Dong-Qing Zhang

Characterization of water-extractable organic matter during the biostabilization of municipal solid waste.

For the purpose of characterizing the evolution of water-extractable organic matter (WEOM) during the biostabilization of municipal solid waste (MSW) and investigating the correlation between biostability and WEOM characteristics, this study conducted 100-day investigation on biostabilization of MSW and applied various analytical approaches to characterize WEOM. The results showed that the respirometric activity of MSW was reduced by 93% to 10.7 mg kg(-1), and the dissolved organic carbon concentration of WEOM kept steady at around 4.0 g kg(-1) after day 44, when the MSW was considered to be stabilized. Moreover, the aromaticity of WEOM significantly increased and the high-molecular weight fraction became the main part of WEOM. Being highly related to the biostability of MSW, the excitation-emission matrix spectra indicated the stability by the fluorescence regional integration technique either by the presence of specific Ex/Em maxima (with wavelength pair of approximately 288/455) or by the rapid increase of normalized excitation-emission area volumes with respect to the humic substances.

Bio-drying and size sorting of municipal solid waste with high water content for improving energy recovery

Bio-drying can enhance the sortability and heating value of municipal solid waste (MSW), consequently improving energy recovery. Bio-drying followed by size sorting was adopted for MSW with high water content to improve its combustibility and reduce potential environmental pollution during the follow-up incineration. The effects of bio-drying and waste particle size on heating values, acid gas and heavy metal emission potential were investigated. The results show that, the water content of MSW decreased from 73.0% to 48.3% after bio-drying, whereas its lower heating value (LHV) increased by 157%. The heavy metal concentrations increased by around 60% due to the loss of dry materials mainly resulting from biodegradation of food residues. The bio-dried waste fractions with particle size higher than 45 mm were mainly composed of plastics and papers, and were preferable for the production of refuse derived fuel (RDF) in view of higher LHV as well as lower heavy metal concentration and emission. However, due to the higher chlorine content and HCl emission potential, attention should be paid to acid gas and dioxin pollution control. Although LHVs of the waste fractions with size <45 mm increased by around 2x after bio-drying, they were still below the quality standards for RDF and much higher heavy metal pollution potential was observed. Different incineration strategies could be adopted for different particle size fractions of MSW, regarding to their combustibility and pollution property.

Effect of inoculation time on the bio-drying performance of combined hydrolytic–aerobic process

The study aimed at investigating the effects of inoculation time on the bio-drying performance of combined hydrolytic-aerobic process. Results showed that the addition of inoculating material at different time exhibited various effects not only on the degradation rate of total organics, but also on the performance of water removal and water content reduction. The beginning of aerobic stage (day 5) was suggested to be the optimal time for inoculation. Under this operational condition, 815 g/kg-W(0) (W(0)=initial water content) was removed and the water content reduced from the initial 72.0% to 48.5%. Adding inoculating material at the start of hydrolytic stage (day 0) reduced water removal and water content reduction rates. The addition of inoculating material at day 7 or 9 could not improve the bio-drying performance significantly. Additionally, the inoculation at days 0, 5, 7 and 9 enhanced lignocelluloses degradation rate by 3.8%, 11.6%, 7.9% and 7.7%, respectively.

Release of volatile organic compounds during bio-drying of municipal solid waste

Three treatments were tested to investigate the release concentrations of volatile organic compounds (VOCs) during the bio-drying of municipal solid waste (MSW) by the aerobic and combined hydrolytic-aerobic processes. Results showed that VOCs were largely released in the first 4 days of bio-drying and the dominant components were: dimethyl disulfide, dimethyl sulfide, benzene, 2-butanone, limonene and methylene chloride. Thus, the combined hydrolytic-aerobic process was suggested for MSW bio-drying due to fewer aeration quantities in this phase when compared with the aerobic process, and the treatment strategies should base on the key properties of these prominent components. Malodorous sulfur compounds and terpenes were mainly released in the early phase of bio-drying, whereas, two peaks of release concentrations appeared for aromatics and ketones during bio-drying. Notably, for the combined hydrolytic-aerobic processes there were also high concentrations of released aromatics in the shift from hydrolytic to aerobic stages. High concentrations of released chlorinateds were observed in the later phase. For the VOCs produced during MSW bio-drying, i.e., malodorous sulfur compounds, terpenes and chlorinateds, their release concentrations were mainly determined by production rates; for the VOCs presented initially in MSW, such as aromatics, their transfer and transport in MSW mainly determined the release concentrations.

Biodrying of municipal solid waste with high water content by combined hydrolytic-aerobic technology

The high water content of municipal solid waste (MSW) will reduce the efficiency of mechanical sorting, consequently unfavorable for beneficial utilization. In this study, a combined hydrolytic-aerobic biodrying technology was introduced to remove water from MSW. The total water removals were proved to depend on the ventilation frequency and the temporal span in the hydrolytic stage. The ventilation frequency of 6 times/d was preferable in the hydrolytic stage. The hydrolytic span should not be prolonged more than 4 d. At this optimal scenario, the final water content was 50.5% reduced from the initial water content of 72.0%, presenting a high water removal efficiency up to 78.5%. A positive correlation was observed between the organics losses and the water losses in both hydrolytic and aerobic stages (R = 0.944, p < 0.01). The evolutions of extracellular enzyme activities were shown to be consistent with the organics losses.

