Article pubs.acs.org/est
Evaluation of Handling and Reuse Approaches for the Waste Generated from MEA-based CO2 Capture with the Consideration of Regulations in the UAE Laila Nurrokhmah, Toufic Mezher, and Mohammad R. M. Abu-Zahra* Masdar Institute of Science and Technology, P.O. Box 54224, Abu Dhabi, United Arab Emirates S Supporting Information *
ABSTRACT: A waste slip-stream is generated from the reclaiming process of monoethanolamine (MEA) based PostCombustion Capture (PCC). It mainly consists of MEA itself, ammonium, heat-stable salts (HSS), carbamate polymers, and water. In this study, the waste quantity and nature are characterized for Fluor’s Econamine FGSM coal-fired CO2 capture base case. Waste management options, including reuse, recycling, treatment, and disposal, are investigated due to the need for a more environmentally sound handling. Regulations, economic potential, and associated costs are also evaluated. The technical, economic, and regulation assessment suggests waste reuse for NOx scrubbing. Moreover, a high thermal condition is deemed as an effective technique for waste destruction, leading to considerations of waste recycling into a coal burner or incineration. As a means of treatment, three secondary-biological processes covering Complete-Mix Activated Sludge (CMAS), oxidation ditch, and trickling filter are designed to meet the wastewater standards in the United Arab Emirates (UAE). From the economic point of view, the value of waste as a NOx scrubbing agent is 6 561 600−7 348 992 USD/year. The secondary-biological treatment cost is 0.017−0.02 USD/ton of CO2, while the cost of an on-site incinerator is 0.031 USD/ton of CO2 captured. In conclusion, secondary biological treatment is found to be the most economical option.
1. INTRODUCTION Among the identified CO2 capture methods, postcombustion capture (PCC) deploying a solvent-absorption system, e.g., monoethanolamine (MEA), is the most predominantly applied for both coal-fired and natural-gas-fired power plant.1−3 This is due to several advantages of MEA, e.g., high reactivity and absorption capacity toward CO2, affordability, and being recoverable by reclaiming processes.3,4 Nonetheless, it is able to form unregenerate carbamate and other degradation products leading to the discharge of a liquid−solid waste mixture.5,6 These products exist due to the reaction between solvent and some impurities contained in the flue gas (O2, NO2, and SO2), makeup water, or corrosion agent.7−10 The degradation products found in the reclaimer are generally classified into oxidative and thermal degradation based on laboratory investigation. Oxidative degradation refers to the transformation of solvent due to the presence of oxidizing materials such as oxygen (O2) and metal or radical impurities.8−10 In contrast, thermal degradation is known as carbamate polymerization from the reaction of MEA and CO2 under high temperature conditions.8,11,12 Another concern is nitrosamine compounds, which have a carcinogenic nature, induced by the presence of NOx in the flue gas.8,13 In order to address the waste problem from the reclaimer, incineration is commonly applied due to the hazardous nature of heavy metal content.14−17 A more environmentally sound © 2013 American Chemical Society
treatment for reclaimer waste consisting of reuse, recycle, and other cleaner methods such as biological processes have been investigated, yet there is a need for a study specific to the waste nature and associated regulations. The economic point of view is also a vital aspect to consider for implementation. In this study, the waste is first characterized and quantified to show the effect of different impurities on the waste content. As waste nature is specific to the technology and its operating condition, Fluor’s Econamine FG ultrasupercritical coal plant of 750 MWe generation and the other supporting findings are adapted. The waste reuse, recycling, biological treatment, and disposal are initially assessed on the basis of the current findings and further evaluated on the basis of UAE regulations. Secondary biological treatment as a means of waste handling is favored and specifically designed due to its environmental benefit. Last, the economic feasibility is performed to define the most cost-effective method conforming to environmental standards. Received: Revised: Accepted: Published: 13644
June 24, 2013 September 15, 2013 November 12, 2013 November 12, 2013 dx.doi.org/10.1021/es4027198 | Environ. Sci. Technol. 2013, 47, 13644−13651
