Yiying Jin

Combined alkaline and ultrasonic pretreatment of sludge before aerobic digestion

Alkaline and ultrasonic sludge disintegration can be used as the pretreatment of waste activated sludge (WAS) to promote the subsequent anaerobic or aerobic digestion. In this study, different combinations of these two methods were investigated. The evaluation was based on the quantity of soluble chemical oxygen demand (SCOD) in the pretreated sludge as well as the degradation of organic matter in the subsequent aerobic digestion. For WAS samples with combined pretreatment, the released COD levels were higher than those with ultrasonic or alkaline pretreatment alone. When combined with the ultrasonic treatment, NaOH treatment was more efficient than Ca(OH)2 for WAS solubilization. The COD levels released in various sequential options of combined NaOH and ultrasonic treatments were in the the following descending order: simultaneous treatment > NaOH treatment followed by ultrasonic treatment > ultrasonic treatment followed by NaOH treatment. For simultaneous treatment, low NaOH dosage (100 g/kg dry solid), short duration (30 min) of NaOH treatment, and low ultrasonic specific energy (7500 kJ/kg dry solid) were suitable for sludge disintegration. Using combined NaOH and ultrasonic pretreatment with optimal parameters, the degradation efficiency of organic matter was increased from 38.0% to 50.7%, which is much higher than that with ultrasonic (42.5%) or with NaOH pretreatment (43.5%) in the subsequent aerobic digestion at the same retention time.

Investigation of accelerated and natural carbonation of MSWI fly ash with a high content of Ca.

The application of accelerated carbonation and natural carbonation to treat municipal solid waste incinerator (MSWI) fly ash was presented. The influence of reaction time and the liquid-to-solid (L/S) ratio, which both affect the reactivity of CO(2), was evaluated to determine their impact on the quality of carbonation. The optimum carbonation reaction of fly ash was found to occur when an L/S of 0.25 was used. Carbonation decreased the leaching of Pb, Cu, Zn and As, but increased the leaching of Cd and Sb. Based on the leaching of these six heavy metals, the optimum pH of the carbonated fly ash was 9.5-10.5. The release of soluble salts such as SO(4), Cl and F changed little following carbonation, and their release occurred independently of pH. The release potential and leaching behavior of carbonated fly ash were further evaluated using the three-step sequential extraction procedure proposed by the commission of the European Communities Bureau of Reference (BCR). The results of the BCR analyses revealed that carbonation exerted a significant effect on the heavy metal fractions in steps 1 and 2, but little effect on the distribution of heavy metals in step 3 and residue fraction. Physical and chemical changes induced by carbonation were presented and discussed.

Development of lightweight aggregate from dry sewage sludge and coal ash

In this study, dry sewage sludge (DSS) as the principal material was blended with coal ash (CA) to produce lightweight aggregate. The effects of different raw material compositions and sintering temperatures on the aggregate properties were then evaluated. In addition, an environmental assessment of the lightweight aggregate generated was conducted by analyzing the fixed rate of heavy metals in the aggregate, as well as their leaching behavior. The results indicated that using DSS enhanced the pyrolysis-volatilization reaction due to its high organic matter content, and decreased the bulk density and sintering temperature. However, the sintered products of un-amended DSS were porous and loose due to the formation of large pores during sintering. Adding CA improved the sintering temperature while effectively decreasing the pore size and increasing the compressive strength of the product. Furthermore, the sintering temperature and the proportion of CA were found to be the primary factors affecting the properties of the sintered products, and the addition of 18-25% of CA coupled with sintering at 1100 degrees C for 30 min produced the highest quality lightweight aggregates. In addition, heavy metals were fixed inside products generated under these conditions and the As, Pb, Cd, Cr, Ni, Cu, and Zn concentrations of the leachate were found to be within the limits of Chinas regulatory requirements.

Application of alkaline treatment for sludge decrement and humic acid recovery

A new method was introduced to reduce waste activated sludge and extract humic acid for liquid fertilizer. Sludge was disintegrated with NaOH (0.4 g/g dry solid, 8 h) and then centrifuged to obtain the supernatant. The residual sludge was then dewatered, while the supernatant was used to extract humic acid with an ultrafiltration membrane. The results showed that the alkaline treatment dissolved more than half of the sludge organic matter, which was composed of 24% humic acid by mass. After the supernatant was concentrated 20 times using a membrane with a molecular weight cutoff of 1000, the retentate contained 94.5% of the dissolved organics and could be used to produce humic acid fertilizer. Additionally, only 26% of the NaOH was consumed and the residual NaOH in the permeate flux could be reused. Due to the removal of water and organics, the dewatered sludge could be reduced by 60% when compared to samples that did not receive the alkaline treatment.

Impact of hydrothermal process on the nutrient ingredients of restaurant garbage

In order to recover the nutrient resource from restaurant garbage, a complete trial with 2 factors on 5 levels of experiments was carried out. The temperature and heating time are the main factors influencing on hydrothermal process (HP) by which improves the degradability and digestibility of the restaurant garbage favorably to make animal feeds or fertilizer. The results showed the variation of protein, saccharide, and oil in the garbage. It showed that protein dissolved and liquefied during hydrothermal process, which made organic nitrogen in solid phase transfer to liquid phase. After heating at 180 degrees C for 60 min, organic nitrogen in liquid phase began to transform into ammonia. It also showed that hydrothermal process could promote the dextrinization, dissolution of the starch and its hydrolysis to reducing sugar, due to that starch in the restaurant garbage decreases and reducing sugar increases. When the temperature reached 140 degrees C, the reducing sugar started to decrease due to chemical reactions. The cellulose was stable at 100--180cC. The floatable oil increased markedly in the hydrothermal process. The suitable condition for de-oil was observed at 160 degrees C heating for 80 min. Furthermore, the extraction of grease from the solid phase accords with first-order reaction dynamic model.

