Combustion characteristics of sewage sludge in an incineration plant for energy recovery

A new type of sewage sludge incinerator that combines a pressurized fluidized bed combustor and a turbocharger driven by flue gas was proposed. In this study, the operation and combustion characteristics of a demonstration plant were clarified, and the design data for a commercial plant were obtained. The steady operation exceeded 600 h in total. CO, NO_x, and N₂O emissions in the flue gas were less than half those of a conventional plant. At an incineration capacity of 100 t/day, an energy savings of approximately 50% can be achieved compared with a conventional plant because the forced draft fan (FDF), the induced draft fan (IDF) and the feed water pump are unnecessary. Also, pressurization allowed reduction of the combustor volume, so about 25% of supplementary fuel can be reduced. Consequently, CO₂ emissions originating from electric power consumption and supplementary fuel is expected to be reduced by about 40% annually compared with emissions from a conventional plant; in addition, the cost of fuel and electricity can be reduced by 23 million yen. Therefore, this advanced incinerator for sewage sludge can realize energy recovery and savings as well as a low environmental impact.     

城市污泥处理利用现状研究

城市污水处理厂污泥产量巨大且成分复杂,如何把污泥进行合理利用已经越来越受人们的关注。描述了目前城市污水处理厂污泥处理与综合利用的现状。并介绍了几种新兴的污泥处理工艺。为今后污泥的资源化利用提供参考。     

Selectivity of polyamide nanofiltration membranes for cations and phosphoric acid

The results shown in this paper are part of a study to investigate the use of nanofiltration membranes for the recycling of phosphorus from sewage sludge. It especially contains the results of an experimental study about the influencing factors pressure, pH and feed composition on the selectivity of nanofiltration membranes for phosphoric acid and multivalent cations. For the use of pretreated sewage sludge an effective pressure of 11 bar could be identified as an effective operational point. The selectivity turned out to increase with decreasing pH which was studied in a pH-interval of pH 1-4. An increasing concentration of multivalent cations in the feed solution had a negative effect on the permeability of phosphoric acid. Accordingly it could be observed that the nanofiltration worked significantly more effective with sewage sludge ash eluates of the concentration 10 g/l than eluates of the concentration 100 g/l.     

Thermal Gasification: A Feasible Solution for Sewage Sludge Valorisation?

The proliferation of wastewater treatment plants has become essential to ensure the protection of the natural environment. As a consequence, the production of waste by‐products, namely sewage sludge, has increased together with both the unit efficiency levels and with the number of wastewater treatment plants. Since dumping is no longer allowed in the EU, the processes mainly used to treat sewage sludge are agricultural spreading and incineration. However, due to the constant increase in production, it has became essential to find alternative methods of treatment. Considering the chemical composition of sludge, some new thermal processes of treatment have recently been considered by researchers. Thermal gasification is one method that could be of significant interest. Dried sludge composition is sufficiently close to the that of classical biomass (such as wood, rice, etc.) to make sludge treatment in a gasifier feasible. In this paper we discuss how gasification technologies could be transposed to the treatment and valorisation of sludge. Although this process seems attractive, it will also be demonstarted that much experimentation remains to be conducted on a laboratory and pilot scale.     

污泥土地利用需注意的问题

污泥是城市污水处理厂不可避免的产物。随着工业的发展和城市化进程的加快,城市污泥的产量急剧增加。城市污泥中含有丰富的有机质及氮、磷等营养元素,但同时也含有种类较多含量不等的重金属、微量的有机污染物和病原物。所以将产生量巨大、成分复杂的污泥经过科学处理后,使其减量化、无害化、资源化,已成为我国乃至全世界环境界深为关注的课题之一。目前,国内外主要的污泥处置方法有填埋、焚烧和土地利用等。污泥土地利用是城市污水污泥资源化利用的一个重要技术途径,为了保证污泥土地利用更加安全有效,本文提出了污泥土地利用应注意的问题。     

