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Sustainable Hydrogen Peroxide Production at Wastewater Treatment Plants
BEE 4870: Sustainable Energy Systems Instructor: Dr. Lars Angenent
Department of Biological and Environmental Engineering Cornell University, Ithaca, NY
Keywords: bioprocessing, bioenergy, sustainability, wastewater treatment Time: 3-‐6 lecture periods, or equivalent This module requires students to access a computer in class and at home. Module Educational Level: Senior environmental, biological, or chemical engineers; graduate students in engineering. Prerequisites: engineering thermodynamics, general chemistry, general biology or microbiology.
I. Learning Objectives By the end of this unit, you should be able to:
1. Estimate the required inputs and outputs for on-‐site production and commercial purchasing of hydrogen peroxide (H2O2) and its use at wastewater treatment plants (WWTP).
2. Access scientific journals using library resources to find articles relating to the commercial production of hydrogen peroxide.
3. Apply the Economic Input-‐Output Life Cycle Assessment (EIO-‐LCA) model to quantify emissions of pollutants for the two processes.
4. Discuss life cycle assessment (LCA) outputs in the evaluation of the two processes.
II. Background Information A. Wastewater Treatment in the United States Annually in the United States, over $25 billion is spent to treat in excess of 11 trillion gallons of municipal and industrial wastewater [1]. A majority of wastewater treatment plants (WWTPs) utilize the activated sludge process to remove organic matter (Fig. 1). In this process, microorganisms convert organic matter to biomass and carbon dioxide under aerobic conditions in an aeration tank. During this process, the wastewater must be aerated to facilitate bacterial growth; this aeration can account for up to 60% of the total energy expenditure of wastewater treatment. Following the aeration tank, the biomass is separated from low organic matter effluent in a settling tank. The resulting settled biomass is called waste activated sludge (WAS) and it is the major waste product of the activated sludge process. A portion of the WAS is recycled back to the beginning of the activated sludge process to retain microorganisms, while the remainder must be treated. The treatment and disposal of WAS can account for up to 60% of operating costs at a WWTP [2, 3]. Anaerobic digesters (ADs) are frequently used to treat WAS, and biogas produced during this process can be combusted in a generator to offset heating and
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electrical costs at WWTPs. The amount of energy generated onsite varies greatly based on a wide variety of factors (i.e., plant size, AD efficiency, use of co-‐digestates), but can account for anywhere from 17% to 100% of the total energy needs of the plant [4]. Residual biosolids in AD effluent are dewatered and shipped to landfills at high costs ($52.92/ton [5]). A 2007 report by the North East Biosolids and Residuals Association indicated that the United States produces approximately 6.5 million dry metric tons of residual biosolids annually [6].
B. Improving treatment and decreasing costs Recently, a novel process has shown that the addition of hydrogen peroxide (H2O2) to ADs at WWTPs can decrease costs associated with biosolids disposal [5]. In this process, H2O2 is added to the recycle stream between two ADs (Fig. 1; red arrow); there, the H2O2 is combined with ferrous iron (Fe2+) (already in the waste stream from a previous treatment step) to form hydroxyl radicals (OH•) in a Fenton’s reaction (Reaction 1).
!"!! + !!!! → !"!! + !" ∙ +!"! Reaction 1 The resulting hydroxyl radical is a powerful oxidant that breaks up otherwise recalcitrant WAS materials, thus, increasing the organic matter available for conversion to methane via anaerobic microorganisms in ADs. Bench-‐scale tests have confirmed the viability of this process, where the five day biological oxygen demand (BOD5) of total biosolids treated with this process increased by 193% [5]. When installed at a full-‐scale WWTP using commercially purchased H2O2, this process resulted in numerous benefits, including: 1) 11.5% reduction in residual biosolids for disposal; 2) 13% increase in specific biogas production; and 3) 13% higher heat output from energy cogeneration. The combination of these benefits translated to a net economic benefit of $78,000 annually at a WWTP treating 8 MGD ($9750/MGD treated) [5].
