Logistic model-based study on fouling thermal resistance evolution and anti-fouling strategies for bathing wastewater-source heat pump heat exchangers

Bathing wastewater-source heat pumps can recover low-grade waste heat, but complex contaminants may cause heat-exchanger fouling and deteriorate heat-transfer performance. This study investigated fouling evolution and control strategies for an immersed copper-tube heat exchanger using physical filtration and composite chemical treatment. Fouling thermal resistance was evaluated using the thermal-resistance difference method and fitted using the Kern–Seaton (KS) and Logistic models. During a 72 h test with untreated wastewater, the overall heat-transfer coefficient decreased by 28.21%, while the overall thermal resistance increased by 39.4%. Baseline filtration at 0.52 mm reduced the steady-state overall thermal resistance by 18.08%, whereas further filtration to 0.25 and 0.04 mm provided limited additional improvement. The Logistic model better described the observed fouling evolution than the KS model, with R 2 increasing from 0.953 to 0.976 and RMSE decreasing by 28.4%. The fitted asymptotic fouling thermal resistance decreased from 5.56 × 10 −4 m 2 ·K/W without filtration to 2.13 × 10 −4 , 1.85 × 10 −4 , and 1.15 × 10 −4 m 2 ·K/W at filtration cut-offs of 0.52, 0.25, and 0.04 mm, respectively. Water-quality indicators suggested a low CaCO₃ scaling tendency and substantial microbial activity in the bulk water; however, these indirect indicators did not establish the dominant fouling mechanism. Orthogonal screening identified a formulation containing 8 mg/L ATMP, 10 mg/L PAA, 30 mg/L GLDA, 30 mg/L TDBAC, and 5 mg/L BTA. Combined with 0.52 mm filtration, this treatment maintained stable heat-transfer performance during a 72 h verification test. These results demonstrate the potential of combining baseline filtration with chemical treatment for fouling control in bathing wastewater heat exchangers.