Nitrification in continuous culture : the effect of pH and surface growth
A detailed kinetic analysis was made of growth of Nitrosomonas europaea and Nitrobacter sp. in chemostat culture. Steady states were established at a number of dilution rates as measured by substrate concentration and cell number. Biomass concentration was also estimated at each dilution rate and these data were used to evaluate the kinetic growth constants; maximum specific growth rate (m), saturation constant (Ks), true growth yield (Yg) and maintenance energy coefficient (me). The transient response to dilution rate changes was monitored and step increases in dilution rate always resulted in substrate overshoots which were mirrored by cell number undershoots. Step reductions in dilution rate resulted in monotonic changes in substrate concentration to lower steady state levels. Nitrobacter exhibited different growth characteristics when cultured continuously in an airlift column fermenter. This was considered to result from biomass settling within the column, resulting from inadequate mixing. Kinetic growth constants for culture of Nitrobacter in the airlift column fermenter were determined. The optimum pH for nitrite oxidation in batch culture was pH 7.5 with no growth at pH values less than 6.0. Nitrite steady states were established in continuous culture at pH 8.0, 6.0 and 5.5, with washout occurring at pH 5.0 and transient nitrite undershoots were observed following pH changes imposed. Surface growth of Nitrobacter was investigated in batch and continuous culture. Attachment and growth on glass and anion exchange resin surfaces resulted in the development of attached microcolonies. Biofilm development on anion exchange resin surfaces was facilitated by slime production which may assist in the irreversible attachment of cells. The specific rate of nitrite oxidation of cells attached to glass surfaces in batch culture was 20-25% greater than that of freely suspended cells. The enhanced activity of attached cells was independent of pH and pH-activity curves of free and attached cells were similar. Similarly cells attached to anion exchange resins in continuous culture exhibited increased oxidation rates per cell, compared to free cells cultured in the same system. Attachment to surfaces significantly lowered the minimum pH for nitrite oxidation and increased protection against low pH, Reduction in pH to 3.5 prevented nitrite oxidation, however cells remained viable and a steady state was subsequently established at pH 4.5.