Catalytic effect of metal oxides on pyrolysis of sewage sludge

The effect of metal oxides (Al₂O₃, CaO, Fe₂O₃, TiO₂, and ZnO) on the pyrolysis of sewage sludge was investigated. The experiments were performed in a thermogravimetric analyzer (TGA) to check the pyrolysis behavior of raw sludge, demineralized sludge and demineralized sludge with metal oxides added, respectively. The results showed that the presence of Fe₂O₃ and ZnO probably inhibited the decomposition of organic matters in demineralized sludge samples to generate more solid residues, while Al₂O₃, CaO, and TiO₂ promoted the degradation of organic matters throughout the whole pyrolysis temperature ranges. All the metal oxides studied accelerated the initial decomposition of sludge samples. Al₂O₃ and TiO₂ might decrease the total pyrolysis time, while CaO, Fe₂O₃, and ZnO prolong pyrolysis time. The structure of demineralized sludge samples might be changed due to the addition of CaO, TiO₂, and ZnO. Between 550 K and 750 K, the conversion of organic matters (mainly cellulose and lignin) in sludge samples was enhanced by Al₂O₃ and TiO₂, but inhibited by CaO, Fe₂O₃, and ZnO. The effects of metal oxides on the weight loss rate of cellulose in demineralized sludge samples presented the following decreasing order of DE-ZnO > DE-TiO₂ > DE-SS > DE-Al₂O₃ > DE-Fe₂O₃ > DE-CaO.     

Optimal Control of Solar Sludge Dryers

Optimal control (OC) methodology is used to develop a control policy for a batch-operated solar sludge dryer with ventilation as the dominant control. Batch performance criteria and instantaneous optimization criteria are developed for two economic environments: quota limited and capital limited. The general formulation, as well as simplifications resulting from the multiplicative form of the evaporation (drying) rate model, are presented. The multiplicative form results in a single constant number, called here “control intensity,” which is used to guide the on-line control decisions. These decisions turn out to be independent of the dry solids content (DSC) of the sludge. Further simplification, by assuming a strictly constant weather, is used to demonstrate the general effects of the economic and physical (weather) environment on the solution.

The OC approach is used to address, via simulation under realistic weather conditions, several design, operation, and pricing problems. In particular, the following have been considered: sizing of the ventilation fans, determination of a fair fee for sludge disposal, finding the best final DSC, and evaluating the effect of the price of electricity. Regarding the particular location represented by the data, it has been shown that the installed capacity of the ventilation fans should be increased and that there seems to be sufficient economic incentive for solar drying.     

Reduction of superheater corrosion by co-firing risky biomass with sewage sludge

Iron and aluminium sulphates are added to processes producing sewage sludge (SWS), and SWS also contains aluminium silicates. These compounds can destroy alkali chlorides and prevent Cl deposition on superheaters. Sulphation of chlorides requires formation of SO₃ in the furnace. Raw and digested SWSs were mixed with a fuel containing bark and recycled fuel (REF). Combustion experiments were conducted with a pilot-scale bubbling bed reactor, and concentrations and deposition of critical elements (Cl, Na, K, S…) in the furnace were measured. Change of the iron and aluminium sulphates added to the SWS to phosphates and hydroxides evidently reduced the power of the sulphur to form SO₃. Nevertheless, addition of SWS to the biomass feedstock effectively decreased alkali chloride concentration and Cl deposition, and the alkali sulphates found in the fine fly ash confirmed the contribution of sulphation to alkali capture. The importance of aluminium silicate reaction in alkali capture was indicated.     

固定化微生物技术处理乳品废水的影响因素研究

本文采用海藻酸钠和聚乙烯醇(PVA)为包埋剂对微生物进行包埋,并应用于乳品废水处理。以三种不同的载体固定化方法进行单因素实验,分别研究了三种主要影响因素pH、温度、小球浓度对乳品废水去除率的影响,并评价了各种包埋剂的处理效果。     

Preparation of carbon-based adsorbents from sewage sludge pyrolysis to remove metals from water

The objective of this study was to evaluate the use of cheap carbon-based adsorbents from sewage sludge pyrolysis to remove Na⁺, K⁺, Ca²⁺ and Mg²⁺ from saline water. Four model solutions of NaCl, KCl, CaCl₂ and MgCl₂ that simulated seawater composition were used. The model solutions were 456.54 mmol_c L⁻¹ NaCl, 9.72 mmol_c L⁻¹ KCl, 19.96 mmol_c L⁻¹ CaCl₂ and 111.09 mmol_c L⁻¹ MgCl₂. Two carbon adsorbents, one with chemical activation, were prepared by a new method and evaluated for ion adsorption. The results indicated that carbon adsorbent without chemical activation was the most effective in removing ions from different solutions and the removal of metals followed the sequence: Na⁺ (76.78−69.66) >K⁺ (66.0−57.80) >Mg²⁺ (44.84−42.85) >Ca²⁺ (35.12−12.38). Results showed that it is to possible prepare carbon-based adsorbents from sewage sludge following inexpensive and environmentally acceptable methods.     

