Letter Cite This: ACS Sustainable Chem. Eng. XXXX, XXX,pubs.acs.org/journal/ascecg XXX-XXX
A Roadmap for Achieving Energy-Positive Sewage Treatment Based on Sludge Treatment Using Free Ammonia Qilin Wang*,†,‡ †
Griffith School of Engineering, Griffith University, Nathan Campus, QLD 4111, Australia Centre for Clean Environment and Energy, Griffith University, Gold Coast Campus, QLD 4222, Australia
‡
S Supporting Information *
ABSTRACT: This letter proposes an innovative roadmap for achieving energypositive sewage treatment based on sludge treatment using free ammonia (FA, i.e., NH3). This FA technology is able to enhance anaerobic energy recovery in the form of methane via pretreatment of primary sludge and/or secondary sludge. It can also achieve stable mainstream nitrogen removal via nitrite instead of nitrate, thereby increasing organics availability for energy recovery. Energy evaluation suggests that the FA technology could transform sewage treatment plants from energy consumers (energy consumption at 0.27 kWh/m3 sewage treated) to energy exporters (energy export at 0.14 kWh/m3 sewage treated). Economic and environmental evaluations indicate that the FA technology would reduce sewage treatment cost and CO2 emission by $0.056/m3 sewage treated and 0.40 kg CO2/ m3 sewage treated, respectively. This FA technology is a sustainable and closedloop technology, which requires negligible chemical/energy input with FA being a byproduct of sewage treatment. It is also easy to implement in any existing and new sewage treatment plants by adding a simple sludge mixing tank. KEYWORDS: Free ammonia, Energy recovery, Sludge treatment, Sewage treatment, Methane, Nitrogen removal via nitrite
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removal without compromising energy recovery,5,6 STPs need to be significantly retrofitted, and therefore, substantial capital cost will be required. Also, stable partial nitritation is still difficult to achieve.6 The free nitrous acid (HNO2 or FNA) technology potentially provides a solution to the above barriers because this technology (1) improves secondary sludge biodegradability and (2) alleviates the organics competition between energy recovery and nitrogen removal by the establishment of nitrogen removal via nitrite.7 Nevertheless, a side-stream nitritation reactor is needed to produce FNA required for this technology, and such a reactor does not exist in most of the current STPs. Therefore, implementing FNA technology would become complicated considering the installation of such reactors. Recent studies demonstrated the biodegradability of primary sludge and secondary sludge could be improved after sludge pretreatment by free ammonia (FA, i.e., NH3) at >250 mg NH3− N/L.8,9 Wang et al.10 also demonstrated that much more nitriteoxidizing bacteria (NOB: producing NO3− from NO2−) were inactivated by FA at ∼250 mg NH3−N/L compared with ammonium-oxidizing bacteria (AOB: producing NO2− from NH4+). These discoveries drove the development of an FA-based sludge treatment technology, which could potentially transform sewage treatment from an energy-consuming process to an energy-generating process. Importantly, this technology is
INTRODUCTION Sewage treatment consumes substantial amounts of energy, which is approximately 1800 TJ/year in Australia.1 At the same time, sewage also contains a significant amount of energy at approximately 10,000 TJ/year in the form of organics in Australia.1 Therefore, the energy consumption for sewage treatment only represents 1/6−1/5 of the chemical energy contained in sewage. While energy recovery has been considered in some sewage treatment plants (STPs) where the primary sludge and/or secondary sludge is anaerobically digested to produce methane, the recovered energy only occupies a few percent of the chemical energy contained in sewage. For instance, if this process is applied to all the sewage produced in Australia, the recovered energy would be 500−1000 TJ/year, accounting for 5−10% of the chemical energy available.1 One barrier for achieving maximal energy recovery is the poor sludge biodegradability.2 To this end, sludge pretreatment is often required prior to anaerobic digestion to increase sludge biodegradability, thereby maximizing energy recovery. Various technologies have been developed, including chemical, thermal, and mechanical treatments.3,4 However, all of these require either intensive energy input or large chemical consumption, incurring substantial operational costs. Another barrier for attaining maximal energy recovery is the competition for organics between anaerobic energy recovery and biological nitrogen removal because both processes require organics. Although mainstream autotrophic nitrogen removal via partial nitritation and anammox could achieve satisfactory nitrogen © XXXX American Chemical Society
Received: July 31, 2017 Revised: October 15, 2017 Published: October 23, 2017 A
DOI: 10.1021/acssuschemeng.7b02605 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering
Figure 1. Roadmap of the closed-loop free ammonia (FA) technology in a sewage treatment plant (STP) for achieving energy-positive, economically favorable, and environmentally friendly sewage treatment. Option 1: Pretreatment of primary sludge with FA for enhancing energy recovery. Option 2: Pretreatment of secondary sludge with FA for enhancing energy recovery. Option 3: Sludge treatment using FA for establishing mainstream nitrogen removal via nitrite, thereby increasing organics availability for energy recovery. The three options can be applied jointly or separately. FA is directly acquirable from anaerobic digester effluent of the STP, and therefore, negligible chemical/energy input is required.
