Enhancement of sewage sludge thickening and energy self-sufficiency with advanced process control tools in a full-scale wastewater treatment plant
Graphical abstract
Introduction
Wastewater treatment is evolving to create a new resource recovery paradigm, transforming wastewater treatment plants (WWTPs) into water resource recovery facilities (WRRFs) (Larriba et al., 2020). This change has motivated the development of a wide range of new technologies that can deliver high-quality effluents with lower energy demand and operational costs, thereby generating rapid progress toward becoming WRRFs, which are sustainable and economically feasible (Soares, 2020). However, most existing WWTPs are based on the conventional activated sludge treatment process. In such traditional WWTP configurations, aeration requirements for nutrient removal (NuR) account for 45–75% of the total energy demand of the plant (Rosso et al., 2008). Moreover, during water treatment, large volumes of primary sludge (PS) and secondary or waste activated sludge (WAS) are generated, which are collectively referred to as sewage sludge (SS). In the sludge line of WWTPs, PS and WAS are usually independently thickened to an average total solids (TS) content in the range of 2–8%, and subsequently mixed. In WWTPs of a certain size, the mixed SS is then sent to anaerobic digestion (AD) reactors to produce biogas (energy), reduce the volatile solids (VS) content, and stabilise the sludge from a biological perspective (Borzooei et al., 2019). Finally, the thickened or digested sludge is de-watered up to 15–30% of the TS, and appropriately managed for disposal in landfills, incineration, composting, or applications in agriculture. It is estimated that the treatment and management of sewage sludge account for ∼50% of the WWTP's operational costs (Lorenzo-Toja et al., 2016). Fortunately, when available, the biogas produced from AD can be recovered and used to supply 40–60% of the on-site energy needs (Bezirgiannidis et al., 2020).
With the aim of enhancing the sustainability of the water cycle, WWTPs are attempting to achieve energy self-sufficiency by reducing their energy consumption and maximising on-site power generation (Macintosh et al., 2019a). The optimization of AD plays a key role in achieving this ambitious objective (Jenicek et al., 2013). Several technologies can be applied to improve AD performance, such as SS pre-treatments (Carrère et al., 2010; Romero-Güiza et al., 2019), anaerobic co-digestion (Mata-Alvarez et al., 2014), use of additives (Liu et al., 2021; Romero-Güiza et al., 2016), or two-step AD (Romero-Güiza et al., 2021). Although co-digestion is a feasible alternative to increase the energy self-sufficiency of WWTPs (Macintosh et al., 2019b), WWTP-specific singularities or local regulation restrictions sometimes exclude the option of co-digestion. Moreover, alternatives such as pre-treatments or two-step AD may also consume significant amounts of energy or require large capital expenses (CAPEX), which can make them economically or energetically unfavourable (Çelebi et al., 2021).
Several studies have suggested that enhancing sludge thickening plays an important role in achieving energy self-sufficiency (Ruffino et al., 2014). Jenicek et al. (2013) showed that improvements in PS and WAS thickening significantly increased biogas production to 12.5 m3 per population equivalent per year (p.eq y−1). Similarly, Borzooei et al. (2019) showed that the introduction of an optimised thickening stage has a positive impact on the energy and greenhouse gas balance of WWTPs. A high TS content (above 4%) in the thickened sludge could be sufficient for achieving a positive energy balance in two-step AD processes (Bolzonella et al., 2007). Moreover, Ge et al. (2011) showed that 4% of TS in the feed of anaerobic digesters is enough to fully offset heat requirements with the heat produced in a co-generation engine (focused on electricity production). Furthermore, these requirements were largely exceeded if the feed concentration increased to 6% TS.
Consequently, many efforts have been made to improve sludge thickening, especially in WAS. Some examples of these developments/technologies include (i) modified hydro-cyclones that can help to increase sludge thickening (but are still in the early stages of development, according to Senfter et al., 2021) and (ii) membrane technologies, such as forward osmosis, which can enhance WAS thickening up to 50 g L−1 (Sun et al., 2019; Yi et al., 2021) or (iii) high-rate dissolved air flotation systems (Cagnetta et al., 2019). However, all of these alternatives have some drawbacks, such as large investment for implementation.
