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Dewatering of Chemical Sludges from Enhanced Coagulation, Arsenic Treatment and Innovative Clarification Processes

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New drinking water regulations for the removal of arsenic and organic precursors that form disinfection by-products will require water treatment facilities to add increased quantities of coagulant to achieve the desired process performance. This will impact surface water treatment plants and groundwater plants that utilize chemical precipitation techniques. The impact of chemical addition will result in changes in sludge characteristics. Surface water treatment solids, particularly from low turbidity waters, will be composed of predominantly chemically precipitated solids instead of natural silts clays and other inorganics. Sludges from groundwater plants removing arsenic will consist of virtually 100% chemically precipitated solids. Though these enhanced processes have been researched thoroughly to date and have demonstrated the capacity to achieve arsenic and organics removal, the ability to successfully dewater these solids not been verified nor is it well understood. This paper presents the results from several studies that evaluated the dewaterability of iron and alum residuals from water treatment.

Typically, the final process in a WTP residuals handling facility prior to landfill disposal is generally dewatering. Dewatering can be accomplished by using either solar drying beds (with or without an underdrain system), centrifugation, belt filter press, or plate and frame filter press. The ability to effectively dewater the thickened solids from enhanced coagulation and arsenic removal processes is an important consideration in overall process selection.

Residuals from conventional treatment coagulation/microfiltration, dissolved air flotation, and ballasted flocculation demonstration testing units with a TSS concentration greater than 2.5 percent were tested using a centrifuge and a plate and frame filter press for dewaterability. These processes were selected because of their widespread use in the water industry and its compact nature for pilot testing. Source waters included Colorado River surface water at the City of Phoenix, AZ Union Hills WTP and groundwater in Fallon, NV. The plate and frame filter press can be tested with limited residuals quantities, which is a constraint during pilot testing. Testing was conducted using a 1 cubic foot, 320 mm manual filter press. A local manufacturer's representative made arrangements for use of the testing equipment. The filter cloth mesh size was 20 micron and the unit was operated at 320 psi. Bench scale and full scale testing of centrifuges were performed using high solids units.

The results from these testing activities indicate that dewaterability decreases as the chemical content of the sludge increases. In some instances, chemical stabilizers such as lime and soda ash are required to generate acceptable dewatered solids characteristics. In general these chemical sludges are more hydrophilic when compared to typical water treatment plant sludges, which results in more complex techniques to achieve satisfactory dewatering results. This is a very important consideration when planning for arsenic removal and enhanced coagulation treatment facilities.

The research activities presented in this paper were completed for the City of Phoenix under the Lake Pleasant Water Quality Testing Study and Water Quality Master Plan Update projects.
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Document Type: Research Article

Publication date: 2003-01-01

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