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2. MEA-BASED RECLAIMER WASTE 2.1. MEA Degradation Products. One of the identified degradation processes is MEA oxidation during MEA-CO2 absorption. The main oxidizing agent is O2, which yields aldehyde as the initial product. This product is further oxidized and reacted with another MEA compound resulting in heatstable salts (HSS), e.g., glycolic acid (HOCH2COOH), oxalic acid (HOCOCOOH), formic acid (HCOOH), and acetic acid (CH3COOH).7 The other side products are ammonia (NH3) and water (H2O).7,12,13,16 An intermediate product (i.e., formaldehyde) may be generated and transformed into hydroxyethyl formamide (HEF) and 1-(2-hydroxyethyl) imidazole (HEI).12 Besides O2, the other compounds found in the makeup water or corrosion inhibitor such as Fe (CN)63−, ClO2, Mn2+, Cr, Ni, and Mo can act as oxidizing agents.7−9 The latter process is thermal degradation generating carbamate polymers from MEA-CO2 desorption at a high temperature. Initially, MEA reacts with CO2 to form carbamate as a normal pathway for CO2 capture. However, this carbamate is suspected to go through condensation and form 2oxazolidone. Oxazolidone subsequently reacts with another molecule of MEA and forms 1-(2-hydroxyethyl) imidazolidone (HEIA). The imidazolidone then hydrolyzed to form the second degradation product: N-(2-hydroxyethyl) ethylenediamine (HEEDA). Another component resulting from polymerization is cyclic urea of the MEA trimer, 1-[2-[(2hydroxyethyl)amino]ethyl]-2-imidazolidone.11,12 Besides these two degradation processes, the presence of NOx in the flue gas induces nitrosamine yield.8,13 Two wellknown nitrosamines from MEA and NOx reactions are nitrosodiethanolamine (NDELA) and nitrosodimethylamine (NDMA).8 NDELA is confirmed to be semivolatile; however, there is a different argument on NDMA’s volatility.12,18 On the basis of these findings, some portions of these two components are expected to present in the waste slip-stream. In addition to the impurity from flue gas, trace elements from the coal such as copper (Cu), zinc (Zn), selenium (Se), and arsenic (As) can also be found in the reclaimer bottom, yet they only present in negligible amounts (i.e., below 17.4 ppm).5,19 The analytical tests on the MEA waste from some pilot plants show some differences with the findings from the laboratory scale. The common materials found in the reclaimer bottom from the CO2 capture plant−IMC Chemicals Facility are HEIA, 2-oxazolidone, ammonia, acetic acid, and MEA.5 Another analysis on a MEA-rich solution aged 11 weeks at the Esbjerg pilot plant only indicates HEIA and 2-oxazolidone as similar products.20 In addition, the analysis from the Aker Kvaerner pilot facility focuses on organic compounds comprising nitrate (NO3−), ammonium (NH4+) or ammonia (N-NH3), sulfate (SO42−), and acetic acid (C2H4O2).21,22 2.2. MEA and Reclaimer Waste Biodegradability. E. coli K12 is verified to be able to decompose MEA in soil or bioreactors into ammonium, acetate, ethanol, and nitrogen due to coenzyme B12. This enzyme is recognized to be a major cofactor initiating this decomposition.23,24 However, there is concern about the nitrosamine content due to its being carcinogenetic or its toxicity. Some investigations indicate nitrosamine biodegradability by axenic strain bacteria including Methylosinus trichosporium OB3b, Mycobacterium vaccae JOB5, and Pseudomonas mendocina KR1.25 NDMA decomposes under anoxic soil conditions by 80% within 132 days.26 On the other hand, NDELA
decomposes in natural water by 80% within 56 days of incubation.27 The degradation products of nitrosamine are expected to be formaldehyde, formate, methylamine, nitric oxide, nitrite, and nitrate, which are easier to break down. Some experiments support the finding of amine waste biodegradability. An experimental work indicates that the degradation rate of amine waste is 1.64 g/dm3·h.23 This leads to a strong possibility of biological process integration for carbon capture waste. On the basis of the degradation rate, a 600 MWe power plant with a 1560 ton/year of CO2 capture capacity requires at least a 109 m3 reactor volume for a continuous biological process.23 A continuous-anaerobic bioreactor was set for processing the amine waste from the Aker Kvaerner pilot plant at Kårstø. This results in steady state biogas (methane) production over 50 days of operation and 93% Chemical Oxygen Demand (COD) reduction in 60 days.21 The process can be enhanced by the codigestion concept, for instance, combining the waste with readily degradable acid compounds such as fruit-based waste. A recent application of moving bed biofilm for predenitrification was carried out by Hauser et al. Within 7 h, MEA is converted into ammonia along with the oxidation of organic material. The biofilm reduce 98 ± 1% of MEA and 72 ± 16% of nitrogen without an additional substrate (i.e., electron donor).28 The conventional postdenitrification of which denitrification occurs after ammonium oxidation (nitrification) shows a similar result with predinitrification system.29
