Rate determination of supercritical water gasification of primary sewage sludge as a replacement for anaerobic digestion

Hydrogen production and phosphorus recovery via supercritical water gasification of sewage sludge in a batch reactor

In this study, gasification of sewage sludge in supercritical water using a batch reactor was investigated. The effects of temperature, retention time, and the oxidation coefficient on gas composition, gas yield, total organic carbon removal efficiency (X_TOC), gasification efficiency (GE), carbon gasification efficiency (CE), and phosphorus release rate (X_p) were investigated. The experimental results indicated that the yields for hydrogen, methane, and carbon dioxide increased with the increase in temperature from 380 °C to 460 °C. A maximum hydrogen molar fraction of 55.72% and a yield of 19.86 mol/kg were obtained at 460 °C and 27 MPa after 6 min. The GE, CE, X_TOC, and X_p also increased with the increase in temperature. An extension of the retention time promoted the gasification of sludge, thereby resulting in an increase in the hydrogen and methane molar fraction, yield, GE, CE, X_TOC, and X_p. Under the conditions of 420 °C and 27 MPa after 6 min, with an increase in the oxidation coefficient from 1.5 to 2.5, the oxidation reaction became dominant in the supercritical water. Hydrogen and methane were converted to carbon dioxide and water with an excess of hydrogen peroxide, which resulted in a lower hydrogen yield. However, the decomposition of organic compounds in the sludge was promoted with the addition of hydrogen peroxide, thereby resulting in an increase in the GE, CE, X_TOC, and X_p. When the oxidation coefficient reached 2.5, a maximum GE of 131.6% and X_p of 98.74% were obtained.

Supercritical water gasification of sewage sludge in continuous reactor

In this study, a process for the continuous recovery of phosphorus and generation of gas from sewage sludge is investigated for the first time using supercritical water gasification (SCWG). A continuous reactor was employed and experiments were conducted by varying the temperature (500-600 °C) and residence time (5-60 s) while fixing the pressure at 25 MPa. The behavior of phosphorus during the SCWG process was studied. The effect of the temperature and time on the composition of the product gas was also investigated. A model of the reaction kinetics for the SCWG of sewage sludge was developed. The organic phosphorus (OP) was rapidly converted into inorganic phosphorus (IP) within a short residence time of 10 s. The gaseous products were mainly composed of H₂, CO₂, and CH₄. The reaction followed first order kinetics, and the model was found to fit the experimental data well.

Treatment of municipal sewage sludge in supercritical water: A review

With increasing construction of wastewater treatment plants and stricter policies, municipal sewage sludge (MSS) disposal has become a serious problem. Treatment of MSS in supercritical water (SCW) can avoid the pre-drying procedure and secondary pollution of conventional methods. SCW treatment methods can be divided into supercritical water gasification (SCWG), supercritical water partial oxidation (SCWPO) and supercritical water oxidation (SCWO) technologies with increasing amounts of oxidants. Hydrogen-rich gases can be generated from MSS by SCWG or SCWPO technology using oxidants less than stoichiometric ratio while organic compounds can be completely degraded by SCWO technology with using an oxidant excess. For SCWG and SCWPO technologies, this paper reviews the influences of different process variables (MSS properties, moisture content, temperature, oxidant amount and catalysts) on the production of gases. For SCWO technology, this paper reviews research regarding the removal of organics with or without hydrothermal flames and the changes in heavy metal speciation and risk. Finally, typical systems for handling MSS are summarized and research needs and challenges are proposed.

Integrated carbon dioxide/sludge gasification using waste heat from hot slags: syngas production and sulfur dioxide fixation

The integrated CO2/sludge gasification using the waste heat in hot slags, was explored with the aim of syngas production, waste heat recovery and sewage sludge disposal. The results demonstrated that hot slags presented multiple roles on sludge gasification, i.e., not only a good heat carrier (500-950 °C) but also an effective desulfurizer (800-900 °C). The total gas yields increased from 0.022 kg/kgsludge at 500 °C to 0.422 kg/kgsludge at 900 °C; meanwhile, the SO2 concentration at 900 °C remarkably reduced from 164 ppm to 114 ppm by blast furnace slags (BFS) and 93 ppm by steel slags (SS), respectively. A three-stage reaction was clarified including volatile release, char transformation and fixed carbon using Gaussian fittings and the kinetic model was analyzed. Accordingly, a decline process using the integrated method was designed and the optimum slag/sludge ratio was deduced. These deciphered results appealed potential ways of reasonable disposal of sewage sludge and efficient recovery of waste heat from hot slags.

Treatment of sewage sludge in supercritical water and evaluation of the combined process of supercritical water gasification and oxidation

Influences of temperature and oxidation coefficient (n) on sewage sludge treatment in supercritical water and its corresponding reaction mechanism were studied. Moreover, the combined process of supercritical water gasification (SCWG) and supercritical water oxidation (SCWO) was also investigated. The results show that ammonia nitrogen, phenols and pyridines are main refractory intermediates. The weight of solid products at 873K and n=4 is only 3.5wt.% of the initial weight, which is lower than that after combustion. Volatile organics in solid phase have almost released at 723K and n=0. Highest yield of combustible gases was obtained at n=0, and H2 yield can reach 11.81mol/kg at 873K. Furthermore, the combination of SCWG at 723K and SCWO at 873K with a total n=1 is feasible for its good effluent quality and low operation costs.

Supercritical water gasification of sewage sludge: gas production and phosphorus recovery

In this study, the feasibility of the gasification of dewatered sewage sludge in supercritical water (SCW) for energy recovery combined with P-recovery from the solid residue generated in this process was investigated. SCWG temperature (400°C, 500°C, 600°C) and residence time (15min, 30min, 60min) were varied to investigate their effects on gas production and the P recovery by acid leaching. The results show that the dry gas composition for this uncatalyzed gasification of sewage sludge in SCW mainly comprised of CO2, CO, CH4, H2, and some C2-C3 compounds. Higher temperatures and longer residence times favored the production of H2 and CH4. After SCWG, more than 95% of the P could be recovered from the solid residue by leaching with acids. SCWG combined with acid leaching seems an effective method for both energy recovery and high P recovery from sewage sludge.

A review of wet air oxidation and Thermal Hydrolysis technologies in sludge treatment

With rapid world population growth and strict environmental regulations, increasingly large volumes of sludge are being produced in today's wastewater treatment plants (WWTP) with limited disposal routes. Sludge treatment has become an essential process in WWTP, representing 50% of operational costs. Sludge destruction and resource recovery technologies are therefore of great ongoing interest. Hydrothermal processing uses unique characteristics of water at elevated temperatures and pressures to deconstruct organic and inorganic components of sludge. It can be broadly categorized into wet oxidation (oxidative) and thermal hydrolysis (non-oxidative). While wet air oxidation (WAO) can be used for the final sludge destruction and also potentially producing industrially useful by-products such as acetic acid, thermal hydrolysis (TH) is mainly used as a pre-treatment method to improve the efficiency of anaerobic digestion. This paper reviews current hydrothermal technologies, roles of wet air oxidation and thermal hydrolysis in sludge treatment, and challenges faced by these technologies.