POST TREATMENT OF GROUNDWATER DENITRIFICATION FLUIDIZED-BED REACTOR EFFLUENTS TO ACHIEVE DRINKING-WATER QUALITY

Post treatment of effluents from heterotrophic groundwater denitrifica tion fluidized bed reactors (FBR) designed to achieve drinking water q uality has been investigated. The denitrification process adds to the dissolved organic compounds, biomass and bacteria in the effluent. The y are also lacking dissolved oxygen. Effluents from the process were t reated in combined post treatment processes based on either a tricklin g filter and sedimentation unit ('TF combination') or contact floccula tion ('CF combination'). Both processes were followed by sand filtrati on, granular activated carbon (GAC) and chlorination. Results regardin g total suspended solids (TSS) and turbidity removal showed an advanta ge to the 'CF combination', and the target turbidity (NTU <1) was alwa ys achieved when the alum dose was 10 or 20 mg L-1. Backwash of the sa nd filter and GAC column was required after 27 h of operation (average value). An average total reduction in dissolved organic carbon (DOC) of 40% was observed with a final DOC of 3.5-4 mg L-1. Most of the remo val of the DOC occurred in the sand filter (28%), while the GAC contri bution was smaller (18%). No regrowth potential was observed using the Werner method when a pure culture of Pseudomonas fluorescens P17 was used as inoculum in samples of chlorinated effluent (post chlorination ). When a mixed culture of indigenous bacteria was used as inoculum, a high regrowth potential was observed. Installing an additional chlori nation unit before the sand filter column (pre and post chlorination) resulted in effluent with no regrowth potential for both Pseudomonas f luorescens P17 and indigenous bacteria.