3. METHODOLOGY The reference case of Fluor’s Econamine FG ultrasupercritical coal plant generating 750 MWe of energy contains 12.21% CO2 in the flue gas, and the plant is able to capture 546.8 ton of CO2/h.15 The expected waste quantity is 0.003 m3/ton of CO2 capture or 3.2 kg/ton of CO2 capture.30−32 In addition, it requires 1.6 kg of MEA/ton of CO2 capture.32 Table 1 Table 1. Reference Case of MEA Reclaimer Waste Composition waste component water MEA total nitrogen NH4+ degradation products NDMA NDELA
quantity
reference
20−33.9 wt % 25 wt % (dry basis) 14% (dry basis) 0.04% (dry basis) assumed to be the remaining part of the waste that consist of 40% oxidative degradation product and 60% thermal degradation product 5 ηg/kg MEA 10 ηg/kg
15, 40 40
31 52
elucidates the proportion of water, MEA, nitrogenous compound, and degradation products. These data are used to estimate the composition of reclaimer waste and to assess the hazard as well as biological characteristics. The waste estimation and characterization were further used for the following aspects: • Waste reuse and recycling for industrial or other activities. The economic potential of the waste is defined based on the market price of the material for a similar purpose. • Design of secondary biological treatment in order to achieve the water quality for irrigation based on the standard in Abu Dhabi, UAE.33 The kinetic parameters for calculating the 13645
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4. WASTE QUANTIFICATION AND CHARACTERIZATION The total CO2 capture for the reference case is 13 123.2 ton of CO2/day. The calculated waste generation is 41 994.24 kg/day or 39.37 m3/day in a flow rate (Q) basis. In estimating the waste component, a percent weight dry basis proportion is used that refers to the quantity of each component after subtracting the water content. In order to meet the standard unit of the regulation, the mass of each compound over the total waste volume (g/m3 waste) is calculated. We obtain 35 974 g/m3 of thermal degradation compounds as the greatest component. These compounds are basically heavy nitrogenous compounds covering 1-(2-hydroxyethyl) imidazolidone-2 (HEIA), N-(2hydroxyethyl)-ethylenediamine (HEEDA), and N-N-di(hydroxyethyl) urea (DHU), as shown in Table 3.
reactor design and operational requirement are covered in the Supporting Information. • Preliminary economic evaluation that consists of CAPEX and OPEX. Specifically, CAPEX entails direct and indirect cost. The direct cost is calculated on the basis of the approach of the U.S. EPA Technical Report Construction Cost for Municipal Wastewater Treatment Plants elucidated in Table 2.34 This Table 2. Direct Cost Component
34
cost component
modela
correlation
mobilization site work excavation piling electrical control & instrumentation yard piping process piping equipment concrete steel influent pumping effluent outfall unit operation activated sludge oxidation ditch trickling filter land requirement (m2)b
63400Q0.69 196000Q0.66 133000Q0.64 66000Q0.57 167000Q0.73 77800Q0.78 115000Q0.71 151000Q0.82 596000Q0.69 502000Q0.79 82200Q0.9 131000Q0.63 61000Q0.77
0.8 0.82 0.79 0.73 0.86 0.81 0.82 0.77 0.7 0.83 0.79 0.77 0.78
519000Q0.75 468000Q0.57 366000Q0.46 1.4665Q0.9853
0.84 0.81 0.78 0.838
Table 3. Estimation and Characterization of MEA Reclaimer Waste water content (33.9%) dry basis component:
14236 kg/day 27758 kg/day quantity (g/ RCRA status53 and proportion m3 waste) biodegradability12a
total nitrogen 14% 9870.93 NH4+ 0.04% 1.08 MEA 25% 677.34 NDMA 2.67 × 10−04 NDELA 5.33 × 10−04 degradation product 59957.3 (remaining product) with the following breakdown: a and b a. oxidative degradation product (40% of degradation product) formate 6.8% 20865.14 thiocyanate 4.6% 1630.84 acetate 1.2% 1103.2 thiosulphate 0.2% 287.8 oxalate 0.2% 47.97 sulfate 87% 47.97 b. thermal degradation product (60% of degradation product) 1-(2-hydroxyethyl) imidazolidone-2 (HEIA) N-(2-hydroxyethyl)35974.38 ethylenediamine (HEEDA) N-N-di(hydroxyethyl) urea (DHU)
Q: wastewater flow rate (million gallon/day). bLand cost in the UAE: 280 USD/m2.
