Hybrid Energy Systems for Wastewater Treatment
As mentioned throughout this book, in recent years, the use of RES within the energy sector has been increasing, currently accounting for 13.7% of the global energy supply. At present, however, about 80% of all primary energy in the world is still derived from fossil fuels, namely, 31.7% from oil, 28.1% from coal, and 21.6% from natural gas . Although great amounts of energy can be obtained from fossil fuels, their use has a high environmental impact. Furthermore, climate change linked to fossil fuel combustion presents an important long-term impact on the availability and quality of water worldwide . Just like for desalination, several options are also available for hybrid RE systems for wastewater treatment. These hybrid systems help decarbonize wastewater industry.
The study by Del Moral and Petrakopoulou  presented the simulation and analysis of a 20 MW hybrid solar/biomass power plant combined with an advanced wastewater facility for a. most demanding areas of the Iberian Peninsula, the Spanish region of Andalusia. This plant aimed to provide the area with potable water and electricity. The plant used molten salts as the working fluid and included two thermal storage tanks of a total capacity of 600 MWh for operational autonomy of up to 4hours. The biomass (olive pomace) combustor was used as an auxiliary system, in the event of limited solar radiation or inadequate thermal energy in the storage tanks. The water treatment plant consisted of a direct potable reuse system with the objective of treating urban sewage and producing clean water for the selected region. The specific treatment train consisted of physical, biological, and chemical procedures including membrane bioreactors, ozonization, biological and granular activated carbon, RO, UV radiation, and chlorination.
The study found that the biomass combustor had the highest exergy destruction among all plant components due to the chemical reaction taking place there. Some heat exchangers, such as the feedwater preheaters, also displayed relatively low exer- getic efficiencies due to the mixing of streams with great temperature differences. A sensitivity analysis was conducted to determine the feasibility of the cogeneration of electricity and water in the area. With a capacity factor of 85% and an annual operation of 7,446 hours, the hybrid solar/biomass power plant generated 148.92 GWh. Exergetic analyses were realized for two extreme cases: exclusive use of the solar block and exclusive use of the biomass system. The full-load global exergetic efficiency of the power plant was found to be 15% when the solar part is used and 34% when the biomass support system was required. The net capital investment required for the construction and implementation of the plant was found to be 211,526,000 euro. This cost included both the hybrid power plant with thermal storage and the required advanced treatment technologies used to supply the region with potable water. The levelized cost of electricity of the combined plant was found to be 0.25 EUR/kWh, a value somewhat higher than existing solar tower power plants. Lastly, a sensitivity analysis of the water reclamation plant was conducted in order to evaluate the feasibility and viability of the project. The required selling price of the generated potable water was found to be 14.61 EUR/m. Accounting for present conditions, this cost is considered relatively high. However, foreseen future water limitations are expected to drive water prices up, especially in arid regions with intense lack of water resources. In essence, the combination of RE plants with water generation processes will provide a valuable and profitable alternative for local communities to create environmentally safe facilities with continuous and stable power supply and the additional sustainable management of fresh water resources. Bustamante  also evaluated a solar-biomass hybrid energy generation system for self-sustainable wastewater treatment.
Soni et al.  examined wind/solar power hybrid system for household waste- water treatment. In this study, an adaptable, affordable, and sustainable wastewater treatment system powered by wind/solar energy was proposed based on proven theory and technology. A household in India was singled out to illustrate the workings of the proposed system, where the wastewater was recirculated through a hybrid of water purifiers powered by solar/wind energy. The system demonstrated in this study was specifically designed for small-scale applications, i.e., for a single household. The solar still was divided into four stages. Partial vacuum is created inside the still so as to obtain boiling point temperatures of 70°C, 67°C, 62°C, and 50°C in the four stages. Dhanbad, India 23.79°N, 86.43°E, with an average solar intensity of 850 W/m2 for 6hours a day, was used for this study. A lumped parameter mathematical model was developed for this study. With an aperture area of 2.5 m2, the total amount of water distilled was found to be 43.3 kg/d. The system proposed is more efficient than existing systems as it was able to achieve efficiencies as high as 53%. The effect of wind speed on distillate output yield was also examined.
