**4. Benefit of energy management in WWTP**

Reducing variability in renewable energy is crucial in managing the peaks in WWTP.As a result, this strategy is dispensable for employing energy storage systems charging during off peak times and injecting energy into smart grids during peak times. Benefits can be estimated from the low price at night, cost of basic contract fee of electricity, and effect of frequency regulation.

Results of the economic benefit assessment are shown in **Table 2**. We assume that renewable energy's purchase price is 0.143 USD. Renewable energy is assumed to be fully fed back to the grid. About 80% of the total RLIB is used as night storage, and the cost of RLIB is 133USD unit kWh. In the case of Dihua plant, the calculation of RLIB demand is 32,106 kWh, which is roughly equivalent to 3200 pure EV battery pack. This value is also about 1/20 of the total number of domestic sales of EVs from 2011 to 2016 in Taiwan. The initial cost of RLIB packs is 4.3 million USD. However, only the sales of renewable energy power into the grid based on feed-in tariff (FIT) are 1.68 million USD. The annual electricity rate difference at noon and night is 140.6 million NTD, and the annual income at noon and night is 4.69 million USD. Therefore, the plant can break even in 2 years and continue to profit each year without considering the installation fee. Other plants also show similar profitable results such as Dihua plant in **Table 2**.


**Table 2.** Benefit assessment of 5 WWTPs applying RLIB (NTD; 30NTD = 1USD).
