Which of the following is NOT required for septic tank design?

In septic tank design, annual inspections are not a structural requirement for the tank itself, but rather, they are a recommendation for maintenance and functionality assessment over time. Inspections ensure that the system operates efficiently and to identify potential issues before they escalate into more serious problems. However, they do not influence the physical design of the septic tank itself.

On the other hand, the other options relate directly to the fundamental requirements for septic tank construction and design. For instance, having two compartments is a common practice to enhance efficiency by allowing for better solids separation and improved effluent quality. A liquid volume proportional to the number of bedrooms ensures adequate waste treatment capacity to manage household waste effectively. Additionally, combined tank designs, which integrate various treatment processes in one structure, can be beneficial in optimizing space and improving waste processing efficiency.

Thus, while inspections are crucial for long-term system health and compliance with regulations, they do not constitute a requirement in the design phase of septic tank systems.