MASSBALANSBERÄKNING SOM VERKTYG FOR DRIFTOPTIMERING AV SEDIMENTERING / Mass balance calculations as a tool for optimizing the operation of secondary sedimentation

This paper presents the results of using mass balance calculations for the sludge transport to a secondary sedimentation tank after an activated sludge process with high sludge concentration. An activated sludge process with nitrogen removal often have high sludge concentration and a sludge with sludge settling characteristics. These factors give high sludge load on the sedimentation tank and the dominant functions become the hand
ling and the thickening of the sludge. In many cases the sedimentation tank is operated near and over the critical Solid Flux load. To gain new understanding and to find easy tools for the operation of this tank a simple mass-balance calculation was used. The incoming sludge mass is defined as (Q,n+ Q,) *X, and outgoing sludge mass as Qr * X,.. To evaluate the tool field studies and solid flux measurements were performed at Helsingborg Bromma and Henriksdahl WWTP. Based on mass balance calculations the sludge transport can be followed, and as a result a better understanding of many operational factors. It is easily showed that controlling the pumping of return sludge is not a method for increasing the capacity on the sedimentation tank. The strategy for
the return sludge pump should instead be focusing of the effects of the biological process. Is it better to have a high sludge concentration or should the sedimentation tank be »empty» to avoid process as release of phosphorus or denitrification? With the mass balance and on-line meters a control strategy can easily be made. The mass balance is also important for the design of new plants and for the evaluation when upgrading existing treatment plants. The requirements of volume for sludge storage and the effects of an increase of the sludge concentration can easily be approximated by the used of mass balance calculations.