Continuous aerobic processing of piggery effluent: a new approach to quantifying the fate of the nitrogen component

摘要

The primary objective was the preparation of a complete mass balance around an aerobic treatment system for pig slurry, to quantify the various forms of nitrogen entering and leaving under different conditions. The purpose of this was to assess the effect of such treatment conditions in terms of the amount of polluting forms of nitrogen generated from the slurry. A laboratory scale reactor (designed for this study) was operated under three separate residence times of 2, 4, and 8 days, and aeration level indicated by a redox value in the range of E(_{Ag/AgCl}) = +100 to +200 mV; the latter two giving nitrifying conditions. Emissions of di-nitrogen gas are a major component of a nitrogen mass balance, yet one which has been often neglected because of difficulties in distinguishing it from that in the atmosphere. A novel technique was developed in which atmospheric N(_2) in the reactor headspace was removed by flushing the system with an 80/20 gas mixture of argon/oxygen. This left microbially derived N(_2) available for collection and analysis by mass spectrometry. Established methods were applied for the measurement of other gaseous nitrogen emissions (NH(_3), N(_2)O, NO) and other forms of nitrogen in the slurry (organic-N, NH(_4^+), NO(_2^-) and NO(_3^-)). The steam distillation technique for nitrite and nitrate was found to be unreliable, therefore, high performance liquid chromatography was used as an alternative. The existence of the intermediate nitrified N form of hydroxylamine is postulated but was not quantified in this study. The presence of unidentified components in raw slurry was investigated using HPLC, but only chloride and acetate could be recognised with a high degree of confidence. Mean N(_2) concentrations measured were 774 mg l(^{-1}) in the 4 day treatment and 523 mg l(^{-1}) in the 8 day treatment. Emissions of the environmentally damaging N(_2)O gas were quantified as being 514 mg l(^{-1}) in the 4 day treatment and 219 mg l(^{-1}) in the 8 day. The lower emissions from the 8 day treatment are attributed to improved contact between oxygen and slurry, reducing the prevalence of zones favourable for denitrification. In the final mass balance study, overall nitrogen leaving the system equalled 86 (±18) % of that entering in the 2 day treatment, 113 (±10) % in the 4 day treatment, and 104 (±21) % in the 8 day treatment. The variation in values was attributed to errors in the liquid phase analysis of slurry nitrogen compounds.