Factors that must be considered in designing conventional aerobic digesters include method of operation, tank volume and detention time, tank design, and aeration and mixing equipment. Typical design criteria for aerobic digestion are presented in Table 4.1.
Method of Operation The two primary operational modes for conventional aerobic digesters are batch or continuous operation, referring to the manner in which supernatant is withdrawn from the process. Figure 4.3 shows cross sections of digesters with the two modes of operation. Batch operation is typically used for small-capacity digestion systems because of the relative simplicity of operation. The continuous mode allows regular operation without interrupting the oxygenation and mixing equipment. Although baffles and a stilling well can be incorporated inside the digester to separate the supernatant, as shown in Figure 4.3, a separate settling tank is recommended, as shown in Figure 4.2, because of the mixing-induced turbulence carrying into the stilling well. Design of separate settling basins is similar to the design of gravity thickeners or flotation thickeners for WAS. For gravity thickeners, surface loading rates ranging from 25 to 50 kg/m2-d (5 to 10 lb/ft2-d) are used. For flotation thickeners, much higher loading rates of 50 to 100 kg/m2-d (10 to 20 lb/ft2-d) are allowed.
TABLE 4.1 Design Criteria for Aerobic Digester
U.S. Customary Units
SRTa At 20°C At 15°C Volatile solids loading Oxygen requirements Cell tissue applied' BOD in primary sludge destroyed Energy requirements for mixing Mechanical aerators Diffused air mixing Dissolved O2 residual in liquid Reduction of VSS
hp/103 ft3 cfm/103 ft3
Source: Adapted from Metcalf & Eddy, 2003.
" To meet pathogen reduction requirements (PSRP) of 40 CFR Part 503 regulations. b With complete nitrification.
Temperature °C x Sludge Age, days
Figure 4.5 Volatile solids reduction versus liquid temperature and sludge age.
Tank Volume and Detention Time The tank volume of an aerobic digester is governed by the detention time necessary to achieve the desired volatile solids reduction. Solids destruction has been shown to be primarily a direct function of both liquid temperature in the digester and the SRT (sludge age), as illustrated in Figure 4.5. This figure is a plot of volatile solids reduction versus the parameter degree-days (temperature x sludge age) and is derived from data taken from both pilot- and full-scale studies on several types of municipal wastewater sludges. In the past, aerobic digesters were designed for a detention time ranging from 10 to 20 days to achieve a 38% volatile solids reduction. However, to meet the pathogen reduction requirements of 40 CFR Part 503 regulations, the SRT criteria of 40 days at 20°C and 60 days at 15°C (see Table 4.1) take precedence over the vector attraction criteria of 38% volatile solids reduction.
In some of the extended aeration wastewater treatment facilities with excessive detention time, 38% volatile solids reduction may not be achievable. In such instances, vector attraction reduction of aerobically digested sludge can be demonstrated, following the 503 regulations, by aerobically digesting a portion of the digested sludge from the digester having a solids concentration of 2% or less in a laboratory bench-scale unit for 30 days at 20°C. Vector attraction reduction is achieved if the volatile solids reduction is less than 15% from the beginning to the end of the 30-day period. In addition, vector attraction reduction, also based on Part 503 regulations, can be achieved by using the specific oxygen uptake rate (SOUR) criteria of less than 1.5 mg of oxygen per hour per gram of total solids at a temperature of 20°C.
The volume of an aerobic digester can be determined by using the equation (WEF, 1998)
V = volume of aerobic digester, m3 (ft3)
Qi = average flow rate to digester, m3/d (ft3/d) Xi = influent suspended solids, mg/L
Y = portion of influent BOD consisting of primary solids, % S = influent BOD, mg/L
X = digester suspended solids, mg/L Kd = reaction rate constant, d-1 Pv = volatile fraction of digester suspended solids, % SRT = solids retention time, d
The term YSt can be disregarded if no primary sludge is included in the load to the digester. If the aerobic digestion process is operated in a staged configuration with two or three tanks in series, the total SRT should be divided approximately equal among the stages.
