Nitrification–denitrification in waste stabilisation ponds: a mechanism for permanent nitrogen removal in maturation ponds

A pilot-scale primary maturation pond was spiked with ¹⁵N-labelled ammonia (¹⁵NH₄Cl) and ¹⁵N-labelled nitrite (Na¹⁵NO₂), in order to improve current understanding of the dynamics of inorganic nitrogen transformations and removal in WSP systems. Stable isotope analysis of δ¹⁵N showed that nitrification could be considered as an intermediate step in WSP, which is masked by simultaneous denitrification, under conditions of low algal activity. Molecular microbiology analysis showed that denitrification can be considered a feasible mechanism for permanent nitrogen removal in WSP, which may be supported either by ammonia-oxidising bacteria (AOB) or by methanotrophs, in addition to nitrite-oxidising bacteria (NOB). However, the relative supremacy of the denitrification process over other nitrogen removal mechanisms (e.g., biological uptake) depends upon phytoplanktonic activity.