0.22 to 0.41−0.53 d−1), respectively.9 Therefore, the FA technology can enhance energy recovery from secondary sludge. Enhancing Energy Recovery by Achieving Mainstream Nitrogen Removal via Nitrite. FA technology can be implemented on the sludge recycling line to attain mainstream nitrogen removal through nitrite (NH4+ → NO2− → N2) rather than nitrate (NH4+ → NO3− → N2) (Option 3 in Figure 1), thereby increasing organics availability for sewage energy recovery. The recent study showed mainstream nitrogen removal via nitrite was rapidly attained (in ∼40 days) with a nitrite accumulation percentage (NO2− − N/(NO2− − N + NO3− − N) × 100%) of above 90% after treating part of the recycling sludge using FA at ∼250 mg NH3−N/L for 1 day.10 Microbial population analysis demonstrated that NOB population decreased by above 95% after incorporating FA treatment, indicating the elimination of NOB.10 Mainstream nitrogen removal via nitrite can theoretically save the organics requirement by 40% compared with nitrogen removal through nitrate.11 This would enable primary settlers to be reinstalled in the STPs to separate more upfront organics for anaerobic methane production, thereby maximizing energy recovery while meeting nitrogen discharge regulatory standard. In contrast, many current STPs have to abolish primary settlers to avoid upfront organics separation in order to achieve satisfactory nitrogen removal, which compromises energy recovery. Therefore, the FA technology can also enhance energy recovery through the establishment of nitrogen removal via nitrite. It should be noted that FA would be diluted significantly by 3−4 orders of magnitude and also be quickly removed biologically in the bioreactor after being recycled to the bioreactor along with the FA-treated sludge. Therefore, the biological activities in the bioreactor would not be affected. It is also noteworthy that the sludge requires to be thickened before being treated by FA. This
sustainable and requires negligible chemical/energy input with FA being a renewable chemical of the STPs. This letter proposes the roadmap of the FA technology. Energy, economic, and environmental evaluations of the FA technology were also carried out.
