---

BACKGROUND

The Trancas Water Pollution Control Plant (WPCP) is located at 6338 Paseo Canyon Drive Malibu, California. The County of Los Angeles Department of Public Works operates the TrancasWPCP, which treats domestic wastewater on behalf of the Malibu West Community and Lechuza Community in the City of Malibu.

WASTEWATER TREATMENT PROCESSES

The Trancas WPCP is a secondary wastewater treatment facility with filters. It was designed to treat 85,000 GPD (average daily dry weather flow) of domestic wastewater and maximum daily wet weather flow of 220,000 GPD. Treatment processes include comminution, extended aeration biological treatment with anoxic zone for denitrificaiton, secondary clarification, sand filtration, and chlorine disinfection. The treated wastewater is discharged into leach fields for disposal. Figure 3-1 is a Flow Diagram of the treatment process. The following is a brief discussion and description of the treatment plant processes.

Figure 3-1 Flow Diagram of treatment process

Preliminary Treatment

Preliminary treatment consists of an influent channel, comminutor and ultrasonic flow meter. The flow enters the treatment plant through an 8-inch gravity line into the influent channel and flows to the comminutor. The comminutor cuts and shreds large pieces of material into smaller pieces. This minimizes clogging and plugging problems in the downstream treatment processes. A by-pass channel with a manual bar rack is provided if the comminutor needs to be serviced. The influent flow is measured by a 3 inch parshall flume downstream from the comminutor and the bar rack.

Secondary Treatment

Secondary treatment consists of a dual-train activated sludge package plant with integral secondary clarifiers and anoxic chambers. The wastewater is pumped from the influent pump station to the anoxic tanks, an oxygen deficient environment, where it undergoes a denitrificaiton process. Flow continues to the aeration tanks where the wastewater is aerated by course bubble diffusion. The aeration tanks provide a location where biological treatment of the wastewater takes place. In these tanks, microorganisms remove organic material from wastewater and form settleable solids. After the aeration tanks, the flow enters the secondary clarifier basins where the solids are settled and pumped by submersible pumps to either the anoxic tank (return activated sludge) or to the aerobic sludge holding tank (waste activated sludge).

Filtration

The secondary effluent flows into a dual-media sand filter for the removal of suspended solids. The filters are sized so that filtration rates at the design maximum daily flow of 85,000 gpd are as follows:

All filters in service 2.90 gpm/sf

One filter out of service 4.35 gpm/sf

The filter has three individual filtration cells containing dual-media (a layer of sand with anthracite on top). Unique to anthracite is its very large void ratio and ample solids holding capacity. Therefore, water passing through the filters dual-media consistently yields a good filtrate quality. The filters were not designed to produce a tertiary effluent quality. The main function of the filters is to enhance the percolation of the effluent into the leach field disposal system.

Disinfection

The filtered effluent is pumped to the chlorine contact tank where a Sodium Hypochlorite (typically 12% NaOCl solution) is dispensed, killing the remaining microorganisms in the filtered effluent. Chlorinated filter effluent is processed through the chlorine contact tank, which provides a contact time of approximately 50 minutes at the maximum daily flow of 220,000 gpd.

Effluent Disposal

Following disinfection, the final effluent is discharged to a leach field disposal system located south of the plant where it is allowed to percolate into the ground.

Sludge Disposal

Sludge generated from the plant goes into the sludge holding tank. The sludge is hauled in tanker trucks to the City of Los Angeles-owned Tillman Water Reclamation Plant for treatment and disposal.