Tank Design Aerobic digesters can be designed with rectangular, circular, or annular geometry. In circular tanks where draft-tube air mixing is used, bottom slope in the tanks typically range from 1 : 12 to 1 : 4. Digesters where air diffusers are used for mixing and oxygenation, floors are generally flat, although a gentle slope may be provided to a small sump to facilitate biosolids removal or draining the tank if required. Side water depths are similar to those provided for activated sludge systems, but with freeboards in excess of 1 m (3 ft) to contain excessive foaming that may occur. At least two tanks should be provided to permit draining and equipment repair. Multiple units are especially important in batch operation systems to provide digester capacity during supernatant discharge and biosolids removal cycle.
Aerobic digesters are typically uncovered tanks of concrete construction. Steel tanks are normally used in small plants. Several recent installations have included covered tanks, especially in extremely cold regions, in consideration of the temperature-dependent nature of the process.
Aeration and Mixing Equipment Several types of aeration devices have been used successfully to provide the oxygenation and mixing requirements of aerobic digesters. These include diffused air, mechanical surface aeration, mechanical submerged turbines, draft-tube aeration, jet aeration, and combined systems.
The design of diffused air systems is similar to that used in aeration basins in conventional activated sludge systems. Coarse-bubble orifice-type diffusers can be located along one side of the tank near the tank bottom in rectangular tanks to produce a spiral or cross-roll pattern. Floor-mounted grid systems with coarse- or fine-bubble diffusers can also be used. Advantages of diffused air systems include the ability to control oxygen transfer by varying the air supply rate, and addition of heat from compressed air into the digester with a resulting increase in the rate of biological activity. However, diffused air systems can have recurring clogging problems, particularly in batch systems where solids are allowed to settle.
Mechanical surface aerators are used primarily in large tanks. They are typically floating, pontoon-mounted devices of either low- or high-speed design. Low-speed design is more common. Mechanical surface aerators have relatively high oxygen transfer efficiencies and are low-maintenance devices. The main disadvantages are their lack of control of the oxygenation rate and their potential ability to destroy the structure the solids flocs.
Mechanical submerged aerators combine several advantages and eliminate some disadvantages of the diffused air and surface aeration devices. Draft tube aeration devices are similar to the gas mixing devices used in anaerobic digesters. They are used in circular tanks. Jet aeration devices have somewhat higher oxygen transfer efficiency than submerged turbines. However, problems with plugging of the devices have occurred in the past where liquid flow paths have not been large enough to pass stringy solids commonly found in aerobic digesters. Combined systems of diffused air and submerged mixers may be required in digesters with high solids concentra tions to keep the solids in suspension. Additionally, the submerged mixers in such a system can be operated as a mixer only, thereby promoting denitrification.
Design Example 4.1 Design a batch-operated aerobic digester system to treat waste activated sludge with 4000 lb/d (1814 kg/d) of solids to achieve a minimum 40% volatile solids reduction in winter. Compare it with a continuous-flow system. Assume the following operational parameters:
minimum digester liquid temperature in winter: 15° C maximum digester liquid temperature in summer: 23° C concentration of thickened WAS from gravity thickener: 2.5% VSS in feed sludge: 78% of TSS in feed sludge reaction rate constant Kd: 0.06 d-1 at 15° C average solids concentration of liquid in digester:
batch-operation system: 70% feed sludge concentration continuous operation system (with decanting and recycle): 3%o
Neglect the specific gravity of sludge.
4000 lb/ d
1. Thickened sludge flow rate =
(8.34lb/gal)(0.025) = 19,184 gpd = 2565 ft3/d (73 m3/d)
2. SRT must be a minimum of 60 days to meet the pathogen reduction requirements (see Table 4.1); therefore, required digester volume = (2565 ft3/d)(60 d)
Compute the required volume using equation (4.10).
Use the larger of the two volumes, 153,900 ft3 (4359 m3). Provide two tanks of 76,950 ft3 (2179 m3) each, 70 ft (21.3 m) in diameter and 20 ft (6.1 m) in sidewater depth with 3 ft (0.9 m) of freeboard; or 62 ft (19.0 m) square and 20 ft (6.1 m) in sidewater depth with 3 ft (0.9 m) of freeboard.
3. VSS reduction: For winter conditions, degree-days = (15°C)(60 d) = 900 degree-days. From Figure 4.5, the VSS reduction for 900 degree-days = 45%. This exceeds the winter requirement of 40%, hence OK.
