Worldwide, the transfer from the level of knowledge to operation at full scale is often difficult due to the expectation of some problems associated with new technologies that usually cannot be identified and solved on a full scale. Therefore, aspects of start-up times and methodologies become even more relevant when starting to implement any (or any project) processing system in wastewater treatment. Thus, this work aimed to develop and validate a concept for starting up a laboratory-scale lagoon system. This system typically contains a series of three continuous-flow lagoons to treat 50 Liters in the day from a municipal sewage facility in Al Rumaitha City, located north of Al Muthanaa Province in Iraq. Further, the influence of Hydraulic Detention Time (HRT) on the hydraulic lagoon performance depending on site-specific conditions and determining factors influencing actual hydraulic residence time was evaluated by investigating the start-up of three different HRTs: 7 days, 14 days, and 21 days, as changing it altered the depth and, thus, the effective volume. The start-up experiment involved two different phases of experiments, and they were conducted for four months, distributed in two different periods: the first period of start-up experiments was characterized to develop a proper microbial floc as quickly as possible and also to select the appropriate HRT of the lagoon. The results from these experiments led to the selection of the best three-cell lagoons design, which had an HRT of 7 days because it remained more stable concerning COD removal. After that, the second period of the experiments began, devoted to helping performance assessment of these facilities in continuous mode via providing basic information about the treatment processes occurring in a lagoon and summarizing performance expectations until a steady state was reached. Throughout the period of this experiment, average removal efficiencies were found to be 73.34% for COD, 76.54% for NH4+-N, 36.06 % for TN, and 38.30% for TP.