Suspended Solids Removal

Framework for Dredged Material Management

May 2004

Contaminants in these streams will present a wide array of concentrations

depending on their source, and individual sources are often highly variable in

concentrations and flows. Most of the contaminants for these streams (with the exception

of leachate) are associated with the suspended solids and will be removed by effective

suspended solids removal. Another characteristic of these streams is their variety of

contaminants, both organic and inorganic, as well as potentially toxic contaminants.

These characteristics may require more than one treatment process. Commonly used

wastewater treatment processes are available to achieve effluent limits for most

contaminants. However, application of treatment processes for dredged material effluent

has been generally limited to removal of suspended solids and contaminants associated

with these particulates.

Liquid treatment technologies can be classified as metals removal processes,

organic treatment processes, and suspended solids removal processes. Many of these

processes concentrate contaminants into another phase, which may require special

treatment or disposal. This discussion focuses on suspended solids, toxic organics, and

heavy metals. Conventional contaminants, such as nutrients, ammonia, oxygen-

demanding materials, and oil and grease, may also be a concern for dredged material

effluents. Most of the processes for dissolved organics removal are suitable for these

contaminants.

5.4.2.1 Suspended Solids Removal

Suspended solids removal is the most important liquid streams technology

because it offers the greatest benefits in improving effluent quality not only by reducing

turbidity but also by removing particulate-associated contaminants. Suspended solids

removal processes differ from dewatering processes because for this application the

solids concentration is much lower than for a dredged material slurry. Settling

mechanisms for these streams are characterized by flocculent settling rather than zone or

compression settling. For CDF liquid streams, the solids remaining will be clay or

colloidal size material that may require flocculants to promote further settling in clarifiers

or sedimentation ponds. Chemical clarification using organic polyelectrolytes is a proven

technology for CDF effluents (Schroeder 1983). Filtration, permeable dikes, sand-filled

weirs, and wetlands have also been used on occasion for CDF demonstrations or pilot

evaluations. More detailed guidance on suspended solids removal processes as applied to

CDFs is available (USACE 1987; Cullinane et al. 1986).

5.4.2.2 Metals Removal

Metals removal processes that may be considered for application at CDFs are

similar to those commonly used for industrial applications. Flocculation is effective for

removal of metals associated with particulate matter. Polymers and inorganic flocculants

have been demonstrated to be effective for removal of suspended solids from dredging

effluents, but removal of dissolved heavy metals has not been evaluated in field

applications. Ion exchange and precipitation are probably two of the more efficient

metals removal processes, but they must generally be designed for specific metals and