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ROADMAP OF FA TECHNOLOGY The roadmap of the FA technology in a STP is shown in Figure 1. Enhancing Energy Recovery from Primary Sludge. FA can be used to pretreat primary sludge to improve energy recovery in the form of methane (Option 1 in Figure 1). It was recently reported that FA pretreatment at 250−680 mg NH3− N/L for 1 day was able to improve anaerobic methane production from primary sludge by 5−15% (from 317 to 340− 365 L CH4/kg volatile solids (VS)) at 15 days of digestion time.8 Also, it was revealed that enhanced methane production was because of the 8−17% improvement in biochemical methane potential (i.e., from 331 to 357−387 L CH4/kg VS), although the decreased hydrolysis rate of 24−38% was observed (i.e., from 0.29 to 0.18−0.22 d−1).8 Consequently, the FA technology can enhance energy recovery from primary sludge. Enhancing Energy Recovery from Secondary Sludge. Energy recovery can also be enhanced by implementing FA pretretament on secondary sludge (Option 2 in Figure 1). Our recent study revealed anaerobic methane production from secondary sludge was enhanced by 20−30% (from 144 to 172−187 L CH4/kg VS) by FA pretreatment at 250−680 mg NH3−N/L for 1 day during the 15 days of digestion time.9 Further analysis by the model indicated that the enhanced methane production was attributed to the improved biochemical methane potential and hydrolysis rate, which increased by 13− 22% (from 160 to 180−195 L CH4/kg VS) and 80−140% (from B
DOI: 10.1021/acssuschemeng.7b02605 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering
Table 1. Energy Evaluation of Sewage Treatment Plant (STP) Before and After Implementing FA Technology (Figure S1 and Figure 1)a STP before implementing FA technology
Energy consumption for aeration (kWh/m3 sewage) Energy consumption for mixing and pumping (kWh/m3 sewage) Energy recovery from methane (kWh/m3 sewage) Energy consumption for sludge thickening, dewatering and heating in anaerobic digester (kWh/m3 sewage) Net energy (kWh/m3 sewage treated)
−0.22 −0.10 +0.09b −0.04 -0.27
STP after implementing FA technology
Energy consumption for aeration (kWh/m3 sewage) Energy consumption for mixing and pumping (kWh/m3 sewage) Energy recovery from methane (kWh/m3 sewage) Energy consumption for sludge thickening, dewatering and heating in anaerobic digester (kWh/m3 sewage) Net energy (kWh/m3 sewage treated)
−0.13 −0.11 +0.46b −0.08 +0.14
a The three options in Figure 1 are assumed to be applied jointly in the evaluation. Calculation details are shown in Table S1. bSTP with FA technology has a significantly higher methane production in comparison with STP without FA technology, which is because of the improved sludge biodegradability and the established nitrogen removal via nitrite.
Table 2. Economic and Environmental Evaluations of STP Before and After Implementing FA Technology (Figure S1 and Figure 1)a STP before implementing FA technology (Economic evaluation)
Benefit because of energy recovery from methane ($/m3 sewage) Cost of sludge transport and disposal ($/m3 sewage) Cost of aeration ($/m3 sewage) Cost of mixing and pumping ($/m3 sewage) Cost of sludge thickening, dewatering and heating ($/m3 sewage)
+0.014 −0.020 −0.033 −0.015 −0.006
STP after implementing FA technology (Economic evaluation)
Benefit because of energy recovery from methane ($/m3 sewage) Cost of sludge transport and disposal ($/m3 sewage) Cost of aeration ($/m3 sewage) Cost of mixing and pumping ($/m3 sewage) Cost of sludge thickening, dewatering and heating ($/m3 sewage) Cost for FA technology implementation ($/m3 sewage) Saving with FA technology ($/m3 sewage treated)
+0.069 −0.024 −0.019 −0.016 −0.012 −0.002 +0.056
STP before implementing FA technology (Environmental evaluation)
Avoided CO2 emission because of methane production (kg CO2/m3 sewage) CO2 emission from aeration (kg CO2/m3 sewage) CO2 emission from mixing and pumping (kg CO2/m3 sewage) CO2 emission from sludge thickening, dewatering and heating (kg CO2/m3 sewage)
−0.10 +0.23 +0.10 +0.04
STP after implementing FA technology (Environmental evaluation)
Avoided CO2 emission because of methane production (kg CO2/m3 sewage) CO2 emission from aeration (kg CO2/m3 sewage) CO2 emission from mixing and pumping (kg CO2/m3 sewage) CO2 emission from sludge thickening, dewatering and heating (kg CO2/m3 sewage) CO2 emission for FA technology implementation (kg CO2/m3 sewage) Reduced CO2 emission with FA technology (kg CO2/m3 sewage treated)
−0.48 +0.15 +0.11 +0.08
a
+0.01 +0.40
The three options in Figure 1 are assumed to be applied jointly in the evaluations. Calculation details are shown in Table S2.
