Summary

Aerobic digestion is practiced in many industrial and a few municipal treatment facilities. However, because specific or technical data are lacking, the inference is that aerobic digestion is a new, unproven technique for solids handling.

However, the aerobic digestion process has many potential benefits and warrants attention by design engineers.

Advantages

Aerobic digestion has the following advantages:

1. A biologically stable end product is produced.

2. The stable end product is relatively noxious; hence, land disposal by holding lagoons or spray irrigation is recommended.

3. Due to simplicity of construction, capital costs for the aerobic system are low compared with anaerobic digestion and other solid handling schemes.

4. Aerobically digested sludge generally possesses efficient dewatering characteristics. It drains well when placed on a sand bed and resists rewetting during rainfall.

5. Volatile solids reduction equivalent to the anaerobic digestion results is possible with aerobic systems in treating secondary sludges.

6. Supernatant (floating) liquors from aerobic digestion possess a lower BOD content than those from anaerobic digestion. The aerobic supernatant commonly possesses a BOD of less than 100 mg/l. This advantage is significant because many conventional biological treatment plants are overloaded due to recycling of high-BOD supernatant liquors from anaerobic digestors.

7. Fewer operation problems occur in aerobic digestion processes than in the more complex, anaerobic process due to the higher stability of the aerobic system. Therefore, lower maintenance costs and less skillful labor are needed with an aerobic facility.

8. Aerobically digested sludge has a higher fertilizer value than that resulting from anaerobic digestion.

Disadvantages

The process has the following disadvantages:

1. High power costs generate higher operating costs compared to anaerobic digestion. The difference in operating cost is not significant with smaller treatment plants but is important with large facilities.

2. Gravity thickening processes following aerobic digestion generate high solids concentrations in the supernatant.

3. Some aerobically digested sludges do not dewater easily in vacuum filtration equipment.

4. No methane gas is produced as a by-product because the process is aerobic.

5. The solids reduction efficiency of the aerobic digester can vary with extreme changes in ambient temperature, which subsequently affects the aeration basin temperature.

The aerobic digestion process is well suited for industrial sludge treatment and small, municipal, activated-sludge plants. The industrial community favors aerobic digestion because of the low capital investment and simple operation. Industry often uses mechanical aerators in inexpensive open tanks followed by holding or disposal lagoons. Although a difference in emphasis exists at municipal waste treatment plants with regard to economics, logically, aerobic digestion should be evaluated, particularly for activated-sludge facilities.

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