Figure 1. A process flow diagram for the activated sludge treatment process. Organic matter in wastewater influent is converted to biomass by aerobic microorganisms in the Aeration Tank. The biomass settles and the water is clarified in the Clarifier-‐Settler; from there treated water is sent to the disinfection step of the wastewater treatment process. A portion of the settled biomass, called waste activated sludge (WAS), is recycled back to the aeration tank to maintain the proper microorganism population. A majority of the WAS is sent to anaerobic digesters (ADs), where it is
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converted to biogas by a diverse community of anaerobic microorganisms. Two ADs are operated – a primary AD (AD1) and a secondary AD (AD2). A portion of the biosolids is recycled from AD2 to AD1; the addition of hydrogen peroxide occurs in this recycle stream, and is denoted by a red arrow in the figure. Residual biosolids from AD2 are dewatered and shipped to a landfill.
C. Commercial H2O2 production is unsustainable However, there is still room to increase the net economic benefit while improving sustainability of the case-‐study WWTP. Currently, this WWTP purchases hydrogen peroxide at $20,000/year, which is a major cost for this system [5]. In addition, the commercial method of hydrogen peroxide generation, which is called anthraquinone oxidation (AO), is far from sustainable [7]. The AO process that produces H2O2 requires a slow, energy intensive, multi-‐step process utilizing a variety of organic solvents to extract H2O2 [7]. Furthermore, there is a large amount of waste generated via this process and the transport, storage, and handling of highly concentrated H2O2 is hazardous and expensive. Hydrogen peroxide can also be produced via electrolysis, an environmentally sustainable process that can be operated on-‐site. However, electrolysis produces hydrogen peroxide at concentrations much lower (up to 50X) than the anthraquinone process. While it is not feasible to replace commercial hydrogen peroxide manufacturing with electrolysis, it may be possible to generate hydrogen peroxide on-‐site for some applications, such as use at a WWTP, using electrolysis.
Exercise 1 (~30 minutes): Read ‘Hydrogen Peroxide Synthesis: An Outlook beyond the Anthraquinone Process’ by J.M. Campos-‐Martin et al. [7]. Then, construct a process diagram showing the required inputs and outputs (i.e., energy, raw materials, waste products) for hydrogen peroxide production via the commercial anthraquinone process. Exercise 2 (~20 minutes): Read the Tech Brief from Eltron Water Systems, LLC on their PeroxEgen system, which produces hydrogen peroxide electrochemically. Construct a process diagram showing the required inputs and outputs for this system. Exercise 3 (In class, ~20 minutes): Discuss the two methods of hydrogen peroxide production from the readings. Use the process diagrams to identify inputs and outputs, and make a list of potential advantages and disadvantages. Discuss why the PeroxEgen system could be an ideal solution for application at a WWTP, and why it is not suited to replace commercial hydrogen peroxide production for use in other industries.
D. Life cycle analysis provides a robust analysis for evaluating processes Due to the low price of fossil fuels and inability of traditional economic analysis to consider environmental concerns, it is unlikely that this electrochemical process will ever be able to compete with commercial H2O2 generation based on economics alone. However, the use life cycle assessment (LCA) considers all aspects of a products life, and accounts for factors missing from traditional economic analysis, including the environmental impacts of the production, use, and disposal of a product. This type of analysis can elucidate the benefits of a sustainable, on-‐site, method of H2O2 production. In the following exercises, we will examine two processes using LCA to examine all aspects of: 1) using commercially produced H2O2 at WWTPs, and 2) generating H2O2 on site at WWTPs.
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III. LCA Tools The “Economic Input Output-‐Life Cycle Assessment” (EIO-‐LCA) [8], which is accessible at http://www.eiolca.net, is a tool for evaluating processes using an approach that estimates the “materials and energy resources required for, and the environmental emissions resulting from, activities in our economy.” It identifies the material and energy resources and environmental emissions based on the product output (in total dollars). The website includes a tutorial detailing the use of the EIO-‐LCA tool that can be accessed by following the ‘Tutorial’ link on the left side of the homepage.
Exercise 4 (In class, groups of 3, ~40 minutes): Open the EIO-‐LCA website at http://www.eiolca.net and answer the following questions:
a. Identify the following aspects associated with $1M of hydrogen peroxide production:
i. Greenhouse Gas Emissions ii. Energy Usage iii. Resource Conservation and Recovery Act (RCRA) materials (i.e., Hazardous
Waste) iv. Water Withdrawals
b. Discuss the results from Part A. Are there any surprises? c. Explore data for other industrial manufacturing processing in the EIO-‐LCA model.