a
method shows the correlation between waste flow rate and the cost of construction activity as well as the capital cost of secondary biological treatment derived from 737 projects in the U.S. The waste flow rate in this study is used to determine the direct cost that is further adjusted by the Marshall Swift Index to reflect the latest state: 1536.5 in 2011 and 545.3 in 1978. On the contrary, indirect cost is considered to be a proportion of total direct cost as follows: 5% is engineering cost; 1% is construction fee; 10% is contingence.35 The CAPEX is then annuitized using an 8% discount factor for 25 years of plant lifetime.36 The operating time of biological treatment is assumed to be the same as that of the capture plant (i.e., 80% hours in a year).37 Last, the total cost is obtained from the summation of CAPEX and OPEX. • In this study, incineration as a disposal option is only evaluated on the basis of the economic performance. The CAPEX is obtained from the market price of an incinerator that is suitable for the waste quantity. Similar to the economic evaluation for biological treatment, CAPEX is annuitized using identical assumption. The OPEX for incineration also adapts the assumption used in the economic evaluation of the biological process. Sensitivity analysis for biological treatment and on-site incineration is finally performed by applying 5% and 10% discount factors. Moreover, 10% escalation of the operational cost per year was utilized as the worst case scenario.38,39 The results are compared with the other references and with the local waste handler (i.e., treatment and incineration) cost.
a
NH, B NH, B H, B H, B
H, B NH, B NH, B NH, B NH, B H, B
B
NH: Not Hazardous. H: Hazardous. B: Biodegradable.
The evaluation based on the Resource Conservation and Recovery Act (RCRA) of the U.S. Environmental Protection Agency (U.S. EPA) indicates the hazardous nature of formic acid, formaldehyde, acetaldehyde, nitrosodiethanolamine (NDMA), and nitrosodiethylamine (NDELA). In addition, the evaluation from a Quantitative Structure−Activity Relationship (QSAR) conducted by SINTEF shows that almost all the waste components are biodegradable except for nitrosodiethanolamine (NDELA) and nitrosodimethylamine (NDMA).12 Nevertheless, axenic bacteria are still able to decompose these components as discussed before.25,27 13646
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5. RECLAIMER WASTE REUSE, RECYCLING, AND HANDLING Waste reuse, recycling, and handling (i.e., treatment and disposal) are assessed on the basis of the current application and the feasibility, as shown in Table 4. The feasibility is
The reuse and recycling activities are highly encouraged in the UAE. Law No. 21/2005 regarding Waste Management in the Emirate of Abu Dhabi particularly stipulates the responsibility for the waste generator to conduct waste reduction efforts by means of reuse and recycling. Prior to these implementations, the Federal Environmental Law 24/ 1999 obligated all industrial activity including waste reuse and recycling activity to obtain the Operating Environmental Permit (OEP). Besides that, air pollution monitoring on the combustion activity is necessary based on Decree 42 of 2009 Abu Dhabi−Emirate Environment, Health and Safety Management System (EHSMS) Regulatory Framework. 5.2. Secondary Biological Treatment. As characterized before, the reclaimer waste exists in a liquid−solid form containing organic and inorganic materials. Secondary biological treatment can be applied for treating the reclaimer waste to meet the wastewater standard of Abu Dhabi Environment, Health and Safety Management System (AD EHSMS) in the UAE. The treated wastewater can further be discharged to the environment or reused for other applications, e.g., landscape irrigation.45 In this study, the waste treatment is proposed by three secondary biological processes covering Complete-Mix Activated Sludge (CMAS), Oxidation Ditch, and Trickling Filter. COD is an important indicator for this process, representing the amount of oxygen required to oxidize the organic compound. Biodegradable−soluble COD is more favorable, as it indicates the material that can be oxidized by biomass. For this study, the degradable COD is 1359 g/m3.46 In order to meet the irrigation water standard, COD should decrease to 150 mg/L while, ammonia nitrogen (NH4+) should decrease to 5 mg/L.33 Table 5 illustrates the result of the CMAS and Oxidation Ditch design. The important parameters to evaluate are the food to microbe ratio (F/M) and volumetric loading (Lorg). The value of F/M for CMAS and oxidation ditch falls within the range of design criteria of 0.2−0.6 g of BOD/g of MLVSS-d and 0.05−0.3 g of BOD/g of MLVSS-d, respectively.47 This conformance indicates that the food is sufficient to feed the
