Biological Treatment

Generally, the colloidal and soluble substances found in contaminated stormwater can contain biological oxygen demand (BOD) that cannot be removed through sedimentation or filtration/ straining. In order to remove these substances biologically, the stormwater would need to contact with microorganisms that through metabolic processes remove the BOD. Unlike wastewater treatment, in which a continuous supply of waste is available to be treated, the discontinuous supply of contaminated stormwater presents a unique problem that needs to be addressed in order to make biological treatment applicable. The biological treatment process relies on the supply of contaminants within the wastewater as a food source to keep the microorganisms alive and thriving. Hence, the lack thereof during dry weather could cause the failure of such a treatment system.

Several methods of maintaining a biological treatment system during dry weather include (a) during dry weather using the system to treat sanitary flows, (b) using a combination of storage and treatment to maintain the system, and (c) if another biological treatment system is on site, using the waste biological solids to maintain the level of microorganisms. In the first case, the substitution of sanitary flow for storm flow, if adequate sanitary flow is present, would maintain tlud<l hHll hthiln tlud<l hHll hthiln

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Figure 13 Air flotation unit (from Ref. 43).

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Figure 13 Air flotation unit (from Ref. 43).

the biology of the system. During dry weather, the treated sanitary discharge could be made to the existing sanitary municipal system if available.

In the second case, storage is used along with a biological treatment system sized to work in a manner that keeps it running during dry periods. This treatment system would operate during dry weather using the contaminated stormwater captured during wet weather within the storage tank. Naturally, the system would operate at average to peak capacity during a storm.

In the third case in which a sanitary treatment system is on site or nearby, the level of active organisms would be maintained by importing organisms from the other facility. This method would likely be subject to the operation of the facility supply, the microorganisms, and may not be as reliable as the other two strategies. Biological systems that may have better application for contaminated stormwater treatment include trickling filters, rotating biological contactors, and aerobic/facultative lagoons. These are illustrated in Fig. 14 [46], Fig. 15 [43], and Fig. 16 [43]. In certain cases, land/wetland application may also be applicable. All of these systems have the common element of long residence time for solids and ability to maintain biomass. Expected treatment efficiencies range from 75-95% BOD5 reduction. Details for the performance of these systems are widely available in EPA publications and other chapters in this handbook.

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