30−560 mg NH3−N/L (i.e., pH 7.5−8.6, 1.0−2.0 g of NH4+− N/L and 33 °C).12 This is sufficient for the FA technology. In the case that the FA concentration in anaerobic digester effluent cannot reach the required concentration, a moderate amount of base could be added to the FA treatment unit to increase pH and therefore raise the FA concentration accordingly. Therefore, this FA technology is a sustainable and closed-loop technology, which requests negligible chemical/energy input. After the FAtreated sludge is added to the anaerobic digester and/or mainstream bioreactor, the activity of methanogenic archaea and the performance of the mainstream bioreactor would not be negatively affected. Energy Evaluation. The energy performance of the STPs before and after implementing FA technology is assessed and
would reduce the sludge volume and thus decrease the amount of FA to be added. In that case, the potential negative effect of the returned FA on the bioreactor performance could be avoided. How To Implement FA Technology. The implementation of the FA technology only requires installing a simple, small sludge mixing tank operated at atmospheric pressure and temperature. Therefore, it will be easy to implement in any existing and new STPs. The three options proposed in Figure 1 (i.e., enhancing energy recovery from primary sludge and secondary sludge and by achieving mainstream nitrogen removal via nitrite) can be applied jointly or separately in the STPs. In addition, the key chemical (i.e., FA) required for this technology is a byproduct of sewage treatment and is acquirable from anaerobic digester effluent of the STPs, which contains FA at C
DOI: 10.1021/acssuschemeng.7b02605 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
ACS Sustainable Chemistry & Engineering
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shown in Table 1. The three options in Figure 1 are assumed to be applied jointly in the energy evaluation. The net energy consumption in the traditional STPs (Figure S1, Supporting Information) without FA technology is 0.27 kWh/m3 sewage treated (Table 1 and Table S1). In contrast, the STPs would be able to export energy at 0.14 kWh/m3 sewage treated after implementing FA technology (Table 1 and Table S1). This reveals that implementing FA technology would transform STPs from energy consumers to energy exporters. The increased energy recovery from methane (i.e., 0.37 kWh/m3 sewage) and decreased energy consumption for aeration (i.e., 0.09 kWh/m3 sewage) are the main reasons for the improved energy performance, although the combined energy consumption for mixing, pumping, sludge thickening, dewatering, and heating increase by 0.05 kWh/m3 sewage after including FA technology. This FA technology is only applicable to the STPs with anaerobic digesters in order to achieve energy-positive sewage treatment. This is because that energy recovery cannot be achieved in the absence of anaerobic digesters. Economic and Environmental Evaluations. The economic and environmental viability of the STPs before and after implementing FA technology are also assessed and shown in Table 2. The three options in Figure 1 are assumed to be applied jointly in the economic and environmental evaluations. Compared to the STP without FA technology (Figure S1), the STP with FA technology can save sewage treatment cost by $0.056/m3 sewage treated (Table 2 and Table S2). The saving is contributed by the improved energy recovery (i.e., $0.055/m3 sewage) and decreased energy consumption for aeration (i.e., $0.014/m3 sewage) in spite of the cost for implementing FA technology (i.e., $0.002/m3 sewage) and the raised cost of mixing, pumping, sludge thickening, dewatering, heating, and disposal (i.e., $0.011/m3 sewage). Therefore, the FA technology would be economically favorable. FA technology can also deliver substantial environmental benefits. For example, the CO2 emission would decrease by 0.40 kg CO2/m3 sewage treated after including FA technology. This is primarily because the raised methane production decreases CO2 emission (i.e., 0.38 kg CO2/m3 sewage). Methane can be used to produce both heat and power energy for the operation of the STP. Therefore, the STP could reduce the input of external energy generated from, for example, coal, and the production of external energy would lead to CO2 emission. In addition, the decreased CO2 emission from aeration (i.e., 0.08 kg CO2/m3 sewage) also plays an important role, although the combined CO2 emissions from mixing, pumping, sludge thickening, dewatering, heating, and FA technology implementation increase by 0.06 kg CO2/m3 sewage. Consequently, the FA technology is environmentally friendly.
