-Taking Beijing as an Example
Zhang Yi-An, Gao Ding, Chen Tong-Bin*, Zheng Guo-Mei, Li Yan-Xia
Center for Environmental Remediation, Institute of Geographic Sciences and Resources, Chinese Academy of Sciences, Beijing 100101
Abstract: To estimate the costs and benefits of landfilling, incineration and composting under different electricity tariffs and transportation distances. p>Abstract: Taking Beijing as an example, the cost of urban sludge treatment and disposal by landfilling, incineration and composting under different electricity prices and transportation distances is estimated, based on which the prospects of various treatment and disposal options are discussed, and the way out for sludge treatment and disposal in Beijing is envisioned. Landfill will be the main treatment and disposal method for a certain period of time, but the proportion will gradually decrease; composting is a more economically feasible treatment and disposal method, which is suitable for promotion; with the improvement of economic strength and technical level, incineration can be applied to individual special locations. Meanwhile, the impact of government subsidies on the benefits of sludge treatment and disposal is analyzed.
Keywords: municipal sludge; treatment and disposal cost; landfill; incineration; composting
Chinese Classification Number: X703 Literature Identifier: A Article Number: 1672-2175(2006)02-0234-05
Municipal sludge is a by-product of sewage treatment, and its volume accounts for 0.3%~0.5% of the treated sewage in terms of the water content of 97%[1]. 0.5% [1], and the volume of sludge produced by deep treatment will increase by 50%~100%. At present, China discharges about 1.3×106 t of dry sludge annually, and is increasing at a rate of about 10%.
The planned sewage discharge in the whole area of Beijing is 330×104 m3/d, of which the sewage discharge in the urban area is about 230×104 m3/d in 2003 [2]. With the planning and construction of 14 sewage treatment plants, the sewage treatment capacity is expected to exceed 320×104 m3/d in 2015, and the treatment rate will exceed 90%. By 2008, Beijing will add nine new water treatment plants, and the deep treatment capacity will increase from the current 1 × 104 m3/d to 47.6 × 104 m3/d, by which time the annual generation of 80% water content city sludge will exceed 80 × 104 m3. Beijing's largest sewage treatment plant - Gaobeidian Sewage Treatment Plant The cost of sludge outbound transportation accounts for 1/3 of the operating cost of the whole plant [3].
The massive generation of urban sludge has caused increasingly severe secondary pollution and has become a bottleneck in the urban sewage treatment industry. One of the very important reasons for the low rate of sludge treatment and disposal is the limitation in terms of investment and operation cost. However, up to now, there is no economic analysis about different sludge treatment and disposal programs, which leads to a large blindness of different units and designers in the choice of programs. In this paper, we take Beijing as an example to conduct economic analysis on several typical urban sludge treatment and disposal methods in order to provide reference basis for the selection of urban sludge treatment and disposal technologies.
1 Municipal sludge treatment and disposal cost estimation
1.1 Estimation method
Taking 1 t of dry sludge (DS) as the basis for calculation, the comprehensive cost = running cost + equipment discount cost. The running cost is estimated by the more mature treatment and disposal methods at present.
The effectiveness of mechanical dewatering of sludge in Beijing is usually around 80%. The cost estimation in each program involves or includes incineration, transportation, landfill and other 3 processes; equipment discount cost is taken as 15 a service life, annual depreciation of 7%, the social interest rate of 10%, that is, 17% of the annual depreciation, and the annual working hours of the equipment is 8000 h. Therefore, the discount cost of equipment = the cost of equipment, which is the same as the cost of the mechanical dewatering of sludge in Beijing. Therefore, the equipment discount = equipment price × index × 0.17/8000.
1.2 Estimation details
(1) unit cost
Landfill: sanitary landfill of domestic waste costs about 60 ~ 70 ￥ / t, sludge landfill in accordance with the compaction of domestic waste: soil: sludge volume to weight ratio of 0.8 : 1 : 1, sludge landfill cost of 48 ~ 56 ￥ / t, take 52 ￥ / t.