Effect of Cl/S molar ratio on theoretical partitioning of heavy metals under waste combustion conditions

A simulation calculation was carried out to predict the behavior of heavy metals during waste incineration according to the variation of chlorine content by using a thermodynamic equilibrium model. To predict the behavior of heavy metals in incineration of wastes, chlorine content in wastes was changed up to 3.00 on a Cl/S molar ratio basis. Then the partitioning characteristics of heavy metals with Cl/Metal molar ratio were investigated as solid, liquid, and gas phases of metals. For analysis of emission characteristics, incineration temperature, chlorine content and air-fuel ratio (λ_r) were chosen as major operating parameters. It was found that the distribution characteristics of heavy metals were not significantly affected by air-fuel ratio, but evaporation rate of heavy metals increased with operating temperature. Most of the heavy metals remained in solid phase of metal oxides, such as CdO, CrO₃, CrO₂, CuO and ZnO, except for lead which existed as PbSO₄ in the given operating conditions (i.e., T_b=800 °C, λ_r=1.3, and Cl/S=0). It was found that most of the heavy metals in solid phase changed to gaseous Cl compounds, which have a high volatility with regard to increasing the Cl/S molar ratio under the same conditions. However, Cr compounds were almost not affected as chlorine level increased. This paper is dedicated to Professor Dong Sup Doh on the occasion of his retirement from Korea University.     

Arsenic interactions during co-combustion processes based on thermodynamic equilibrium calculations

Arsenic emissions are currently considered to be one of foremost importance. Arsenic volatility is higher than most of trace elements, but its vaporization behaviour is strongly dependent on the atmosphere composition. In this sense, thermodynamic equilibrium calculations, using HSC-Chemistry 5.0 software, were performed to evaluate the influence of different compounds in the distribution and mode of occurrence of arsenic in co-combustion processes. The influence of different parameters influencing arsenic behaviour, such as temperature, pressure, trace element concentration and flue gas composition on equilibrium composition were also evaluated. Predicting arsenic species, based on combustion conditions and fuel composition, will be useful to choose the best available control technology to reduce arsenic emissions. Finally, the possible interactions between arsenic and different trace elements (TE), mercury, cadmium and antimony, relevant from an environmental point of view, have also been studied; these interactions are not usually considered in thermodynamic studies; however, TE’s interactions affects the behaviour of a single TE, not only as a result of the formation of new species, but also, because of the different reactivity of TEs towards different elements which may affect TE’s volatilization behaviours. From results obtained in this study it may be concluded that in most cases, arsenic is mainly captured in ashes as a result of the formation of thermally stable species both from interactions with bulk ash and TE’s interactions. Nevertheless, the presence of some compounds (silicon, chlorine and sulphur) may enhance arsenic volatilization.     

Study of bio-oils and solids from flash pyrolysis of sewage sludges

The aim of this study was to evaluate the production of pyrolysis oil from three types of sewage sludges. The flash pyrolysis was performed at 500 °C, the maximum oil yield was 43.1%, and the water content in bio-oils obtained from secondary sludges was relatively low - 10.3% and 17.0%. GC-MS results showed that pyrolytic bio-oils of studied sludges dominantly contained fatty acids and nitrogenous compounds with potential added value.Obtained solids had high ash content and low calorific value which make them unattractive for use in incineration. FT-IR results showed that solids gave similar IR features as notified alumino silicates; utilization of these solids as adsorbents could be a potential valorization.     

Prediction of the distribution of alkali and trace elements between the condensed and gaseous phases in a municipal solid waste incinerator

The FactSage thermodynamic calculation package and databases have been used to predict the equilibrium composition of the solid, liquid and gaseous products from municipal solid waste incineration. One series of calculations has considered the equilibrium for municipal waste combustion over a range of temperatures (850-1350 °C) that includes the typical combustion temperature of 950 °C and covers excursions to both lower and higher temperatures. A second set covers cooling of the combustion gas to 300 °C and the formation of condensates. However, it is recognised that the lower the temperature the greater the importance of kinetics and no attempt is made to deal with such topics as nucleation by pre-existing airborne particulates. Consideration is given to the effect of oxygen potential together with alkali, chlorine, and water content on the equilibrium products. Further calculations conceptualising “local equilibrium” around specific metal-containing items in the waste stream, for example a CRT tube, have illustrated how lack of global equilibrium, arising from non-uniformity in the burning bed could lead to atypical local metal distributions. Combined, these calculations provide a greater understanding of the behaviour of metals in municipal waste incineration than previous studies, accounting to some extent for the great variation in waste composition and local combustion conditions in a complex and variable industrial system.     

