The influent, after screening and decanting is stored to the flow and organic load equalization tank and then inflows at a rate of QF = 80m3 d-1 to the SNdN activated sludge system. The SNdN system actually functions as an autonomous treatment plant where oxidation of organic pollutants, nitrification/denitrification and suspended solids sedimentation are taking place. Furthermore a sludge recycling from the secondary clarifier to the aeration tank is performed so as to ensure the required microbiological load (5000 ± 450 mg L-1). The aeration tank of the SNdN activated sludge system operates under low D.O. concentration (between 0,2 to 0,7 mg L-1). According to Mercalf and Eddy (2003), ...view middle of the document...
The theoretical result is no excess sludge production. In practice, there will be excess sludge but a quantity will be far less than with other activated sludge processes (Corbitt, 1998). Systems having high SRT values produce low sludge yields (Loosdrecht and Henze, 1999; Stephenson et al., 2000). Consequently enhanced processes performance at PCMAS system’s bioreactors is achieved not only by having high SRT, HRT and D.O. concentration on aeration but mainly due to high biomass concentration (Lubbeke et al., 1995). The PCMAS system has high HRT (2,25 d and 0,75 d in aeration and anoxic compartments respectively), high D.O. concentration conditions on aeration tank (D.O. > 4 mg L-1), relative high D.O. concentration in the preanoxic compartment (D.O. = 0,2 – 0,5 mg L-1). Increase of oxygen concentration in activated sludge flocs contributes to excess sludge minimization (Abbassi et al., 1999). Experimental results showed that the reduction of the excess sludge production can be achieved by raising the concentration of dissolved oxygen in the mixed liquor. The increase of the oxygen concentration in the mixed liquor leads to a deep diffusion of oxygen which subsequently leads to an enlargement of the aerobic volume inside the flocs resulting, the hydrolysed microorganisms in the floc matrix to be degraded (Abbassi et al., 1999).
Furthermore the recycle ratio of PCMAS system, at steady state operation conditions, was regulated over 600% resulting small condensation of sludge (suspended solids concentration in return stream, Xu < 1,3 MLSS). Like in our case, if the concentration of suspended solids is very high, sedimentation because of gravity is blocked thus, the downward of the solids is only due to recycling flow, which can be determined and set, and that is why is preferred for controlling the whole process (Mercalf and Eddy, 2003). In case of settlement problems on the PCMAS sedimentation tank, a chemical precipitation tank is integrated on the system.
The influent and effluent characteristics of SNdN and PCMAS systems are presented in Table1. Relative small standard deviation in influent measured characteristic can was expected due to the use of an influent equalization tank.
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