What goes around comes around, in the realm of San Fernando Valley waste water. A flush of the toilet in Tarzana or the water from a shower in Sylmar may end up floating dinghies in Lake Balboa or nourishing the exotic plants in the Japanese Gardens, both of which are part of the Sepulveda Basin.

After leaving homes and businesses, most Valley waste water travels along some of the system's 6,400 miles of sewers on its way to the Donald C. Tillman Water Reclamation Plant. Thereafter approximately 11 hours of treatment it is ready to be reused. Before Lake Balboa opened last year, most of the treated waste water ended up flowing down the Los Angeles river to the harbor. Now the water refills the lake.

Other uses under consideration include irrigating nearby golf courses and making the water clean enough to drink by pumping it into the Valley's underground aquifer for natural filtration. Tillman is one of four waste water treatment plants operated by the Los Angeles Bureau of Sanitation. Together they treat waste produced by 4 million people in a 600-square-mile area. The plant, which opened in 1985, operates around the clock and employs more than 100 people. It processes 65 million to 70 million gallons of raw sewage daily. The Los Angeles-Glendale plant at the other end of the Valley treats waste water from the Glendale/Burbank area.