Integrated drying and incineration of wet sewage sludge in combined bubbling and circulating fluidized bed units

An original integrated drying and incineration technique is proposed to dispose of sewage sludge with moisture content of about 80% in a circulating fluidized bed. This system combines a bubbling fluidized bed dryer with a circulating fluidized bed incinerator. After drying, sewage sludge with moisture less than 20% is transported directly and continuously from the fluidized bed dryer into a circulating fluidized bed incinerator. Pilot plant results showed that integrated drying and incineration is feasible in a unique single system. A 100 t/d Sewage Sludge Incineration Demonstration Project was constructed at the Qige sewage treatment plant in Hangzhou City in China. The operational performance showed that the main operation results conformed to the design values, from which it can be concluded that the scale-up of this technique is deemed both feasible and successful.     

Circulating fluidized bed incineration of industrial solid wastes

A pilot scale test facility of a circulating fluidized bed incinerator was established to generate design and operation data and help assess the technical feasibility for industrial applications. The use of high turbulence in the combustion zone and feeding an acid-capturer directly into the incinerator to absorb acid gases eliminates the costly afterburner and scrubber. This paper presents some systematic incineration tests of uniform industrial wastes such as paper mill sludge cake, rubber waste and petroleum coke. Sludge cakes with high moisture and low heating values can be treated to a low emission level by co-firing with coal. The high superficial gas velocity has improved combustion efficiency but increases NO_x emissions. However, sulphur content has almost no influence on sulphur retention. The problems of CO/NO_x emissions and circulation stability are also discussed.     

Study on optimal energy efficiency of a sludge drying-incineration combined system

Thermal drying is a frequently used technology to further remove the water in dewatered sludge. However, it is an expensive solution due to its highly energy consumption. The combination of sludge drying and incineration system, in which, the energy generated from sludge incineration is reused to sludge drying, can largely save the energy consumption of sludge treatment facilities. A bench-scale paddle sludge dryer was built to study the drying characteristics of sludge. Results show that, a significant fluctuation of sludge drying rate and stirring power emerges at the moisture content of 55–65 %. An energy model was established based on a sludge drying and incineration project. The most reasonable dryness of sludge outlet from sludge dryer and input to sludge incinerator was analyzed, in the purpose of achieving optimal energy efficiency. The mono-incineration of dry sludge can be achieved at 850 °C combustion temperature, when sludge lower heating value (LHV) is about 11213 kJ/kg and moisture content is about 60 % w/w. The effect of operation conditions, including sludge moisture content, LHV, and operation load were analyzed based on the energy model. This energy model could be applied for the improvement of energy efficiency of sludge drying and incineration combined system.     

Method development and implementation for co-planar polychlorinated biphenyls (PCBs)

The Emission Measurement Center (EMC) in the Environmental Protection Agency's Office of Air Quality Planning and Standards was directed to conduct an emissions test program at a sewage sludge incinerator in support of a Maximum Achievable Control Technology (MACT) standard. One pollutant category of concern at these facilities was polychlorinated biphenyls, or PCBs. An objective of the test program was to measure co-planar PCBs in the incinerator emissions, the sewage sludge introduced to the incinerator, and the scrubber water effluent used in controlling the incinerator emissions. Co-planar PCB congeners are those having four or more chlorine atoms with only a few substitutions in the ortho positions, i.e. positions designated 2, 2', 6, and 6'. Thirteen PCB compounds are sometimes referred to as the "WHO PCBs," because the World Health Organization (WHO) has derived toxic equivalency factors for these congeners. Studies have shown that these dioxin-like compounds can react with the aryl hydrocarbon receptor. This same reaction is believed to initiate adverse health effects for dioxin and furan congeners. In order to conduct the co-planer PCB testing, the EMC had to develop analytical methods that could measure the 13 co-planar PCBs. The purpose of the test program was to develop, evaluate, and implement analytical test methods capable of measuring co-planar PCBs in three matrices: incinerator stack gases, sewage sludge, and scrubber water effluents. The paper summarizes the initial development work that was performed in preparation of analytical test protocols that could measure co-planar PCBs in air, water, and sludge matrices. Following the method development, a MACT emissions test program was conducted at a sewage sludge facility in July 1999 and these data are also summarized in the paper.     

