The addition of solids recycle can be a reliable method of producing an effluent meeting stringent ammonia limits. With the addition of recycle, a lagoon is converted to a low mixed liquor suspended solids (MLSS) activated sludge system. This can be accomplished using an external clarifier and adding a pump to return solids to the headworks. The BIOLAC® process uses an internal clarifier. Effluent from the clarifier is discharged to disinfection or routed through subsequent cells of the lagoon system.
Successful operation of a low-MLSS activated sludge system requires that the recycled solids be kept in suspension. The aerated lagoon must be kept completely mixed. In most cases, a portion of the existing lagoon is partitioned into a complete mix cell because the power required to mix the cell is far greater than that required to reduce the BOD or nitrify the ammonia. The remaining portion of the system is used for polishing the effluent or to store the water before discharge.
Because the recycle system is an activated sludge variation, it can be designed and operated with traditional activated sludge design methods. Floating baffle curtains with exit ports are frequently used for cell partitioning. Excess sludge wasting can be accomplished in a separate holding pond, or downstream cells of the existing lagoon can be used to store and treat sludge for disposal.
TABLE 4.23 Average Values
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