# Application and Evaluation of Energy Conservation Technologies in Wastewater Treatment Plants
## 2 Green Energy Utilized in WWTPs
### Energy Recovery of WW&S
#### 2.3.2 Biomass and Its Derivative Energy  
Biomass energy is a kind of clean and safe energy with abundant reserves. It comes from biomass that stores solar energy as chemical energy. Biomass is expected to contribute to half of the EU renewable energy targets according to the Renewable Energy Directive, while a large amount of biomass energy comes from WW&S[73; 74]. The application technology of biomass and its derived energy in WWTPs mainly includes the following aspects: cultivation of microalgae in wastewater, production of biogas by anaerobic digestion (AD) and microbial fuel cell technology (MFC).  
The wastewater treatment with microalgae has recently attracted more and more attention for its energy conservation, low cost, effective nutrient removal ability, fixing CO\({}_{2}\) ability, and its good economic benefits. Cultivation of microalgae has low requirements for water quality, and it absorbs nutrients (N, P) and the organic carbon source during the growth process. [PERSON] et al. used ordinary chlorella to simulate the removal of N from the secondary treatment effluent. After 2 days, the N removal rate reached 90%, and the P removal rate was 94%[75]. The microalgae also have economic benefits and are widely used in fertilizer, feed, and other agricultural fields. The bio-oil produced by hydrothermal liquefaction of microalgae has also attracted extensive attention. Two demonstration-scale algae-based facilities of the USA are expected to produce 5 million gallons of fuel per year[76].  
AD, the most popular sludge stabilization technology, is carried out under the conditions of an anaerobic environment. The process transforms sludge organic solids to biogas, which is a mixture of CH\({}_{4}\), CO\({}_{2}\), and traces of other gases at medium temperature (30-36 \({}^{\circ}\)C) or high temperature (50-55 \({}^{\circ}\)C)[62; 77]. Biogas contains 50-70% CH\({}_{4}\) and 30-50% CO\({}_{2}\), traces of other gases (such as H\({}_{2}\), H\({}_{2}\)S, and N\({}_{2}\))[47]. The calorific value of the biogas produced by AD is about 21-25 MJ/m\({}^{3}\)[6; 78]. This biogas can be utilized as a source of energy in the production of electricity. The total installed capacity of the EU's biogas plants in 2010 exceeded 6.1 \(GW\), of which 9.9% came from WW&S[79]. The study of [PERSON] et al. shows that biogas slurry contains nutrients necessary for the growth of microalgae, which is suitable for microalgae cultivation[80]. It can solve the problem of the treatment of biogas slurry produced by AD effectively. Data from Spanish WWTPs show that AD can recover 52% of the energy in sludge through biogas[81].  
The microbial fuel cell (MFC) is a novel wastewater treatment technology that uses microorganisms as biocatalysts to oxidize organic matter and transfer electrons from substrate oxidation to the anodic surface for bioelectricity production[82]. Mixed bacteria can be used to biodegrade organic wastes into CO\({}_{2}\), H\({}_{2}\)O, and generate electricity at the same time. [PERSON] et al. applied MFCs to a municipal WWTP and tracked it for a long time, finding it removed 65-70% COD at 11 h and reduced about 50% SS[83]. [PERSON] et al. investigated the possibility of the application of an MFC in sludge treatment and achieved a maximum voltage of 517 mV through anaerobically digested[84]. Due to the low voltage generated by MFC, there is still a long way to go for the large-scale application of MFC power generation.