The wind-solar hybrid system presented by Soni et al.  was a self-sustaining system for pumping domestic wastewater from ground level using wind energy and making it reusable with the aid of solar energy. The system was designed in such a way so as to be installed in households without disturbing the existing plumbing systems. In most Indian households, the household water, excluding the sewage, collects at a common point so as to be disposed of to drains. The system presented in this study did not incorporate sewage waste, due to sanitary and environmental issues. This wastewater, excluding sewage, needed to be pumped from ground level to roof level where it was distilled in order to obtain pure water. Thus, there were two major tasks to be performed by the system: first, lifting water from ground level and, second, making it consumable by purifying it. Water was lifted from ground level with the help of a wind pump. If the wind did not blow for a couple of days, the storage tank may get empty. To counter this, a crank was attached to the wind driven rotor. When the wind did not blow, a hand-driven wheel achieved the rotary motion of the rotor.
The distillation system presented in the study was a combination of flat plate collector, tubular heat exchanger, and an evaporative condenser unit. The distiller consisted of multiple reservoirs created by a stacked array of distillation trays that acted as condensers for the tray below. Each stage consisted of an extruded cylindrical opening which was used to create the required partial vacuum. A valve was provided in each stage in order to regulate the pressure. There were multiple stages inside the distiller. Perfect sealing was maintained between the stages to prevent any vapor loss through the contact surfaces between the stages. Sunlight captured by the glass cover was concentrated on a black surface, heating the water in the topmost chamber and, thus, evaporating it. This vapor was then condensed to form droplets. Wastewater was fed into each stage from the tank. Each stage consisted of an evaporator and a condenser surface.
There was a pressure gradient inside the chamber. The pressure decreased in each stage in order to obtain a lower boiling point in upper chambers. Based on economic and productivity analysis, the study concluded that optimum number of stages to be four. These temperatures were used as the solar still worked best in this range [167-171]. A pump was used to create a partial vacuum inside the distillation chamber. To achieve high temperatures, heat was supplied to the wastewater in the lowest reservoir via a heat exchanger through which water was circulated from the solar collector, passed through the heat exchanger tubes, and then returned back to the solar collector again. Cold water got heated and moved to the surface so that it can evaporate faster. Vapor generated in the lower stage condenses on the bottom surface of the intermediate stage, thus giving its heat to the water in the intermediate stage. Vapor from the intermediate stage condenses on the upper stage, transferring its latent heat of condensation to the water in the upper stage. Water in the top-most reservoir, which was painted black to maximize radiation capture, was also heated directly by solar radiation. In the intermediate stages, heat transfer apart from radiation and convection occurred by evaporation and condensation, thus utilizing the latent heat of condensation and improving the system’s efficiency. Radiation and convection constitute minor energy transfer between the stages and, hence, were ignored.
Reddy et al.  showed that the gap between stages should be 10cm. Evacuated solar stills have already been demonstrated in the past but no one had used a pressure gradient along the stages to get a variable evaporating point of water in the stages. The suggested concept can improve the efficiency of the existing systems to a great extent. The only problem associated with the working of the still was that the plates need to be cleaned daily. To prevent algae and scaling on inner black surfaces, a still would be required to be dried completely once a week. Bleaching or chlorination can also be used to prevent algae formation. Ahmed et al.  experimentally investigated the multistage evacuated solar still. The productivity of this new system was found to be about threefold greater than the maximum productivity of the basin type solar still. While designing the above system, some keys points were taken into consideration: (a) simple, appropriate technology was adopted, as an overly complex system would be challenging due to its maintenance problems; (b) the system was flexible as per the demand of the people in a particular area, i.e., its cost varied as per the quantity of water required; (c) the system operated in a sustainable manner. This means being funded, owned, and operated by the individuals using the water supply; (d) the system was independent of any external power source. The basic principle used was reducing the air pressure in order to reduce the boiling point of water.
The water purification system presented above utilized two modes of RE, solar, and wind. Wind energy was used to drive a vacuum pump which reduced the air pressure inside the system. The number of stages was optimized to be four, as any further increase in the number of stages was not justifiable from an economic point of view. The vapor pressures maintained in the successive stages of the still were 31, 27, 20, and 18kPa. Constricting nozzles were used to connect the household drain with the recirculating loop and still. Solar energy was used to heat the water lying in the chambers of the still. Solar energy was transferred to the system from the bottom chamber via a heat exchanger and from the top by direct heating of the water. The fresh water production capacity of the investigated solar-collector four-stage solar still, when operated for 6hours a day at a constant flux of 850 W/m2, was found to be 17.4 kg/m2/d at Vw = 1 m/s, which was greater than conventional multistage solar stills [168-171]. The annual cost of the system was approximately Rs. 7450, with the per unit water cost in the range of 0.5-1.2 Rs/kg for the wind speed range of 1-5 m/s.
