Thursday, December 13, 2012

Sediment Disposal from the Washington Aqueduct Water Treatment Plants

Drinking water systems may obtain their water supply either directly from the rivers, lakes, reservoirs for surface water or from wells for ground water or like DC Water, Arlington and Falls Church may purchase finished water from wholesalers like the Washington Aqueduct. Raw water is treated to produce finished drinking water. During the treatment of source water, water treatment plants, WTPs, remove contaminants by screening, sedimentation, flocculation and filtration. The waste streams generated from these steps are water treatment residuals. In a 2011 report the US Environmental Protection Agency, EPA, estimated that approximately 31% of the WTPs directly discharge to surface water, 7% transfer residuals to waste water treatment plants and the remainder is disposed of on land.

Solid residuals from water treatment plants include sludge, schmutzdecke (biological surface layer in slow sand filtration units), and spent treatment media. Residuals contain contaminants removed from the source water and treatment chemicals added by the WTP. Prior to final disposal residuals from the source water treatment operations can be treated on site by the WTP. Washington Aqueduct has a newly constructed residuals management facility that disposes of the solids by contract hauling.  The residuals from the Aqueduct are being used for reclamation and backfilling under a Maryland surface mining permit. The residuals are permitted to be used  offsite as clean fill. The volume and characteristics of the residuals depend on the source water, drinking water production rate, efficiency of source water treatment, and type of source water treatment used. The goal of all residuals treatment/ solids removal systems is to decrease the volume of water while increasing solids content. This process creates two waste streams the liquid and the solid.

EPA’s Filter Backwash Recycle Rule, FBRR, established requirements to ensure that WTPs do not compromise the quality of finished drinking water when recycling water from residuals management. The FBRR requires WTPs that reuse certain wastewater liquid residuals (filter backwash, thickener supernatant, and dewatering process liquids) the water must be returned to a point in the water treatment process where it will be treated by coagulation and filtration.

Water treatment residuals solids contain naturally occurring suspended and dissolved solids from the source water, as well as precipitated solids generated by chemical treatment as well as residual contaminants from chemical treatment. The naturally occurring solids include sediment and soils that are carried to the Potomac in run off from rain and snow melt. These solids are regulated under the Resource Conservation and Recovery Act, RCRA, regulations and are classified as hazardous or nonhazardous. A waste is characterized as hazardous or nonhazardous based on its ignitability, corrosivity, reactivity, and toxicity. Generally speaking, these wastes are not toxic and are often sold and used as soil amendments in agriculture or disposed of by contract hauling to a permitted disposal facility. The solids residual from water treatment generally contains the river sediment, traces of the algaecides and flocculants.

Sludge generated by water treatment plants is not subject to regulation under the Biosolids Rule. The Biosolids Rule (part of the Clean Water Act Amendments of 1987) was created to protect public health and the environment from any anticipated effects from recycling of sewage sludge Biosolids. The toxicity of solid residuals from sewage treatment is assessed by the Toxicity Characteristic Leaching Procedure (TCLP), which is a soil sample extraction method for chemical analysis. If contaminant concentrations in the TCLP leachate are below those listed in the Land Disposal Restrictions of RCRA, the solid residual is classified as non-hazardous and can be disposed at a municipal landfill or other location. There is tremendous controversy associated with potential impacts of Biosolids and the land disposal or reuse of Class B and even Class A Biosolids. I am not aware of any controversy associated with agricultural use of solids residual (predominately river silt) of water treatment plants.

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