Pick one process and compare the greenhouse gas emissions, energy use, RCRA (hazardous waste), toxic releases, and water withdrawals with those of hydrogen peroxide production. Discuss the results and the use of the EIO-‐LCA tool.
Note: The EIO-‐LCA tool is most accurate for economic activity greater than or equal to $1M. To calculate emissions/usage for economic activity under $1M, run the EIO-‐LCA tool for $1M, and then scale the resulting emissions/usage proportionally. Hint: Instructions for using the EIO-‐LCA tool:
1. Access www.eiolca.net 2. On the left side of the page, click ‘Use the Tool’ 3. Select the ‘US National Producer Price Models/US 2002’ in box 1. 4. Select the industry and sector. For hydrogen peroxide, select the industry
‘Petroleum and Basic Chemical’ and the sector ‘All other basic inorganic chemical manufacturing’.
5. Enter the amount of economic activity in box 3. 6. Select the desired results from the dropdown menu in box 4. 7. Click ‘run the model’.
IV. Homework Assignment 1. Use campus library resources (http://www.library.cornell.edu/) to access 2-‐5 articles discussing
the commercial process currently used for hydrogen peroxide production. After thoroughly reading the articles, prepare a 500-‐1000 word summary of commercial hydrogen peroxide production. This should include, at a minimum, a discussion of the processes sustainability, energy efficiency, and economics. Be sure to properly cite the articles.
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2. Use the EIO-‐LCA model to compare the following two scenarios on a per-‐year basis: a. One year supply of commercial production of H2O2 ($20,000, not including shipment of
H2O2 from the production site to the plant). b. One year of on site production of H2O2 with an electrochemical system. Use the
following assumptions about the electrochemical system: i. 0.36 kW (3 A at 120 Vac) to operate the system, and a cost of $0.10/kWh ii. 3.6 gal/day of H2O at a price of $0.44/gal iii. 0.16 lb/day of sodium sulfate as a electrolyte at $5/lb Hint: To do this, calculate the financial amount of energy, water, or chemical usage – then use the EIO-‐LCA tool to calculate the associated emissions/usage for each input over a year and sum the three inputs to get the total emissions/usage.
Compare the two processes using the following metrics: i. Greenhouse gas emissions in tons of CO2 equivalent per year. ii. Energy requirements iii. Hazardous Waste emissions iv. Water usage
Present the results of the analysis in a table comparing the two processes. Provide a description of your analysis, including any assumptions. Summarize the differences between the two processes, and advantages/disadvantages of each. Discuss any drawbacks of LCA analysis, and suggest possible improvements.
V. References 1. Clean safe water for the 21st century. 2001, Water Infrastructure Network. 2. Zhao, Q.L., and G. Kugel, Thermophilic/mesophilic digestion of sewage sludge and organic waste.
Journal of Environmental Science and Health Part a-‐Toxic/Hazardous Substances & Environmental Engineering, 1997. 31: p. 2211-‐31.
3. Horan, N.J., Biological wastewater treatment systems : theory and operation. 1990, Chichester ; New York: Wiley. viii, 310 p.
4. Law-‐Flood, A. Tapping Energy and Revenue Potential in Our Waste Streams: A Discussion of Anaerobic Digestion and Combined Heat and Power in MA. in Massachusetts Water Pollution Control Association Quaterly Meeting. 2011. Boxborough, MA.
5. Lozano, J. Enhanced anaerobic digestion using Fenton's reagent. in 83rd Annual Water Environment Federation Technical Exhibition and Conference (WEFTEC). 2010. New Orleans, LA.
6. Association), N.N.E.B.a.R., A National Biosolids Regulation, Quality, End use and Disposal Survey—Preliminary Report, April 14, 2007. North EastBiosolids and Residuals Association Web. 2007.
7. Campos-‐Martin, J.M., G. Blanco-‐Brieva, and J.L. Fierro, Hydrogen peroxide synthesis: an outlook beyond the anthraquinone process. Angewandte Chemie International Edition, 2006. 45(42): p. 6962-‐84.
8. Hendrickson, C., et al., Economic input-‐output models for environmental life-‐cycle assessment. Environmental Science & Technology, 1998. 32(7): p. 184a-‐191a.
