| The activated sludge process is the most widely used biological treatment formunicipal and industrial wastewater worldwide. This process uses microorganisms totransform dissolved and colloidal organic substance in the wastewater into biomass orcarbon dioxide (CO2) and water. The production of the major byproduct, the wasteactivated sludge (WAS), is a serious disposal problem for treatment plants. The WAScontains a considerable amount of hazardous organic and inorganic materials, such aspathogens, parasite eggs, and a number of heavy metals. The mixture is frequentlysubjected to treatment prior to disposal to avoid posing a significant threat to theecological system.The main treatment of the excess sludge presently employed in China dependson the landfill operation after coagulation filtration. The conventional method to treatexcess sludge is concentrate, sludge dewatering and stabilization, and then landfill,incineration, land utilization and sewage outfall. However, the disposal of the excesssludge by these operations is not effective and has defects. The costs associated withthe treatment of the excess sludge may cover up to25%-65%of the total plantoperation cost. High energy consumption not only affects the sewage treatment costs,it also has effects on the sustainable utilization of energy resources and theenvironmental pollution associated with the energy production processes. So, it isnecessary to develop a new method to treat excess sludge and reduce the energyconsumption. The investigations are mainly involved in the following respects:(1) Thermophilic bacteria substantially enhanced the reduction of excess sludge.A thermophilic strain was isolated from the waste activated sludge and identified as anew species of Bacillus by16S rRNA gene sequence analysis, named Bacillus sp.Hnu. Bcillus sp. Hnu is able to release protease that can help in dissolving sludge.Dissolve experiments indicated that higher temperature and more oxygen supply wasadvantageous to the VSS removal ratio with the same effect to that of protease activity.The maximum VSS removal ratio was achieved at21.5%,42.5%, and55%after108h digestion at pH6.9and60°C under anaerobic, microaerobic and aerobic conditons. VSS removal ratio and protease activity were only slightly affected by the pH. Thekinetic study showed that both the hydrolysis of sludge with Bacillus sp. Hnu and thecontrol test followed the first-order kinetic equation (except at60°C). The hydrolysisrate constants (Kh) for the anaerobic, microaerobic, and aerobic conditions were3,4.8,and7times (40°C) and3.5,9.8, and11.8times (50°C) and2.7,7.2, and10.3times(60°C) higher than that of the control test. The above results prove that the Bacillus sp.Hnu and the oxygen supply help in accelerating the hydrolysis rate.(2) Solar energy is used as a hot source to enhance excess sludge reduction. Themagnitude of solar energy is largely related to the season of the year. The maximumremoval efficiency of total solid and total volatile solid was37%and44%as thesludge retention time was10days in summer, respectively. The maximum removalefficiency of total solid and total volatile solid was29.3%and27.5%in autumn andin winter as the sludge retention time was10days and the removal efficiency was18.5and24.8%, respectively. Sludge retention time affects the sludge removalefficiency greatly. As the sludge retention time increased from10days to20days,total solid decreased from14g/L to7.1g/L, and total volatile solid decreased from8g/L to3.7g/L. In the meantime the removal efficiency increased from29.3%and27.5%to49.3%and53.4%, respectively. The meteorological conditions determinethe temperature in the reactor, while the temperature affects the sludge dissolution. Inclear days the temperatures were18°C,17.4°C, and10°C higher, while in rainy daysthe temperatures were12.5°C,6.1°C, and4°C higher inside the reactor than outside insummer, fall, and winter, respectively.(3) Magnesium ammonium phosphate is produced from digested sewage sludgeby means of adsorption. Modified bagasse can be used to remove heavy metals fromthe sludge supernatant efficiently. The highest metal ion removal efficiencies were96%,95%,98%, and97%for Cu, Cd, Zn and Pb ions, respectively, under modifiedbagasse dosage0.5g/L. The process follows the pseudo-second model. pH was animportant parameter in the simultaneous removal of NH4+and PO43–. Optimumreactions for NH4+and PO43–removal were observed at pH9.4and9.6, respectively.The effect of excess dosages of Mg ions on NH4+removal was greater than that ofPO43, increasing from75%to90%as the Mg/N/P molar ratio increased from1:1:1to1.3:1:1. SCOD removal rate also increased from38%to48%as the Mg/N/P molar ratio increased from1:1:1to1.3:1:1. The XRD pattern generated from theprecipitated matters matched with the database model for standard MAP standard, i.e.,the position and intensity of the peaks. It can thus be concluded that the precipitatesobtained were MAP crystals. The produced MAP fulfilled all the ecological indicesfor fertilizers used in China, and is recommended for use as a high-quality fertilizer inthe Chinese market.(4) Remove and recover phosphorus during anaerobic digestion of excess sludgeby adding waste iron scrap. The results of the current study indicate that WIS couldeffectively remove phosphorus from excess sludge. The highest phosphorus removalefficiency at WIS dosages of1,2and3g/L is39%,93%and99%, respectively. Themechanisms of phosphorus removal by WIS are surface adsorption onto WIS,hydrolysis and bio-reduction of WIS, and precipitation of phosphorus by ferrous ironsresulting from hydrolysis bacteria and iron-reducing bacteria. The first and mostimportant mechanism of phosphorus removal using WIS is hydrolysis bacteria, whichreduce the pH of excess sludge to corrode the WIS, followed by precipitation ofphosphorus using ferrous irons. Phosphorus adsorption to WIS is the secondmechanism, which has less than16%,34%and38%at WIS dosages of1,2and3g/L,respectively. This type of material may have broader applications because of its highphosphorus removal efficiency, abundant supply, and low cost. The removedphosphorus is recovered by56%using a magnet. This method is characterized byhigh removal efficiency, easy and steady operation, low cost, recovery and reuse,making it better than other physical and chemical treatments. |
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