Treatment of domestic wastewater in a 9 L well defined conventional biotechnologytype reactor was investigated over a range of stirrer speeds (8.3 to 16.7 s-1) and retentiontimes (8 to 12 h). Parameters of reactor oxygen transfer coefficient and shear werefound to be close to conditions used for pure cell culture in industrial applications ratherthan typical wastewater treatment conditions.The major treatment effects measured were carbonaceous load removal andnitrification. Carbonaceous load removal was found to be highest at low stirrer speedswith short retention times. Ammonia removal was greatest at stirrer speeds of 15 s-1with 12 h retention time. Most of the ammonia was converted to nitrite, this agreed withreports in the literature of temperature; retention time and free ammonia inhibitionpromoting nitrite build up. Specific nitrification rates of up to 35 mg(N)g-1h-1 (at 15 s-110 h retention time) were achieved in the reactor, found to be close to those observed inpure culture experiments. An inverse correlation was observed between ammonia andCBOD5 removal.The temperature increased with stirrer speed and also had a strong effect in thenitrification rate. The interaction between temperature and stirrer speed was investigatedusing a control unstirred reactor and multiple linear regression technique. It was foundthat while the temperature and stirring were correlated, separate effects could bediscerned. The stirrer effects were further investigated by varying the impeller type. Tipspeeds were matched to the disk turbine for a low and a high shear impeller. The lowershear LE20 impeller gave promising results that required a much lower power input toachieve the treatment.Finally an anoxic reactor was added to denitrify the stirred tank effluent. It was found tosuccessfully denitrify when sufficient nitrite and nitrate were supplied by the stirredtank. The combination of a stirred nitrifying tank followed by a denitrifying stage couldmake be an attractive alternative wastewater treatment method providing the stirred tankpower requirements can be reduced.
展开▼