Characterization of PCDD/Fs and heavy metals from MSW incineration plant in Harbin

The characterization of PCDD/Fs and heavy metals in the flue gas and fly ash of Harbin municipal solid waste (MSW) incineration plant, located in the northeast of China, was investigated in this study. The MSW was treated in a twin internal fluidized (TIF) bed incinerator. The results indicate that the emission of PCDD/Fs into the environment is 0.02 ng I-TEQ/m3 and the level of PCDD/Fs in the fabric filter fly ash is 0.7982 ng I-TEQ/g. The leachability levels of Pb, Cd and Hg in the fly ash are below the limits of environmental protection standard in China. However, the contents of Cu, Zn, and Hg are high in the fly ash. This suggests that the fly ash is a hazardous waste that requires special treatment and disposal. The practice of more than four years of operation shows that the TIF bed incinerator is very suitable and practical for China.     

Simulation of co-incineration of sewage sludge with municipal solid waste in a grate furnace incinerator

Incineration is one of the most important methods in the resource recovery disposal of sewage sludge. The combustion characteristics of sewage sludge and an increasing number of municipal solid waste (MSW) incineration plants provide the possibility of co-incineration of sludge with MSW. Computational fluid dynamics (CFD) analysis was used to verify the feasibility of co-incineration of sludge with MSW, and predict the effect of co-incineration. In this study, wet sludge and semi-dried sludge were separately blended with MSW as mixed fuels, which were at a co-incineration ratios of 5 wt.% (wet basis, the same below), 10 wt.%, 15 wt.%, 20 wt.% and 25 wt.%. The result indicates that co-incineration of 10 wt.% wet sludge with MSW can ensure the furnace temperature, the residence time and other vital items in allowable level, while 20 wt.% of semi-dried sludge can reach the same standards. With lower moisture content and higher low heating value (LHV), semi-dried sludge can be more appropriate in co-incineration with MSW in a grate furnace incinerator.     

Gasification of dried sewage sludge: status of the demonstration and the pilot plant

The disposal of sewage sludge from municipal waste water treatment plants is suffering from raising costs.The gasification is an alternative way of treatment, which can reduce the amount of solid residues that must be disposed from a water treatment plant. The produced gas can be used very flexible to produce electrical energy, to burn it very cleanly or to use it for upgrading.The gasification in the fluidised bed and the gas cleaning with the granular bed filter has shown successful operation. A demonstration plant in Balingen was set up in 2002 and rebuilt to a larger throughput in 2010. As a next step a demonstration plant was built in Mannheim and is now at the end of the commissioning phase. Nowadays the product gas is blended with biogas from sludge fermentation and utilized in a gas engine or combustion chamber to produce heat. In the future the process control for a maximized efficiency and the removal of organic and inorganic impurities in the gas will be further improved.     

流化床富氧焚烧含油污泥技术经济性分析

介绍了流化床富氧焚烧含油污泥技术的流程和优势,计算了富氧焚烧含油污泥系统主要设备的电耗和技术经济指标。流化床富氧焚烧含油污泥技术可实现烟气及其他污染物零排放,产生的蒸汽可直接供应油田生产和生活使用,产生的液态CO2可直接用于油井驱油,烟气中的SO2和NOx可转化为硫酸和硝酸。采用日处理200 t含油污泥的流化床锅炉年处理含油污泥量约73 kt,每年减少排污费7 300万元;锅炉年产蒸汽量约177 kt,每年节约蒸汽费用1 380.6万元,合计每年节约成本8 680.6万元。回收得到质量分数为40%的稀硫酸5.56 t/d,质量分数为37%的硝酸0.708 t/d,年回收CO2约99 kt。     

