Real-time control of aerobic/anoxic digestion for waste activated sludge

Experiments were conducted to study the performance characters of aerobic/anoxic （A/ A） digestion of sludge at 30± 1 ℃, while the sludge retention time （SRT） was kept 16 d. The varia tions of oxidation reduction potential （Eh ） and pH were continuously monitored during the A/A di gestion and the conversions of ammonium and nitrate were investigated. Important features on both Eh and pH profiles were identified to develop process control strategy. Since the feature point on Eh profile where d2 Eh/dt^2 =0 is very stable during anoxic cycle, it can be used to determine the end of denitrification. The end of nitrification can be identified according to dpH/dt = 0. A real-time control strategy of A/A digestion of sludge was developed and tested with pH and Eh as control parameters. It is shown that the performance of the real-time control strategy is better than that of a fixed time control strategy. While the real time controlled A/A digestion system can achieve a similar volatile suspended solids （VSS） destruction efficiency of 35.2 % as a continuously aerated system, it im proves the supernatant quality in a shorter aeration time（7. 75 d for a 20 d period）.

Temporal-spatial variations of euphotic depth of typical lake regions in Lake Taihu and its ecological environmental significance

By using the data of underwater irradiance measured in the different lakeregions of Lake Taihu during 1998—2004 and total suspended solids (TSS), wind speeds of the total13 stations in typical lake regions during 1993—2003, this paper analyzes the factors ofinfluencing on PAR (pho-tosynthetically available radiation) attenuation, euphotic depth, andpresents the temporal and spatial variations of euphotic depth of typical lake regions in LakeTaihu, and the spectral distributions of euphotic depth at station 2. The results show that theconcentration of TSS is the most important factor impacting PAR attenuation, followed by chlorophylla; chromophoric dissolved organic matter (CDOM) has little impact on the PAR euphotic depth. During1993—2003, the mean yearly PAR euphotic depths of the typical lake regions ranged from 1.04 to1.95 m with a mean value of 1.35±0.23 m. The PAR euphotic depth fell into 3 spatial zone types:Type Ⅰ, the lowest, including the lake center and the inflows of rivers; Type Ⅱ, intermediate,including Meiliang Bay, Wulihu Lake and Gonghu Bay; Type Ⅲ, the greatest, including the East LakeTaihu; corresponding mean depths were approximately 1.1, 1.4, 2.0 m, respectively. The seasonalvariations of euphotic depths were not quite the same in different lake regions. In the lake center,the mean values of PAR euphotic depth in summer and autumn were significantly greater than those inwinter and spring; in the Meiliang Bay, winter means were significantly greater than in the otherthree seasons; in the East Lake Taihu, winter means were significantly less than in the other threeseasons. However, no distinct seasonal change was recorded in the Wulihu Lake, Gonghu Bay and theinflows of rivers. The spectral distributions of euphotic depth present a minimal value at the bluelight wave band of 400 nm, and a peak at the green light wave band of around 580 nm. In 1998 and1999, based on the seasonal many-day continuous measurements, the PAR euphotic depths at station 2were recorded 2.00±0.21, 2.52±0.45, 1.58±0.24, 2.00 ±0.15 m in spring, summer, autumn andwinter, respectively. The peak value of 440 nm absorbed by phytoplankton corresponded to a euphoticdepth of only 0.81—1.47 m (mean 1.07±0.29 m), which was much lower than the mean PAR euphoticdepth of 1.98±0.41 m.