PRECIPITATION POTENTIAL AS A MAJOR FACTOR IN THE FORMATION OF GRANULAR SLUDGE IN AN UPFLOW SLUDGE-BLANKET REACTOR FOR DENITRIFICATION OF DRINKING-WATER

The effects of the chemical composition of water on granular sludge fo rmation and characteristics in a denitrifying upflow sludge-blanket (U SB) reactor were studied. Denitrification of drinking water showed dif ferent biomass sludge characteristics when the reactor was fed with gr oundwater as opposed to surface water. USB reactors fed with groundwat er produced granules with good settling characteristics, SVI (sludge v olume index) values lower than 30 ml/g, and high reactor biomass conce ntrations (20-25 g/l), while surface-water-fed reactors exhibited lowe r biomass concentrations (10-15 g/l) due to poor settling characterist ics (SVI values of 50-90 ml/g). Sludge granules from the reactor fed w ith surface water had a low mineral content of between 10% and 20% as compared to a mineral content of 25%-50% in the groundwater reactor. T he larger mineral content in the groundwater-fed reactor was due to a greater precipitation potential, i.e. higher concentrations of calcium and alkalinity present in groundwater combined with the release of al kalinity and subsequent increase in pH caused by biological denitrific ation. Verification for this phenomenon was established by enriching s urface water with calcium and alkalinity, which increased the reactor' s precipitation potential from 15 mg/l to 40 mg/l (as CaCO3). The gran ules obtained from the reactor fed with enriched surface water had a h igh mineral content of between 40% and 50% and very low SVI values, co ntributing to improved granule-settling characteristics and reactor st ability.