Effect of sludge disintegration on nutrient removal
One intrinsic drawback of biosolids (or sludge) reduction processes is that it potentially hampers biological phosphorus removal. It is simply because biological phosphorous removal relies on the excess biological removal from the system. On the contrary, nitrogen removal is not significantly affected when ozone is used in sludge disintegration process. It is because the disintegrated sludge recycled back to the biological system not only contains nitrogen but also contains enough amount of readily biodegradable COD. In this process, the disintegrated sludge is fed to anoxic tank to denitrify the nitrate recirculated from aeration tank. The remaining TKN goes to aeration tank, where it is oxidized before recirculated to anoxic tank (nitrogen removal mechanisms are discussed here).
It has been reported that the sludge disintegration by ozone does not affect nitrogen removal efficiencies as long as biological nitrogen removal is performed using anoxic tanks (Yasui, 1996; Sakai, 1997). This observation in a municipal wastewater treatment plant was also confirmed by mathematical modeling based on ASM#1 (Yoon 2005b). According to the modeling study, total nitrogen (TN) including NH4-N and NO3-N in the effluent marginally increases when A/O process is combined with sludge disintegration (A/O-SD) as shown in Fig. 1. In this study, the volumes of anoxic and aeration tanks were assumed at 5 L and 10 L, respectively, and the influent flow rate was at 30 L/d. The COD of the influent was assumed at 300 mg/L and TKN at 30 mg/L. The parameter ‘beta’ in Fig. 1 indicates the conversion efficiency of non-biodegradable matters to biodegradable during the disintegration process. Perhaps the beta is higher with chemical sludge disintegration methods rather than with mechanical methods. But, regardless of the beta, TN increase is marginal until the sludge disintegration becomes excessive so that biological nitrification is affected negatively.
Fig. 1. Effect of the sludge disintegration rate on total nitrogen (TN) in effluent, where A/O-SD indicates the activated sludge process with anoxic and aeration tanks combined with sludge disintegration (Yoon 2005b).
Although there is an intrinsic drawback in phosphorous removal in sludge reduction processes, it can be somewhat relieved by using enhanced biological phosphorus removal (EBPR) processes. At least in theory, phosphorous removal can be maintained at a reasonable level by enhancing “luxury uptake of phosphorus“, while maintaining the percentage of sludge reduction at a compromised level such as 50-70%. However, little experimental results have been published yet.
© Seong Hoon Yoon