Effect of temperature over start-up of a groundwater-denitrifying submerged filter inoculated with psychrotolerant bacteria

Abstract

Inoculation of a submerged filter was carried out using three bacterial strains previously selected on the basis of their psychrotolerance and high denitrifying activity with the aim of apply selective inoculation to a submerged filter system for the denitrification of groundwater. Laboratory-scale assays were carried out at 5, 10 20 and 30°C. Surface scanning microscopy was used to evaluate the capacity of each inoculant to colonise the support. In all cases a biofilm in the initial stages of development was observed, with abundant connection material and cells in division. Increase in temperature had a negative effect on colonisation evolution, motivated by the use of psychrotolerant bacteria. Each inoculant presented a different colonisation optimum, but always at temperatures under 20°C. To monitor system setup, concentrations of total nitrogen, nitrate and nitrite after treatment were measured. In most cases, the stabilisation phase was observed to be longer at lower temperatures, independently of the inoculant employed. However, at 5°C, only one of the inoculants reached steady-state phase with total nitrogen elimination. In all the assays, an accumulation of nitrite was observed during stabilisation phase. At lower temperatures, maximum concentrations of nitrite were greater and were reached after longer operation times. Use of selective inoculants was shown to promote subsequent development of a stable biofilm achieving efficient elimination of nitrate from the influent. This occurs regardless of the inoculant employed, except at a temperature of 5°C, at which the type of inoculant conditions system setup. However, colonisation capacity of the inoculant at low temperatures is not a determining factor.