# Application and Evaluation of Energy Conservation Technologies in Wastewater Treatment Plants
## 4 Performance Evaluation of Energy Conservation Technologies and WWTPs
### Evaluation Indexes  
The appropriate evaluation index can not only evaluate energy the performance of WWTPs horizontally but also evaluates the differences of processes vertically, which can provide guidance for technological improvement. Besides, it can help researchers confirm the application value of energy conservation technologies [149]. The diversity of energy-saving technologies makes some indexes that can comprehensively compare their differences more valuable. In Section 1.2.1, EI is used to describe energy performance. Energy consumption per volume of treated wastewater and energy consumption per unit of pollutant removal are two widely used energy indexes, both of which have some problems [150]. Energy consumption per unit of pollutant removal needs to consider a variety of pollutants, which will cause difficulties, while energy consumption per volume of treated wastewater ignores the difference of wastewater (water quality, scale, process, etc.). This problem has been described in Section 1.2.1. The premise of using energy consumption per volume of treated wastewater is that the fluctuation of influent water quantity and water quality (COD, BOD, SS, TN, and TP, etc.) of sewage treatment plants is relatively small, and the situation of influent water quality and water quantity of each sewage treatment plant is comparatively close. In general, energy conservation technology needs to consider energy, feasibility, treatment capacity, economy, environment, policy, water supply safety, risk, and other factors. This is also the premise of energy-saving applications such as multi-objective optimization and optimal control. There are various performance evaluation indexes for WWTPs. Table 7 shows the indexes used by some researchers. These different indexes with different concerns can show the various situations of WWTPs. This makes the data on energy performance, application value and popularization potential of energy-saving technology more reliable. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Types** & **Technologies** & **Achievements** & **Reference** \\ \hline Solar energy & PV power generation & Provide 10\% energy needs & [144] \\ \hline Heat energy & Wastewater source heat pump & Electricity equivalent 2.18 kWh/m\({}^{3}\) & [144] \\ \hline Biomass energy & Anaerobic digestion & Provide 50\% energy needs & [144] \\ Microbial fuel cell & Achieve 0.360 kWh/kg COD & [145] \\ \hline Chemical energy & Pyrolysis & Recover 59.2-79.3\% sludge energy & [146] \\  & Incineration & Provide 20\% energy needs & [62] \\ \hline Algorithms & Fuzzy algorithm & Save about 40\% of the energy & [111] \\  & Model Predictive Control & Reduce over 25\% in power usage & [147] \\ \hline Equipment & Use the direct drive turbine & Save more than 35\% energy & [125] \\  & Use variable frequency driver & Reducing energy consumption by up to 50\% & [126] \\ \hline Control strategy & ASM2d model & Save 2.2 to 3.3\% of energy & [148] \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Energy consumption of WWTPs using common green energy technologies. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Objectives** & **Indexes** & **Reference** \\ \hline Water quality & Total oxygen demand, Oxygen consumption potential, EO index & [151] \\ \hline Economy & Energy use per unit emission reduction, Cost per unit emission reduction, Environmental benefit per unit investment, Ratio of energy consumption from sewage treatment to national total energy consumption, Ratio of cost of sewage treatment to the Gross Domestic Product, Structural Coordination degree of sewage treatment, and Scale harmony degree of sewage treatment, Financial payback time & [152; 153] \\ \hline Wastewater recovery \& utilization & Wastewater Circonomics Index, Waste Production Efficiency Indicator, Composite Wastewater Reuse indicator, Wastewater recycling indicator & [154] \\ \hline Energy & Material flow, Energy flow, Specific energy consumption, Chemical energy potential & [155] \\ \hline \hline \end{tabular}
\end{table}