Table 4. Overview of MEA Reclaimer Waste Reuse, Recycle, and Handling handling activity reuse recycle treatment disposal
purpose/ technique
reclaimer waste from MEA-based PCCa
NOx scrubbing agent Co-firing in coal burner biological treatment incineration
××× ×× ××× ×××
reference 40, 54, 55 43 22−24, 28, 29, 46, 56, 57 14−17
×××, Proven investigation. ××, Strong possibility to apply. ×, Need further test/investigation. a
categorized into activity that has been strongly proven through previous investigation, activity that has strong potency to apply, and activity that needs further study. The technical feasibility of NOx as a scrubbing agent has been proven on the laboratory scale, yet the economic potential needs to be confirmed. In contrast, the waste recycling into the coal burner needs further technical evaluation. Although biological treatment for amine waste has been proven, a fullscale design of the biological process has not been performed. The last option of waste handling that has been longestablished is destruction by incineration, which needs to be economically assessed. 5.1. Potential Waste Reuse and Recycling in the UAE. Ammonia (NH3) and nitrogenous materials found in the reclaimer waste can be useful for the NOx emission control process. A previous experiment of NOx reduction employing MEA reclaimer waste in a selective noncatalytic reduction (SNCR) process shows a satisfactory result: 96% NO x reduction achieved with the input of 50% MEA reclaimer waste solution and a ratio of total nitrogen to NOx of 8.5 under an operating temperature of 950 °C.40 The high thermal conditions allow MEA contained in the waste to further degrade into NH3 and a NHi radical.41 This process is expected to be accelerated when temperature conditions are above 130 °C.11 Theoretically, all nitrogen-containing compounds (i.e., degradation products) pursue this process during combustion. There is a concern of additional NOx emission from the use of the reclaimer waste; however it is deemed to be insignificant as NO generation occurs at a higher temperature of 1500 °C.42 Co-firing is a combustion process for two different materials simultaneously, for example, the cocombustion of coal and waste (e.g., sewage sludge, municipal solid waste, refused derived fuel, and many more). This option can be considered as an option for recycling reclaimer waste into a coal burner based on the preliminary study for the Kingsnorth Carbon Capture and Storage Project.43 The reclaimer waste has a calorific value of 16.3 MJ/kg.40 It falls within the calorific value of coal (14 to >30 MJ/kg) that indicates a good characteristic for combustion. During 1 MWh electricity production, around 56 kg of waste is generated. In contrast, the amount of coal needed to produce 1 MWh of electricity is 295.82 kg.44 This shows that the quantity of waste is insignificant compared to the coal feed; therefore the issue of additional emission can be considered negligible.
Table 5. CMAS and Oxidation Ditch Design aeration design aspect SRT (d) volume of aeration (m3) depth (m) area (m2) HRT (hr) oxygen demand (kg/h) air flow rate (m3/min) F/M (g/g-d) L org (kg/m3 d) NOr (g/d) anoxic NOr (g/d) anoxic time required (h)
CMAS
oxidation ditch
9 136.04 4 34.01 3.46 67.65 23.45 0.2 0.4
12 157.44 3 52.48 4 68.27 23.66 0.14 0.34 388417.85 13588.57 28.58
clarifier recycle ratio hydraulic rate (m3/m2 d) diameter (m) area (m2) solid loading rate (kg/m2 h) 13647
0.6 16 1.5 1.77 4.45
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biomass and sustain its growth in the system. In addition, the Lorg value is within the range of design standards: 0.3−1.6 kg of BOD/m3-d and 0.16−0.8 kg of BOD/m3-d, respectively.47 This value shows the mass of organic matter per day for a specific volume of aeration tank, which also determines the amount of oxygen needed. Table 6 illustrates the result of the trickling filter design targeting to reduce 90% nitrogen as well as organic material.
