## 1 Introduction  
Treated sewage, which contains a large amount of urban low-rank energy, is an ideal heat and cooling source [1; 2]. Its temperature (20-30 \({}^{\circ}\)C) is ideal for maintaining a high efficiency for heat pump operations [3; 4; 5]. Treated sewage source heat pump systems (TSSHPS) can reuse the waste energy contained in the treated sewage for building heating and cooling [6], and many practical projects, such as the 2008 Beijing Olympic Village TSSHPS, have come into service and been proved to be efficient [7; 8]. However, the high bacterial concentration and the rich nutrient environment in the treated sewage cause biofilm formation on the stainless steel plates in the plate heat exchangers [9; 10; 11], which are important components in the TSSHPS to transfer heat from the treated sewage to the clean working medium due to their compactness and high thermal efficiency [12]. Biofilms reduce the heat transfer coefficient of the heat exchangers and the coefficient of performance of the whole heat pump system. A previous study has reported that biofilms were formed within 40 days of operation in a TSSHPS and the coefficient of performance of the system decreased by 50% [13]. Biofilms, defined as a complex community of microbes settled to a solid substrate and encased in the extracellular polymeric substance [14; 15], or the EPS, is a refractory problem for heat-exchanging systems, food processing equipment, medical instruments, and hull surfaces, etc. [16; 17; 18; 19]. Regarding the formation process of the biofilms, first, microorganisms, such as bacteria, attach to the biotic or abiotic surfaces. Then the microorganisms form microbial colonies and secret EPS, which is mainly composed of polysaccharides, proteins, nucleic acids, etc., and acts as a \"glue\". Finally, the biofilm develops into a matured porous structure, which protects the microorganisms from hazard external environments, such as high concentration of bactericides and oligotrophic conditions, and help them gain ecological advantages [20; 21; 22]. Due to the serious losses caused by biofilms, innovative biofilm inhibition and elimination strategies are constantly being sought [23; 24]. Physical approaches, such as coatings, micro-patterned surfaces and gas discharge plasmas, have been reported to inhibit bacteria settlement and disturb biofilm structures [25; 26; 27]. Antimicrobial agents have been tested against biofilms and were found effective in killing the indwelling organisms [28; 29]. Various peptides and enzymes have been used to inhibit biofilm formation [30]. However, many of these strategies are limited when applied to the TSSHPS. Micro-patterned surfaces and biofilm-inhibition coatings are limited as they are costly and might lead to extra thermal resistance in the heat exchangers. Due to the enclosed environment of the plate heat exchangers, external biofilm-inhibition forces, such as gas discharge plasmas, would be difficult to be used. Additonally, as the treated sewage is in large flow rate and drained to nearby rivers after the heat transfer process, the application of chemical and biological antimicrobial agents is limited due to the large consumption and environmental protection concerns [11]. Among the many strategies, thermal shock at accessible temperatures (\(\leq\)80 \({}^{\circ}\)C) might prove to be a more feasible and effective approach towards biofilm inhibition in plate heat exchangers. Thermal shock could be applied to the biofilms just by adding another pathway to the plate heat exchanger and inletting hot water at regular intervals during TSSHPS operation. Compared with the majority existing biofilm-inhibition approaches, thermal shock is easy to realize and would not lead to high costs or harms to the environment. Thermal shock is commonly used for bacteria inactivation. Pasteurization for milk and liquid foods sterilization has been used for over a century [31] and autoclaving (>120 \({}^{\circ}\)C) has been used for biofilm sterilization on medical equipment surfaces [32].