Improving dewaterability of waste activated sludge through advanced oxidization process

Abstract

The production of excess sludge in biological wastewater treatment processes has been a serious issue for the operation of wastewater treatment plants (WWTPs) on both economic and environmental sides. To reduce the volume of sludge to be disposed of, the sludge dewaterability needs to be improved through conditioning processes. Many conditioning methods have been developed and applied for this purpose. Among them, the advanced oxidization process is a promising technique due to its high efficiency, and economical and environmental advantages. The aim of this thesis is to apply a series of novel advanced oxidization techniques on sludge conditioning process for the improvement of sludge dewaterability, such as the indigenous iron activated peroxidation and zero-valent iron (ZVI)/hydrogen peroxide, zero-valent iron/persulfate. Also, the role of extracellular polymeric substances (EPS) during the advanced oxidization process was also investigated to expand the knowledge of basic mechanism on the sludge dewaterability. Capillary suction time stands for the time needed for completing the filtration of sludge, is used as quantitative indexes for the evaluation of the sludge dewaterability. The indigenous iron was firstly utilized as the substitute for externally supplied iron salt to improve dewaterability of waste activated sludge through catalysing the hydrogen peroxide. Significant improvement in sludge dewaterability was attained after the addition of hydrogen peroxide at 30 mg/g total solid (TS) and 28 mg/g TS under acidic conditions (pH = 3.0), with the highest reduction of capillary suction time being 68% and 56%, respectively, for sludge containing an iron concentration of 56 mg Fe/g TS and 25 mg Fe/g TS, respectively. The observations were due to Fenton reactions between the iron contained in sludge (indigenous iron) and hydrogen peroxide. For the sludge with an insufficient level of indigenous iron, the addition of iron salt was found to be able to improve the sludge dewaterability. To expand the understanding of the indigenous iron activated advanced oxidization process, the effect of indigenous iron activated peroxidation on methane production from waste activated sludge was also investigated. Pre-treatment of waste activated sludge for 30 min at 50 mg H2O2/g TS and pH 2.0 (iron concentration in sludge was 7 mg/g TS) substantially enhanced waste activated sludge solubilization. Biochemical methane potential tests demonstrated that methane production was improved by 10% at a digestion time of 16 d after incorporating the indigenous iron activated peroxidation pre-treatment. The combined conditioning process with zero-valent iron and hydrogen peroxide was applied to improve dewaterability of a full-scale waste activated sludge. The combination of ZVI and hydrogen peroxide at pH 2.0 substantially improved the waste activated sludge dewaterability due to Fenton-like reactions. The highest improvement in waste activated sludge’s dewaterability was attained at 500 mg ZVI/L and 250 mg hydrogen peroxide/L, when the capillary suction time of the waste activated sludge (WAS) was reduced by approximately 50%. Particle size distribution indicated that the sludge flocs were decomposed after conditioning. Economic analysis showed that combined conditioning with zero-valent iron and hydrogen peroxide was a more economically favorable method for improving WAS dewaterability than the classical Fenton reactions.However, no obvious enhancement of methane production from waste activated sludge was attained after the zero-valent iron/hydrogen peroxide pre-treatment. EPS is believed to influence the sludge dewaterability largely due to its binding ability with water molecules. Through the conditioning using Fenton’s reagent (Fe(II) + H2O2), the EPS structure changed, which resulted in the improvement of sludge dewaterability. It is still unclear about the correlation between EPS structure/property changes and improved dewaterability although several mechanisms have been proposed to date. The characteristics of both extracted EPS (primarily loosely-bound EPS) from waste activated sludge and the sludge itself before and after the treatment with the Fe(II) activated peroxidation process, i.e. Fenton’s conditioning was investigated. It was found that the decrease of protein and polysaccharide from extracted EPS after Fenton’s conditioning was less than the increase of soluble protein and polysaccharide from treated sludge, which was accompanied with the increase of dewaterability. The increased dewaterability was thus likely achieved through the destruction of both EPS (including loosely-bound and tightly-bound EPS) and cells by Fenton’s conditioning. The results also suggested that other mechanisms such as flocculation and oxidization of floc particles played a secondary role. A novel conditioning method for improving waste activated sludge dewaterability by combination of persulfate and ZVI was used. The combination of ZVI (0-30g/L) and persulfate (0-6g/L) under neutral pH substantially enhanced the sludge dewaterability due to the advanced oxidization reactions. The highest enhancement of sludge dewaterability was achieved at 4g persulfate/L and 15g ZVI/L, with which the capillary suction time was reduced by over 50%. The release of soluble chemical oxygen demand during the conditioning process implied the decomposition of sludge structure and microorganisms, which facilitated the improvement of dewaterability due to the release of bound water that was included in sludge structure and microorganism. Economic analysis showed that the proposed conditioning process with persulfate and ZVI is more economically favorable for improving WAS dewaterability than classical Fenton reagent.