Introduction

The growth of the population and the development of economy make water pollution more common than ever before. Nearly 80% of the world’s population is facing serious water security threats, and the world will face a 40% water deficit by 2030 [1,2]. To deal with the challenge of water pollution, the European Union promulgated the Water Framework Directive to provide some guidance in water basin management and water quality restoration [3]. However, the instruction can only provide guidance, and sewage still needs to be treated to achieve water quality recovery. In order to protect the living environment of human beings, it is necessary to purify the wastewater and remove the pollutants in the wastewater.

According to the different treatment objectives, wastewater treatment processes can be divided into three levels to select the appropriate treatment methods. The primary treatment uses physical methods like screening, interception, and gravity separation to remove suspended solids (SS), fat, and grease in wastewater [4]. The removal rate of biochemical oxygen demand (BOD) is only 30–40% and SS is 50–60% in primary treatment [5,6,7]. Secondary treatment, the main body of wastewater treatment, uses biological treatment and some chemical treatment methods to remove degradable organic matter and some colloidal pollutants from wastewater [8]. The activated sludge method is a biochemical combination method, which is the most widely used method for secondary treatment of wastewater [9]. Tertiary treatment is used to remove pollutants that cannot be removed by secondary treatment such as P, N, and biodegradable organic pollutants.

WWTPs enrich pollutants into sludge, which contains many organic substances and pathogens while purifying wastewater. Sludge treatment is a process of sludge stabilization, reduction, and harmless treatment, including sludge drying, aerobic, anaerobic digestion, and adding chemical agents. Sludge disposal is the process of reasonable storage and resource utilization [6]. Sludge disposal mainly includes sludge landfill, land use, composting and thermochemical treatment technology. Common wastewater and sludge (WW&S) treatment processes are shown in Figure 1.

Figure 1. Flow chart of common wastewater and sludge (WW&S) treatments.

Green Energy Utilized in WWTPs

In order to solve the problem of high energy consumption, the application of green energy to WWTPs is a direct and effective treatment scheme. Green energy includes not only external energy such as solar energy and wind energy but also energy recovered from wastewater and sludge. The application of green energy is an important means for WWTPs to reduce energy consumption [44,45,46,47].

Solar Energy and Photovoltaic (PV) Energy

Solar energy refers to the solar radiation that reaches the earth in the form of the electromagnetic waveform. As an important energy green resource, solar energy is abundant, renewable, clean, safe and easy to access. Solar energy is one of the most widely used green energy sources in WWTPs [48]. The form of solar energy used in WWTPs is shown in Table 2.

Table 2. Applications of solar energy in wastewater treatment plants (WWTPs).

Wind Energy

Wind energy, a significant part of green energy, is kinetic energy produced by air flow under solar radiation. Wind energy is widely distributed and abundant in reserves. It is especially important for islands and areas that are far away from the main power grid. For the poor stability and randomness of wind energy itself, it is difficult to use it alone for the power supply of WWTPs. To make up for the shortcomings of wind energy and solar power generation, wind power is mainly combined with solar energy in WWTPs under the current technical conditions, which can realize wind-solar complementary power generation to supply energy to WWTPs [58,59]. This technology has been widely used in the field of wind energy utilization [60,61].