Letter
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02605. Details of energy, economic, and environmental evaluations and schematic diagram of the traditional sewage treatment plant. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: qilin.wang@griffith.edu.au. ORCID
Qilin Wang: 0000-0002-0346-2396 Notes
The author declares no competing financial interest.
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ACKNOWLEDGMENTS Dr. Qilin Wang acknowledges the supports of Australian Research Council (ARC) Discovery Early Career Researcher Award (DE160100667) and ARC Discovery Project (DP170102812).
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REFERENCES
(1) Lazarova, V.; Choo, K.-H.; Cornel, P. Water-Energy Interactions in Water Reuse; IWA Publishing, 2012. (2) Appels, L.; Baeyens, J.; Degreve, J.; Dewil, R. Principles and potential of the anaerobic digestion of waste-activated sludge. Prog. Energy Combust. Sci. 2008, 34, 755. (3) Wang, Q.; Wei, W.; Gong, Y.; Yu, Q.; Li, Q.; Sun, J.; Yuan, Z. Technologies for reducing sludge production in wastewater treatment plants: State of the art. Sci. Total Environ. 2017, 587−588, 510. (4) Neumann, P.; Pesante, S.; Venegas, M.; Vidal, G. Developments in pre-treatment methods to improve anaerobic digestion of sewage sludge. Rev. Environ. Sci. Bio/Technol. 2016, 15, 173. (5) Kartal, B.; Kuenen, J. G.; Van Loosdrecht, M. C. M. Sewage treatment with anammox. Science 2010, 328, 702. (6) Cao, Y.; van Loosdrecht, M. C. M.; Daigger, G. T. Mainstream partial nitritation−anammox in municipal wastewater treatment: status, bottlenecks, and further studies. Appl. Microbiol. Biotechnol. 2017, 101, 1365. (7) Wang, Q.; Hao, X.; Yuan, Z. Towards energy positive wastewater treatment by sludge treatment using free nitrous acid. Chemosphere 2016, 144, 1869. (8) Wei, W.; Zhou, X.; Xie, G.; Duan, H.; Wang, Q. A novel free ammonia based pretreatment technology to enhance anaerobic methane production from primary sludge. Biotechnol. Bioeng. 2017, 114, 2245. (9) Wei, W.; Zhou, X.; Wang, D.; Sun, J.; Wang, Q. Free ammonia pretreatment of secondary sludge significantly increases anaerobic methane production. Water Res. 2017, 118, 12. (10) Wang, Q.; Duan, H.; Wei, W.; Ni, B.; Laloo, A.; Yuan, Z. Achieving stable mainstream nitrogen removal via the nitrite pathway by sludge treatment using free ammonia. Environ. Sci. Technol. 2017, 51, 9800. (11) Turk, O.; Mavinic, D. S. Preliminary assessment of a shortcut in nitrogen removal from wastewater. Can. J. Civ. Eng. 1986, 13, 600. (12) Cervantes, F. J. Environmental Technologies to Treat Nitrogen Pollution; IWA Publishing, 2009.
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CONCLUSION This letter proposes an innovative, sustainable, and closed-loop technology based on sludge treatment using FA. This FA technology would potentially transform sewage treatment from an energy-consuming process to an energy-generating process. This technology would also substantially reduce sewage treatment cost and achieve “green” sewage treatment by decreasing CO2 emission. Moreover, the FA technology requires negligible chemical/energy input because FA is a byproduct of sewage treatment. Also, this technology is easy to implement in any existing and new sewage treatment plants by only adding a simple sludge mixing tank. However, this technology is still in its early stage, and full-scale tests are required in the future. D
DOI: 10.1021/acssuschemeng.7b02605 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
