Titanium-salt flocculation and its sludge resource recovery to photocatalyst for advanced water treatment

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This research embraces several objectives targeting different aspects of environmental concern in terms of wastewater flocculation, sludge disposal and removing of persistent organic pollutants from water and wastewater. The production of a large amount of sludge using coagulants of iron (Fe) and aluminium (Al) salts that needs disposal is considered as the most costly and environmentally problematic challenge in wastewater treatment. Titanium (Ti) salt used as an alternative coagulant reduces the cost of sludge disposal and protects the environment by producing titania photocatalyst from the incinerated sludge. However, titania photocatalyst is only a UV light responsive and its pollutant-specific photocatalytic degradation for various organic pollutants has not being examined. Thus, the main objectives of this study are: 1. Increase the efficiency of Ti-salt flocculation in terms of organic matter removal and sludge reduction by using a natural polymer of chitosan as a coagulant-aid. 2. Trace the seasonal variation in the characteristics of Ti-salt flocculation and as prepared-titania photocatalyst. 3. Produce pollutant-specific titania photocatalyst by synthesising titanate nanotubes (TN) and thiourea (CSN2H4) doped-titanate nanotubes (TD-TN) photocatalysts through the hydrothermal treatment of as prepared-titania. 4. Investigate the pollutant-specific photocatalytic activity of as prepared-titania and the synthesised titania photocatalysts in photodegrading of organic pollutant of gaseous acetaldehyde, humic acid (HA), dichloroacetic acid (DCA), rhodamine B (RhB), metsulfuron methyl (M&M) and phenol under UV, visible and solar light irradiation. 5. Then, select the best pollutant-specific titania for removing of 16 micropollutants of pharmaceutical and personal care products (PPCPs) and endocrine disrupting chemicals (EDCs) in water. Our findings indicated that Ti-salt flocculation exhibited more reduction in turbidity, and colour of wastewater compared with Fe- and Al-salt flocculation. In addition, the use of chitosan was very efficient for enhancing the performance of Ti-salt flocculation. Ti-salt and chitosan flocculation improved significantly the turbidity and organic removal of wastewater up to 85%, considerably reduced the optimum dose of Ti-salt from 25 mg/L to less than 5 mg/L, solved the low pH of Ti-salt flocculation, and achieved in 40% reduction of Ti-salt flocculation sludge. The change in wastewater characteristics in different seasons has a negligible influence in the characteristics of Ti-salt flocculation and as prepared-titania. During the whole seasons, the turbidity and orthophosphate removal of Ti-salt flocculation were varied from 70 to 90% and 96 to 99%, respectively. As prepared-titania photocatalyst exhibited predominant anatase structure, high BET surface area and insignificant change in its photocatalytic activity. The photocatalytic degradation of HA and RhB was varied from 85 to 90% and 91 to 98%, respectively. The modified TD-TN photocatalyst exhibited superior photocatalytic activity than as prepared-titania and TN photocatalysts for photocatalytic degradation of the tested organic pollutants under visible and solar light. TD-TN photocatalyst was selected to photodegrade a set of 16 micropollutants of PPCPS and EDCs in water, exhibited a complete photocatalytic degradation of 7 micropollutants at TD-NT concentration of 50 mg/L. The photocatalytic degradation significantly increased with increasing in TD-NT concentration and complete photocatalytic degradation was achieved at TD-NT concentration of 500 mg/L after 90 minutes. The high photocatalysis activity of TD-NT in decomposing persistent organic pollutants and micropollutants would increase the attention to use of Ti-salt coagulant and titania photocatalyst as alternatives in water and wastewater treatment.
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