The cost of the landfill of domestic waste: soil: sludge, the cost of landfill, the landfill cost of 48 ~ 56 ￥ / t, take 52 ￥ / t.
Drying: drying energy consumption is directly proportional to the amount of dehydration. Gas heating efficiency of 85%, boiler thermal efficiency of 70%, process heat loss of 5%, the evaporation of water energy consumption of 150 (kW?h)/t, per hour to remove 1 t of water equipment investment of 180 × 104 ￥ [4].
Incineration: Currently more fluidized bed technology, every h incineration of 1 t dry sludge equipment costs 528 × 104 ￥, sludge by dry mass reduction of 60%. Incineration of operating costs 24 ￥ / t, flue gas treatment consumption of NaOH amount of about 37 kg / t, discounted about 128 ￥ / t [5].
Electricity prices: Beijing's industrial electricity prices in the peak period, the flat section of the area, the trough period were 0.278, 0.488, 0.725 ￥ / (kW?h). The electricity price is set at 0.30, 0.60￥/(kW?h) according to different subsidy programs.
Freight: The transportation price in Beijing is between 0.45~0.65￥/(t?km), sludge is a special solid waste, which needs to be transported by special box trucks, and the price is at the high end. In addition, there is a rising trend of transportation price in recent years. Therefore, the transportation cost is taken as 0.65 ￥/(t?km).
In addition, drying and incineration are added 30% of the cost of material consumption and labor management fees and civil support costs according to the cost of equipment.
(2) sludge moisture content
Sludge organic matter and moisture content is high, there are a series of problems in landfill, the current main concern is the mechanical properties of the soil, when the moisture content is higher than 68% need to be mixed into the soil according to the ratio of m (soil) ∶ m (sludge) ＝ 0.4 ~ 0.6 [6-8]. There is a sudden change in sludge properties when the water content is reduced, so the landfill dewatering target is set at 80% and 30%.
Moisture content is a key factor in sludge incineration treatment. High organic matter content, low moisture content is conducive to maintaining spontaneous combustion, reduce the sludge moisture content is essential to reduce sludge incineration equipment and treatment costs. Generally, when the sludge water content is reduced to the ratio of volatile matter content less than 3.5, spontaneous combustion can be formed [9]. The organic matter content of sludge in Beijing is below 45%, so the moisture content of sludge to maintain spontaneous combustion incineration should be less than 61.2%. Zhu Nanwen summarized several foreign sludge thermal drying technologies that can dry sludge to 10% moisture content [10]. The comprehensive cost of sludge incineration changes dynamically with the degree of drying, the higher the degree of drying, drying energy consumption rises, incineration equipment and operating costs decline. For the sake of simplicity, this paper takes the sludge to keep the heat balance combustion as the estimation premise, and no longer carries out the cost estimation of adding heavy oil under high moisture. Therefore the drying targets for sludge incineration are set at 60% and 10%.
Table 1 Overview of landfill sites in Beijing [11] and closest distance to wastewater treatment plants
Table 1 Description of landfill sites and wastewater treatment plants
Landfill sites Landfill site location Treatment size/(t?d-1) Estimated. Closure time Nearest wastewater treatment plant Nearest straight-line distance/km 1)
Bei Shenshu Tongxian Jiqiu Township 980 2006 Gaobeidian 20
Anding Daxing District Anding Township 700 2006 Xiaohongmen 36
Liu Li Tuen Haidian District Yongfengtun Township 1500 2017 Qinghe 15
Gao Antun Chaoyang District Louzizhuang Township 1000 2018 Gaobeidian 15
Asuwei Xiaotangshan Township, Changping District 2000 2012 Qinghe and Beixiaohe 40
Jiaojiapo Yongding Township, Mentougou District 600 2011 Lugouqiao 15
1) The nearest-distance data are measured by the authors
To summarize the above, the treatment and disposal methods of the sludge are as follows: composting, drying to 80% of the water content respectively, landfill when drying to 30% water content, and incineration when drying to 60% water content and 10% water content, respectively.
1.
