Optimization of a biofilter system (C.R.O.P.[trademark]) for future wastewater treatment

Abstract

Temperature and dissolved carbon dioxide effects on the C.R.O.P. [registered trademark] filter and possible encrustation of pumice were investigated in two different operation modes. Temperature effects at 15 degree C, 28 degree C and 35 degree C were examined in continuous mode over 101 days, on three filter units each. Complete nitrification took place in all units, since nitrite could only be detected sporadically and in low concentrations in the filtrate. At 28 degree C plus/minus 0.6 the average conversion rates of ammonium nitrogen to nitrate nitrogen was highest (3110.19 mgNO3-N / l) in comparison to the cooled and the heated units. All continuously operated filters showed a stable acid content with a pH value below 5 over the complete observation period. Higher temperatures (34.9 degree C plus/minus 0.8) inhibited the filter units more than lower temperatures (15.7 degree C plus/minus 0.4). Automatic CO₂ gas injection depending on pH value in fed batch operated filters was only activated during increased nitrogen addition and did not work due to defective equipment. Low nitrogen additions (1 l / d) did not affect the acidity of the filtrate, while high additions (2 l / d) disturbed the system and the pH value of the filtrate rose above 8. Manually activated temporary injection did not influence the pH value in the long-term, it only showed a decrease of dissolved oxygen concentration in the filtrate. A comparison of both operation modes based on conversion rates, pH and technical effort showed that continuous operation is more suitable for the C.R.O.P. [registered trademark] filter than fed batch operation. Marked pumice was stored for 94 days in the top and bottom steel baskets of the filter tubes, to analyze changes on the pumice surface. Encrustation occurred on pumice stored at the top of the filter tube, independent of temperature or operation mode, while none of the bottom stored pumice pieces showed any deposits. Encrustations consist of nine different components and can affect the filter performance negatively by suppression of nitrifying microorganisms.