Dongjie Niu

Enhanced dewaterability of sewage sludge in the presence of Fe(II)-activated persulfate oxidation

The potential benefits of Fe(II)-activated persulfate oxidation on sludge dewatering and its mechanisms were investigated in this study. Capillary suction time (CST) was used to evaluate sludge dewaterability. Both extracellular polymeric substances (EPS) and viscosity were determined in an attempt to explain the observed changes in sludge dewaterability. The optimal conditions to give preferable dewaterability characteristics were found to be persulfate (S(2)O(8)(2-)) 1.2 mmol/gVSS, Fe(II) 1.5 mmol/gVSS, and pH 3.0-8.5, which demonstrated a very high CST reduction efficiency (88.8% reduction within 1 min). It was further observed that both soluble EPS and viscosity played relatively negative roles in sludge dewatering, whereas no correlation was established between sludge dewaterability and bound EPS. Three-dimensional excitation-emission matrix (EEM) fluorescence spectra also revealed that soluble EPS of sludge were degraded and sludge flocs were ruptured by persulfate oxidation, which caused the release of water in the intracellular pace and subsequent improvement of its dewaterability.

Novel insights into enhanced dewaterability of waste activated sludge by Fe(II)-activated persulfate oxidation

The potential of Fe(II)-activated persulfate (S(2)O(8)(2-)) oxidation on enhancing the dewaterability of sludge flocs from 3-full scale wastewater treatment plants (WWTPs) were investigated. Normalized capillary suction time (CST) was applied to evaluate sludge dewaterability. Both extracellular polymeric substances (EPS) and metabolic activity of microorganisms were determined to explore the responsible mechanism. Fe(II)-S(2)O(8)(2-) oxidation effectively improved sludge dewaterability. The most important mechanisms were proposed to be the degradation of EPS incorporated in sludge flocs and rupture of microbial cells. Three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy confirmed that the powerful SO(4)(-) from Fe(II)-S(2)O(8)(2-) system destroyed the particular functional groups of fluorescing substances (i.e., aromatic protein-, tryptophan protein-, humic- and fulvic-like substances) in EPS and caused cleavage of linkages in the polymeric backbone and simultaneous destruction of microbial cells, resulting in the release of EPS-bound water, intracellular materials and water of hydration inside cells, and subsequent enhancement of dewaterability.

Synergetic pretreatment of waste activated sludge by Fe(II)–activated persulfate oxidation under mild temperature for enhanced dewaterability

The potential benefits of Fe(II)-activated persulfate (S(2)O(8)(2-)) oxidation under mild temperature in enhancing the dewaterability of waste activated sludge were investigated. Capillary suction time (CST) was used to characterize sludge dewatering. Zeta potential, particle size distribution, three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy, fourier-transformed infrared (FT-IR) spectroscopy and scanning electronic microscopy (SEM) were employed to explore influencing mechanisms. The results indicated that the dewaterability was deteriorated with single thermal treatment, but significantly enhanced in the presence of Fe(II)-S(2)O(8)(2-) oxidation and further advanced together with thermal treatment. EEM and FT-IR analysis indicated that combined thermal and Fe(II)-S(2)O(8)(2-) oxidation pretreatment led to degrading of tyrosine and tryptophan protein-like substances in extracellular polymeric substances (EPS) and cleavage of linkages in polymeric backbone. SEM images further revealed the rupture of sludge flocs at the colloidal scale, which contributed to the release of EPS-bound water and interstitial water trapped between flocs, and subsequent enhanced dewaterability.

Enhanced dewatering characteristics of waste activated sludge with Fenton pretreatment: effectiveness and statistical optimization

In this work, the enhanced dewaterabing characteristics of waste activated sludge using Fenton pretreatment was investigated in terms of effectiveness and statistical optimization. Response surface method (RSM) and central composite design (CCD) were applied to evaluate and optimize the effectiveness of important operational parameters, i.e., H2O2 concentrations, Fe2+ concentrations and initial pH values. A significant quadratic polynomial model was obtained (R2= 0.9189) with capillary suction time (CST) reduction efficiency as the response. Numerical optimization based on desirability function was carried out. The optimum values for H2O2, Fe2+, and initial pH were found to be 178 mg·g−1 VSS (volatile suspended solids), 211 mg·g−1 VSS and 3.8, respectively, at which CST reduction efficiency of 98.25% could be achieved. This complied well with those predicted by the established polynomial model. The results indicate that Fenton pretreatment is an effective technique for advanced waste activated sludge dewatering. The enhancement of sludge dewaterability by Fenton’s reagent lies in the migration of sludge bound water due to the disintegration of sludge flocs and microbial cells lysis.

Copper leaching of MSWI bottom ash co-disposed with refuse: Effect of short-term accelerated weathering

Co-disposal of refuse with municipal solid waste incinerator (MSWI) bottom ash (IBA) either multi-layered as landfill cover or mixed with refuse could pose additional risk to the environment because of enhanced leaching of heavy metals, especially Cu. This study applied short-term accelerated weathering to IBA, and monitored the mineralogical and chemical properties of IBA during the weathering process. Cu extractability of the weathered IBA was then evaluated using standard leaching protocols (i.e. SPLP and TCLP) and co-disposal leaching procedure. The results showed that weathering had little or no beneficial effect on Cu leaching in SPLP and TCLP, which can be explained by the adsorption and complexation of Cu with DOM. However, the Cu leaching of weathered IBA was reduced significantly when situated in fresh simulated landfill leachate. This was attributed to weakening Cu complexation with fulvic acid or hydrophilic fractions and/or intensifying Cu absorption to neoformed hydr(oxide) minerals in weathered IBA. The amount of total leaching Cu and Cu in free or labile complex fraction (the fraction with the highest mobility and bio-toxicity) of the 408-h weathered IBA were remarkably decreased by 86.3% and 97.6% in the 15-day co-disposal leaching test. Accelerated weathering of IBA may be an effective pretreatment method to decrease Cu leaching prior to its co-disposal with refuse.

