Which characteristic is important for poured-in-place concrete septic tanks?

The characteristic that makes poured-in-place concrete septic tanks particularly important is their watertight nature and the requirement for them to be designed by a Professional Engineer (PE). Ensuring that these tanks are watertight is crucial because they need to prevent any effluent from leaking into the surrounding soil, which could lead to contamination of groundwater and other environmental issues. Furthermore, a Professional Engineer's involvement ensures that the design meets all necessary structural and code requirements, optimizing performance and longevity while adhering to safety standards.

Overall, the watertight nature of the tank is essential for its function, and professional design provides necessary expertise to create a system that is both effective and compliant with regulations. While other options may seem attractive in different contexts, they do not meet the critical needs for septic systems, which prioritize safety, functionality, and regulatory compliance.