1.3 Landfill cost
Landfill cost = energy cost + transportation cost + landfill cost + equipment discount cost
Energy cost = [1/(1 - η0) - 1/(1 - ηe)]×150 × α×Pele
Transportation cost = 0.65 × L /(1-ηe)
Landfill cost = βPf /(1-ηe)
Equipment discount = [1/(1-η0)-1/(1-ηe)]×180 × α × 0.17 × 104/ 8000
Wherein, η0, ηe for the treatment and disposal of the beginning and end of the water content, respectively; Pele for the price of electricity, ￥ / (kW?h); L for the transportation distance, km; α for the construction and labor supporting cost index, 1.3; β for the volume coefficient, the water content of ≥ 68% in the water content rate of 1.4 ~ 1.6 between 1.5, take 1, the water content rate of <68% take 1; Pf for the landfill landfill prices
Sludge landfill transportation distance: Beijing existing landfill capacity is not enough to meet the demand for domestic waste disposal, even after the completion of the planned landfill, the surplus landfill capacity is also very limited, sludge landfill need to look for additional land for a new landfill. With the development of the city and the geological conditions of the landfill, the transportation distance will be farther and farther. Referring to Table 1, the transportation distance of the sludge
landfill will be more than 40 km, so when estimating the landfill cost in the future, we will take 50 km and 100 km as the transportation distance of the landfill in the near future and the long term, respectively.
1.4 Composting Costs and Benefits
Land use of municipal sludge after harmless treatment by composting is a common treatment and disposal method internationally. Forced-ventilated static stack composting treatment is the mainstream technology of sludge composting, and its treatment cost is related to the initial moisture content of sludge, the treatment scale, the distance between the composting plant and the wastewater treatment plant, as well as the origin of the equipment, etc. The composting plant should be built in the same place as the wastewater treatment plant. The composting plant should be built around the sewage treatment plant, and the transportation cost is counted as 0. The composting cost is mainly composed of blower, drying, screening energy consumption, conditioning agent and equipment discount cost. At present, the market price of compost products is 350~500￥/t, after deducting 15% moisture content, take 500￥/t DS.
Using CTB composting automatic control system [12,13] to carry out forced ventilation of static stack composting in Luohe City, Henan Province, the application of municipal sludge composting plant results show that, when the sludge moisture content is not higher than 80%, the air blower energy consumption of 40~60 ( kW?h)/t DS, and 60 (kW?h)/t DS was taken.The price of CTB conditioner was 300 ￥/t, and the loss rate was generally 5% [14]. After 10-14 d of composting, the sludge dry matter was reduced by 30% with 45% water content. Using thermal drying technology to dry to 15% water content, dewatering load 0.45 t/t DS; conditioning agent in the drying of pre-screening and then natural drying, need to screen energy consumption; screening load ***9.3 t/t DS, screening capacity of 1 t/h, the power of 3 kW. the whole process of energy consumption of 95 (kW?h)/t DS, taking into account the unknown energy consumption, take 100 (kW?h)/t DS.
Equipment discount: the sludge composting plant with a capacity of 0.3×104 t/a has an equipment investment of about 7 million ￥, and the equipment discount is 182 ￥/t DS (including land cost), taking 200 ￥/t DS.
1.5 Incineration cost
Considering the incineration of emissions and other issues, it is preferable to incinerate the incineration more than 30 kilometers away from the incineration plant, taking 30 kilometers away from the incineration plant; incineration of the incineration is based on the dry matter of 60% reduction, burning residues. Reduced by 60%, the burning residues need to be transported to the landfill, the transportation distance is taken as 50 km. reference to Table 3 can be seen, dry to 10% incineration cost is lower than dry to 60%. The higher the degree of drying, the smaller the footprint of the incineration plant, so it is appropriate to dry to 10% before incineration.
1.6 Costs of drying for agricultural use
There are application safety hazards for unstabilized treated sludge, which are no longer estimated given the poor stabilization and limited safety of drying.
