1 Improving Wastewater Management Using Free Nitrous Acid (FNA)
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Improving wastewater management using free nitrous acid (FNA) Haoran Duana,b, Shuhong Gaoc, Xuan Lia, Nur Hafizah Ab Hamidb, Guangming Jiangd, Min Zhenga, Xue Baia, Philip L. Bonda, Xuanyu Lua,b, Mariella M. Chisletta, Shihu Hua, Liu Yeb, Zhiguo Yuana,* aAdvanced Water Management Centre, The University of Queensland, St Lucia, QLD 4072, Australia bSchool of Chemical Engineering, The University of Queensland, St. Lucia, QLD 4072, Australia cInstitute for Environmental Genomics, Department of Microbiology and Plant Biology, University of Oklahoma , Norman , Oklahoma 73019 , United States. dSchool of Civil, Mining and Environmental Engineering, University of Wollongong, Wollongong, NSW 2522, Australia *Corresponding author: Tel.: +61 7 3365 4726; fax: +61 7 3365 4726; E-mail address: [email protected] (Z. Yuan). 1 Table of content Abstract ...................................................................................................................................... 2 1. Introduction ........................................................................................................................ 4 2. Inhibitory and biocidal effects of FNA............................................................................... 6 2.1 The chemical property of FNA ................................................................................... 6 2.2 Inhibitory and biocidal effects of FNA on microbes ................................................... 6 2.3 Proposed inhibitory and bactericidal mechanisms of FNA on microbes .................. 11 3. The use of FNA in sewer system ...................................................................................... 14 4. The use of FNA in wastewater treatment system ............................................................. 19 4.1 Achieving energy and carbon efficient nitrogen removal in mainstream wastewater treatment .............................................................................................................................. 19 4.2 Enhancing sludge reduction and energy recovery in sludge management ................ 25 4.3 Production of FNA from wastewater ........................................................................ 29 4.4 The FNA-based applications are feasible and economically viable in wastewater treatment systems ................................................................................................................. 30 5. The use of FNA in membrane and algae systems ............................................................ 34 6. Knowledge gaps and opportunities ................................................................................... 36 6.1 Fundamental knowledge on the chemistry of FNA .................................................. 36 6.2 The impact of FNA sludge treatment on biological phosphorus removal ................ 37 6.3 Constraints of lab-scale studies call for pilot- and full-scale trials ........................... 38 6.4 Alternatives for acid addition .................................................................................... 38 6.5 Intrinsic limitations and opportunities....................................................................... 39 7. Conclusions ...................................................................................................................... 40 References ................................................................................................................................ 42 List of Abbreviations Abstract Free nitrous acid (FNA), the protonated form of nitrite, has historically been an unwanted substance in wastewater systems due to its inhibition on a wide range of microorganisms. However, in recent years, advanced understanding of FNA inhibitory and biocidal effects on microorganisms has led to the development of a series of FNA-based applications that improve wastewater management practices. FNA has been used in sewer systems to control sewer corrosion and odor; in wastewater treatment to achieve carbon and energy efficient nitrogen 2 removal; in sludge management to improve the sludge reduction and energy recovery; in membrane systems to address membrane fouling; and in wastewater algae systems to facilitate algae harvesting. This paper aims to comprehensively and critically review the current status of FNA-based applications in improving wastewater management. The underlying mechanisms of FNA inhibitory and biocidal effects are also reviewed and discussed. Knowledge gaps and current limitations of the FNA-based applications are identified; and perspectives on the development of FNA-based applications are discussed. We conclude that the FNA-based technologies have great potential for enhancing the performance of wastewater systems; however, further development and demonstration at larger scales are still required for their wider applications. Keywords: Free nitrous acid; sewer; wastewater treatment; sludge management; membrane fouling; algae; 3 1. Introduction Wastewater management, including wastewater collection, transportation, treatment and associated sludge management, is a most critical feature in modern societies, as it provides essential human health and environment protection. There has been extensive research and development in this area in recent times, however, there are still many challenges and improvements to be made. Sewer systems receive and transport wastewater via underground pipelines to wastewater treatment plants (WWTPs) for pollutant removal before environmental discharge. However, sewer systems are largely concrete constuctions and are under serious threat of corrosion, which costs billions of dollars annually and has been identified as a main cause of global sewer deterioration (Pikaar et al. 2014). In WWTPs, there are three generally accepted objectives guiding its operation: satisfactory pollutant removal, sludge reduction and resource & energy efficiency. A few obstacles in achieving the goals are: 1) conventional nitrogen removal process is carbon and energy demanding. Nitrification requires significant aeration energy and denitrification consumes a large amount of carbon source. In most conventional WWTPs, aeration takes up approximately 50-60% of all energy consumption (Gu et al. 2017); 2); waste activated sludge (WAS) is poorly degradable, resulting in high sludge handling cost, which represents 20–60% of the overall costs for wastewater treatment, and low biogas production (Foladori et al. 2010). In addition, there are many other challenges within wastewater management, such as membrane fouling impeding the development of membrane-based wastewater treatment technologies (Meng et al. 2009), lack of cost-effective algae harvesting approach that retards the application of algae technology for wastewater treatment (McGarry and Tongkasame 1971, Milledge et al. 2013). Extensive research endeavours have been taken to address those challenges in wastewater management. In recent years, Free Nitrous Acid (FNA), the protonated form of nitrite, has shown great potential in addressing some of the above-mentioned challenges. FNA was previously regarded mainly as an unfavourable inhibitor in wastewater management. As a compulsory intermediate of nitrification and denitrification, nitrite frequently accumulates during biological wastewater treatment. In nitrification, many environmental conditions, e.g., low dissolved oxygen (DO), short sludge retention time (SRT), high temperature, could lead to nitrite accumulation, that can be equivalent of up to 100% of the ammonium oxidized. The 4 conditions favouring the nitrite accumulation during nitrification were reviewed elsewhere (Cao et al. 2017, Ge et al. 2015, Ma et al. 2016, Peng and Zhu 2006). In denitrification, the presence of easily biodegradable organic carbon (e.g., volatile fatty acids), high C/N ratio and high pH could lead to nitrite accumulation (Cao et al. 2013, Ge et al. 2012, Glass and Silverstein 1998, Oh and Silverstein 1999). It is also reported that as high as 80% of the nitrite reduced from nitrate could accumulate stably in wastewater treatment (Du et al. 2015, Ma et al. 2017a). FNA has been a concern as is known to inhibit a wide range of microorganisms in wastewater management, including nitrogen removal bacteria (nitrifiers, denitrifier and anammox), phosphorus removal bacteria and methanogenic archaea, thus deteriorating the performance of WWTPs. Thus, much effort has been placed to understand and mitigate these inhibitions caused by FNA (Zhou et al. 2011). Readers can refer to Zhou et al. (2011) for an overview of FNA- caused inhibition of key microorganisms in wastewater treatment processes. Recently there has been improved knowledge of the inhibitory and biocidal effects of FNA, and in contrast to FNA being a problem, there are instances where FNA can be used beneficially to improve wastewater management. The use of FNA has attracted attention for various purposes, these include reducing sewer odor and corrosion, enhancing nitrogen removal in wastewater treatment, to aid sludge reduction, for fouling control in membrane- based systems and for improving algae harvesting. Despite the obvious enhancements to wastewater management, and the rapidly increasing number of publications, there has not been an attempt to critically review all the FNA-based technologies, which is required to support further technology development and applications. Therefore, this paper aims to review the current status of the uses of FNA in wastewater management and, to provide