Farm waste management: the effects of fine bubble aeration on cattle slurry and the fate of ammonia during the treatment process

Abstract

The aim of this investigation was to study the effect of fine bubble aeration on the relative amounts of N species, specifically NH 3 and NI-I4 within cattle slurry. Research into farm waste treatment systems has mainly concentrated on pig waste, yet cattle slurry treatment presents a challenging and relatively neglected research area. Farrell (1996) developed a fine bubble
aeration treatment system and studied the effects of aeration on physical, chemical and fertiliser quality of slurry. He noted substantial loses of total Kjeldahl N from aerated slurry. This is a major concern as stripped NH3 can present a significant atmospheric pollutant.
Here aeration treatment was studied at laboratory-scale (1 litre) and pilot-scale (100 litres). Air was externally supplied to tubular fine bubble diffusers. Initially a settleable floculative microbial culture was developed using batch-to-batch aeration that was continuously aerated for a maximum of 5 days. Treatment conditions were varied to study effects of aeration rate,
temperature, pH and the addition of an adapted microbial culture upon N species within the slurry. The exhaust gases from the aeration bioreactors passed through a chemical capture system. Slurry samples were obtained during treatment to establish the chemical and biological changes. Finally, the effect of aeration on the fertiliser value of the treated slurry was studied.
Experiments at laboratory-scale showed that N was released as NH 3 within the exhaust gases and that there were major changes in the amount and type of N species within the slurry. Increasing the pH of the slurry increased the quantity of NH3 released. Similar increases in rates of NH 3 release were shown for aeration rates up to 0.2 vvm and temperature up to 60 °C. The optimum conditions for NH3 stripping were found to be alkaline pH, high aeration rate and high
temperature. At low aeration rates and low temperatures, there was a 96.59 % conservation of NH3 . The addition of an adapted microbial culture minimises the loss of N from the system, through the conversion of volatile N into organic microbial N fraction. In conserving N it was found that problems with sedimentation occurred. To improve rates of pollutant reduction it was necessary to increase temperature and aeration rates. Increasing temperature from 50 °C to 60 °C was found to have a negligible additional effect upon the biological degradation of the organic material within the slurry. This treatment strategy requires a capture system to trap the released NH3 . The aerobic treatment of cattle slurry was not found to have a significant effect upon the growth of maize in comparison to that of unaerated cattle slurry. Recommendations are made for
operating conditions of an aerobic treatment system that will enable N to be either stripped from, or conserved within the slurry.