Linking organic matter removal and biogas yield in the environmental profile of innovative wastewater treatment technologies
Introduction
In general, current wastewater treatment plants (WWTPs) meet environmental requirements in terms of organic matter, nitrogen and phosphorus removal. However, it is becoming increasingly evident that wastewater technologies must address more complex challenges such as the safe disposal of emerging contaminants such as recalcitrant compounds and pathogens, as well as efficient operation with less resource consumption (Barbosa et al., 2016; Gu et al., 2018). It is widely known that one of the hotspot in wastewater treatment is the energy consumption in aeration for the biological process (Gikas, 2017). Conventional nitrification-denitrification is carried out under aerobic, anaerobic and anoxic conditions. In the first stage (nitrification), energy consumption can vary between 0.3 kWh/m3 to 0.6 kWh/m3 (Wan et al., 2016), while in the denitrification, an external organic matter (OM) source may be necessary, which entails higher operational costs (Jiang et al., 2019). Additionally, the sludge has low methanisation potential because only 30–50% of volatile solids are converted into biogas (Cao and Pawłowski, 2013).
Currently, many efforts have been made to explore new technologies aiming to follow the principles of circular economy. In this framework, the Anammox process, in which ammonium is directly converted together with nitrite in dinitrogen gas, was a significant advance in wastewater treatment. Energy in aeration can be reduced, an external source of OM is not necessary, which finally renders into low sludge generation (Morales et al., 2015). The Anammox process has proven its efficiency in the treatment of effluents from the anaerobic digestion process (Lotti et al., 2019; Vázquez-Padín et al., 2014). However, this process is currently being developed and applied in the mainstream (Gu et al., 2018; Yang et al., 2018), with different modifications: integrated fixed film activated sludge (IFAS) (Malovanyy et al., 2015a), ELAN® (from the Spanish autotrophic nitrogen removal) (Pedrouso et al., 2018) or partial nitrification process (SHARON) (Van Dongen et al., 2001). These technologies encounter limitations in the case of streams with a high percentage of solids or a huge C/N ratio (Xu et al., 2015).
The problems associated with the C/N ratio and the percentage of solids can be solved with the application of a new treatment strategy that consists in recovering the OM in primary treatment and removing nitrogen with a partial nitrification-annamox process. Primary sludge is more biodegradable than secondary, so the methanisation factor can increased. This implies greater energy production of biogas that is transformed into electricity and heat, making WWTPs more self-sufficient in terms of energy (Pérez-Elvira and Fernández-Polanco, 2012).
There are several technologies such as high rate activated sludge (HRAS), rotating belt filter (RBF), chemical enhanced primary treatment (CEPT) which have been implemented as primary treatments (Gu et al., 2018; Rahman et al., 2019; Ruiken et al., 2013) and others more widespread such as upflow anaerobic sludge blanket (UASB) (Malovanyy et al., 2015b). The choice of one or another technology and its combination depend on several factors. For example, RBF can be combined with technologies such as HRAS and CEPT but not with the Anammox process due to the high solids content (Ruiken et al., 2012).
Sludge management is another decisive element in the operation of WWTPs according to the circular economy approach. Although the most applied methods are incineration and land application (Kelessidis and Stasinakis, 2012; Tomei et al., 2016), other options such as gasification, thermal process or supercritical water oxidation have been explored such as sludge disposal alternatives (Garrido-Baserba et al., 2015). Its use as an additive in cement production (Bertanza et al., 2016) or its conversion into granular activated carbon or bio-oil are also of interest (Kacprzak et al., 2017; Mu’Azu et al., 2019). Among the different alternatives, incineration or gasification have higher costs related to electricity consumption and, in general, a greater impact on the climate change category (Judex et al., 2012; Murakami et al., 2009). Several authors demonstrated that the application of sludge in agriculture is a low-cost valorisation option that can provide nutrients to the soil (Pradel and Aissani, 2019; Raheem et al., 2018). However, heavy metals and other uncontrolled harmful substances may cause surface and groundwater pollution problems. This implies that their concentrations must be monitored to ensure that the discharge of heavy metals present in the sludge complies legislation requirements; otherwise, it will be necessary to implement treatment technologies to handle these streams safely (Cieślik et al., 2015).