Table 7. CAPEX for Biological Treatment cost (USD)
direct cost mobilization site work excavation piling electrical control & instrumentation yard piping process piping steel influent pumping effluent outfall equipment concrete unit operation total for year 1978 land cost total direct cost (2011) indirect cost engineering contractor’s fee contingency total indirect cost total direct + indirect cost annuitized CAPEX (USD/ year)
Table 6. Trickling Filter Design design aspect
value
depth (m) Rn (g/m2 d) area of packing (m2) volume of packing (m3) filter area (m2) diameter (m) volumetric oxidation rate (kg/m3 d) clarifier depth (m) flow rate (m/h) area (m2) diameter (m)
6 1.96 178257.09 1782.57 49.52 7.94 0.93
CMAS
oxidation ditch
trickling filter
41 183 23 550 28 427 175 533 15 316 509 920.54
4381 15 212 11 154 7259 9884 3794 7354 6307 2519 11 420 3092 41 183 23 550 51 470 198 577 15 316 574 849
61 631 144 005 15,316 421 081
25 496 2550 50 992 79 037 588 958 55 172
28 742 2874 57 485 89 101 663 950 62 198
21 054 2105 42 108 65 267 486 348 45 560
component
4 1.64 1.56 1.5
The packing area functions for the medium of attached biomass that is estimated to be around 178 257 m2 with a diameter of ±7.9 m. In conclusion, the designed volumetric oxidation rate meets the standard (0.4−1.3 kg/m3 d).48 In the emirates of Abu Dhabi, the entire water industry is controlled by the Regulation and Supervision Bureau (RSB). It stipulates the obtainment of a license that should be renewed yearly for wastewater treatment installment and application including the permit to discharge the treated wastewater to a sewerage system.49
need for oxygen transfer using an aerator, as shown in Table 8. The specifications for the aerator are known to include a Table 8. OPEX for Biological Treatment
6. ECONOMIC POTENTIAL AND EVALUATION 6.1. Economic Potential of Waste as NOx Reducing Agent. The economic potential of the reclaimer waste can be estimated on the basis of the current price of common reagents for NOx scrubbing, such as anhydrous ammonia, aqueous ammonia, and urea. The cheapest reagent is urea, which costs 500−560 USD/ton based on the most updated market price in 2013.50 The waste generation is approximately 1330.5 ton/year. Therefore, the potential revenue of reusing the reclaimer waste could reach 6 561 600−7 348 992 USD/year. 6.2. Economic Evaluation and Comparison. The U.S. EPA Technical Report Construction Cost for Municipal Wastewater Treatment Plants approach is based on the wastewater flow rate.34 Since the flow rate is the same for all biological processes, the results for most of the direct costs are also identical, such as mobilization, site work, control, and instrumentation. Nonetheless, the unit process costs differ from each other, as demonstrated in Table 7. We can see that the highest cost is incurred from site work, excavation, and equipment. Furthermore, CAPEX (i.e., Total Direct Cost) is adjusted by Marshall Swift Index 2011. As explained in the methodology, the indirect cost is calculated as a proportion of the total direct cost. Finally, annuitized cost is acquired after calculating the total direct and indirect cost that results in 45 560−62 198 USD per year. One of the OPEX components is energy cost. For CMAS and Oxidation Ditch, the energy requirement comes from the
cost (USD/year) component
cost reference
energy cost total labor cost total administration and supervision maintenance overhead
total OPEX
0.04 USD/kWh 5.4 USD/h 51 8.1 USD/h 51
58
4% of FCI (CAPEX) 60% of labor + administration & supervision + maintenance 35
oxidation ditch
trickling filter
6339
5710 13 500 20 250
3278
23 558 34 385
26 558 36 184
19 454 31 922
98 032
102 203
88 404
CMAS
35
dynamic efficiency of 6.5 kg/kWh and an oxygen transfer of 0.35 kg/h. The total power needed for CMAS and oxidation ditch is 500.85 kWh/day and 451.14 kWh/day, respectively. In contrast, 258.98 kWh/day is needed to pump the water into a trickling filter. The total energy cost is calculated on the basis of the electricity tariff in the UAE. Moreover, the labor, administration, and supervision costs are considered identical for all processes. On the basis of the calculation, the main operational expenses are maintenance and overhead cost. The total cost of annuitized CAPEX and OPEX is presented in Table 9. It summarizes the cost of wastewater (USD/m3 and USD/ton CO2 captured) with the order from the highest to the lowest being: oxidation ditch, CMAS, and trickling filter. The 13648
