Carbon-Neutrality in Wastewater Treatment Plants: Advanced Technologies for Efficient Operation and Energy/Resource Recovery

1. Introduction

Recently, “carbon neutrality”, the state of net-zero carbon dioxide emission, has been one of the most frequently used terms in the sustainable development of environmental and energy industries. Indeed, many efforts have been made to advance novel technologies for resource recovery, material synthesis, and energy saving from various municipal and industrial wastes [1,2,3,4,5,6,7,8,9]. In particular, the development of energy-efficient water technologies and resource recovery from wastewater has become a non-trivial problem as (i) almost 4% of the global non-CO₂ greenhouse gases (GHGs) are emitted from wastewater treatment plants (WWTPs) [10] and (ii) wastewater contains various organic/inorganic resources and even heat energy [11]. Therefore, many efforts have been put into improving energy self-sufficiency in sustainable WWTPs and developing sustainable processes for the recovery of many useful resources from WWTPs such as chemicals, nutrients, energy, and even water itself.

In the aspect of theoretical studies, Borzooei et al., suggested that the introduction of an advanced thickening stage and sludge pre-treatment can positively influence the energy-saving and GHGs emission from WWTPs [12]. Hao et al. investigated the environmental impact of resource recovery from WWPTs by life cycle assessment (LCA), which concluded that (i) water reuse via effluent recycling is not sufficient for net-zero impact, and (ii) thermal energy recovery can play a significant role on environmental impact while phosphorus recovery limited the contribution [13]. Another LCA study concluded that (i) ozonation has the lowest life cycle costs compared to other options such as photo-Fenton and granular activated carbon and (ii) pyrolysis has the best profit compared to those of anaerobic digestion, wet air oxidation, and composting [14].

In the aspect of experimental studies, a pyrolysis product, known as biochar, from various organic wastes in agricultural activities and sludge of WWTPs, has been also considered as a promising up-cycled material for the treatment of wastewater. In particular, the efficient removal of organic and heavy-metal contaminants [15,16] can reduce the GHGs emission from agricultural and environmental industries and produce clean water economically. Microbial fuel cells (MFCs) have also been extensively studied in the field of wastewater treatment as they can reduce the sludge disposal cost (accounting for almost 50% cost of WWTPs) [17]. Moreover, the additional possibility not only for the production of bioelectricity, but also the recovery of high-value products such as hydrogen (H₂), methane (CH₄), and hydrogen peroxide (H₂O₂), has brought more attention to the WWTPs related researchers [18].

As discussed above, the sustainable development of WWTPs has become a hot topic worldwide. Thus, this Special Issue aimed to collect the state-of-the-art for the advances in WWTPs fields. Indeed, the papers published in this Special Issue cover a broad range of important issues; biochar application for wastewater treatment [19,20,21], LCA and cost optimization studies of WWTPs [22,23,24], enhanced anaerobic digestion [25], MFC of swine wastewater [26,27], and removal of radioactive and pharmaceutical pollutants by chemical processes [28,29]. In the next section, we provide a brief review of the papers published, roughly classifying them according to the thematic areas mentioned above.

2. Short Review of the Contributions in This Issue

Many papers can be grouped under the broad heading of Science and Technology.

The first topic is the recycling of carbon wastes for wastewater treatment. Many novel biochars were introduced in this Special Issue for the wastewater treatment contaminated by various aqueous pollutants. Kim et al. synthesized magnetic biochar from steel slag waste and pine sawdust (MSSB) for efficient NH₄-N removal in an aqueous phase. They found that NH₄-N was adsorbed on the MSSB surface via co-precipitation of Mg(NH₄)PO₄ from magnesium and phosphate in biochar suspension [21]. The experimental results showed that the adsorption capacities of NH₄-N can be influenced by many parameters such as NH₄-N concentration, the ratio of steel slag to pine sawdust, and pyrolysis temperature. Another biochar was prepared by Lee et al. by using ground coffee residue with NaOH activation (GCRB-N). The BET area of GCRB-N was almost 100 times higher than that of GCRB without NaOH treatment [20], which resulted in the enhanced removal of three herbicides (i.e., Alachlor, Diuron, and Simazine) significantly. Zhao et al. succeeded the synthesis of optimized biochar derived from apple tree branches at different pyrolysis temperatures [19]. The optimized biochar prepared at 500 °C showed the efficient sorption capacity toward aqueous Cr(II) and Zn(II), which was proceeded via surface precipitation, ion exchange, and cation–pi interaction.

The second topic is the optimization of energy saving and cost-effectiveness in WWTP. Szulc et al. showed the overall environmental impact of a WWTP in Poland by analyzing the LCA from the database in 2019 [22]. They concluded that energy components (e.g., WWTP process demand and electricity production) were the key factors to determine the sum of environmental impact. Karolinczak et al. investigated the cost analysis of the septage pre-treatment in subsurface vertical flow constructed wetland (SS-VF) prior to the municipal treatment plant [23]. The high cost-effectiveness of septage pre-treatment process in SS-VF (particularly in the rural area) compared to the conventional WWTP resulted from the low construction and operation costs coming from the reduction in the electricity consumption in biological section operation and lower cost of sludge management. Similarly, Gretzschel et al. concluded that implementing electrolysis operated with regenerative energy in combination with micropollutant removal using ozonation and activated carbon filter is a reasonable and sustainable option for climate and water protection [24]. Damtie et al. revealed that co-digestion of microalgae and primary sludge in thermophilic aerobic conditions can increase the methane yield up to 36%, while the anaerobic pretreatment did not show any synergistic effect on the co-digestion process [25]. Therefore, the authors suggested the careful optimization of operating conditions during the production of methane from biological pretreatment of co-digestion.

The third topic is the MFC study. Ni et al. investigated the microbial community of anodic biofilm during the microbial fuel cells (MFCs) operation with swine wastewater [26,27]. They prepared three different modified anode carbon clothes and concluded that acid thermal modification can increase the power density up to 350% rather than that without modification [26]. The modified anode also showed a significant impact on the microbial community, which was beneficial to improve the performance of MFCs in this study. Furthermore, they found that different concentrations of swine wastewater resulted in changes in the output voltage of MFC and COD removal rate, and microbial community diversity [27].

The last topic is the efficient chemical treatments for radioactive pollutant (i.e., cesium, Cs) and pharmaceutical contaminated wastewaters. Kim et al., demonstrated that the desorbed Cs from hydrobiotite could be treated by a sequential treatment method, i.e., pre-chemical precipitation (addition of Ca(OH)₂) of non-radioactive cations and heavy metals, then post-Cs removal by sodium tetraphenylborate [29]. Lee et al. showed the enhanced oxidation of 17α-ethinylestradiol, ibuprofen, and atenolol by microbubble ozonation process [28]. This was due to the higher specific area of ozone microbubbles than that of millibubbles, which could improve the mass transfer at the gas–water interface.

3. Conclusions

This Special Issue addresses the state-of-the-art findings in the field of WWTPs for sustainable development of WWTPs and implementation of carbon-neutrality. In particular, the contributions include the development of efficient and eco-friendly materials, assessment of cost and energy, energy-saving anaerobic digestion, and efficient chemical treatment. These papers can advance our fundamental knowledge of efficient management and operation of WWTPs and provide new insight to step forward for the accomplishment of carbon-neutrality of WWTPs in near future.