How far must distribution lines be from sidewalls in seepage beds and sand filters?

The correct answer, which is 18 inches, reflects the standard practice established in Illinois for the installation of distribution lines within seepage beds and sand filters. This distance is crucial because it helps ensure that effluent can adequately disperse into the surrounding soil without risking the structural integrity of the filtering systems or the potential for contamination.

Maintaining an 18-inch separation allows for the proper functioning of the system while also providing adequate air circulation and reducing the risk of soil saturation around the distribution lines. This is important for promoting effective filtration and minimizing health and environmental risks. The specific distance is established based on extensive research and experience in the field, ensuring optimal performance of septic systems while protecting public health and local water sources.

In contrast, the other distances listed do not meet the recommended practices for effective effluent distribution and can lead to operational issues or system failures. For instance, being too close to the sidewalls could impair the flow of effluent or hinder the treatment process, leading to the potential for groundwater contamination.