Size-fractionation and characterization of landfill leachate and the improvement of Cu2+ adsorption capacity in soil and aged refuse

Leachate was collected from an anaerobic lagoon at Shanghai Laogang refuse landfill, the largest landfill in China, and the sample was separated into six fractions using micro-filtration membranes, followed by ultra-filtration membranes. Several parameters of the samples were measured, including chemical oxygen demand (COD), total organic carbon (TOC), total solids (TS), pH, total phosphate (TP), total nitrogen (TN), fixed solids (FS), NH4+, orthophosphate, color, turbidity, and conductivity. These parameters were then quantitatively correlated with the molecular weight cutoff of the membrane used. Organic matter in the dissolved fraction (MW<1kDa) predominated in the leachate, accounting for 65% of TOC. Thermal infrared spectroscopy was used to characterize the filter residues. Asymmetric and symmetric stretching of methyl and methylene groups, and of functional groups containing nitrogen and oxygen atoms, were observed. In addition, the ability of two different samples to adsorb heavy metals was tested. Cu2+ was chosen as the representative heavy metal in this study, and the samples were soil; aged refuse, which had spent 8 years in a conventional sanitary landfill; and samples of soil and aged refuse treated for 48h with leachate in the ratio of 5g of sample per 50ml of leachate. Cu2+ uptake by the raw soil was approximately 4.60microg/g, while uptake by the leachate-contacted soil and leachate-contacted aged refuse were 5.66 and 5.11microg/g, respectively. These results show that the organic matter in the leachate enhanced the capacity of aqueous solutions to adsorb Cu2+.

A comprehensive overview of rural solid waste management in China

This paper aims to conduct a comprehensive review of rural solid waste (RSW) in terms of characteristics, management and legislation. Survey results show that RSW generation rates range from 0.25 to 2.1 kg ∙(capita∙d)−1 in different rural villages across regions of China. Total RSW generation has been increasing, which is far higher than official data in 2014. RSW are dominated by food residue and coal ash/cinder/dust (at approximately 70%). Most of RSW are still discarded randomly without any treatment in China. Scattered RSW generation sources, imperfect legislation system, poor infrastructure on treatment and disposal are identified as the biggest challenge for RSW management currently in China. To improve RSW management, increasing financial resources, establishing sorting collection and transportation network, promoting sorting collection and recycling, improving treatment technology are proposed as the possible solution.

Source-Separated Collection of Rural Solid Waste in China

The rapid urbanization progress and the continuous improvement of rural residents’ living standards are contributing to the increase in rural solid waste (RSW) in China. RSW generation rates range from 0.25 to 2.3 kg (capita d)−1 in different rural areas, and the real total RSW generation amount was far higher than official data in 2014. RSW is dominated by food residue and coal ash/cinder/dust in rural China, and most of it is discarded randomly without any treatment. In this work, rural household behaviors toward RSW treatment and their perceptions in terms of awareness and attitudes on the source-separated collection of RSW are investigated with a questionnaire survey consisting of 518 valid samples. The results indicated that some rural households had spontaneously separated the recyclable waste and food waste to some extent. The public were aware of the importance of RSW separation through various media, and more than half of households were willing to participate in a separation program. The dominant barriers to participation were the lack of awareness of separation, inconvenience, and an insufficient separation facility (53.7%). 62.5% of rural households had a positive willingness to pay (WTP) for RSW separation and management, and the mean WTP was estimated to be 3.8 USD/year. Age, annual household income, and location significantly influenced the respondents’ WTP. More positive policy is necessary to encourage the local government to devote efforts to provide collection service and improve RSW management by combining the governmental financial budget and rural household payments.

NaHCO3-enhanced sewage sludge thin-layer drying: drying characteristics and kinetics

ABSTRACT Drying sewage sludge is a highly energy-extensive process. For this reason, this work seeks to identify a reagent that can enhance the effectiveness of the drying process. In this study, drying experiments of sewage sludge were conducted at drying temperatures ranging from 100 to 160°C. NaHCO3 was selected as the drying reagent, which was added to the sludge before drying. The thin-layer drying characteristics of the sludge and sludge/NaHCO3 mixtures were later investigated and compared. Various mathematical models were used to simulate the sludge drying curves. It was found that adding 2 and 6% (wet basis) of NaHCO3 to the sludge was effective in improving the moisture diffusion during the drying process, whereas the drying rate of the sludge/NaHCO3 mixtures decreased when the addition of NaHCO3 was further increased to 10% (wet basis). When the addition ratio was 2%, the increase in the maximum drying rate was the largest. With coefficients of determination (R2) over 0.9999, the modified Midilli model proposed in this study was observed to be the most suitable model to describe thin-layer drying of sludge relative to the other models examined in terms of R2, reduced χ2, root mean square error, and residual sum of squares. The values of the diffusion coefficients at each temperature were obtained using Fick’s second law of diffusion, which varied from 3.700 × 10−9 to 1.085 × 10−8 m2/s over the temperature range (i.e., 100–160°C). The activation energy of moisture diffusion was determined to be 27.57 kJ/mol. Scanning electron microscope images of the dried sludge and sludge/NaHCO3 mixtures indicated that the porosity of the sludge after drying increased with an increase in the NaHCO3 addition ratio. Overall, the results suggested that NaHCO3 is a suitable reagent to improve the drying efficiency of the sludge.