Biological denitrification of greenhouse leachates was investigated in laboratory scale upflow sludge blanket (USB) reactors. Two simulative solutions, representing greenhouse leachates at “high concentration” (200 mg/L NO3–N and 140 mg/L PO43–) and “low concentration” (120 mg/L NO3–N and 15 mg/L PO43–) ranges were used in the experiments. Ethanol was added as the carbon and electron source for bacterial metabolism. The USB reactor successfully removed 99% of the nitrate at retention times from 1 hour to 10 minutes. Nitrogen removal rates reached 16.7 g NO3–N/L · d for the “low concentration” reactor and 26.8 g NO3–N/L · d for the “high concentration” reactor. Due to the large increase in alkalinity resulting from the denitrification, precipitation of calcium compounds occurred which resulted in sludge with good settling characteristics (SVI=6.5 to 8 ml/g). The mineral fraction of the granular sludge was higher than 50%. High concentrations of biomass (100g/L TSS and 40 g/L VSS) were also observed. From the mineral analysis of the granular sludge, the main precipitating compound in the “low concentration” reactor was identified as calcium carbonate. In the “high concentration” reactor, the mineral analysis did not give a clear answer as to the composition of the precipitating compound. Using theoretical Ksp values together with the remaining soluble concentrations of calcium, phosphate and pH in the effluent, amorphous Ca3(PO4)2 was suggested as the main precipitating compound. The calcium phosphate rich sludge was shown to be unstable and dissolved when the influent concentrations were changed. Modeling the biological and chemical processes in the reactor was done using AQUASIM modified to include chemical systems. When experimental Ksp values were used, the model gave good agreement with reactor results.

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