Effect of Activated Sludge Composition on its Dewaterability and Sticky Phase

摘要

This research is aimed at solving two major operational issues the wastewater treatment plant of Monsanto in Antwerp was coping with at the end of 2004. At first, inexplicable periodic deteriorations of the activated sludge settling resulted frequently in sludge losses from the final clarifier. And secondly, the Centridry® installation, used to reduce the waste sludge volume by the combination of mechanical dewatering and thermal drying, was performing significantly below its designed capacity. The latter was more specifically caused by two bottlenecks, that is (i) the high vibrations of the decanter centrifuge and (ii) the unexplainable phenomenon of dryer fouling by build-up of drying solids on the dryer wall, inducing substantial operational downtime. Moreover, the investigation of this peculiar fouling phenomenon is seriously impeded by the design of the Centridry® - the design which makes it, at the same time, also a unique technology. By combining both unit operations, dewatering and drying of the sludge, in one enclosed machine, the Centridry® inherently rules out sampling and, hence, analysis of the dewatered cake leaving the centrifuge and thus entering the drying stage. The lack of information on this intermediate product forms a serious drawback from a research and optimization point of view. A 3-year study of the varying Ca++ and Na+ concentrations in the wastewater has been conducted, in relation to the changing sludge settling characteristics. Two mechanisms are demonstrated to exist by which Ca++ ions influence the bioflocculation and so the sludge settleability: (i) in accordance with the Divalent Cation Bridging Theory (DCBT), the divalent Ca++ ions exchange the monovalent Na+ ions from the sludge flocs, bridging the individual flocs into larger and stronger flocs that settle better; (ii) the low soluble CaCO3, formed in the biodegradation basin, precipitates from the water, and is enmeshed within the floc structure, making the flocs heavier and so settling better. Hence, in the case of a Ca++ depletion from the wastewater (when the only Ca++ supplying plant on site goes in shutdown), the sludge settleability starts to worsen over time as a result of (i) the floc disintegration, and (ii) the continuous wasting of the (old) heavy (CaCO3) flocs, being replaced by new grown sludge flocs without the heavy solids incorporated in the floc structure. The first objective of this research, avoiding the sludge settling to deteriorate to such an extent that solids losses manifest in the clarifier overflow, is being accomplished by adding CaCl2 to the wastewater in order to avoid the complete floc disintegration to happen when the influent is for a longer period deficient in Ca++. Conclusively, Ca++ ions are of upmost importance for the bioflocculation, and, hence, for the sludge settleability.In the second part of this research, two lab tests are introduced to (i) simulate the sludge dewatering in the decanter centrifuge (that is, to simulate the important intermediate product of the Centridry®) and (ii) to map the sticky behavior of drying sludge (that is, to make the fouling issue in the Centridry® tangible). A 4-year follow-up of the centrifugal dewaterability of the Centridry® sludge feed has been conducted at lab scale. It is emphatically demonstrated that the dryness of the dewatered sludge cake is mainly governed by the sludge CaCO3 fraction, with a high sludge inorganic fraction yielding a higher cake dryness. Up to a sludge inorganic fraction of 55%, the cake can be made ‘wet’ again by reducing the amount of inorganic clay conditioner to the Centridry® feed. Furthermore, the lab protocol to map the stickiness of the sludge provides information of equal importance to better understand the fouling issue in the dryer. The drying of sludge is, worldwide, cumbersome because of its sticky phase. When partially dried it behaves as a sticky, gluey substance that likes to cling on a dryer wall. An explanation for this physical consistency is proposed to be found in the 3-dimensional structure of extracellular polymeric substances (EPS) surrounding the micro-organisms, which achieves its strongest (plastic, pasty) configuration when the sludge is partially dried. The sticky phase, i.e., the range of dryness wherein it behaves most sticky, depends on the sludge inorganic fraction as well, with a high sludge inorganic fraction (and thus a low EPS content) resulting in a higher sludge dryness ‘needed’ to come in the sticky phase. Further, it is shown that the intrinsic stickiness of sludge can be reduced by adding pure polyaluminiumchloride (PACl) to waste sludge. It is postulated that this salient feature of PACl conditioning is due to the large Al13-clusters and their associated hydrated water. When present on the exterior of the drying flocs, these (artificially added) clusters of bound water (water shields) act as some kind of lubricant for the intrinsically sticky solids. This stickiness mitigating effect of pure PACl addition to waste sludge is a totally new application of PACl. In addition to both lab tests, a statistical model is obtained for the fouling propensity, based on a 5-year data base with operational experience. This empirical model clearly demonstrates that the sludge characteristics, resulting from the cations in the incoming wastewater, govern the fouling probability, with sludge characterized by a high inorganic fraction (excellent dewaterability) corresponding with a higher chance of dryer fouling. The second objective of this research, increasing the Centridry® capacity, is then accomplished as follows. The vibrational bottleneck is eliminated by substantially reducing the centrifuge speed of rotation, from 3165 rpm to 2500 rpm. Next, the fouling of the sludge dryer is managed by a two-fold control strategy: (i) for sludge with an inorganic fraction below a threshold of approximately 55%, a wet cake after the centrifuge is aimed for, by lowering the inorganic clay addition to the sludge feed (making ‘wet cake’); (ii) for sludge that dewaters to such a high extent that making ‘wet cake’ is not an option anymore, pure PACl is added to the sludge feed. The latter strategy is dealing with reducing the sludge’s intrinsic stickiness, whereas the former is ‘post-poning’ the sludge’s sticky phase to occur only after the drying solids have passed the most sensitive places at the entrance of the dryer before behaving most sticky. By doing so, the Centridry® capacity could be increased with 107%.In conclusion, this PhD study made the optimization of the industrial-scale Centridry® installation possible by unraveling the relation between the micro-scale activated sludge floc structure and the changing cation concentrations in the wastewater.