Circadian periodicity of biological oxidation under three different operational conditions

Daily cycles of biological oxidation efficiency were studied in three different biofilters: high-load trickling filter A, 64 kg fish m −3, BIO-NET ® material (Norddeutsche Seekabelwerke), 260 m 2 active surface area per m 3 volume; low-load trickling filter B, 1·2 kg fish m −3, lower section Hydropak ®-foil (Friedrich Uhde GmbH), 200 m 2 m −3, upper section BIO-NET ® material, 260 m 2 m −3; and low-load submerged rotating contactor (SRC), BIO-NET ® material, 380 m 2 m −3. The dissolved BOD 5 removal efficiency of trickling filter A was dependent on the pH and on the space-load of organic matter. The total ammonia-nitrogen oxidation efficiency decreased directly after feeding from 60% to just over 20% and returned 4 h later to the earlier oxidationrate of 60% fluctuating between 60% and 30% (initial total ammonia-nitrogen concentrations ranged between 0·78 and 2·89 mg N litre −1 8 h after feeding). This decreasing efficiency was caused by an increasing initial carbonaceous (BOD) level from 4 to 20 mg O 2 litre −1. In the low-load trickling filter B the total ammonia oxidation efficiency ranged between 35% early in the morning to 60%, 10 h after the first feeding. The removal efficiency in the SRC increased constantly from nearly 2% to more than 40% 7 h after the first feeding and decreased during night time to values of about 4%. The degradation efficiency of total nitrogen in both trickling filters fell drastically after feeding (from 75 to 23% and from 88 to 42%). The SRC showed a relatively constant increase from 28% directly after feeding to 58% 7 h later.