(32,33) Protein-like fractions can be distinguished from humic and fulvic acids, with different subfractions potentially responding differently to biological treatment. To this end, fluorescence excitation and emission matrix (EEM) spectroscopy is a sensitive technology capable of distinguishing between fractions of fluorescent organic matter in water. (30,31) Tracking the removal of different DOM fractions from natural waters is challenging due to a lack of affordable analysis techniques for measuring specific organic matter fractions at the concentrations encountered in surface water. Therefore, the optimum EBCT for organic matter removal must be balanced against water demand and considered at the plant design stage.ĭOM removal by drinking water biofilters is traditionally assessed using bulk parameters like ultraviolet (UV) absorbance at 254 nm ( A 254), specific absorbance (SUVA), color, and total or dissolved organic carbon (TOC or DOC, respectively). Because huge amounts of water pass through drinking water treatment plants, a small change in EBCT can have a substantial influence on filtered water volumes. From an operational perspective, long EBCTs require large filters containing more filter media. (27) reported a 30% increase in DOC removal by the pilot-scale biofilters when the EBCT was increased from 5 to 30 min, beyond which no improvement was observed. (26) concluded from column tests that the removal of humic material was unaffected by EBCTs of >25 min.

(24,25) There are also reports that increasing the EBCT beyond 15 min does not significantly improve effluent water quality. It is generally considered that DOM removal increases up to an EBCT of ∼30 min, with longer EBCTs having little additional impact. Organic matter removal by biofilters can often be improved by increasing the EBCT. (7)Ī critical parameter for DOM removal by biofilters is the empty bed contact time (EBCT), i.e., the residence time of the fluid in the filter calculated as though the entire volume of the filter media is occupied by water. These biofilm microbes utilize the biodegradable fraction of DOM as a carbon source for their growth, metabolism, and reproduction, yielding effective DOM removal. (5,6) In all biofilters, DOM adsorbs onto a naturally occurring biofilm or is biodegraded by microbes living within the biofilm. First-stage biofilters receiving coagulated water accumulate more flocs and particles during a filtration cycle. The position of the filter in the treatment chain also affects performance. Biofilter performance depends upon multiple factors such as water quality (e.g., DOM concentration and characteristics, pH and turbidity, and temperature) and design/operation parameters (e.g., type of medium, filtration rate, retention time, and backwash frequency). First-stage biofilters treat pretreated (e.g., coagulated) water, and second-stage biofilters are used to “polish” prefiltered water. “Direct” biofilters are mostly used to remove turbidity, particles, and biological organic matter before membrane filtration.

Biofilters serve different purposes in different treatment chains.

The results suggest that long contact times (>30 min) have advantages for the operation of some biological filters, especially if placed ahead of barriers that are sensitive to biofouling, e.g., membranes.īiological filtration is an efficient treatment technology for removing biodegradable DOM compounds. Protein-like fluorescence is associated with labile organic matter fractions, which are known to be removed poorly by drinking water treatment barriers apart from biological filters.

Other DOM fractions were largely unaffected by biofiltration, or at slightly higher concentrations in the effluent. The removal efficiency of organic matter fractions increased with EBCT for dissolved organic carbon (DOC) and microbial humic-like (F 290/420) and protein-like (F 280/340) fluorescent organic matter. During a two-month experiment, contact times in four parallel biofilters were switched to and maintained at 15, 30, 50, and 80 min by manipulating the hydraulic loading on each filter. The study aimed to locate the optimal operating conditions via experimental manipulation of the biofilter empty bed contact time (EBCT) during full-scale operation at the plant. The biofilters treat urban and agriculturally impacted river water using a commercial non-adsorptive, expanded-clay filter medium. A study was conducted at a water treatment plant to optimize parallel rapid gravity biofilters for dissolved organic matter (DOM) removal.

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