Water evaporated in four different stages, each separated by a distance of 10cm. In the absence of wind, a hand-driven wheel can be used to drive the reciprocating pump to propel the water from ground to roof level. The suggested multistage solar desalination system can meet the fresh water needs of rural and urban communities by distilling 25-45 kg/d, considering wind speed is in the range of 1-5 m/s.
Gandiglio et al.  examined enhancement of energy efficiency of wastewater treatment plants (WWTPs) through codigestion and FC systems. The study provided an overview of technological measures to increase the self-sufficiency of WWTPs, in particular, for the largely diffused activated sludge (AS)-based WWTP. The operation of WWTPs entails a huge amount of electricity. Thermal energy is also required for preheating the sludge and sometimes exsiccation of the digested sludge. On the other hand, the entering organic matter contained in the wastewater is a source of energy. Organic matter is recovered as sludge, which is digested in large stirred tanks (anaerobic digester) to produce biogas. The onsite availability of biogas represents a great opportunity to cover a significant share of WWTP electricity and thermal demands. Especially, biogas can be efficiently converted into electrical energy (and heat) via high-temperature FC generators. The final part of this study reported a case study based on the use of sewage biogas into a solid oxide FC. However, the efficient biogas conversion in CHP devices was not sufficient. Self-sufficiency required a combination of efficient biogas conversion, the maximization the yield of biogas from the organic substrate, and the minimization of the thermal duty connected to the preheating of the sludge feeding the anaerobic digester (generally achieved with prethickeners). Finally, the codigestion of the organic fraction of municipal solid waste into digesters treating sludge from WWTPs represents an additional opportunity for increasing the biogas production of existing WWTPs, thus helping the transition toward self-sufficient plants.
Maktabifard et al.  examined methods for achieving energy neutrality in WWTPs through energy savings and enhancing RE production. WWTPs consume high amounts of energy which is mostly purchased from the grid. During the past years, many ongoing measures have taken place to analyze the possible solutions for both reducing the energy consumption and increasing the RE production in the plants. This review contained all possible aspects which may assist to move toward energy neutrality in WWTPs. The sources of energy in wastewater were introduced and different indicators to express the energy consumption were discussed with examples of the operating WWTPs worldwide. Furthermore, the pathways for energy consumption reductions were reviewed including the operational strategies and the novel technological upgrades of the wastewater treatment processes. Then, the methods of recovering the potential energy hidden in wastewater were described along with application of renewable energies in WWTPs. The available assessment methods, which may help in analyzing and comparing WWTPs in terms of energy and GHG emissions, were introduced. Eventually, successful case studies on energy self-sufficiency of WWTPs were listed and the innovative projects in this area were presented.
Finally, Tee et al.  outlined various hybrid wastewater treatment processes that can be effective pollutant removal and simultaneously generate bioenergy. The study classified hybrid wastewater systems which typically include physical-biological hybrid, physical-chemical hybrid, chemical-biological hybrid, and physical-chemical-biological hybrid system. The study showed that based on the literature, hybrid systems have demonstrated some potential advantages compared to stand-alone systems such as more stable and sustainable in the voltage generated, better overall treatment efficiency, and energy savings all leading to decarbonization of wastewater industry. The study showed that hybrid treatment system can be a great choice of treatment options in wastewater in terms of effectiveness. Nevertheless, the overall cost of the hybrid system has to be taken into consideration in terms of capital costs, the operating costs, and maintenance costs . Most costs are very site specific, and for a full-scale system, these costs strongly depend on the flow rate of the effluent, the configuration of the reactor, the nature (concentration) of the effluent as well as the pursued extent of treatment. Hybrid system with a combination of physical treatment such as membrane may pose a challenge because of the high operating cost in terms of energy consumption. If the hybrid system is not designed in a way to have positive energy gained in the overall system, membrane-based hybrid system may not be worth to invest. Besides, membrane technologies may require high maintenance cost depending on fouling frequency and the particular application of the membrane-based hybrid system. Frequent membrane replacement can be very costly.
The physical-chemical precipitation is simple to implement, reliable, and efficient but presents several disadvantages such as increased operating costs due to the consumption of chemical reagents and corrosiveness of some of the coagulants which may lead to other problems . Besides, excess sludge production through coagulation, occultation, and precipitation may lead to problem in terms of disposal unless the sludge produced is able to be recycled for other purposes. Some of the biological process combinations such as AS process may require large amount of oxygen supply for the biological process. Such hybrid system may require high operating cost which makes the overall hybrid system not worth to be invested. There must be a balance made between the energy consumed with the energy gain from the hybrid system in order to make the overall system worthwhile.