Characterization of denitrifying granular sludge treating soft groundwaterin an upflow sludge-blanket reactor

Nitrate removal from soft groundwater using ethanol as a carbon source in a n upflow sludge-blanket reactor containing denitrifying granular sludge was investigated. At a hydraulic retention time of 0.83 h, influent nitrate wa s increased stepwise from 20 to 145 mg N/l (volumetric loading rates (VLRs) , 0.60 to 4.2 g N/l/d, respectively) and sludge was periodically wasted to maintain a sludge bed of about 2/3 the liquid volume. Complete nitrate remo val was achieved at influent nitrate concentrations up to 75 mg N/l(2.1g N/ l/d). MLSS increased from 20 g/l at a VLR of 0.6 g N/l/d to 51 g/l at a VLR of 1.9 g N/l/d, above which it decreased. VSS increased from 11 g/l to a m aximum of 25 g/l at a VLR of 2.1g N/l/d. Settling velocities showed the sam e trend with maximum values in a VLR range of 1.5 to 2.1 g N/Nd. However, g ranule size, calcium and magnesium contents of the granular sludge and prot ein, carbohydrate and nucleic acid contents of extracellular polymers decre ased steadily with an increase in VLR throughout the range of testing. With in the VLR range of 0.6 to 2.1g N/l/d, corresponding to complete nitrate re moval and efficient sludge retention, the granular sludge had a high calciu m content of 24 to 22%, magnesium ranged from only 0.7 to 0.1%, proteins fr om 3.2 to 1.3%, carbohydrates from 4.2 to 1.4%, and nucleic acids from 0.34 to 0.05% of the sludge dry weight. These results suggest an optimum operat ional VLR in terms of nitrate removal and sludge retention of about 2 g N/l /d.