# Application and Evaluation of Energy Conservation Technologies in Wastewater Treatment Plants
## 5 Challenges and Perspectives for Water and Energy Nexus
### Challenges  
However, with the rapid development of wastewater treatment, some problems and challenges are gradually exposed. These challenges can be roughly divided into cost issues, technology issues, management issues, and external issues [18; 47; 174]. These issues are often intertwined with each other.  
The expansion of the city and the development of industry make the existing WWTPs and pipe networks difficult to meet the requirements. In developing countries, the wastewater treatment capacity cannot fit the wastewater discharge volume. The gaps is still further expanding, which makes the problem fiercer [173]. Wastewater treatment facilities have a high investment and account for a large proportion of the national economy. In the 1980s, the investment in wastewater treatment of China, the United States, Japan, Germany, and France respectively accounted for 0.02-0.034%, 0.80%, 0.75%, 0.88%, and 0.53% of the gross national product (GNP) [175]. Similarly, the operation of WWTPs requires a lot of money. The cost of construction and operation has become a bottleneck restricting the development of the wastewater treatment industry [19]. Considering the conflict between the large gap of wastewater treatment and the high cost of construction, a feasible scheme is to adopt a suitable wastewater management mode. Distributed wastewater management is an important solution for developing countries [176]. Some studies have shown that the rational allocation of infrastructure can minimize costs [177; 178]. However, this work needs a lot of data to support. Similarly, wastewater treatment cost forecasting has important values in reducing costs. Accurate forecasting can help the government to make reasonable plans [27].  
The core of technological challenges is the high energy consumption. To meet the increasing water quality requirements, municipal WWTPs should increase aeration volume, prolong aeration time and increase sludge return flow, which leads to the increase of energy consumption. Sludge disposal and resource utilization have not received enough attention. The vast majority of sludge is simply landfilled and incinerated, which can also cause serious environmental pollution. At present, less than 10% of WWTPs in the United States are producing surplus biogas for commercial use [41; 47]. Advanced wastewater treatment technologies are not widely used in WWTPs. This may be due to technical barriers or high upgrade costs. Reducing GHG emissions is also a major technical challenge to WWTPs [39; 133]. To reduce GHG emissions, monitoring, model, emission reduction technology, cleaner production, and other technologies need to be paid enough attention [129; 179; 180; 132].  
For the wastewater treatment industry, external challenges mainly come from the change of pollutants. The emergence of new pollutants, such as micro plastics, hormone, and medicine, has brought external challenges to the wastewater treatment industry [181; 182]. [PERSON] believes that antibiotics as an important water pollutant have not received enough attention in India [183]. Wastewater treatment plants usually do not deal with pollutants occurring in the sub-mg/L, which makes micro pollutants accumulate in drinking water and pose a threat to human health. Therefore, the monitoring and treatment of micro pollutants in key water bodies must be paid enough attention, which is a technical and managerial challenge [184].