Practical Applications of Static Low Density Media Filters for Use in the Treatment of Domestic Wastewater

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ABSTRACT
In the treatment of domestic wastewater, both biological and physical contaminant removal mechanisms must occur to facilitate compliance with NPDES permit discharge requirements. Static low density media (SLDM) filters can be used to perform both of these duties allowing for the consolidation of various unit operations into one component. Previous studies have shown that both organic and solids removal can occur concurrently in a single SLDM filter, without the
need for a secondary clarifier (Wagener et al., 2002). This paper investigates the practical use of SLDM filtration on domestic wastewater from a small commercial facility and an intrastate rest area located in Louisiana. The performance was based largely on CBOD5, TSS, and TAN removal with attention given to fine solids capture.

INTRODUCTION
In response to increasingly strict regulations on effluent quality and other factors such as space and operational limitations, many existing domestic wastewater treatment systems require upgrades. Static low density media (SLDM) filters can provide a low cost, robust, and easily manageable treatment upgrade alternative. SLDM filters can be used to perform multiple duties by consolidation of unit operations into one single structure. The functions of biological treatment, secondary clarification, and tertiary treatment are accomplished in one bioclarifier. Previous studies have shown that both organic and solids removal can occur concurrently in a single Static Low Density Media (SLDM) filter, without the need for a secondary clarifier (Wagener et al., 2002). The small footprint required for placement of a SLDM filter makes it an easy addition to an existing treatment plant with minimal interference to the current treatment train. The SLDM bioclarifiers can operate with minimal supervision and require little maintenance. This paper investigates the practical uses of SLDM filters at two different locations in Louisiana each with unique wastewater characteristics. Each filter tested differed slightly in physical design but operated with an identical fundamental bioclarification treatment strategy. The focus was largely on CBOD5, TSS, and TAN removal with attention given to greater fine solids capture to improve effluent quality.

Static Low Density Media Filters
Static low-density media (SLDM) filters are known in the aquaculture community as Floating Bead Filters (FBFs). The floating bead filters (FBF’s) are expandable granular filters that display a bioclarification behavior similar to sand filters (Malone et al. 2000). The units are now widely employed as clarifiers or bioclarifiers in support of high-density recirculating production and holding systems for fish, reptiles, and crustacea (Malone and Beecher, 2000; DeLosReyes
and Lawson, 1996).

The units are normally operated with the floating bed in a packed or static mode. In the packed bioclarification mode, the units concurrently provide solids capture, carbonaceous BOD removal, and nitrification. During the packed or filtration mode, influent wastewater enters below the media bed. At the end of each pass through the media bed, the water is returned to the polishing chamber 80 to 90 times before discharge. Recirculation with an airlift provides external aeration in addition to multiple pass removal of particulate and soluble CBOD5 making it a very critical
management tool with one pass retention times between 30 seconds to 1.5 minutes per pass.

The beds are periodically expanded for removal of accumulated solids and excess biofilm (Malone and Beecher, 2000; Cooley, 1979). Backwashing or expansion of the bead bed can be accomplished by hydraulic, pneumatic or, mechanical means. Figure 3-1 illustrates the two modes of operation in a SLDM filter treating domestic wastewater using a pneumatic backwashing mechanism.

One drawback to granular medium filters, particularly with newer submerged biofilters, is the build up of headloss in the carrier material (Ødegaard et al, 1994). The head loss and caking problems associated with granular packed beds are minimized in SLDM filter applications using high-frequency backwashing. Increased head loss through the filter bed can cause biofouling and inhibit filter performance. A pneumatic backwashing technique used in SLDM filters effectively reduces bed and screen head loss thus permitting high-rate recirculation via an airlift.

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