How far away must distribution lines be from the sidewalls of a seepage bed?

The correct distance for distribution lines to be set from the sidewalls of a seepage bed is 18 inches. This requirement is established to ensure adequate spacing for effective drainage and to prevent potential obstruction of wastewater flow. Maintaining this distance allows for proper distribution of effluent, reduces the risk of clogging, and facilitates adequate soil interaction, which is essential for the treatment process. Following these standards also helps in managing the system's capacity and longevity.

The other options present distances that either do not provide sufficient space for effective seepage bed functioning or exceed what is typically necessary for optimal performance and maintenance. Consistently adhering to regulation guidelines, such as the 18-inch standard, is critical for the sustainable operation of septic systems.