With regard to these new systems, several questions arise: Are these systems more environmentally friendly and economical than conventional systems? Is it possible to improve efficiency through energy production? In order to assess the sustainability of these wastewater treatment technologies, the life cycle assessment (LCA) methodology is considered as a good choice for calculating and quantifying the environmental impacts throughout the entire cycle of a product or process (ISO 14040, 2006). This methodology has been widely used for evaluating and comparing the environmental profile of different technologies or wastewater treatment schemes (Bertanza et al., 2017; Rashidi et al., 2018). In addition, environmental methodology was combined with economic impacts to look for more efficient and economic options (Garrido-Baserba et al., 2014; Piao et al., 2016). Furthermore, the multiple-criteria decision analysis can support the evaluation of different wastewater treatment alternatives (Achillas et al., 2013). In this sense, a general approach of environmental, economic, social and technological aspects can be carried out for different wastewater configurations (Bertanza et al., 2018). In this context, the main goal of this manuscript is to evaluate from an environmental and economic perspective the incorporation of the strategy to treat wastewater through OM recovered followed by a partial nitrification-Anammox unit in three different wastewater treatment configurations and verify if these schemes are more efficient than the conventional scheme.
Section snippets
Materials and methods
In this study, the LCA methodology was implemented as a quantitative tool to assess the negative effects that a process or product create during its life cycle (ISO 14040, 2006). The procedure is divided in four main stages: i) goal and scope definition; ii) life cycle inventory (LCI); iii) life cycle impact assessment (LCIA) and iv) interpretation. In addition, environmental categories will be transformed into environmental costs (De Bruyn et al., 2018) which were added to the total plant
Environmental comparison between the different wastewater treatment schemes
The environmental results are presented as a comparison between the different scenarios considered (Table 3). The best scenarios are Scenario 1 (UASB + IFAS configuration) followed by Scenario 2 (HRAS + IFAS configuration) because there is more electricity production than in the others (Tables S3 and S4 of Supplementary Material). In addition, there is not chemical consumption in these wastewater units (primary and secondary technologies). However, Scenario 3 (RBF + CEPT + IFAS scheme), which
Improving wastewater treatment efficiency
The main objective of the new wastewater configurations is to achieve more efficient systems, mainly through the recovery of OM to enhance the biogas yield. In this context several questions should be answered: i) do new schemes improve efficiency with respect to conventional systems? and ii) how does energy recovery affect to the environmental profile? Would these new configurations make sense if energy was not recovered?
For studying the efficiency of these new configurations, and indicator
Conclusions
In this study, a new treatment strategy focused on organic matter recovery was evaluated from an environmental and economic perspective. Three schemes based on this strategy: (i) UASB + IFAS; (ii) HRAS + IFAS, and, (iii) RBF + CEPT + IFAS) were compared with a conventional treatment scenario (PC + CAS). The UASB and HRAS followed by an IFAS unit had a better environmental profile than the conventional technology. In addition, energy consumption in aeration can be reduced by 13% when IFAS is
CRediT authorship contribution statement
A. Arias: Conceptualization, Investigation, Writing - original draft. G. Feijoo: Conceptualization, Investigation, Writing - review & editing. M.T. Moreira: Conceptualization, Investigation, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was supported by the UE projects: Pioneer_STP (PCIN-2015-22 (MINECO)/ID199 (WaterJPI). The authors belong to the Galician Competitive Research Group GRC ED431C 2017/29 and to the CRETUS Strategic Partnership (ED431E 2018/01). All these programmes are co-funded by FEDER (EU).
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