Anne Kihn

Impact of temperature on nitrification in biological activated carbon (BAC) filters used for drinking water treatment

The impact of temperature on nitrification in biological granular activated carbon (GAC) filters was evaluated in order to improve the understanding of the nitrification process in drinking water treatment. The study was conducted in a northern climate where very cold water temperatures (below 2 degrees C) prevail for extended periods and rapid shifts of temperature are frequent in the spring and fall. Ammonia removals were monitored and the fixed nitrifying biomass was measured using a method of potential nitrifying activity. The impact of temperature was evaluated on two different filter media: an opened superstructure wood-based activated carbon and a closed superstructure activated carbon-based on bituminous coal. The study was conducted at two levels: pilot scale (first-stage filters) and full-scale (second-stage filters) and the results indicate a strong temperature impact on nitrification activity. Ammonia removal capacities ranged from 40 to 90% in pilot filters, at temperatures above 10 degrees C, while more than 90% ammonia was removed in the full-scale filters for the same temperature range. At moderate temperatures (4-10 degrees C), the first stage pilot filters removed 10-40% of incoming ammonia for both media (opened and closed superstructure). In the full-scale filters, a difference between the two media in nitrification performances was observed at moderate temperatures: the ammonia removal rate in the opened superstructure support (more than 90%) was higher than in the closed superstructure support (45%). At low temperatures (below 4 degrees C) both media performed poorly. Ammonia removal capacities were below 30% in both pilot- and full-scale filters.

Impact of backwashing on nitrification in the biological activated carbon filters used in drinking water treatment

Abstract Nitrification during biological filtration is currently used in drinking water production to remove ammonia, which is the source of several water quality problems during treatment and distribution. We evaluated here the impact of backwashing on nitrification efficiency in filters used for drinking water treatment. Two different granular activated carbon (one open and one closed carbon superstructure) were tested. Ammonia removal and fixed nitrifying bacterial biomass before and after backwashing were compared in first‐stage pilot filters and full‐scale second‐stage filters. Backwashing has a greater impact on nitrification on first‐stage than on second‐stage filters. Backwashing improved the ammonia removal in warm (≥ 18°C) water in a first‐stage filter containing an open‐superstructure granular activated carbon, whereas a closed‐superstructure support showed a removal capacity that is less after a regular backwashing than before, or similar to it. In cold water (≤ 4°C), backwashing had a negative impact on nitrification capacity in an open‐superstructure medium first‐stage filter. In full‐scale second‐stage filters, backwashing had a slight negative impact on filter performances for both open‐ and closed‐superstructure media at temperatures between 8°C and 12°C. In colder waters (≤ 3°C), nitrification was very poor both before and after backwashing. Sampling of fixed, nitrifying biomass in the pilot filter columns showed that in all cases (low and high expansion backwash; both media tested) and at 20±3°C, the overall biomass levels remained unchanged before and after backwash. In the full‐scale second‐stage filters, nitrifying biomass was similar before and after backwashing for both types of media tested.

Measurement of potential activity of fixed nitrifying bacteria in biological filters used in drinking water production

Nitrification during biological filtration is being used more and more in drinking water production to remove ammonia, which can be the source of several water quality problems during distribution. In this process, ammonia is converted into nitrite and then into nitrate by fixed autotrophic nitrifying bacteria. The purpose of this work was to develop a technique to estimate fixed nitrifying biomass (sum of ammonia- and nitrite-oxidizing populations). The quantification of autotrophic nitrifying biomass was determined by potential nitrifying activity measurement. The production of oxidized forms of inorganic nitrogen (nitrates and nitrites) was measured after an incubation of 2 cm3 of colonized solid support in the presence of a 5-ml nitrifier medium containing 10 mg N-NH4 L−1 for 30 min at 32°C. The production rate of oxidized nitrogen in optimal conditions was measured and converted into nitrifying biomass by using the maximum specific oxidizing activity. This technique was shown to be appropriate for conditions encountered in the biological filters used in drinking water production and sufficiently simple to be used for routine measurements. Journal of Industrial Microbiology & Biotechnology (2000) 24, 161–166.