The results of investigations of natural groundwater treatment containing increased manganese and ammonia nitrogen concentrations are presented. The main aim of the work was to show the influence of changes in water composition on manganese and ammonia nitrogen removal from groundwater on second-stage mature quartz sand filters. According to the results obtained, the efficiency of manganese elimination from water did not depend on periodic changes in ammonia nitrogen or dissolved oxygen concentrations. However, if low oxygen concentrations (lower than 2 mg/dm3) were maintained for a longer time, the manganese concentration in the filtrate gradually increased. The ammonia nitrogen concentration in the filtered water appeared to be the most relevant factor affecting the efficient removal of manganese and the dissolved oxygen concentration was less important. The nitrification process appeared to be vulnerable to changes in the concentrations of the substrates: ammonia nitrogen (either an increase or decrease) and oxygen (an decrease). It was observed that it is necessary for nitrifying bacteria to adapt (lag phase) to rapidly changing water composition. Due to the sorption process, the inhibition of nitrification on the filtering beds did not significantly influence the efficiency of ammonia nitrogen removal.

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