How Long Do Septic Lagoons Last? Septic lagoon systems are similar to septic tanks in that they’re both designed specifically for treating wastewater. However, lagoon maintenance is different from tank maintenance in several ways. Septic lagoons can last for 20-30 years before needing renovations or replacement. Regular maintainance is required to ensure it's longevity. See what’s involved with taking care of your lagoon and why regular septic maintenance is essential. Septic Lagoon ...