2 Discussion and Analysis
2.1 Treatment Costs and Economic Benefits
Table 2 Costs and their benefits of treating and disposing 1 t of municipal sludge (dry mass)
Table 2 Comparison of the estimated cost and benefit of sewage sludge treated and/or disposed by different ways
Filling and burying
Drying Transportation Landfilling Comprehensive cost/$
Target Energy consumption/$ Equipment discount/$ Distance/km Freight/$ Filling ratio Cost/$
80% 0 0 50 163 50% 390 5531). 5532)
30% 2091), 4182) 178 50 46 0 74 5071), 7162)
80% 0 0 0 100 325 50% 390 7151), 7152)
30% 2091), 4182) 178 100 93 0 74 5541), 7632)
< p>incineration
Drying Incineration Burning Burning residue Comprehensive cost/￥
Target Energy consumption/￥ Equipment discount/￥ Operation/￥ Equipment discount/￥ NaOH/￥ Freight/￥ Landfill/￥
60% 1461), 2932) 124 60 365 128 13 20 8561), 10022)
10% 2281)
Composting
Energy consumption/$ Equipment discount/$ Conditioning agent loss/$ Total cost/$ Sales/$ Total benefit/$
391), 782) 200 75 3141), 3532) 410 961), 572)
1) Electricity price is taken as 0.30 ￥/(kW?h); 2) Electricity price is taken as 0.60 ￥/(kW?h)
The process of estimating the treatment cost of various treatments and the results are shown in Table 2. As can be seen from Table 2, sludge treatment and disposal in the form of composting cost
lowest, about 300~350 ￥ / t DS; landfill about 500~760 ￥ / t DS. incineration cost the highest, about 800~1000 ￥ / t DS. Composting cost is lower than landfill, significantly lower than incineration, with the increase in transportation distance landfill cost is significantly higher than the cost of composting. In addition, sludge incineration has a large one-time investment and the highest operation and maintenance costs.
Various treatment methods, sludge landfill without resource recovery, the benefit of zero; considering the level of sludge calorific value, the possibility of recovery of incineration heat energy is low, the net benefit is not significant; sludge drying can play a dewatering effect, but the effect of stabilization is limited, coupled with the drying process is prone to explosions and fertilizer effect of slow and other issues, it is not appropriate to advocate; in the case of good sales of the product, according to the price of electricity is different, the Composting treatment can be profitable 50~100￥/t DS.
2.2 Advantages and disadvantages of various treatment and disposal technologies
Most of the existing landfill design and construction standards are low, lack of pollution control measures, there are poor stability and other problems, resulting in the emission of gases and odors, contamination of groundwater, and can not guarantee the safety of landfilled waste, but only to slow down the contamination but not to eliminate the pollution eventually. In order to minimize the above problems, some countries have set minimum standards for the physical properties of sludge to be treated, which greatly increases the treatment cost of landfilled sludge. Germany, for example, requires landfilled sludge to have a dry basis content of not less than 35%. In order to avoid groundwater pollution caused by decomposition of organic matter in sludge, Germany issued the Technical Outline for the Control and Disposal of Municipal Waste in 1992, which requires that from 2005 onwards, any material to be disposed of in a landfill should have an organic content of no more than 5% [15], which implies that sludge, even after drying, does not satisfy the requirements for landfilling. Sludge landfills face multiple pressures from landfill sites, the public, and regulations, and the cost of landfills will gradually increase, and in recent years the proportion of sludge disposed of in landfills in foreign countries has become smaller and smaller [6].
Whether to promote the composting treatment of municipal sludge, first of all, the potential environmental risks of sludge composting should be effectively assessed. Du Bing et al [16] showed that compared with foreign countries a typical sewage treatment plant in Beijing phenols, phthalates, PAHs are at a lower level of pollution. The sustained high temperature of composting treatment can ensure the killing of pathogens and the safety of sludge for agricultural use. Chen Tongbin et al [17] on China's urban sludge heavy metal content and its trend of change in the research results show that the average content of China's urban sludge is generally low, the metal content is basically not more than the standard for agricultural use [18], and shows a gradual decline in the trend. Relevant studies in recent years have also proved that scientific and reasonable agricultural use of urban sludge will not cause heavy metal pollution problems in soil and agricultural products [19]. The environmental risk of heavy metals from land use of municipal sludge in China is not as serious as people think.