VI. Appendix I – Eltron Water Systems, LLC. Tech Brief
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Sustainable Hydrogen Peroxide Production at Wastewater Treatment Plants
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Solutions to exercises and homework problems
These solutions were prepared by Elliot Friedman (Graduate Teaching Assistant) and Dr. Lars Angenent (Associate Professor)
Department of Biological and Environmental Engineering Cornell University, Ithaca, NY
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Solutions to exercises Section II.C Process flow diagram for commercial hydrogen peroxide production: Example Solution 1:
Example Solution 2:
Process flow diagram for electrochemical hydrogen peroxide production: Example Solution 1:
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Example Solution 2:
Section III
a. Identify the following aspects associated with $1M of hydrogen peroxide production: i. Greenhouse Gas Emissions (all in tons of CO2e)
Total = 2720 tons CO2 fossil = 2140 tons CO2 Process = 98.4 tons CH4 = 216 tons N2O = 203 tons HFC/PFCs = 66.1 tons
ii. Energy Usage
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Total Energy = 45.3 TJ Coal = 8.11 TJ Natural Gas = 16.9 TJ Petroleum = 8.43 TJ Bio/Waste = 8.39 TJ Non-‐Fossil Electric = 3.44 TJ
iii. RCRA (Hazardous Waste) Total Waste = 10.8 X 106 short tons
iv. Water Withdrawals Total water withdrawal = 49,800,000 gal
b. Discuss the results from Part A. Are there any surprises? c. Explore data for other industrial manufacturing processing in the EIO-‐LCA model. Pick one
process and compare the greenhouse gas emissions, energy use, RCRA (hazardous waste), toxic releases, and water withdrawals with those of hydrogen peroxide production. Discuss the results.
Category
$1M Hydrogen Peroxide Production
$1M Petroleum Refinery
Greenhouse Gas
Emissions
Total (tons CO2e) 2720 2790
CO2 fossil (tons CO2e) 2140 1800
CO2 process (tons CO2e) 98.4 242
CH4 (tons CO2e) 216 734
N2O (tons CO2e) 203 6.39
HFC/PFCs (tons CO2e) 66.1 11.3
Energy Use
Total Energy (TJ) 45.3 31.7 Coal (TJ) 8.11 2.59 Natural Gas (TJ) 16.9 12.9 Petroleum (TJ) 8.43 13 Bio/Waste (TJ) 8.39 1.15 Non-‐fossil Electric (TJ) 3.44 2.09
Hazardous Waste Total (short tons) 10,800,000 412,000 Water Withdrawal Total (kgal) 498,000 9,410
Discussion: While aggregate greenhouse gas emissions (total tons CO2e) are comparable, $1M of hydrogen peroxide production requires ~50% more energy than the same amount of activity at a petroleum refinery. Additionally, the hydrogen peroxide production industry generates over 25 times the amount of hazardous waste and uses 50 times the amount of water.
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Homework Solutions Part 2:
Use the EIO-‐LCA model to compare the following two scenarios on a per-‐year basis: i. Commercial production of $20,000 worth of H2O2 shipped to the Wastewater
Treatment Plant (WWTP). ii. An equivalent amount of hydrogen peroxide produced on site with an
electrochemical system. Use the following assumptions about the electrochemical system:
i. 0.36 kW (3 A at 120 Vac) to operate the system, and a cost of $0.10/kwh
ii. 3.6 gal/day of H2O at a price of $0.44/gal iii. 0.16 lb/day of sodium sulfate as a electrolyte at $5/lb
Compare the two processes using the following metrics: i. Greenhouse gas emissions in tons of CO2 equivalent per year. ii. Energy requirements iii. Hazardous waste emissions iv. Water usage
Present the results of the analysis in a table comparing the two processes. Provide a description of your analysis, including any assumptions. Summarize the differences between the two processes, and advantages/disadvantages of each. Discuss any drawbacks of LCA analysis, and suggest possible improvements.
