Sewer wastewater has a temperature between 10-25°C, which is an attractive source of energy, and it can be recovered by installing a heat recovery system (heat exchanger and heat pump) inside or outside sewer. The viable conditions for effective heat recovery and proper functioning of heat systems are wastewater temperature, flow rate, heat exchanger system, and heat pump. Viability conditions for heat recovery have been developed and applied to the Antwerp case study. Three methods perform heat recovery estimation: temperature drop method, Specific performance value (kW/L/s/m2 and kW/m2), and heat balance equation which is applied to the Antwerp case study of 29 and 1697 pipes to estimate heat recovery for selected pipes from the case study. Excel software has been used to performs calculations for heat recovery methods. IF Function is done additionally, to check with the Antwerp case study results as 200kwh/pipe. 29 pipes heat recovery was around 0.45-3.6kwh for the temperature drop method for 15 modules of heat exchanger. Specific values results are 1.04kw/L/s/m2 and 2.4-8.75kw/m2 for 29 pipes scenario. Selected pipes from 1697 results show 0.45-2.7kwh for temperature drop method for 15 modules, whereas specific values are 0.15-0.65kW/L/s/m2 and 2.4-8.75kw/m2.