The reduction by incineration is the most significant, and the volume reduction rate after incineration of sludge containing 80% water is more than 90%. However, sludge contains a variety of organic matter, incineration will produce a large number of harmful substances, such as dioxin, sulfur dioxide, hydrochloric acid, etc., by the domestic incineration technology limitations, dioxin pollution has not yet been well resolved, heavy metal fumes and combustion ash may also cause secondary pollution. In addition, incineration wastes the nutrients in sludge. Comparing the three treatment and disposal methods, sludge incineration has the smallest footprint, but the highest comprehensive cost, high equipment maintenance requirements, and high environmental risks, all of which are unfavorable and limit the wide application of sludge incineration technology.
In summary, the composting process to achieve resource utilization of sludge, scientific and rational application of health and safety and heavy metal safety can be guaranteed, while more economically feasible, is the main development direction of sludge treatment and disposal technology. However, from the marketing point of view, the sales channels of sludge composting products need to be improved. The advantages and disadvantages of various treatment methods are summarized in Table 3 (next page).
2.3 Electricity price impact and government subsidies
Electricity price affects the cost of sludge treatment and disposal. If the price of electricity is reduced from 0.60 ￥/(kW?h) to 0.30 ￥/(kW?h), the comprehensive cost of various treatment methods will be reduced by 40~230 ￥/t DS. If the price of electricity is taken to the price of electricity during the trough period of electricity consumption or lower, the cost can be further reduced.
Table 3 Comparison of advantages and disadvantages of various treatment and disposal technologies
Table 3 Comparison of landfill, composting and incineration for sewage sludge
Treatment and disposal method Income and expenditure balance/(￥?t-1) 1) Technical difficulty Site Requirements Resource utilization Harmlessness degree
Landfill -507~ -763 Simple Large Cannot Delay the pollution, no final elimination of the risk of pollution
Composting 57~96 Simple Smaller Can Achieve the harmlessness of heavy metals below the standard of agriculture
Incineration -771~ -1000 High requirement of technical equipment Small Cannot Tail gas may bring secondary pollution
1) Transportation, transportation and incineration for sewage sludge
The treatment and disposal methods are as follows p>
1) Transportation distance of 100 km, the price of electricity 0.60 ￥ / (kw?h), 80% moisture content of landfill costs slightly lower than 30% moisture content of landfill, but its occupancy for the latter 5.25 times, a comprehensive consideration of 30% landfill
Sludge moisture content of 80% and 60% landfill occupancy of 30% landfill for the 5.25 times, respectively, 1.75 times. The government through subsidies such as reducing the price of electricity and other means of regulation, the sewage treatment inputs reasonably allocated to the sludge treatment unit, can reduce the incineration cost of sludge treatment unit, landfill area, reduce the cost of composting. Government subsidies can play the role of economic leverage, regulation of sludge treatment industry input and output conditions, which is conducive to the healthy development of the sludge treatment and disposal industry. In short, sludge treatment and disposal should have appropriate government subsidies.
3 Conclusion
(1) The cost of sludge composting varies with the price of electricity about 300~350 ￥/t DS, and the sale of compost can compensate for part of the treatment cost, so that the sludge composting reaches the micro-profit level. Reasonable application of compost can provide nutrients and organic matter, which is an important direction of sludge treatment and disposal technology.
(2) Sludge landfill operation is simple, but its cost is about 500~760 ￥/t DS, which is higher than composting treatment. Considering the increasing scarcity of land resources and the problem of secondary pollution, and from the experience of developed countries, sludge landfill will be gradually restricted, so its application ratio should be gradually reduced.
(3) Sludge incineration has the most obvious reduction effect, but it has the highest initial investment and operating costs, with a comprehensive cost of about 771~1000 ￥/t DS. Its equipment maintenance is complicated, and if the tail gas is not properly treated, it will cause secondary pollution.
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