Within the District each city owns and operates its own system of sewage collection lines that discharge into larger District-owned lines. In general, city lines are smaller and serve individual subdivisions.
The District’s collection system consists of approximately 100 miles of sewer line with approximately 1,570 manholes and pipes ranging from 8 inches to 84 inches in diameter. The majority of the lines are constructed of reinforced concrete pipe; the oldest dating back to the mid-1940’s.
Rates of flow vary throughout the day, from weekday to weekend. There are two peak flow periods during the day, in the late morning and in the evening. Although the higher discharges originate in the early morning and afternoon, the higher flows are not received at the plant until up to 10 hours later from the more remote regions because of the time it takes to travel from the point of origin to the treatment plant. All lines, regardless of their size, are designed to create minimum flow velocities of 2 feet per second when flowing full. The flow velocity is important because it is what keeps solids from settling and accumulating in the pipe.
Every six months, District personnel inspect all the manholes in the system, reporting any signs of deterioration, odor, corrosion, obstruction of flow, and other abnormalities so that action can be taken to maintain efficent operation with minimal risk of incident.
The District uses special televising equipment to inspect approximately 130,000 feet of sewer lines each year to check for cracking, infiltration, deterioration, root intrusion, grease build-up, or other conditions indicating needed repair or maintenance. All inspections are conducted by NASSCO (National Association of Sewer Service Companies) certified District collection staff according to NASSCO standards using PACP and MACP. The entire system can be flushed, cleaned, and inspected fully every four years. Flushing and inspecting the lines also minimizes the accumulation of sediment that can contribute to odors and corrosion. Televised inspections are recorded for filing and future reference.
Most pipes, when properly designed, constructed, and maintained, remain in service for 50 years or longer.