Comparison of the fixation effects of heavy metals by cement rotary kiln co-processing and cement based solidification/stabilization

Cement rotary kiln co-processing of hazardous wastes and cement based solidification/stabilization could both immobilize heavy metals. The different retention mechanisms of the two technologies lead to different fixation effects of heavy metals. The same amount of heavy metal compounds were treated by the two types of fixation technologies. Long-term leaching test (160 days), the maximum availability leaching test (NEN 7341) and a modified three-step sequential extraction procedure, proposed by the Commission of the European Communities Bureau of Reference (BCR) were employed to compare the fixation effects of the two fixation technologies. The leaching concentrations in NEN 7341 and long-term leaching tests were compared with identification standard for hazardous wastes (GB5085.3-1996) and drinking water standard (GB5749-2005). The results indicate that the leaching concentrations of the long-term leaching test and NEN 7341 test were lower than the regulatory limits and the leached ratios were small. Both cement based solidification/stabilization and cement rotary kiln co-processing could effectively fix heavy metals. Calcination in a cement rotary kiln and the following hydration that follows during cement application could fix As, Cd, Pb and Zn more effectively and decrease the release to the environment. Cement solidification/stabilization technology has better effect in immobilizing Cr and Ni. Cr wastes are more fitful to be treated by cement solidification/stabilization.

Leaching Characteristics of Heavy Metals in Municipal Solid Waste Incinerator Fly Ash

Abstract The potential release of Pb, Zn, Cr, and Cu in municipal solid waste incinerator (MSWI) fly ash was investigated by batch leaching experiments using sodium acetate solution as the extractant. The concentrations of heavy metals decreased against the increase of liquid-to-solid (L/S) ratio with the exception of Zn. In the case of low L/S ratios, the concentration of Pb and Zn was relatively high. Leaching solvents with a wide pH range (2, 4, 6, 8, 10, and 12) were used to determine the acid neutralizing capacity of MSWI fly ash. The concentrations of heavy metals declined evidently along with the increase of initial pH of leaching solvent when the L/S ratio was fixed at 20. The turning point appeared at the pH value close to 6. When the pH value was higher than 6, the concentrations of heavy metals remained at a low level. Sequential extraction tests were also performed to reveal the long-term leaching behavior. The results showed that Pb, Zn, Cu, and Cr were mainly present in the last three fractions, which were Fe-Mn oxides, organic matters bound and residual fractions and their leachability could be well-controlled in common natural environment. However, the long-term leaching risk would be increased with the presence of reducing agents and the change of the pH value of the surrounding environment.

Alkaline post-treatment for improved sludge anaerobic digestion.

Alkaline post-treatment was tested in order to improve sludge anaerobic digestion. Between the 8th and the 12th hour of a 24-h digestion cycle, 5% of sludge was extracted from a semi-continuous digester with a sludge retention time of 20 days. The sludge was then disintegrated with 0.1 mol/L NaOH and returned to the digester after neutralization. The results showed that alkaline post-treatment increased the level of soluble organic substances in the extracted sludge, particularly of volatile fatty acids and polysaccharides. This process resulted in a 33% enhancement of biogas production in comparison with the control. When the ratio of the recycled sludge was further increased to 10% or 15%, the increment of biogas yield was reduced, due to excessive inactivation of anaerobic bacteria in the digester. Alkaline post-treatment had a minimal impact on the dewaterability of digested sludge.

Effect of hydrothermal treatment on the levels of selected indigenous microbes in food waste

The ability of hydrothermal treatment to reduce or eliminate selected indicator organisms in food waste was assessed in this study. Raw food waste collected from student canteens at Tsinghua University was heat-treated under hydrothermal conditions at 90-140 °C for 10-40 min. Hydrothermal inactivation analyses were carried out on four types of indigenous microbes used as indicators of hygiene: molds and yeasts (MY), total coliforms (TC), total aerobic counts (TAC) and Staphylococcus aureus (SA). Significant reductions in the levels of indigenous microbes in food waste were achieved during the ramping and holding periods of the hydrothermal treatment, and the microbial inactivation effect increased with increasing temperature, increasing time and increasing pressure. Due to the typical properties of food waste, hydrothermal treatment at 120 °C-0.3 MPa for at least 40 min was sufficient to achieve complete sterilization of the food waste. The results showed that hydrothermal treatment could significantly reduce the levels of indigenous microbes and is a potential advanced technique for the sterilization of food waste with a high moisture content in China.

Combustion effects and emission characteristics of SO2, CO, NOx and heavy metals during co-combustion of coal and dewatered sludge

The influences of dewatered sludge blending ratio in coal on flammability index (C) and combustion characteristic index (S) and release of sulfur dioxide (SO2), nitrogen oxide (NOx), carbon monoxide (CO) and heavy metals (Hg, As, Cd, Pb and Cr) were studied. The impact on combustion characteristics could be ignored if less than 20% of dewatered sludge was added in coal. Besides, emission pattern experiments of NOx, SO2, CO and heavy metals were carried out in a high-temperature tubular furnace. Results showed that the conversion rate of NOx and total emission of SO2 reduced with the increase of sludge adding ratio, and a better effect of fixing sulfur could be obtained when the blending ratio reached 30%. Concentrations and distributions of five types of heavy metals in different residues (bottom ash and fly ash) as well as in flue gas were analyzed. It was shown that the characteristics of coal and sludge, as well as the volatilization of heavy metals had a great influence on the distribution of heavy metals.