Research articleTowards energy positive wastewater treatment plants
Graphical abstract
Introduction
It has been estimated that over 20% of the total energy consumption for public utilities by the municipalities is for the operation of wastewater treatment plants (Means, 2004). Conventional treatment of municipal wastewater is indeed an energy intensive process, primarily, due to the need for supplying large quantities of air into the biological process (Shi, 2011, Svedal and Kroiss, 2011). It has been estimated that, approximately 30–35% of total cost of service in North American wastewater treatment facilities is for electric energy (WERF, 2010a). With the increasing cost of electricity, it is wise to explore alternative wastewater treatment process with low energy requirement, and further to explore the potential to utilize the chemical energy content of wastewater for power production. An additional benefit, apart from the reduced energy consumption, will be the reduction of carbon footprint (Tchobanoglous et al., 2009).
Activated sludge process is the norm for municipal wastewater treatment, especially for centralized facilities. However, activated sludge process is energy intensive, as it requires large quantities of air (far beyond the stoichiometric amount) to be pumped into the biological tank. In addition, primary and secondary sludge processing requires the utilization of significant amount of electric energy for sludge pumping, processing and disposal (Tchobanoglous et al., 2003). In middle and large wastewater treatment facilities, anaerobic digestion of primary and secondary sludge is used for partial conversion (up to 60%) of organic carbon into methane and carbon dioxide (Tchobanoglous et al., 2003). The produced anaerobically digested sludge is partially stable; however, further processing is required for safe disposal. Thermal processes have also been exploited for biosolids to energy. However, combustion can be energy positive only if the water content of sludge is below about 30%, as in the opposite case more energy is required for incineration than it is produced through combustion (WEF, 1992). On the other hand, gasification appears advantageous against incineration, and thus it is a preferable process, despite the higher degree of process complexity (Fericelli, 2011). Energy-wise, gasification has been found to have a higher potential for net electric energy production, compared to anaerobic digestion (Gikas, 2014).
Relatively small reduction in energy requirements may be achieved lately by selecting more efficient aerators, by employing more efficient aeration control processes or by improving the sludge management processes (WERF, 2009, Muller et al., 2006). Recently, an increasing number of publications has focused on low energy requirements or even on self sustainable wastewater treatment processes (McCarty et al., 2011, Gude, 2015, Chae and Kang, 2013, Nowak et al., 2011, Wett et al., 2007), most of which involve either complicated technologies, either involve the use of external renewable sources, either the proposed solution involves high capex or opex costs. However, to significantly reduce the energy requirements, the wastewater treatment concept should be viewed from a fundamentally different angle. For example, the role of energy-intensive biological process should be marginalized, while the role of energy-efficient physicochemical processes should be utilized to a maximum, where applicable. If the above is combined with the maximization of the utilization of the chemical energy contained in the wastewater, then an energy positive wastewater treatment process can be possible. It is important though, that the proposed process should have comparable or lower cost with the one of activated sludge system.
The present manuscript presents a novel, energy efficient, wastewater treatment process, with significantly reduced energy requirements (compared to conventional activated sludge process). The process consists of a combination of physicochemical processes for upfront solids removal, along with downstream low energy requirement biological filtration processes for complete wastewater treatment. Moreover, the manuscript assesses the use of the produced biosolids for electric energy production, either by anaerobic digestion or by gasification. Finally, mass and energy balances are employed to show that a positive energy wastewater treatment plant can be possible. The various sub-processes presented in the present manuscript have been investigated in pilot facilities.
Section snippets
Energy content of wastewater and biosolids
As discussed earlier, the wastewater treatment concept should be revolutionized to operate with low energy requirement, and further to recover as much energy as possible from the wastewater constituents. Typical approaches towards energy conservation for wastewater treatment do not involve fundamental changes in the wastewater treatment process, but are rather focused on improving the energy yield of the existing processes (Mo and Zhang, 2010). The present manuscript proposes a more radical
Novel wastewater treatment process
The proposed process (only the wastewater treatment part) is outlined and compared with the conventional activated sludge process in Fig. 2. The process has been investigated in a large pilot facility, with hydraulic capacity of 380 m3/d (Fig. 3), and the experimental findings have been presented in previous studies (Franchi et al., 2012, Zarikas and Gikas, 2014).
The proposed process is based on a number of selected filtration processes, with minimal usage of biological processes. Biosolids
Overall wastewater treatment process-mass and energy balances
A schematic diagram of the novel, energy positive wastewater treatment plant is shown in Fig. 7. In brief, the process consists of bar screen, microsieving, sand or cloth media filtration, denitrification coupled with trickling filtration, tertiary filtration (sand or cloth media) and disinfection (UV or chlorination). The backwash flow of the filters is processed through a lamella clarifier and the produced sludge is dewatered along with the fine sievings in an auger press. The biosolids are
Conclusions
A novel wastewater treatment process has been proposed, based on upfront solids removal, using physicochemical and biological filtration processes, and electric energy production through biosolids gasification. The energy requirements for complete municipal wastewater treatment (including disinfection by chlorination), per volume of inlet raw wastewater, have been calculated at 0.057 kWh/m3, (or 0.087 kWh/m3 if UV disinfection will be selected), about 85% below of the energy requirements of
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