Effect of C/N ratio and support material on heterotrophic denitrification of potable water in bio-filters using sugar as carbon source

Abstract

This work studied heterotrophic denitrification in packed-bed reactors using sugar as electron donor and carbon source. Two pilot-scale bio-filters, loaded with different types and sizes of support media, were operated under sequencing batch (SBR) mode with recirculation. The materials used were silicic gravel placed in two layers (diameter 2.41 and 4.03 mm) within the first filter (R1 – multilayer), and hollow plastic tubes (diameter 1.6 cm) within the second filter (R2 – monolayer). Feed NO3−-N concentrations ranged between 20 and 400 mg/L, while the C/N ratio was kept constant at 13.5. Higher denitrification rates (4.06–8.76 kg N/(m3d)) and total sugar biodegradation were observed in bio-filter R1 than in R2. However, carbon residues were observed in the treated water of bio-filter R1 for feed concentrations above 70 mgNO3−-N/L. Bio-filter R1 was then used to study the effect of C/N ratio on its performance. Experiments showed that a decrease of the C/N ratio from 13.5 to 6.25 resulted in lower carbon residue in the treated water and high denitrification rates (3.21–6.64 kgN/(m3d)), however, significant NO2−-N accumulation was observed. Further reduction of the C/N ratio (i.e., 5.48 and 4.71) resulted in filter malfunction as insufficient removal of NO3−-N was achieved. Results suggest that under the more favourable conditions of C/N:13.5 and silicic gravel support material, satisfactory NO3−-N and COD removal could be achieved for feed concentrations up to 60 mgNO3−-N/L at a cost of 0.019 €/KgN or 1.13 × 10−3 €/m3, which is much lower than ever reported in the literature and confirms the economic viability of the process. To describe the heterotrophic denitrification process, a mathematical model of microbial growth on nitrate and nitrite treated as substitutable substrates and sugar as the carbon source is proposed. Model simulations could be used as a guide for the optimum design and operation of water treatment systems for nitrate removal.