Controlling Toxicity

There are several operational measures that can be used to minimize or prevent toxicity in a biological treatment process. These measures include the following:

• Identify potential toxic dischargers.

• Identify potential toxic wastes.

• Monitor and regulate the discharge of potential toxic wastes.

• Develop simplistic and reliable indicators of toxicity in the treatment process.

• Regulate solids inventory in reaction tanks.

• Use granular activated carbon (GAC).

• Use coagulants and polymers.

Potential toxic dischargers include industrial and commercial establishments. These establishments can be identified by the quantity and quality of wastes discharged to the sanitary sewer, the time of day of discharge, and whether a discharge produced an unacceptable condition such as a high pH or low pH within the sanitary sewer or treatment process. Identification of potential toxic dischargers and toxic wastes should incorporate the periodic review of material safety data sheets (MSDS). The review should include MSDS of all cleaning agents that are used at industrial and commercial establishments including hospital and school cafeterias and hospital laundry facilities.

The discharge of potential toxic wastes should be closely monitored and regulated to prevent toxicity in the treatment process. The discharge should be regulated or equalized to ensure a constant low concentration of the toxic waste enters the treatment process over designated time periods. Careful regulation of the discharge of a toxic waste can acclimate the treatment process to accept higher concentrations of the toxic waste over time.

Acclimation of the treatment process is slow and develops a biomass that can survive and treat wastes efficiently in a new environment or condition such as gradually increasing concentrations of a toxic waste. Acclimation should be quickly terminated if signs of toxicity occur. Shock loads of toxic wastes are unacceptable and should be avoided. If a shock load should occur, the load may be diverted to a temporary storage facility and then slowly discharged to the treatment process.

Simplistic and reliable indicators of toxicity should be developed and used on a routine or as needed basis in order to quickly identify toxicity and initiate appropriate process control measures. These indicators may be biological, chemical, or physical and should be applicable for the activated sludge process (Table 19.14) and the anaerobic digester (Table 19.15).

Microscopic indicators of toxicity can be mimicked for training purposes. For example, healthy mixed liquor should be observed under the microscope and then the mixed liquor can be "spiked" with a suspect toxic waste such as a heavy metal or surfactant that may enter the treatment process as identified by an MSDS. After the healthy mixed liquor has been spiked with the toxic waste, the mixed liquor and toxic waste should be allowed to react during an aeration period and a sample of spiked mixed liquor can then be examined microscopically. The healthy mixed liquor and spiked mixed liquor should be examined for differences in (a) quantity of dispersed growth and particulate material in the bulk solution, (b) shapes, sizes, and strength of floc particles, and (c) activity and structure of protozoa and metazoa.

Solids inventory in the activated sludge process can be regulated to either degrade the toxic wastes entering the treatment process or minimize or prevent tox-icity. Solids inventory can be regulated through two techniques.

First, aeration tanks that typically are not used may be used to address toxicity concerns. Uniform solids concentration should be maintained in all aeration tanks. Aeration tanks (reserve solids or bacteria) that are not needed for daily treatment of wastewater should be placed "off-line" by closing the influent gate to these tanks. The off-line aeration tanks should be aerated and may be rotated daily (Figure 19.17).When a toxic waste enters the activated sludge process the off-line tanks may be placed "on-line" quickly by opening the influent gates. By placing reserve solids or bacteria on-line, the following conditions are established:

• The toxic mass-to-biomass ratio is lowered.

• More bacteria are available to degrade the toxic waste.

• An increase in aeration tank capacity (volume) that is produced by placing more tanks on-line provides for increased dilution of the toxic waste.

Second, reserve solids may be stored in the contact-stabilization mode of operation (Figure 19.18). Solids in the stabilization tanks are maintained at a higher concentration than solids in the contact tanks. Solids in the stabilization tanks are protected from a direct "hit" of toxic wastes.

Although off-line aeration tanks may not be needed to overcome toxicity for relatively long periods of time, the off-line aeration tanks provide for the following benefits:

• Increased volatile solids destruction

• Use of nitrate (NO3-) produced through nitrification for anoxic periods of 1-2 hours to destroy undesired filamentous organism growth

• Use of nitrate ions to strengthen floc particles

FIGURE 19.17 Rotating aeration tanks. Aeration tanks 1 and 2 are taken off-line during a fixed period of time (left), and then aeration tanks 1 and 2 are placed back on-line and aeration tanks 3 and 4 are taken off-line for a period of time (right).

FIGURE 19.18 Contact stabilization mode of operation.

FIGURE 19.18 Contact stabilization mode of operation.

• Cycling of nitrification and denitrification (anoxic) periods to reduce total nitrogen discharged from the treatment process

Granular activated carbon (GAC) can be used to overcome toxicity. GAC improves floc formation and removes many toxic organic compounds and some toxic inorganic compounds before they come in contact with large numbers of bacteria. GAC also helps to develop a dense or highly populated biomass that lowers the toxic mass-to-biomass ration. Coagulants and polymers also improve floc formation and remove some toxic wastes.

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