Comparative evaluation of vacuum sewer and gravity sewer systems

Abstract

Two most common types of sewer systems are gravity sewer system and pressure sewer system. In last 25 years, a relatively new wastewater collection system called vacuum system got acceptance and popularity in certain circumstances (e.g., hilly and rocky terrains, areas with high groundwater table, unstable soil condition). Due to lack of comprehensive understanding and design procedure, very limited studies on vacuum sewer have been reported in the literature. This study complied a complete design procedure for vacuum system and attempted to compare and evaluate capital and operating costs of the vacuum sewer system against gravity counterpart. From the detailed designs of two systems for same areas, the vacuum system found relatively cheaper in terms of total construction and operation costs. The excavation cost for vacuum system is around one-fourth of the excavation cost for the gravity system. However, total manhole cost in gravity system is relatively cheaper (33 %) than the total cost of the similar components in vacuum system (inspecting valves, vacuum valves, division valves and connections). Other costs, like trench filling materials cost, labor cost, dewatering and safety measure costs are also less in vacuum system compared to the similar costs for the gravity system. Total energy cost for vacuum system is around 6.5 % less compared to the total energy cost for the gravity counterpart for same time period. Overall, vacuum system is around 30 % less expensive than gravity system.