Fuel savings in sewage sludge incineration

As a result of a demonstration project partly sponsored by the United States Environmental Protection Agency in cooperation with the Indianapolis Center for Advanced Research, the City of Indianapolis, Indiana, realized a 34% fuel savings for sewage sludge incineration. At the same time, sludge throughput was increased 10%. In addition to these proven savings, operational downtime for repairs was reduced, maintenance costs were reduced, and air pollution was reduced. The air pollution reduction allowed Indianapolis to cancel a $3,000,000 construction program for air pollution abatement.Other cities incinerating sewage sludge in the United States that have initiated the fuel efficient mode of operation have saved even more fuel than Indianapolis. Nashville, Tennessee, reduced its fuel usage 40%; Buffalo, New York, reduced its fuel usage 47% from design expectations; Hartford, Connecticut, reduced its fuel usage 51 %; and Jacksonville, Florida has reduced its fuel usage over 50%.These savings result from installing additional instrumentation and controls (often not required at newer facilities), modifying the incinerator operating methods, and training the operators to operate the facilities more efficiently. At Indianapolis, it cost $20,000 per incinerator for instrumentation and operator training. This was an older plant and required a maximum amount of new controls and instrumentation; however, the payback for this $20,000 was less than three months due to the reduction in fuel use based on an oil price of $0.264 per litre ($1.00 per gallon). In the other cities where instrumentation controls were adequate, the cost for developing the now operating mode and training the operators averaged $75,000 per city.     

Elutriation characteristics of fine particles from bubbling fluidized bed incineration for sludge cake treatment

In this study, measurements of elutriation rate were carried out in a bench scale bubbling fluidized bed incinerator, which was used to combust sludge cake. The particle size distribution and ignition loss were analyzed to study the elutriation characteristics of bubbling fluidized bed incineration. Drawn from the experimental data, the elutriation rate constant K(i)* for fine particles were obtained and correlated with parameters. It was found that most of the solid particles (about 95%) elutriated came from the fluidized medium (inorganic matters), but few came from unburned carbon particles or soot (about 5%). Finally, this paper lists a comparison of K(i)* between this study and the published prediction equations derived or studied in non-incineration modes of fluidized bed. A new and modified correlation is proposed here to estimate the elutriation rate of fine particles emitted from a bubbling fluidized bed incinerator. Primary operation variables (superficial gas velocity and incineration temperature) affecting the elutriation rate are also discussed in the paper.     

Phosphate fertilizer from sewage sludge ash (SSA)

Ashes from sewage sludge incineration are rich in phosphorus content, ranging between 4% and 9%. Due to the current methods of disposal used for these ashes, phosphorus, which is a valuable plant nutrient, is removed from biological cycling. This article proposes the possible three-stage processing of SSA, whereby more than 90% of phosphorus can be extracted to make an adequate phosphate fertilizer. SSA from two Swiss sewage sludge incinerators was used for laboratory investigations. In an initial step, SSA was leached with sulfuric acid using a liquid-to-solid ratio of 2. The leaching time and pH required for high phosphorus dissolution were determined. Inevitably, dissolution of heavy metals takes place that would contaminate the fertilizer. Thus in a second step, leach solution has to be purified by having the heavy metals removed. Both ion exchange using chelating resins and sulfide precipitation turned out to be suitable for removing critical Cu, Ni and Cd. Thirdly, phosphates were precipitated as calcium phosphates with lime water. The resulting phosphate sludge was dewatered, dried and ground to get a powdery fertilizer whose efficacy was demonstrated by plant tests in a greenhouse. By measuring the weight of plants after 6 weeks of growth, fertilized in part with conventional phosphate fertilizer, fertilizer made from SSA was proven to be equal in its plant uptake efficiency.     

Energy recovery from sewage sludge by means of fluidised bed gasification

Because of its potential harmful impact on the environment, disposal of sewage sludge is becoming a major problem all over the world. Today the available disposal measures are at the crossroads. One alternative would be to continue its usage as fertiliser or to abandon it. Due to the discussions about soil contamination caused by sewage sludge, some countries have already prohibited its application in agriculture. In these countries, thermal treatment is now presenting the most common alternative. This report describes two suitable methods to directly convert sewage sludge into useful energy on-site at the wastewater treatment plant. Both processes consist mainly of four devices: dewatering and drying of the sewage sludge, gasification by means of fluidised bed technology (followed by a gas cleaning step) and production of useful energy via CHP units as the final step. The process described first (ETVS-Process) is using a high pressure technique for the initial dewatering and a fluidised bed technology utilising waste heat from the overall process for drying. In the second process (NTVS-Process) in addition to the waste heat, solar radiation is utilised. The subsequent measures--gasification, gas cleaning and electric and thermal power generation--are identical in both processes. The ETVS-Process and the NTVS-Process are self-sustaining in terms of energy use; actually a surplus of heat and electricity is generated in both processes.     

