Effects of shear and mixing on a continuously-fed stirred tank reactor for aerobic, biological wastewater treatment

Treatment of domestic wastewater in a 9 L well defined conventional biotechnology type reactor was investigated over a range of stirrer speeds (8.3 to 16.7 s-1) and retention times (8 to 12 h). Parameters of reactor oxygen transfer coefficient and shear were found to be close to conditions used for pure cell culture in industrial applications rather than typical wastewater treatment conditions. The major treatment effects measured were carbonaceous load removal and nitrification. Carbonaceous load removal was found to be highest at low stirrer speeds with short retention times. Ammonia removal was greatest at stirrer speeds of 15 s-1 with 12 h retention time. Most of the ammonia was converted to nitrite, this agreed with reports in the literature of temperature; retention time and free ammonia inhibition promoting nitrite build up. Specific nitrification rates of up to 35 mg(N)g-1h-1 (at 15 s-1 10 h retention time) were achieved in the reactor, found to be close to those observed in pure culture experiments. An inverse correlation was observed between ammonia and CBOD5 removal. The temperature increased with stirrer speed and also had a strong effect in the nitrification rate. The interaction between temperature and stirrer speed was investigated using a control unstirred reactor and multiple linear regression technique. It was found that while the temperature and stirring were correlated, separate effects could be discerned. The stirrer effects were further investigated by varying the impeller type. Tip speeds were matched to the disk turbine for a low and a high shear impeller. The lower shear LE20 impeller gave promising results that required a much lower power input to achieve the treatment. Finally an anoxic reactor was added to denitrify the stirred tank effluent. It was found to successfully denitrify when sufficient nitrite and nitrate were supplied by the stirred tank. The combination of a stirred nitrifying tank followed by a denitrifying stage could make be an attractive alternative wastewater treatment method providing the stirred tank power requirements can be reduced.