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ditch, CMAS, and trickling filter. These results are subsequently compared with the cost of waste treatment and incineration in the UAE obtained from the quotation: 0.214−0.332 USD/ton CO2 and 3.5 USD/ton CO2, respectively. Furthermore, the references demonstrate that the cost of waste treatment and incineration are 0.23−0.79 USD/ton CO2 15 and 0.61 USD/ ton CO2,17 respectively. We can see that the biologicalsecondary treatment process still indicates the most economical option. Among the three biological treatments, trickling filter is the most cost-efficient unit process.
Table 9. Recapitulation of Cost and Unit Cost of Biological Treatment component
CMAS
oxidation ditch
trickling filter
annuitized CAPEX + OPEX (USD/ year) treated wastewater (m3/year) unit cost (USD/m3) unit cost per ton CO2 (USD/ton CO2)
153 205
164 401
133 965
6.55 0.02
24 606 7.05 0.021
5.71 0.017
■
cost ranges between 5.71 and 7.05 USD/m3 and 0.017 and 0.021 USD/ton CO2 captured. The cost estimation is higher than that calculated in another study for treating 220 000 m3/ day wastewater, which is 0.32 USD/m3 for trickling filter as well as CMAS and 0.37 USD/m3 for oxidation ditch.51 This phenomenon shows the great impact of economies of scale. As one of the options is to dispose the waste into an incinerator, the cost for its installment is considered in this study. Table 10 elucidates the breakdown of capital, opera-
* Supporting Information
The Supporting Information includes biological kinetic parameters and design equations for secondary biological treatment. This information is available free of charge via the Internet at http://pubs.acs.org/.
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59
incinerator cost annuitized capital fuel consumption60 fuel cost labor cost51 hours of labor60 cost for labor maintenance cost total O&M total capital + O&M unit cost waste generation unit cost per CO2 captured
*Tel.: +971 2 810 9181. Fax: +971 2 810 9901. E-mail:
[email protected]. Funding
value
unit
88 000 16 902 75 000 93 116 5.4 2500 13 500 3520 110 136 127 038 0.295 0.1051 0.031
USD USD/year kg/year USD/year USD/h h/year USD/year USD/year USD/year USD/year USD/kg waste kg/ton CO2 USD/ton CO2
This research is funded by Siemens. Notes
The authors declare no competing financial interest.
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Table 11. Sensitivity Test Result cost (USD/ton CO2) 10% OPEX escalation IR 5%
IR 8%
activated sludge oxidation ditch trickling filter on-site incineration
0.018 0.019 0.016
0.020 0.021 0.017 0.031
IR 10%
IR 5%
IR 8%
0.021 0.023 0.018
0.019 0.020 0.017
0.021 0.022 0.018 0.034
REFERENCES
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tional, and maintenance cost. On the basis of the calculation, it costs around 0.295 USD/kg waste or 0.031 USD/ton CO2 to install and operate an incinerator. The sensitivity analysis is applied for biological treatment and on-site incineration to ensure the validity of cost calculation. Table 11 suggests that the higher the interest rate, the more
unit process
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Table 10. Cost for on-Site Incineration component
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S
IR 10% 0.022 0.024 0.019
expensive the present cost of biological treatment. However, the incineration cost is not sensitive to the interest rate variation. The yearly escalation of operational cost certainly increases the cost of all the methods. On the basis of this sensitivity test, the economical state of the biological treatment remains the same from the highest to the lowest cost: oxidation 13649
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