The application of nitrifying trickling filters (NTFs) to potable water treatment is less well understood than their application to wastewater treatment, particularly regarding the effect of low ammonia substrate concentrations and organic carbon loading on filter performance. A large pilot-scale NTF was operated under conditions that simulated the raw water quality of poorly protected catchments typically found in SE Asia, with the objective of reducing the ammonia driven chlorine demand during disinfection. The efficacy of a high rate NTF to remove low concentrations of ammonia (0.5–5.0 mg NH4-N L−1) in the presence of high organic carbon (1–12 mg soluble biochemical oxygen demand (sBOD5) L−1) was investigated. Results demonstrated that 90 to 100% of nitrification was maintained only when the carbon load was less than 0.7 g sBOD5 m−2 d−1 (<4 mg sBOD5 L−1). Once the organic load was increased beyond 0.75 to 2.1 g sBOD5 m−2 d−1 (4.5–12.1 mg sBOD5 L−1), a linear decline in nitrification from 70 to 15% was observed within a timeframe of 8 to 10 d. The impact of high organic loads on the distribution of nitrification down the NTF was also investigated. Results confirmed that carbon loads greater than 0.95 g sBOD5 m−2 d−1 (>5.5 mg sBOD5 L−1), severely suppressed nitrification throughout the entire filter bed.

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