Nitrification potential of attached biofilms in dairy farm waste stabilisation ponds

Abstract

Dairy farm waste stabilisation ponds are a major source of ammoniacal-N to surface waters in New Zealand. Ammoniacal-N is of particular concern in New Zealand where native aquatic invertebrates appear to be very sensitive to ammonia toxicity. This paper investigates improvement of ammoniacal-N nitrification in dairy farm facultative ponds with mechanical aeration and provision of biofilm attachment surfaces. Biofilm was grown on surfaces at different depths (0.1 m, 0.2 m and 0.6 m) under three mechanical aeration regimes (no aeration, night-only aeration and continuous aeration). Nitrification potential of biofilm was determined as the rate of ammoniacal-N removal in bioassays with ammoniacal-N spiked pond water or culture medium under controlled conditions (20°C, pH 7.0, constant stirring, DO 2–3 g m−3, dark). The nitrification potentials (0.30 g N m−2 biofilm d−1 to 2.17 g N m−2 biofilm d−1) of biofilm-coated surfaces were largely controlled by oxygen availability and consistency of supply in the pond. Nitrification potentials were high where oxygen availability was high, such as close to the pond surface where atmospheric re-aeration and algal photosynthesis were prevalent. Nitrification potentials of biofilms incubated at depth were enhanced by mechanical aeration, with higher values achieved under the continuous aeration regime and at more turbulent sites closer to the aerator.