Disintegration of sewage sludge with bifrequency ultrasonic treatment.

A bifrequency ultrasonic generating trough was used to improve disintegration of sewage sludge from thickener tanks. Ultrasonic operating parameters such as frequency, power, and time were optimized. Ultrasonic treatment successfully disintegrated the floc structure of sewage sludge and promoted the release of organic matter and metal ions such as Ca2+ and Mg2+. After ultrasonic treatment, soluble chemical oxygen demand (SCOD) in the sludge solution increased 11.28-49.06%, while the concentrations of Ca2+ and Mg2+ rose 5.5-25.0% and 2.7-19.0%, respectively. The ultrasonic frequency of 25 kHz was most effective in disintegrating sludge, and high energy densities and longer treatment time increased SCOD and metal ion release. The optimal parameters that promoted the increase SCOD were 25 kHz, 75 W/L and 60 s. Under these conditions, double-frequency ultrasonication was more effective than single-frequency ultrasonication.

MINTEQ modeling for evaluating the leaching behavior of heavy metals in MSWI fly ash.

At present, all kinds of municipal solid waste incineration (MSWI) fly ash stabilization technology has been reported and successfully applied in many countries. However, leaching procedures are very different that the technologies lack uniform standard, and it is even impossible to predict the long-term stabilization. Geochemical model can explain the environmental stabilization based on chemical phase and thermodynamic crystal structure, and it is also able to guide the development of environment-friendly stabilization technology and choosing of chemical agents. Both experiment analysis and geochemical modeling were used to study the correlation between leaching behavior of MSWI fly ash and variation of pH. Dissolution/precipitation mechanism was applied in the simulation. The result indicated that the pH-dependent leaching behavior predicted by Visual MINTEQ is well in agreement with the result of pH-dependent test. pH value of leachate can significantly change the leaching behavior of MSWI fly ash. The leaching behavior of heavy metals for Pb and Cd is controlled by dissolution/precipitation mechanism, whereas for Zn and Ni, it is effected by surface adsorption reaction over a special extent of pH value.

Environmental and technical assessments of the potential utilization of sewage sludge ashes (SSAs) as secondary raw materials in construction.

Ashes produced by thermal treatments of sewage sludge exhibit common properties with cement. For example, major elements present in SSA are the same of major elements of cement. Hydraulic properties of SSA are quite the same of cement ones. They may therefore be used to substitute part of cement in concrete or other cementitious materials, provided that technical prescriptions are satisfied and that environmental risks are not significantly increased. The objective of the present study was to determine the appropriate substitution ratios to satisfy both technical and environmental criteria. In a first step, the elemental composition and particle size distribution of the ashes were measured. Then the ashes were used along with Portland cement and sand at different ratios of substitution to produce mortar and concrete which were cured for up to 90 days into parallelepipedic or cylindrical monoliths. The mechanical properties of the monoliths were measured using standard procedures for flexural and compressive strengths, and compared to blanks containing no ashes. The environmental criteria were assessed using leaching tests conducted according to standard protocols both on the ashes and the monoliths, and compared to the blanks. Results showed that the characteristics of the ashes ranged between those of cement and sand because of their larger particle size and higher content in SiO2 as compared to cement. The monoliths made with the highest substitution ratios exhibited a significant decrease in flexural and compressive strengths. However, when the ashes were used in partial substitution of cement at appropriate ratios, the concrete monoliths exhibited similar compressive strengths as the blank samples. The most appropriate ratios were found to be 10% substitution of cement and 2% substitution of sand. The leaching tests conducted on the ashes in their powdery form revealed that amongst the potential contaminants analyzed only Mo and Se were leached at concentrations above the threshold limits considered. The leaching tests conducted on concrete monoliths showed however that none of the contaminants monitored, including Mo and Se, were leached above the limits. In addition, whether concrete recipe incorporated ashes or not, similar concentrations were measured for each potential contaminant in the leachates. This result indicated that mixing ash with cement and sand to produce mortar or concrete induced a stabilization of Mo and Se and thereby constituted in itself a good treatment of the ashes.

Continuous CO(2) capture and MSWI fly ash stabilization, utilizing novel dynamic equipment.

Novel dynamic equipment with gas in and out continuously was developed to study the capture capacity of CO(2). Municipal solid waste incineration (MSWI) fly ash has a high capture rate of CO(2) in CO(2)-rich gas. Fly ash can sequester pure CO(2) rapidly, and its capacity is 16.3 g CO(2)/100 g fly ash with no water added and 21.4 g CO(2)/100 g fly ash with 20% water added. For simulated incineration gas containing 12% CO(2), the capture rate decreased and the capacity was 13.2 g CO(2)/100 g fly ash with no water added and 18.5 g CO(2)/100 g fly ash with 20% water added. After accelerated carbonation, the C and O contents increased, indicating CO(2) capture in the fly ash; CO(2) combines with Ca(OH)(2) to form CaCO(3), which increased the CaCO(3) content from 12.5 to 54.3%. The leaching of Pb markedly decreased from 24.48 to 0.111 mg/L.

[Improvement of disintegration and anaerobic digestion for sewage sludge with ultrasonic generator].

Ultrasonic generating trough was applied to study the impact of ultrasonic on conditioning of sewage sludge. There are few ultrasonic generators for sludge treatment specifically, so we designed a double-frequency ultrasonic generating trough which could adjust frequency, energy density and treating time flexibly. In the research characteristics of sewage sludge treated by single-frequency and double-frequency ultrasonic were compared. According to the results, the digestion biogas of the sewage sludge treated by ultrasonic increase obviously, and the digestion biogas increment of the sewage sludge treated by single-frequency is 40.93%, higher than that treated by double-frequency. But the SCOD of the sewage sludge treated by double-frequency is 23.5%, more than that treated by single-frequency.

[Investigation of the sequestration of CO2 and carbonation process for the stabilization of MSW fly ash].

The sequestration of CO2 and carbonation process for the stabilization of fresh fly ash from municipal solid waste (MSW) incinerator were investigated. The purpose of this paper is to evaluate the influence of fundamental parameters affecting the stabilization of heavy metal of Pb and diffusivity and reactivity of CO2 (i. e. the water content and concentration of CO2). In addition, the major physical and chemical changes in fly ash after carbonation were investigated using XRD and SEM. It shows that 10% or more water content added to fly ash could remarkably accelerate the sequestration of CO2, compared with control samples without water. The sequestration of CO2 is not remarkable within 1 day in the air atmosphere for low content of CO2 (0.03%). The XRD analysis indicates that CO2 could combine Ca(OH)2 to form CaCO3 and CO2 could also combine heavy metal oxides to form heavy metal carbonate (i.e. from PbO4 to PbCO3). Aggregates of crystals of sheet and cylinder can be observed by SEM.