The effects of low dissolved oxygen tension during the aerobic treatment of piggery slurry in completely mixed reactors*

The energy requirement for aerobic treatment of animal excreta is minimal when the dissolved oxygen (DO) level is as low as possible. The efficiency of continuous aerobic treatment at 15°C in terms of the removal of carbonaceous materials was found to be the same at DO levels as low as 1% of saturation as at high DO levels. However, up to 45% of the total input nitrogen and 90% of the soluble nitrogen were lost from the slurry during treatment at low DO. This loss was the result of the simultaneous occurrence of nitrification and denitrification within the completely mixed reactor. It was also observed that low DO levels favoured nitrification during aerobic treatment. This resulted from the activity of denitrifying bacteria which raised the pH and removed the inhibitory effect of acidic conditions upon the nitrifiers. Control of the pH value at pH 7.1 during aerobic treatment resulted in the complete oxidation of the ammonia content of piggery slurry.