Life cycle assessment of sewage sludge co-incineration in a coal-based power station

A life cycle assessment was conducted to evaluate the environmental and economic effects of sewage sludge co-incineration in a coal-fired power plant. The general approach employed by a coal-fired power plant was also assessed as control. Sewage sludge co-incineration technology causes greater environmental burden than does coal-based energy production technology because of the additional electricity consumption and wastewater treatment required for the pretreatment of sewage sludge, direct emissions from sludge incineration, and incinerated ash disposal processes. However, sewage sludge co-incineration presents higher economic benefits because of electricity subsidies and the income generating potential of sludge. Environmental assessment results indicate that sewage sludge co-incineration is unsuitable for mitigating the increasing pressure brought on by sewage sludge pollution. Reducing the overall environmental effect of sludge co-incineration power stations necessitates increasing net coal consumption efficiency, incinerated ash reuse rate, dedust system efficiency, and sludge water content rate.     

Waste paper and clinoptilolite as a bulking material with dewatered anaerobically stabilized primary sewage sludge (DASPSS) for compost production

Environmental problems associated with sewage sludge disposal have prompted strict legislative actions over the past few years. At the same time, the upgrading and expansion of wastewater treatment plants have greatly increased the volume of sludge generated. The major limitation of land application of sewage sludge compost is the potential for high heavy metal content in relation to the metal content of the original sludge. Composting of sewage sludge with natural zeolite (clinoptilolite) can enhance its quality and suitability for agricultural use. However, the dewatered anaerobically stabilized primary sewage sludge (DASPSS) contained a low concentration of humic substances (almost 2%), and the addition of the waste paper was necessary in order to produce a good soil conditioner with high concentrations of humics. The final results showed that the compost produced from DASPSS and 40-50% w/w of waste paper was a good soil fertilizer. Finally, in order to estimate the metal leachability of the final compost product, the generalized acid neutralization Capacity (GANC) procedure was used, and it was found that by increasing the leachate pH, the heavy metal concentration decreased. The application of the sequential chemical extraction indicated that metals were bound to the residual fraction characterized as a stabilize fractions.     

垃圾焚烧发电厂渗滤液处理站污泥危险特性研究

通过对一例垃圾焚烧发电厂渗滤液处理站污泥进行鉴定,表明该部分污泥不属于危险固废,可以纳入焚烧炉与生活垃圾一起处理.鉴别结果为促进固体废物循环利用及同类企业合理处置渗滤液处理站污泥提供了借鉴.     

Use of dewatered municipal sludge on Canna growth in pot experiments with a barren clay soil

The present study evaluated the possibility of using the dewatered municipal sludge for non-agricultural purposes. The sludge was amended with soil and was applied at 0, 165, 330, 495 and 660 t/ha to promote the growth of Canna. The results showed that the Canna growth pattern exhibited a pronounced positive growth response in the range of 165–495 t/ha, and the Canna could not survive at an amendment rate of 660 t/ha. The analysis of chlorophyll fluorescence parameters showed that sludge did no harm to Canna, while under the conditions of barren soil alone, the plants were put into nutrients stress conditions. Due to the application of sludge, the concentration of heavy metals (Cu, Zn, Cr, Cd, Pb and Ni) in soil increased. However, by planting of Canna, contents of Cd, Ni and Zn showed trends of decline; Cd and Ni have shown a significant decline in concentration, while Zn had only limit response. As a result, dewatered sludge might be used to amend the barren soil and Canna could be used for phytoremediation of sludge.     

Environmental assessment of sewage sludge recycling options and treatment processes in Tokyo

Tokyo has historically suffered from a shortage of final disposal sites for the treated sewage sludge. Given this situation, sludge recycling and incineration have been promoted to reduce the volume of treated sludge conveyed to the disposal site, and the recycling options have changed since the late 1990s. This study aims to revisit the sewage sludge treatment and recycling processes in Tokyo and to evaluate different recycling options (brick, aggregate, refuse derived fuel and slag) from the energy consumption perspective by clarifying the complex flow of treated sludge within Tokyo's 23 wards. The study also estimates environmental loads associated with the operation of the whole sludge management system in the area. The environmental loads include: (1) total energy consumption and (2) gas emissions (greenhouse and acidification gases). The estimation was carried out for the years 1995, 1997, 1999 and 2001, during which a drastic change in recycling options occurred. The results indicated that the production of refuse derived fuel was the most energy consuming recycling option while aggregate production is the least energy consuming. They also showed that despite the increasing sludge volume, the energy consumption associated with the operation of the whole system decreased during the period while the gas emissions increased.     