Nevertheless, previous attempts at improving PS thickening have mostly focused on chemical precipitation, such as acidification (Maraschin et al., 2020b), use of aluminium salts, attapulgite, and polyelectrolytes (Maraschin et al., 2020a). For instance, Bezirgiannidis et al. (2020) studied the effects of a chemically enhanced primary treatment in terms of organic carbon removal and biogas production. Their results showed that the chemically enhanced primary treatment minimized oxygen consumption and increased biogas production, thereby achieving annual energy gains of 1.3 GWh for a WWTP of 50,000 p.eq. However, these new applications need to be assessed under full-scale conditions, in terms of economic feasibility and the effect of chemicals in the resulting sludge, as they can affect the final sludge disposal alternatives (Kacprzak et al., 2017).
Advanced process control (APC) is widely used in water lines to control aeration (Rieger et al., 2012), and for nutrient removal (Regmi et al., 2015; Vanrolleghem et al., 2003) or energy optimisation (Palatsi et al., 2021). The main references for process control used in the sludge line are aimed at controlling AD through the development of predictive mathematical models (Draa et al., 2019; Kim et al., 2014; Micolucci et al., 2014), the control of VFA concentrations in reactors (García-Sandoval et al., 2016), or the development of soft sensors (Oppong et al., 2012). However, there are limited literature references on the application of APC in optimising thickening. In previous studies on optimising the water lines of the WWTP under study, APC tools emerged as an affordable alternative to upgrading existing WWTPs (Palatsi et al., 2021). Considering each APC tool as an individual module within the overall control of the WWTP, it is easy to gradually incorporate new control tools (into the sludge line), as well as those interrelated to them (with the water line). Consequently, APC strategies may be a simple and cost-effective solution that can be applied to optimise existing sludge lines, without the need for additional chemical consumption or large CAPEX, which is associated with the implementation of other advanced technologies/equipment.
In this study, we provide practical knowledge of different APC strategies applied in a full-scale WWTP to improve the performance of the sludge line and to increase the energy self-sufficiency and sustainability of the WWTP. APC tools are used to (i) enhance PS and WAS thickening, (ii) improve AD performance, and (iii) reduce chemical consumption in the sludge line. A detailed discussion of the effect of the APC strategies on each process step is provided. Furthermore, a final energy and economic study was conducted to evaluate the contribution of the APC to the energy self-sufficiency of the WWTP, and the economic impact of its implementation.
Section snippets
Wastewater plant description (The sludge line)
The municipal WWTP under study is located in Lleida (northeast of Spain, UTM 489853 m E, 4694716 m N). The current treatment capacity of the WWTP (activated sludge process) is 160,000 p.eq. A complete WWTP description of the water line and the established discharge limits set by the regional/national authority can be found in Palatsi et al. (2021). Regarding the sludge line, the PS is thickened in two thickening settlers (440 m3), while the WAS is thickened using a flotation system composed of
Sludge line performance
For each of the operation phases at the WWTP, the mean values of the process parameters during the entire operation period, with their standard deviation and statistical analysis, were compared. These means were obtained daily and averaged monthly for 2 years for P0 and P2, and 1 year for P1 and P3. This was done to evaluate the effect of the different APC strategies implemented on the WWTP (Table 2). The relatively high standard deviation for some of the inflow parameters in the water line is
Conclusions
The implementation of APC tools represents an easy and cost-effective solution for improving the sludge lines of existing WWTP. With the strategies implemented herein, a 43–51% increase in the WWTP's energy self-sufficiency was achieved. A significant enhancement (9.5%) in the thickening of the sewage sludge was achieved, together with an increase in the methane yield of digesters (10%), a reduction in the consumption of antifoam and iron chloride (53% and 75%, respectively), and a reduction
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.
Acknowledgments
The authors are grateful to the dedication and work of Jordi Lopez on WWTP supervision and maintenance tasks, Gloria Prunera for analytical support, and previous WWTP Managers & Process Managers (J. Millà, J. Giribet, D. Gonzalez, D. Blanch, M. Isidro, and O. Domingo) for their work on operation and control. Acknowledgements are also due to Voltec (L. Perez) and the Createch360 teams (F. Ripoll) involved in the design, implementation, and adjustment of the APC tools. The authors would also like
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