Evolution of heavy metals in municipal solid waste during bio-drying and implications of their subsequent transfer during combustion

Bio-drying has been applied to improve the heating value of municipal solid waste (MSW) prior to combustion. In the present study, evolution of heavy metals in MSW during bio-drying and subsequent combustion was studied using one aerobic and two combined hydrolytic-aerobic scenarios. Heavy metals were concentrated during bio-drying and transformed between different metal fractions, namely the exchangeable, carbonate-bound, iron- and manganese-oxides-bound, organic-matter-bound and residual fractions. The amounts of heavy metals per kg of bio-dried MSW transferred into combustion flue gas increased with bio-drying time, primarily due to metals enrichment from organics degradation. Because of their volatility, the partitioning ratios of As and Hg in flue gas remained stable so that bio-drying and heavy metal speciation had little effect on their transfer and partitioning during combustion. In contrast, the partitioning ratios of Pb, Zn and Cu tended to increase after bio-drying, which likely enhanced their release potential during combustion.

Sorting efficiency and combustion properties of municipal solid waste during bio-drying

One aerobic and two combined bio-drying processes were set up to investigate the quantitative relationships of sorting efficiency and combustion properties with organics degradation and water removal during bio-drying. Results showed that the bio-drying could enhance the sorting efficiency of municipal solid waste (MSW) up to 71% from the initial of 34%. The sorting efficiency was correlated with water content negatively (correlation coefficient, r=-0.89) and organics degradation rate positively (r=0.92). The higher heating values (HHVs) were correlated with organics degradation negatively for FP (i.e. the sum of only food and paper) (r=-0.93) but positively for the mixing waste (MW) (r=0.90), whereas the lower heating values (LHVs) were negatively correlated with water content for both FP (r=-0.71) and MW (r=-0.96). Other combustion properties depended on organics degradation performance, except for ignition performance and combustion rate. The LHVs could be greatly enhanced by the combined process with insufficient aeration during the hydrolytic stage. Compared with FP, MW had higher LHVs and ratios of volatile matter to fixed carbon. Nevertheless, FP had higher final burnout values than MW.

Environmental and economic assessment of combined biostabilization and landfill for municipal solid waste.

Biostabilization can remove considerable amounts of moisture and degradable organic materials from municipal solid waste (MSW), and can therefore be an effective form of pretreatment prior to landfill. The environmental and economic impacts of two combined processes, active stage biostabilization + sanitary landfill (AL), and active and curing stage biostabilization + sanitary landfill (ACL), were compared with sanitary landfill (SL) for MSW with high moisture content. The results indicated that land requirement, leachate generation, and CH(4) emission in the ACL process decreased by 68.6%, 89.1%, and 87.6%, respectively, and the total cost was reduced by 24.1%, compared with SL. This implies that a combined biostabilization and landfill process can be an environmentally friendly and economically feasible alternative to landfill of raw MSW with high moisture content. Sensitivity analysis revealed that treatment capacity and construction costs of biostabilization and the oxidation factor of CH(4) significantly influenced the costs and benefits of the AL and ACL process at an extremely low land price. When the land price was greater than 100 USD m(-2), it became the dominating factor in determining the cost of treatment and disposal, and the total costs of ACL were reduced to less than 40% of those of SL.

Potential gases emissions from the combustion of municipal solid waste by bio-drying.

One aerobic and two combined hydrolytic-aerobic processes were set up to investigate the influence of bio-drying on the potential emissions of combustion gases and the quantitative relationships of potential emissions with organics degradation. Results showed that the bio-drying would result in the increase of the HCl and SO(2) emissions and potential for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) formation, but the decrease of NO(x) emissions in the combustion. The potential emissions of combustion gases were correlated with organics degradation (correlation coefficient, r=0.67 for HCl, r=0.96 for SO(2), r=0.91 for PCDD/Fs and r=-0.60 for NO(x)). Interestingly, the total emissions of combustion gases based on input waste could be minimized by bio-drying. The bio-drying caused a reduction of NO(x) emissions but a negligible variation of total emissions of HCl and SO(2) as well as the potential for total PCDD/Fs formation. Moreover, the bio-drying could significantly improve the ratio of gas emissions to low heating values. The mixed waste after bio-drying was more favorable for combustion and the combined process with insufficient aeration during the hydrolytic stage was proposed for the bio-drying operation.

Effect of leachate inoculum on biopretreatment of municipal solid waste by a combined hydrolytic-aerobic process.

The biopretreatment of municipal solid waste by a combined hydrolytic-aerobic process has become of great interest for biostabilization or biodrying. The study aimed at investigating the effect of leachate inoculum on the biopretreatment. During the hydrolytic stage, the inoculum addition at the ratio of leachate to waste (LWR) of 5% and 7% stimulated the start-up of hydrolysis and enhanced the hydrolysis rate by 27.4% and 24.2%, respectively; whereas the inoculation at LWR of 1% had almost no effect on the hydrolysis rate and the inoculation at LWR of 10% reduced the hydrolysis rate by 12.7%. During the subsequent aerobic stage, the inoculations at LWRs greater than 5% decreased organics degradation rate. As a whole, compared with none inoculation, the total degradation rates of organics for inoculating trials at LWRs of 5%, 7% and 10% decreased by 14.5%, 14.3% and 32.7%, respectively. Correspondingly, their net water removal rates were reduced by 4.4%, 5.8% and 19.0%. The inoculation at LWR of 1% could not significantly affect the biopretreatment. The inoculum addition at LWR of 5% and 7% could shorten hydrolytic stage and thus accelerate the whole combined process. Moreover, the inoculations at LWRs greater than 5% were favorable for lignocelluloses degradation.