Which statement is true regarding the design of septic tanks?

The correct statement regarding the design of septic tanks is that they must have two or three compartments. This design is crucial for effective treatment of wastewater. The multiple compartments facilitate better separation of solids and liquids, allowing for more efficient breakdown of organic material.

With two or three compartments, the first compartment acts primarily as a settling chamber where solids are allowed to settle to the bottom, forming sludge, while the lighter materials, such as fats and grease, float to the top, forming scum. Subsequent compartments help to clarify the effluent further and allow for additional anaerobic digestion, which aids in the treatment process. This multi-compartment design is a standard requirement in many jurisdictions to enhance the functionality and longevity of the septic system.

The other options present features that are not universally true for all septic systems. For instance, while many septic tanks are buried underground to allow for gravity flow and to reduce surface odors, it is not an absolute requirement as some above-ground systems do exist. Stipulating only one compartment oversimplifies the necessary configuration for effective treatment, while restricting materials to only plastic neglects the viability of other materials like concrete or fiberglass often used in tank construction. Therefore, the specification for two or three compartments is integral to the proper design