Rapid Nonspecific Control Measures

Rapid, nonspecific control measures usually are used without the identification of the problematic filamentous bacteria or their operational conditions that permit their undesired growth. These measures consist of (1) adjustment of RAS rate, (2) manipulation of the substrate feed point to the aeration tank, (3) addition of a coagulant to the secondary clarifier influent, (4) addition of a polymer to the secondary clarifier influent, and (5) the addition of a toxicant.

The first four rapid, nonspecific control measures do not control the growth of the filamentous bacteria but, instead, control the operational problems associated with the undesired growth—that is, settleability problems and loss of solids. The RAS rate may be increased to remove solids more quickly from the secondary clarifier. However, an increase in RAS rate may shear floc particles and decreases the hydraulic retention time (HRT) in the aeration tank resulting in decreased treatment efficiency.

The substrate feed point (primary clarifier effluent) to the aeration tank may be manipulated to partition the mixed liquor suspended solids (MLSS) into zones of high solids concentration and low solids concentration (Figure 15.3). With the zone of low solids concentration discharging to the secondary clarifier, the solids loading to the secondary clarifier is decreased and improved settleability occurs in the secondary clarifier. However, manipulating the substrate feed point results in decreased treatment efficiency.

A coagulant or metal salt such as lime (Ca(OH)2) or a cationic polymer can be added to the secondary clarifier influent to improve settleability. Coagulants and polymers improve settleability by (1) increasing the floc particle density, (2) overcoming interfloc bridging and open floc formation, and (3) decreasing the surface area of the floc particle and filamentous bacteria. Coagulants add weight to the floc particles, while polymers remove large quantities of fine solids from the bulk solution.

The only rapid, nonspecific control measure that controls the growth of the filamentous bacteria is the addition of a toxicant. There are two toxicants that

148 FILAMENTOUS BACTERIA Mixed liquor influent

Mixed liquor effluent to secondary clarifier

Return activated sludge (RAS)

Mixed liquor influent

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Mixed liquor effluent to secondary clarifier -►

Mixed liquor effluent to secondary clarifier -►

Return activated sludge (RAS)

FIGURE 15.3 Change in mixed liquor influent feed point to reduce solids loading to the secondary clarifier. By taking one-half of the influent flow (top) and discharging it to the rear portion of the aeration tank (bottom), a hydraulic gradient is established across the aeration tank. The hydraulic gradient results in the partitioning of the solids in the aeration tank into two zones. The first zone at the beginning of the tank contains a high concentration of MLVSS, while the second zone at the end of the tank contains a low concentration of MLVSS. Only the zone of low concentration of MLVSS is discharged to the secondary clarifier. This results in a decrease in solids loading upon the clarifier and reduced settleability problems.

commonly are used for filamentous bacteria control. These toxicants are chlorine and hydrogen peroxide (H2O2). Chlorine is less expensive than hydrogen peroxide and usually is available on-site at wastewater treatment plants as a disinfectant for the final effluent.

Chlorine may be applied as gaseous chlorine (Cl2) mixed with final effluent to produce free chlorine as hypochlorous acid (HOCl) and hypochlorous ion (OCl-). Chlorine also may be applied as a solution of calcium hypochlorite (Ca(OCl)2) or sodium hypochlorite (NaOCl). Chlorine may be introduced into the activated sludge process in (1) the RAS line, (2) a sidestream, (3) the aeration tank, and (4) mixed liquor effluent (Figure 15.4).

(3) directly to the aeration tank, and (4) mixed liquor effluent.

The usual starting dose of chlorine for the control of undesired filamentous bacteria is 2-3 pounds of chlorine per 1000 pounds of mixed liquor volatile suspended solids (MLVSS) per day. To ensure the destruction of an adequate number of filamentous bacteria via chlorination and prevent the overchlorination of the activated sludge process, it is necessary to periodically monitor the impact of chlorine upon the biomass and the final effluent and adjust the dose of chlorine as needed.

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