Author:

Keywords:

0904 Chemical Engineering, Chemical Engineering, 4004 Chemical engineering

Abstract:

The purification of wastewater with activated sludge generates huge amounts of excess sludge, which is most often incinerated after mechanical dewatering and drying. During the dewatering-drying process, i.e. to within a certain range of dry solids content, the sludge passes through a sticky phase. Its sticky consistency causes the sludge to cling to the dryer walls, causing operational downtime. Hence, it is of utmost importance to control the dry solids content of the sludge at the end of the mechanical dewatering step (i.e. before it enters the drying step) to avoid the sticky issues. Ca²⁺ ions play a crucial role in the bioflocculation process and, hence, in sludge dewatering, by means of (i) the exchangeable Ca²⁺ and (ii) the enmeshed CaCO3 solids in the flocs. It is demonstrated that both Ca²⁺ pools improve the cake dryness obtained after centrifugal compaction. More specifically, in the case when exchangeable Ca²⁺ is present in the sludge, in this way stabilizing the floc structure, the cake dryness increases by about 2% DS. An increase of the sludge CaCO3 fraction from 30% to 70% raises the cake dryness by 10% DS. At the Monsanto Antwerp WWTP, Belgium, for sludge characterized by too good a natural dewatering capacity due to the presence of both Ca²⁺ pools (and, in particular, due to the incorporation of the CaCO3 solids in the sludge), the traditional addition of clay mineral as a skeleton builder is reduced in the sludge feed to the centrifuge-dryer apparatus. In this way the cake dryness after the centrifuge is controlled which avoids the stickiness phenomena in the dryer operation.