatment requires review and approval by the appropriate permitting authority to ensure they are not toxic to aquatic organisms. Link here for more information. Dewatering bags, wire-reinforced silt fence enclosures, geotextiles, fiber log berms, rock berms, and other filters can be used effectively for many dewatering applications.

Soil infiltration of dewatering flows is also highly effective. The key to success is to ensure that 1 the filter traps the target size range and desired volume of particles, and 2 flow volumes do not overwhelm the filtering site.

Filters are often used in conjunction with chemical treatment to remove flocculated soil particles prior to final discharge. Frequent cleaning of the filter media will be required. Sedimentation facilities for dewatering include wire-reinforced silt fence enclosures — which perform double duty as both filters and settling devices - sediment basins and traps, tanks, fabricated enclosures, and other settling vessels.

At a minimum, sediment basins and traps should be designed with 3, cu ft of storage volume per acre of upstream drainage area. Sedimentation facilities are often used in conjunction with chemical treatment to remove flocculated soil particles prior to final discharge.

Typical construction site application of flow-through chemical treatment involves the addition of a flocculant e. Flows involving a more complex mix of challenges in terms of pH and pollutants may require more sophisticated methods for treatment. In an advanced flow-through chemical treatment system, stormwater is collected at interception point s on the site and is diverted by gravity or by pumping to a stormwater storage pond or other stormwater holding area.

It is important that the holding area is large enough to provide adequate storage until treatment occurs. Stormwater is then pumped from the stormwater holding area to the chemically enhanced sand filtration system where a coagulant is added. Adjustments to pH may be necessary before coagulant addition. The sand filtration system in an advanced flow-through chemical treatment system continually monitors the stormwater effluent for turbidity and pH.

If the discharge water is out of an acceptable range for turbidity or pH, the water is returned to the untreated stormwater pond where it will begin the treatment process again. Dewatering flows can also be treated in batches, rather than via flow-through contact with the flocculants. Batch treatment is often used for dealing with fine clays, where runoff can be captured and stored, allowing time for chemical mixing and sedimentation prior to discharge.

Batch treatment on construction sites typically involves the use of a sediment pond, trap, or other vessel e. Treated water in the basin or vessel can be discharged from the upper portion of the water column after allowing sufficient time for floc settling. Discharges of treated water can also be routed through a filter for additional sediment removal.

An engineered batch treatment system may be required in cases where complex treatment challenges exist. A typical engineered batch chemical treatment system for these more challenging situations consists of. Prior to chemical treatment, ensure the pH of the stormwater to be treated is within the recommended range for the chemical used typically between 6. The pH of the stormwater can be altered with the application of carbon dioxide acidic or a base such as sodium bicarbonate baking soda.

A batch chemical treatment system consists of four main steps. Multiple treatment cells allow for clarification of chemically treated water in one cell while the other cell s is filling or emptying. Size the stormwater storage system and treatment cells to hold 1. Provide bypass around the chemical treatment system into a settling pond to accommodate larger storm events.

The following is a list of chemicals typically used for treating turbidity subject to local, state, or federal requirements; the appropriate permitting authority should be contacted for additional information and conditions of use.

List of chemicals typically used for treating turbidity table Link to this table. As noted above, sediment filtration can be used in conjunction with gravity settling e. There are two main types of sediment filtration methods: rapid and slow filtration. Slow sediment filtration uses gravity to move water through a system. It is typically used on the treatment line of construction stormwater and does not have a backwash system.