Category $20,000 Hydrogen Peroxide Production
Yearly Hydrogen Peroxide Production with Electrochemical
System % Reduction
Greenhouse Gas Emissions
Total (tons CO2e) 54.4 1.79 96.71% CO2 fossil (tons CO2e) 42.8 1.17 97.26% CO2 process (tons
CO2e) 1.968 0.13 93.64% CH4 (tons CO2e) 4.32 0.39 90.99% N2O (tons CO2e) 4.06 7.89E-‐02 98.06%
HFC/PFCs (tons CO2e) 1.322 2.54E-‐02 98.08%
Energy Use
Total Energy (TJ) 0.906 2.17E-‐02 97.60% Coal (TJ) 0.1622 6.82E-‐03 95.80%
Natural Gas (TJ) 0.338 6.52E-‐03 98.07% Petroleum (TJ) 0.1686 3.29E-‐03 98.05% Bio/Waste (TJ) 0.1678 7.86E-‐04 99.53%
Non-‐fossil Electric (TJ) 0.0688 4.33E-‐03 93.70% Hazardous Waste Total (short tons) 216000 764.27 99.65% Water Withdrawal Total (kgal) 9960 6.25 99.74%
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Discussion: This analysis suggests that the on-‐site production of H2O2 would offer significant non-‐monetary advantages – including more than a 96% reduction in total greenhouse gas emissions, energy usage, hazardous waste generation, and water usage. As this is a preliminary analysis, many assumptions are made. One major assumption is that the EIO-‐LCA values for ‘basic inorganic chemical manufacturing’ are valid for the hydrogen peroxide industry. Additionally, any emissions/usages associated with shipping (i.e., H2O2 for the commercial case or sodium sulfate for the on-‐site case) are not considered because the distance that these items would need to be shipped is unknown. This LCA analysis does not consider any economics, which would need to be evaluated before any further recommendations can be made. The implementation of an electrochemical system would require a significant initial investment, and the rate of return may be very low. Additionally, for this analysis we have not considered the use of biogas from the anaerobic digesters as a source of electrical power. If an electrochemical cell were to be employed, it may be feasible to power its operation using the additional methane generated from using H2O2 in the digesters. However, further data is required to determine whether this will be possible and the cost of grid electricity for the electrochemical cell is so low (< $13/year) that any changes to this analysis would likely be insignificant.
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Description of Calculations for Electrochemical System Operation: 1. First, calculate the annual production costs for the three major inputs
Costs Electricity ($/kwh) $ 0.10 Water ($/gal) $ 0.44 Sodium Sulfate ($/lb) $ 5.00
Usage Electricity (kW) 0.36 Water (gal/day) 3.6 Sodium Sulfate (lb/day) 0.16
Annual Costs Electricity ($) $ 12.92 Water ($) $ 581.45 Sodium Sulfate ($) $ 292.00
2. Then get the data for “Power generation and supply ”, “Water, sewage, and other systems ”, and
“All other basic inorganic chemical manufacturing” by using the EIO-‐LCA tool. Obtain the values for $1M of economic activity and then scale down to the annual costs calculated in the previous step. Add them together to get the total values for yearly hydrogen peroxide production using an electrochemical cell.
Category Electricity Water Sodium Sulfate Total
Greenhouse Gas
Emissions
Total (tons CO2e) 0.12 1.03 0.64 1.79
CO2 fossil (tons CO2e) 0.11 0.59 0.47 1.17
CO2 process (tons CO2e) 4.04E-‐04 0.03 0.10 0.13
CH4 (tons CO2e) 4.47E-‐03 0.35 3.39E-‐02 0.39
N2O (tons CO2e) 7.27E-‐04 5.87E-‐02 1.95E-‐02 7.89E-‐02
HFC/PFCs (tons CO2e) 7.43E-‐04 5.34E-‐03 1.94E-‐02 2.54E-‐02
Energy Use
Total Energy (TJ) 1.43E-‐03 1.08E-‐02 9.46E-‐03 2.17E-‐02 Coal (TJ) 1.01E-‐03 3.21E-‐03 2.59E-‐03 6.82E-‐03 Natural Gas (TJ) 3.19E-‐04 3.11E-‐03 3.10E-‐03 6.52E-‐03 Petroleum (TJ) 6.64E-‐05 2.07E-‐03 1.15E-‐03 3.29E-‐03 Bio/Waste (TJ) 1.25E-‐06 1.22E-‐04 6.63E-‐04 7.86E-‐04 Non-‐fossil Electric (TJ) 3.95E-‐05 2.32E-‐03 1.98E-‐03 4.33E-‐03
Hazardous Waste Total (short tons) 1.62 123.17 639.48 764.27 Water Withdrawal Total (kgal) 3.24 12.14 10.86 26.25
