Current wastewater treatment processes, especially, aerated systems are energy intensive. In developed countries including the USA and the UK, water and wastewater treatment plants and their infrastructure operations together contribute up to 3–5% of national electrical energy demands. This portion can be anywhere between 25 and 40% for small communities which is quite alarming [1]. In general, energy costs account for 5–30% of the total operating costs of water and wastewater utilities worldwide [2]. With increasing population, elevating living standards and escalating environmental pollution issues, it is crucial to consider alternative energy sources and treatment processes to provide sustainable wastewater treatment as this is the most fundamental step towards environmental protection.

A secondary wastewater treatment process includes a primary treatment step followed by a biological treatment process. Primary treatment involves removal of suspended solids and other non-biodegradable contaminants by physical separation in a sedimentation unit and or chemical adjustment or precipitation. A biological process is employed to remove the dissolved and particulate biodegradable organic matter through either suspended medium or attached growth processes. Sedimentation, filtration and disinfection follow the biological treatment process before the effluents are returned to the surface waters. The entire process is called a secondary treatment technique which is a required level of wastewater treatment around the world [3]. Various steps involving energy consumption in wastewater treatment are shown in Figure 1. It should be noted that aeration step in wastewater treatment requires large amounts of energy which also adds to operational costs. Other significant energy-demanding steps include pumping, anaerobic digestion and lighting and building heating and cooling operations. The nature of the energy requirements (electrical and thermal types) provides opportunities for implementation of different renewable energy sources such as solar, photovoltaic, and geothermal and wind energy sources. Figure 1 shows various opportunities for utilization of renewable energy sources in wastewater treatment operations.

Solar thermal energy has been evaluated as a viable energy source for heating requirements in anaerobic digestion process treating waste sources from municipal wastewater, agricultural wastes and other food and industrial operations [4][5][6]. Geothermal energy source was also considered for anaerobic digestion process heating applications [7]. Solar driven photovoltaic energy was also implemented in water and wastewater pumping operations around the world [8]. Solar energy can be utilized to generate both thermal and electrical energy suitable for various applications by employing concentrated solar panels or photovoltaic thermal collectors [9]. It is also important to note that the environmental, energy and economic payback periods for solar energy applications and technologies are very short-term (less than 2.5 years) and generated or expected benefits are long-term (up to 25 years) [10]. Application of wind turbines for water pumping operations was also evaluated [11].

Wastewater contains a multitude of components that can be used to derive energy which is several times higher than the energy requited for its treatment [1]. Biomass or excess sludge produced from wastewater treatment can be purposely utilized to produce different forms of energy [2]. While wastewater treatment can be energy intensive, several options are available for making this process energy-yielding rather than an energy-consuming one. As shown in Figure 2, the embedded energy in wastewater can be recovered in one or more of the following forms [1][3] [12][13]:

• Electrical energy or heat from the utilization of biogas from anaerobic digester which consists mainly of methane and carbon dioxide.
• Electrical energy and heat from thermal conversion of biomass (bio-solids).
• Heating or cooling energy using plant influent or effluent as heat source or sink.
• Biofuel generation from microalgae grown in wastewater.
• Biofuel, biochemical and other valuable metals via bioelectrochemical systems.

Wastewater sanitation continues to be the most fundamental need for sustainable development. The inevitable energy consumption and excess sludge production issues should be dealt together by employing concepts of sustainable production and operations. Co-digestion with other organic wastes and integrated microalgae systems may result in sustainable biofuel and biogas production.

Techno-economics should be considered in detail prior to utilization of renewable energy in wastewater treatment. Sustainable application of these renewable systems is influenced by various factors such as location, size, treatment capacity, available renewable energy sources and support from local communities and government agencies. If these issues can be resolved, renewable energy application may transform wastewater treatment into a sustainable endeavor.

Keywords: Biofuels, Renewable energy, Sustainability, Solar, Wastewater

1. Gude VG. Energy and water autarky of wastewater treatment and power generation systems. Renewable and sustainable energy reviews 2015;45:52–68.   [CrossRef]    
2. Chae KJ, Kang J. Estimating the energy independence of a municipal wastewater treatment plant incorporating green energy resources. Energy Conversion and Management 2013 Nov 30;75:664–72.   [CrossRef]    
3. Gude VG. Energy positive wastewater treatment and sludge management. Edorium Journal of Waste Management 2015 Jul 16;1:10–5.    
4. El-Mashad HM, van Loon WK, Zeeman G. A model of solar energy utilisation in the anaerobic digestion of cattle manure. Biosystems Engineering 2003 Feb 28;84(2):231–8.   [CrossRef]    
5. Yiannopoulos AC, Manariotis ID, Chrysikopoulos CV. Assessment of the effectiveness of a solar system heating an anaerobic bioreactor. Water, Air, & Soil Pollution 2012 May 1;223(4):1443–54.   [CrossRef]    
6. Ren Z, Chen Z, Hou LA, Wang W, Xiong K, Xiao X, Zhang W. Design investigation of a solar energy heating system for anaerobic sewage treatment. Energy Procedia 2012 Jan 1;14:255–9.   [CrossRef]    
7. Diamantis V, Tataki V, Eftaxias A, Iliadis G, Aivasidis A. Geothermal energy valorisation for enhanced biogas production from agro-industrial residues. Environmental Processes 2016 Oct 1;3(1):81–90.   [CrossRef]    
8. Meah K, Ula S, Barrett S. Solar photovoltaic water pumping—opportunities and challenges. Renewable and Sustainable Energy Reviews 2008 May 31;12(4):1162–75.   [CrossRef]    
9. Gude VG, Nirmalakhandan N, Deng S. Integrated PV-thermal system for desalination and power production. Desalination and Water Treatment 2011 Dec 1;36(1–3):129–40.   [CrossRef]    
10. Gude VG, Nirmalakhandan N, Deng S, Maganti A. Low temperature desalination using solar collectors augmented by thermal energy storage. Applied Energy 2012 Mar 31;91(1):466–74.   [CrossRef]    
11. Ramos JS, Ramos HM. Sustainable application of renewable sources in water pumping systems: Optimized energy system configuration. Energy Policy 2009 Feb 28;37(2):633–43.   [CrossRef]    
12. Shilton AN, Mara DD, Craggs R, Powell N. Solar-powered aeration and disinfection, anaerobic co-digestion, biological CO2 scrubbing and biofuel production: the energy and carbon management opportunities of waste stabilisation ponds. Water Sci Technol 2008;58(1):253–8.   [CrossRef]   [Pubmed]    
13. Ghoneim WA, Helal AA, Wahab MA. Renewable energy resources and recovery opportunities in wastewater treatment plants. 2016 3rd International Conference on Renewable Energies for Developing Countries. (REDEC). p. 1–8.