Membrane based treatment processes are very effective in removing salt from wastewater, but are hindered by calcium scale deposit formation. This study investigates the feasibility of removing calcium from treated sewage wastewater using accelerated seeded precipitation. The rate of calcium removal was measured during bench scale batch mode seeded precipitation experiments at pH 9.5 using various quantities of calcium carbonate as seed material. The results indicate that accelerated seeded precipitation may be a feasible option for the decrease of calcium in reverse osmosis concentrate streams during the desalination of treated sewage wastewater for irrigation purposes, promising decreased incidence of scaling and the option to control the sodium adsorption ratio and nutritional properties of the desalted water. It was found that accelerated seeded precipitation of calcium from treated sewage wastewater was largely ineffective if carried out without pre-treatment of the wastewater. Evidence was presented that suggests that phosphate may be a major interfering substance for the seeded precipitation of calcium from this type of wastewater. A pH adjustment to 9.5 followed by a 1-h equilibration period was found to be an effective pre-treatment for the removal of interferences. Calcium carbonate seed addition at 10 g l−1 to wastewater that had been pre-treated in this way was found to result in calcium precipitation from supersaturated level at 60 mg l−1 to saturated level at 5 mg l−1. Approximately 90% reduction of the calcium level occurred 5 min after seed addition. A further 10% reduction was achieved 30 min after seed addition. 相似文献
In wastewater treatment plants (WWTPs), a secondary settler acts as a clarifier, sludge thickener, and sludge storage tank during peak flows and therefore plays an important role in the performance of the activated sludge process. Sludge thickening occurs in the lower portions of secondary clarifiers during their operation. In this study, by detecting the hindered zone from the complete thickening process of activated sludge, a simple model for the sludge thickening velocity, us = aXb (a = 0.9925SSVI3.5, b = 3.541ln(SSVI3.5)+12.973), describing the potential and performance of activated sludge thickening in the hindered zone was developed. However, sludge thickening in the compression zone was not studied because sludge in the compression zone showed limited thickening. This empirical model was developed using batch settling data obtained from four WWTPs and validated using measured data from a fifth WWTP to better study sludge thickening. To explore different sludge settling and thickening mechanisms, the curves of sludge thickening and sludge settling were compared. Finally, it was found that several factors including temperature, stirring, initial depth, and polymer conditioning can lead to highly concentrated return sludge and biomass in a biologic reactor. 相似文献
Surface O3 production has a highly nonlinear relationship with its precursors. The spatial and temporal heterogeneity of O3-NOx-VOC-sensitivity regimes complicates the control-decision making. In this paper, the indicator method was used to establish the relationship between O3 sensitivity and assessment indicators. Six popular ratios indicating ozone-precursor sensitivity, HCHO/NOy, H2O2/ HNO3, O3/NOy, O3/NOz, O3/HNO3, and H2O2/NOz, were evaluated based on the distribution of NOx- and VOC-sensitive regimes. WRF-Chem was used to study a serious ozone episode in fall over the Pearl River Delta (PRD). It was found that the south-west of the PRD is characterized by a VOCsensitive regime, while its north-east is NOx-sensitive, with a sharp transition area between the two regimes. All indicators produced good representations of the elevated ozone hours in the episode on 6 November 2009, with H2O2/HNO3 being the best indicator. The threshold sensitivity levels for HCHO/NOy, H2O2/HNO3, O3/NOy, O3/NOz, O3/HNO3, and H2O2/NOz were estimated to be 0.41, 0.55, 10.2, 14.0, 19.1, and 0.38, respectively. Threshold intervals for the indicators H2O2/HNO3, O3/NOy, O3/NOz, O3/HNO3, and H2O2/NOz were able to identify more than 95% of VOC- and NOx-sensitive grids. The ozone episode on 16 November 16 2008 was used to independently verify the results, and it was found that only H2O2/HNO3 and H2O2/NOz were able to differentiate the ozone sensitivity regime well. Hence, these two ratios are suggested as the most appropriate indicators for identifying fall ozone sensitivity in the PRD. Since the species used for indicators have seasonal variation, the utility of those indicators for other seasons should be investigated in the future work.