For summer conditions, degree-days = (23°C)(60 d) = 1380 degree-days. From Figure 4.5, the VSS reduction for 1380 degree-days = 49%.
4. Mass of VSS reduction:
= 3120 lb/d (1415 kg/d) VSS reduction in winter = (3120 lb/d)(0.45)
= 1404 lb/d (677 kg/d) VSS reduction in summer = (3120 lb/d)(0.49)
5. Oxygen required: From Table 4.1, 2 lb (2 kg) O2 per 1b (kg) of VSS applied is required.
Note: Some designers use 2.0 to 2.3 lb/lb (2.0 to 2.3 kg/kg) VSS destroyed instead of VSS applied. This will not, however, give adequate mixing energy. If primary sludge is also included in the feed to the digesters, additional air is required to oxidize the organic solids in the primary sludge. In such instances, the total air required should be compared to the maximum mixing energy requirement and should be adjusted accordingly.
O2 required = (3120lb/d)(2lb/lb-d) = 6240 lb/ d (2831 kg/d)
6. Air required:
6240 lb/d standard air =
(0.075 lb/ ft3 )(0.232) = 358,621 ft3/d (10,156 m3/d )
After making corrections for plant elevation, ambient temperature, and a and p coefficients, assume an oxygen transfer efficiency of 10%:
airflow rate/unit volume =
153,900 ft3/1000 = 16cfm/103 ft3 (0.02 m3/m3 • min)
Qt= 19,184 gpd
Figure 4.6 Mass balance with decanting and recycling.
Note: This is less than the minimum required that is listed in Table 4.1. Therefore, additional air or supplemental mixing devices must be provided to mix the digester content properly. Consult equipment manufacturers for the type of mixing devices and the horsepower required. Assuming 0.5 hp per 103 ft3 for supplemental mixing, the mixing energy required is 40 hp for each of the two digesters. An alternative is to provide mechanical aerators for oxygen transfer and mixing in lieu of diffused air. This problem is normally encountered when digesters are designed for the 60-day SRT at 20°C per the Part 503 regulations. 7. To design a continuous-flow digestion system, a mass balance with decanting and recycle (Figure 4.6) should be prepared as follows:
non-VSS in feed sludge = (4000 - 3120) lb/d = 880 lb/d total feed solids not destroyed = (3120 x 0.6 + 880) lb/d
From the mass balance diagram,
Assuming a TSS value of 300 mg/L in supernatant and digested sludge concentration of 3.5% yields
Multiplying formula (1) by 0.0025 gives
0.2669Qd = 2703
fraction of feed solids not destroyed =-
Let the digester solids concentration = C, and let the recycle ratio R = Qr/Qf. Then
Therefore, the recycle of thickened biosolids should be 265%, or 35 gpm on a continuous basis, to maintain the 60-day SRT. From Figure 4.1, for 40% VSS reduction, degree-days required = 475
SRT required in winter = -^j- = 32 days total solids in digester SRT =-
total solids removed from the digester/day = total solids in digester total solids removed/day + solids lost in supernatant/day
(volume of digester V) 60 = (solids concentration in digester)
Provide two tanks of 47,728 ft3 (1352 m3) each, 55 ft (16.7 m) in diameter and a 20 ft (6.1 m) sidewater depth with 3 ft (0.9 m) of freeboard; or 50 ft (15.2 m) square and 20 ft (6.1 m) of sidewater depth with 3 ft (0.9 m) of freeboard.
Air required is same as computed before, which is 2490 cfm (35 m3/ m3 • min).
airflow rate/unit volume = 2490 cfm
Note: According to Table 4.1, the airflow rate is adequate for mixing. Conclusion: The digester volume is reduced by 38% with a continuous-flow digester with recycle. 8. The 60-day requirement follows the Part 503 regulations for pathogen reduction requirements. However, an SRT of only 32 days is required for 40% VSS reduction at 15°C. The total volume require for this condition is 82,075 ft3 (2324 m3), which is only 53% of the volume for the 60-day SRT requirement. However, to meet the pathogen reduction requirements, testing is required as described in Section 4.1.
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