The Phetchaburi municipal's sewer system for transporting wastewater to treatment system at The Royal LERD project comprise with a gravity sewer, a Klongyang collection pond and the 18 km-HDPE pressure pipeline that allows for a retention period of the wastewater. Consequently, this research focuses on the treatment of the wastewater during the transportation phase of the sewer system. The results are generally concluded that the municipal wastewater compromises with easily biodegradable

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... matter, mesophile temperature and abundant nutrients that promote self-biodegradation in both anaerobic and aerobic processes. The BOD concentration at the municipal's point sources was found to be around 907 mg/l, as the retention time on the gravitational system allows for both aerobic and anaerobic digestion process to decreases its concentration. During the transportation phase in the HDPE pipeline to The Royal LERD Project found that only anaerobic process occurs. After reaching 12 km in the HDPE the digestion rate trend to be zero with the 35.2 hours retention time. The BOD concentration reaching The Royal LERD project was found to be 52 mg/l with 94% efficiency. The knowledge gained from this experiment proved that the self-purification processes happened in the sewer system, which help supports domestic wastewater treatment.