## Materials and Methods
### Pilot Heat Exchanging System  
A pilot heat exchanging system was used to investigate the biofilm inhibition effect of thermal shock on biofilms in a miniaturized plate heat exchanger. The schematic diagram of the system is shown in Figure 2.  
For the biofilm formation stage, Valves 1, 2, 3, and 4 were open and valves 5 and 6 were closed. Model sewage and clean water were contained in the two water baths (HX-1050, Biocool Tech., Beijing, China) and pumped into the plate heat exchanger (CB10, Alfa Laval, Shanghai, China) to exchange heat. The temperatures of the model sewage and the clean water were set at 25 and 30 \({}^{\circ}\)C, which was a typical summer operating condition in the TSSHPS. Biofilms were formed in the heat exchanger as the model sewage flew through and the heat transfer coefficient would decrease. The flow rates, inlet and outlet temperatures of the model sewage and clean water were monitored by the flow meters and the T thermocouples, and stored on a computer to calculate the heat transfer coefficient of the plate heat exchanger and the thermal resistance of the biofilms. This biofilm formation process lasted for 120 h and every 12 h fresh model sewage was prepared to replace the old model sewage in the water bath.  
Every 24 h, the biofilms were thermal shocked. For the thermal shock stage, the clean water in the third water bath (HX-1050, Biocool Tech., Beijing, China) was preheated to the designed temperature. Then valves 1 and 2 were closed and valves 5 and 6 were opened. Biofilms in the heat exchanger were treated with hot water for the designed exposure time. Then valves 5 and 6 were closed and valves 1 and 2 were opened to continue the biofilm formation process. Different thermal shock protocols with different temperature and exposure times (50 \({}^{\circ}\)C-30 min/24 h, 65 \({}^{\circ}\)C-10 min/24 h, 80 \({}^{\circ}\)C-5 min/24 h and 25 \({}^{\circ}\)C-30 min/24 h as the control group) were investigated.  
The thermal resistance of the biofilms, \(R_{\mathrm{f}}\) (m\({}^{2}\cdot\)K/W), was calculated by:  
\[R_{\mathrm{f}}=\frac{1}{h_{\mathrm{f}}}-\frac{1}{h_{0}} \tag{2}\]  
where \(h_{\mathrm{f}}\) (W/m\({}^{2}\cdot\)K) was the heat transfer coefficient of the heat exchanger with biofilms and \(h_{0}\) (W/m\({}^{2}\cdot\)K) was the heat transfer coefficient of the clean heat exchanger.  
Figure 2: Schematic arrangement of the pilot heat exchanging system in the TSSHPS.  
The heat transfer coefficient, \(h\) (W/(m\({}^{2}\cdot\)K)), was calculated by:  
\[h=\frac{\rho_{\rm{C}}\rm{P}[Q_{V,m}(T_{m,out}-T_{m,in})+Q_{V,c}(T_{c,in}-T_{c,out} )]}{A(T_{c,in}-T_{m,out}+T_{c,out}-T_{m,in})} \tag{3}\]  
where \(T_{m,out}\), \(T_{m,in}\), \(T_{c,out}\), \(T_{c,in}\) were the outlet and inlet temperatures of the model sewage and the clean water (\({}^{\circ}\)C), \(Q_{V,m}\), and \(Q_{V,c}\) were the flow rate of the model sewage and the clean water (m\({}^{3}\)/s), \(\rho\) was the water density (996 kg/m\({}^{3}\)), \(c_{\rm{P}}\) was specific heat at constant pressure of water (4.18 \(\times\) 10\({}^{3}\) J/(kg\(\cdot\)K)) and \(A\) was the equivalent heat transfer area of the heat exchanger (m\({}^{2}\)) [41; 43].