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Aerobic Stabilization Tanks
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8.1. Aerobic Stabilization Tanks
Background
Surplus activated sludge (hereinafter referred to as waste activated sludge or WAS, and secondary sludge), from all three treatment lines is pumped to aerobic stabilization tanks for digestion and stabilization of volatile suspended solids (VSS).
The plant has 4-40 m diameter, circular stabilization tanks, similar to secondary clarifiers, with a working volume per unit of 86,400 m3. The hydraulic retention time, based on a daily total average WAS volume of 24,207 m3/d, is 3.6 days.
Technical Assessment by COWI
COWI reasoned that because of the already high sludge age in the aeration tanks, only a minor additional amount of retention time is necessary for aerobic stabilization. The aerobic stabilization tanks were therefore assessed as being of sufficient capacity and no upgrading or improvements were to be made.
Technical Assessment by SNC-Lavalin
Aerobic digestion of WAS is generally limited to smaller treatment plants with capacities up to about 18,900 m3/d (5 mgd, US gallons). Although, in North America, the process has been successfully used in facilities with capacities up to 189,000 m3/d (50 mgd). The process is very energy intensive and for very high capacities, it becomes inefficient. Some of the advantages and disadvantages of the aerobic stabilization process for waste activated sludge are:
Advantages:
- A relatively inoffensive and biologically stable product;
- Low capital cost;
- Comparatively simple operational control with safe operation;
- Comparatively low odour potential;
- Production of non-explosive gas; and
- A relatively clean recycle stream.
Disadvantages:
- High power costs associated with aeration (the major disadvantage);
- Ammonia is released which requires about 29% of total oxygen for nitrification to nitrates;
- Alkalinity is consumed while oxidizing ammonia, which can lead to an acidic pH;
- Reduced cold weather efficiency;
- Dewaterability of aerobically digested sludge tends to deteriorate with increasing sludge age;
- Production of carbon dioxide rather than methane, which is released to the atmosphere;
- Non-production of methane gas from which energy can be recovered (as in anaerobic digestion);
- Typically high suspended solids (SS) and TKN (Total Kjeldahl Nitrogen) contents of supernatant which is returned to the aeration process.
Some observations of our review of the aerobic stabilization process at the Kyiv Vodokanal wastewater treatment plant include:
- The average sludge age (SRT) of the MLSS (mixed liquor suspended solids) in the aeration basins varies for Lines 1, 2 and 3, and was found to be 14.2 days in Line 1, 5.8 days in Line 2 and 17.7 days in Line 3.
- Based on total solids under aeration in the aeration basins of each treatment train, the weighted average SRL for the plant is 11.3 days. With an additional 3.6 days of retention time in the aerobic digesters, the total SRL available is about 14.9 days. This is insufficient to achieve complete stabilization of WAS solids, and it is concluded that the current aerobic stabilization process is insufficient.
- An assessment of biological sludge quantities and the aerobic stabilized sludge quantities given in the COWI Working Document No. 1, Table 7.13, leads us to conclude that very little VSS are actually digested in the aerobic stabilization process.
- Design guidelines in the 1970s in North America for aerobic digestion of WAS required a minimum SRT of 15 days. More recent regulations by the US EPA (United States Environmental Protection Agency) require:
- VSS destruction for WAS of about 35 to 45%; a minimum of 38% is objective;
- SRT of 40 days for WAS at temperature of about 20oC and 60 days for temperature of 15oC.
- It is observed that the need for very long retention times for sludge solids is due to the potential for vector attraction on sludge solids and the incomplete stabilization of pathogenic microorganisms.
- To overcome the uncertainty associated with SRT as a design criterion, EPA proposed the Specific Oxygen Uptake Rate (SOUR) as a better indicator to measure / assess the degree of sludge stabilization and vector attraction potential of sludge. On this basis, EPA has suggested a SOUR equal to or less than 1 mg of oxygen per hour per gram of TS (sewage sludge solids).
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