Since the loss of water from CM by evaporation was unavoidable, another method, the recycle of biogas coupled to washing
with an ammonia absorber, was carried out in the present study.
The results are shown in Fig. 2b. By means of this method, ammonia was stripped successfully without water loss, and the percentage of ammonia removal reached as high as 82 at the final pH of
8.5. Conversion to ammonia from total Kjeldahl nitrogen was
80%. However, acetate concentration dramatically dropped from
183 to 5 mmol kg-CM1, suggesting the possibility of oxidation of
acetate to carbon dioxide by oxygen contamination during fermentation. Addition of the safety bottle containing ferrite powder for
the removal of oxygen to the rear of the ammonia absorber cured
this problem. This action arrested the decrease of acetate concentration, but ammonia removal was decreased to 61.2% compared
with the method without the oxygen absorber (Fig. 2c). This lower
ammonia removal could be attributed to the lower pH of 7.4,
caused by the accumulation of acetate, compared with the method
without the oxygen absorber (82% at pH 8.9), since lower pH decreases the portion of free ammonia and inhibits its release.