[FeiP0

The readily biodegradable substrate concentration of the influent wastewater will also affect the denitrification rate in an initial anoxic zone. Denitrification is rapid when readily biodegradable substrate is available, but is much slower when only slowly biodegradable substrate is present. This is because the use of slowly biodegradable substrate is controlled by the rate of hydrolysis, which is relatively slow under anoxic conditions. Consequently, if the amount of readily biodegradable substrate entering an initial anoxic zone is insufficient to remove the nitrate-N added, the anoxic zone must be large enough to provide time for the hydrolysis of slowly biodegradable substrate. Hydrolysis and fermentation of slowly biodegradable substrate in an upstream anaerobic zone can produce readily biodegradable organic matter that can pass into an anoxic zone and produce a high rate of denitrification there. 18

Significant fermentation will occur in some wastewater collection systems, resulting in a wastewater that contains sufficient quantities of readily biodegradable substrate (particularly VFAs) to allow efficient biological phosphorus removal and denitrification. Warm temperatures, low velocities, which minimize reaeration, and force main systems, which maintain the wastewater under anaerobic conditions and in contact with the fermentative bacteria that r,row as slimes on the walls of the collection system, provide ideal conditions for fermentation. When this does not occur in the wastewater collection system, the influent wastewater can be treated to convert slowly biodegradable organic matter into a more readily biodegradable form. As discussed in Section 11.1.2, fermentation is a developing technology that can be used to accomplish this conversion. Either the raw wastewater itself can be fermented, or primary solids can be separated and fermented. Solids fermentation is discussed in Chapter 13.