![Fig. 4. Variation of electricity consumption with different sizes of WWTP for representative wastewater treatment processes. Data is taken from 1500 POTWs in USA [52]. Note: MGD denotes million gallons per day.](/figures/fig-4-variation-of-electricity-consumption-with-different-2pq0mix9.webp)
![Fig. 7. Nitrogen cycling network of nitrification, denitrification, and anaerobic ammonium oxidation Microbial fuel cell (MFC). Power generation using MFC is observed in several WWTPs, including both domestic and industrial wastewater treatment [103-110]. MFC technology can transform the chemical energy of excess sludge into electrical energy [106]. Unlike conventional AD, MFC generates electricity directly, which is a](/figures/fig-7-nitrogen-cycling-network-of-nitrification-2bplbhm4.webp)
![Fig. 10. (a) Energy balance of the Strass WWTP in 2005–2007. (b) Energy balance of Wolfgangsee-Ischl WWTP from September, 2009 to August, 2010. Adapted from [28].](/figures/fig-10-a-energy-balance-of-the-strass-wwtp-in-2005-2007-b-36frcrsx.webp)
![Fig. 9. Integrated recovery processes in the HCWTP. Source: [27]](/figures/fig-9-integrated-recovery-processes-in-the-hcwtp-source-27-1mms6lje.webp)
Q2. What can be achieved by incorporating a MFC?
the incorporation of a MFC can make wastewater treatment a zero-energy-input process, and good treatment performance can be achieved in a MFC-based combined system.
Q3. What are the key factors influencing the whether wastewater treatment plants should be centralized or decentralized?
Energy efficiency and operating cost are the key factors influencing the whether wastewater treatment plants should be centralized or decentralized.
Q4. Why is the potential for energy self-sufficient WWTPs increasing?
Due to increasing energy costs and concerns about the environment, the potential for energy self-sufficient WWTPs has become an area of increasing research and innovation.
Q5. What is the importance of a theoretical assessment of a WWTP?
In order to promote engineering applications of energy self-sufficient WWTPs, theoretical assessment of such systems is often required.
Q6. What is the key tool for the operators to optimize the energy recovery sections?
during the actual operating process, the operators need to constantly optimize the energy recovery sections though adjusting key parameters or through the adoption of advanced technologies to achieve energy independence with the lowest cost.
Q7. What are the main factors that affect the energy production in algal-based wastewater treatment system?
the energy production in algal-based wastewater treatment system is affected by various factors such as light, temperature, CO2 availability, HRT, mixing, cultivation mode, algal recycling, nutrients, invertebrates in the system and algal biofuel options [133].
Q8. What is the way to convert biomass from a wastewater treatment system into biofuel?
It was also suggested that converting the biomass produced in HRAP to biofuel through hydrothermal liquefaction processes could achieve the highest conversion efficiency (>70%) compared with other conversion pathways like pyrolysis, AD or gasification [133].
Q9. How much can anet-zero energy WWTP save in China?
It was estimated that anet-zero-energy WWTP with a capacity of 10,000 m3/d can save $100,000 each year in China through energy self-sufficiency [145].
Q10. What is the purpose of the design of a fixed-inverter hybrid heat pump system?
a fixed-inverter hybrid heat pump system was designed to run in a real WWTP in South Korea, aimed at improving load response and maintaining high efficiency levels [140].
Q11. What are the two complementary aspects to realize energy self-sufficiency in WWTPs?
Two complimentary aspects are required to realize energy self-sufficiency in WWTPs: (1) Energy savings via improving efficiency in unit processes.
Q12. What is the effect of sludge digestion on the recovery of energy?
A study on a high-rate contact stabilization reactor system showed that the system operated at high sludge-specific loading rates and lower SRT can recover more energy.
Q13. What is the role of lignocellulose in the energy production of a municipal WW?
co-digestion with lignocellulose from maize or grass in rural areas or urban biomass also can contribute to energy production in municipal WWTPs.
Q14. What are the advantages of using MFCs in wastewater treatment?
electrode materials in MFC are expensive for wastewater treatment and therefore are limited to specific applications of MFCs.[113]
Q15. How many KWh/m3 of energy was produced by the stackable baffled?
This system produced enough energy (0.153 KWh/m3) for the operation of the whole system, resulting in the recovery of 0.055 KWh/m3 of net energy [118].
Q16. How much energy is used for treating the same amount of wastewater?
A case study in California comparing a decentralized and centralized wastewater treatment system based on a life cycle assessment (LCA) showed that the decentralized wastewater treatment system consumes approximately 37,000 J of energy per liter of wastewater, while the centralized system uses only approximately 6,800 J for treating the same amount of wastewater [60].
Q17. What is the main reason for the higher energy consumption of WWTPs in Sweden?
Repeated energy benchmarks since 1990’s have shown a decrease of 30% in energy consumption in WWTPs in Austria through improvements [62].
Q18. How much energy could be saved by thermal pre-treatment?
Another recent hypothetical study of a WWTP showed that 585 kW of energy could be saved by thermal pre-treatment of 3.5% of sludge at 80 oC for 0.5 h [87].
Q19. How many KWh/m3 of electricity can be generated from a single MFC?
If this strategy can be adopted by a WWTP with a capacity of 50,000 m3/d, net electricity of about 3850 kWh could be generated per day [123].