A Novel District Heating System Coupled with Reclaimed Water Source Heat Pump: Energy, Economic, Environmental, and Application Analysis

Abstract

Wastewater treatment plants (WWTPs) are an energy-intensive industry. The high energy consumption of WWTPs is detrimental to achieving carbon neutrality. Wastewater treatment accounts for an estimated 3.4% of annual electricity consumption in the United States, nearly 1% of the total electricity consumption in Germany, and 0.25% of the total electricity consumption in China. [1][2][3] The unit electricity consumption for wastewater treatment in the aforementioned countries is around 0.52, 0.40, and 0.254 kWh m À3 , respectively. [3] The volume and standard of wastewater treatment will be further improved due to the increase in urban population and more stringent environmental protection requirements, [4] which will bring higher energy consumption. To achieve the energy balance of wastewater treatment, it is necessary to recover energy from resources contained in wastewater. [5] In the early period, most of the energy recovery of wastewater concentrated on organic matter in wastewater. Organic energy in wastewater is about 9-10 times higher than that used to treat wastewater. [6] Researchers have explored the potential of biogas generated from sludge for power generation at the national level. [7,8] For example, the vast majority of existing biogas plants in Sweden are located in WWTPs, which almost accounts for 60% of the total biogas production (3 PJ year À1 ). [7] Anaerobic digestion plays an important role for its abilities to further transform organic matter into biogas. [9,10] However, anaerobic sludge digestion systems tend not to be well operated in developing countries. For example, less than 100 WWTPs in China (3% of all plants) use biogas from anaerobically digested sludge to generate heat and electricity. [11] Co-digestion with kitchen or other organic wastes is currently only applied on a laboratory scale in China. [12] The chemical energy extracted from the wastewater depends not only on the amount of biodegradable chemical oxygen demand (COD) in the wastewater, but also on the conversion efficiency of chemical energy in CHP system. Hao et al. think based on a BNR-WWTP (an AAO process; Q ¼ 600 000 m 3 d À1 ), only about 13% (0.20 kWh m À3 ) of the theoretical chemical energy (1.54 kWh m À3 at COD ¼ 400 mg L À1 , in the influent of the evaluated WWTP) can be converted into electricity and heat. [5] The thermal energy recovery potential of wastewater was significantly higher than that of chemical energy. [13] In northern China, the wastewater temperatures are 10-16 °C. [13][14][15] The large amount of low-grade thermal energy from wastewater is not fully utilized. The untreated wastewater will scale at the heat exchanger, which will affect the heat transfer effect, so treated

Opportunities and Challenges for Research on Heat Recovery from Wastewater: Bibliometric and Strategic Analyses

“…The results of research [34,130] on the use of HPs in wastewater heat recovery systems confirm that this technology allows for significant energy savings, and thus financial savings. This applies to both heating and cooling systems.…”

“…The use of wastewater heat pumps can efficiently reduce the energy consumption needed for domestic hot water preparation, space heating and cooling. In addition, properly adapted wastewater heat pumps enable direct cooperation with the district heating, reducing the consumption of energy from conventional sources [130].…”

“…Wang et al [130] Analysis of the feasibility of utilizing reclaimed water from a wastewater treatment plant as a lower energy source for a heat pump to provide heating buildings in a district heating network Energy Technology (Wiley)…”