Mathematical modelling of sewage sludge incineration in a bubbling fluidised bed with special consideration for thermally-thick fuel particles

Fluidised bed combustor (FBC) is one of the key technologies for sewage sludge incineration. In this paper, a mathematical model is developed for the simulation of a large-scale sewage sludge incineration plant. The model assumes the bed consisting of a fast-gas phase, an emulsion phase and a fuel particle phase with specific consideration for thermally-thick fuel particles. The model further improves over previous works by taking into account throughflow inside the bubbles as well as the floating and random movement of the fuel particles inside the bed. Validation against both previous lab-scale experiments and operational data of a large-scale industrial plant was made. Calculation results indicate that combustion split between the bed and the freeboard can range from 60/40 to 90/10 depending on the fuel particle distribution across the bed height under the specific conditions. The bed performance is heavily affected by the variation in sludge moisture level. The response time to variation in feeding rate is different for different parameters, from 6min for outlet H(2)O, 10min for O(2), to 34min for bed temperature.     

Evaluation of bangkok sewage sludge for possible agricultural use.

Bangkok (Thailand) covers more than 1500 km2 and has 10 million inhabitants. The disposal of wastewater is creating huge problems of pollution. The estimated amount of sewage sludge was estimated to be around 108 tonnes dry matter (DM) per day in 2005. In order to find a lasting way of disposal for this sewage sludge, the suitability of the sludge produced from three waste-water treatment plants for use as fertilizing material was investigated. Monthly samplings and analysis of sewage sludge from each plant showed that the composition of sludge varied according to the area of collection and period of sampling, and there was no link to rainfall cycle. Plant nutrient content was high (i.e. total N from 19 to 38 g kg(-1) DM) whereas organic matter content was low. The concentrations of heavy metals varied between sludge samples, and were sometimes higher than the E.U. or U.S. regulations for sewage sludge use in agriculture. Faecal coliforms were present in the sludge from one of the plants, indicating a possible contamination by night soil. In order to decrease this potentially pathogenic population the sewage sludge should be heated by composting. As the C/N ratio of sewage sludge was low (around 6) some organic by-products with high carbon content could be added as structural material to enhance the composting.     

Characterization of a thermal power plant air heater washing waste: a case study from Iran.

In Iran most of the electricity is generated by thermal power plants. As a result of fuel oil burning in winter time, the air heaters of the boilers have to be washed and cleaned frequently. The wastewater originating from air heater washing is then treated in an effluent treatment plant by chemical precipitation followed by dewatering of the sludge produced. The resulting waste is classified as specific industrial waste that should be characterized in detail under the Waste Management Act of Iran. The quantity of this waste produced in the studied power plant is about 20 tonnes year(-1). In the present investigation, the first to be carried out in Iran, seven composite samples of dewatered sludge from air heater washing wastewater treatment were subjected to investigation of the physical properties, chemical composition and leaching properties. The most likely pollutants that were of concern in this study were heavy and other hazardous metals (Cd, Co, Cr, Mn, Ni, Pb, Zn and V). The results revealed that mean pH, wet and dry density and moisture content of the waste were 6.31, 1532 kg m(-30, 1879 kg m(-3) and 15.35%, respectively. Magnetite, SiO2, P2O5, CaO, Al2O3 and MgO were the main constituents of the waste with a weight percentage order of 68.88, 5.91, 3.39, 2.64, 2.59 and 1.76%, respectively. The toxicity characteristic leaching procedure test results for some heavy and other hazardous metals showed that mean elemental concentrations of Cd, Co, Cr, Mn, Ni, Pb, V and Zn in leachate were 0.06, 1.55, 5.49, 36.32, 209.10, 0.58, 314.06 and 24.84 mg L(-1), respectively. According to the Waste Management Act of Iran this waste should be classified as hazardous and should be disposed of in accordance with hazardous waste disposal regulations.