Isothermal section of CaO-SiO2-La2O3 system within specific region at 1673–1473 K

So far, the relevant phase equilibrium relations and the type of equilibrium phase fields in the CaO-SiO₂-La₂O₃ basic slag system phase diagram are still unknown, which restricts the smelting process and application in materials of rare earth elements. In the current work, phase equilibrium relations within specific region of CaO-SiO₂-La₂O₃ system at 1673–1473?K were studied experimentally by using the thermodynamic equilibrium experiment followed by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectrometer (EDS). According to the experimental results, the existence of ternary compound CaO·3SiO₂·2La₂O₃ was determined, it is confirmed to be a solution solid phase. The sub-solidus phase relations between different solid phases were also determined. Finally, the isothermal sections of CaO-SiO₂-La₂O₃ system within specific region at 1673?K, 1573?K and 1473?K were obtained, respectively. The experimental results can not only enrich the phase diagram information of silicate system, but also have practical significance for the application of rare earth in materials.     

Experimental Results and Model for N‐Gas Compound Production in Pure Steam Gasification for Wastewater Sewage Sludge

Sewage sludge produced in wastewater treatment plants represent a significant portion of waste resulting from human activity and it is important to offer new ways of treatment. Thermal gasification of sludge is a promising thermal treatment, particularly because it can lead to a recovery of energy and a drastic reduction in the initial waste volume. However, it is a subject that has rarely been treated in the literature for sewage sludge. In particular, the nitrogen compounds present in significant quantities in the sewage sludge and their reactions during a thermal gasification step, must necessarily be taken into account. In order to study nitrogen behavior, this paper presents experimental results that are included in a model to predict NH₃ emission.     

VOLUME REDUCTION AND BIOLOGICAL STABILIZATION OF SLUDGE IN SMALL SEWAGE PLANTS BY SOLAR DRYING

High investments, high dumping costs and high electrical energy consumption turn waste water treatment in small sewage plants an expensive process. Therefore, the University of Hohenheim and the company Thermo-System have developed a fully automatic low temperature solar drying plant allowing further stabilization and volume reduction of sewage sludge by continuous mixing and aerating. To test the potential of the technology, the drying process was investigated in a commercially operated municipal sewage treatment plant with aerobic sludge digestion. During experiments, the changes of the dry solids concentration and the volatile solids, the climatic conditions and the energy consumption were constantly measured and documented. It was found, that the sludge could be dried from a dry solids concentration of 3 to 93% w/w in 64 (flocculated) or 83 days (non-flocculated). At the same time, the volatile solids content was reduced from 65 to 34% w/w, respectively. The electrical energy consumption was only 22 to 28 kWh per ton of evaporated water, compared to 70 to 110 kWh required for conventional drying processes. The thermal energy for evaporation was fully covered by solar energy.     

VOLUME REDUCTION AND BIOLOGICAL STABILIZATION OF SLUDGE IN SMALL SEWAGE PLANTS BY SOLAR DRYING

ABSTRACT

High investments, high dumping costs and high electrical energy consumption turn waste water treatment in small sewage plants an expensive process. Therefore, the University of Hohenheim and the company Thermo-System have developed a fully automatic low temperature solar drying plant allowing further stabilization and volume reduction of sewage sludge by continuous mixing and aerating. To test the potential of the technology, the drying process was investigated in a commercially operated municipal sewage treatment plant with aerobic sludge digestion. During experiments, the changes of the dry solids concentration and the volatile solids, the climatic conditions and the energy consumption were constantly measured and documented. It was found, that the sludge could be dried from a dry solids concentration of 3 to 93% w/w in 64 (flocculated) or 83 days (non-flocculated). At the same time, the volatile solids content was reduced from 65 to 34% w/w, respectively. The electrical energy consumption was only 22 to 28 kWh per ton of evaporated water, compared to 70 to 110 kWh required for conventional drying processes. The thermal energy for evaporation was fully covered by solar energy.