The value Q is

The value Q is defined as the Quantity of Sewage measured in gallons per day (GPD). This terminology is commonly used in the context of wastewater management and On-Site Sewage Facilities (OSSFs) to quantify the volume of sewage generated or expected to be managed by the system. Understanding the volume of sewage is crucial for designing wastewater treatment systems, ensuring compliance with regulatory standards, and assessing the environmental impact of discharges.

Quality, while an important aspect of sewage, pertains to the characteristics of the sewage water, such as its chemical or biological composition, and is not quantified in gallons per day. Therefore, focusing on "Quantity" correctly captures the essence of what Q represents in this context.