Process Loading Effects

The presence of certain contaminants in a reactor has been shown to depress the value of K_La for systems using porous diffusers. Any chemical, physical, or biological reaction occurring within the aeration tank that results in the removal of these con-taminants will directly affect K_La and alpha. This result is clearly seen in the spatial changes that occur in alpha with the level of treatment obtained. Studies conducted at the Madison, WI treatment plant equipped with dome diffusers revealed significant increases in αSOTE with increasing MCRT (Boyle, 1994). From 1984 to 1985, when the plant was not nitrifying, the MCRT averaged 2.4 days and the average αSOTE was 11.5 percent. In 1987, when the plant was nitrifying, the average MCRT was 14 days and the average αSOTE was measured at 17.1 percent. Rieth et al. (1995) showed that at a volumetric loading of 0.48 kg BOD₅/m³d (30 lb/1000 ft³d), a system that operated at an MCRT of eight days produced a significantly higher value of alpha than one that operated at two days. The wastewater treatment plant at Phoenix increased its MCRT from one day to 14 days to achieve nitrification. The diffusers were dome diffusers in the two parallel tanks. The αSOTE increased from a range of 6.9 to 7.2 percent for the one day MCRT to a range of 11.5 to 12.7 percent for the 14-day MCRT. The corresponding value of alpha increased from 0.24 to 0.39. In

still another study, the Los Angeles–Glendale facility tested the same basin under almost identical operating conditions but with two different MCRTs, 1.6 days and 8.8 days. The lower MCRT operation produced an αSOTE of 7.5 percent versus 11.6 percent for the higher MCRT mode of operation. The corresponding alpha values for these two operating conditions were 0.33 and 0.46 (Groves et al., 1992).

Data from 21 operating ceramic diffuser plants were plotted to illustrate the effect of MCRT (SRT) on alpha SOTE (EPA, 1989) and are shown in Figure 3.32. Although wide variations in system design and operation, as well as wastewater characteristics, are evident at these sites, it appears that a trend does exist between process loading and αSOTE. Nitrification plants have been highlighted to indicate their relative tance to the relationship. Tables 3.13 through 3.17 also illustrate the apparent impor-tance of process loading on alpha using nitrification as the measure of loading.

A review of the dynamics of αSOTE in a number of aeration systems suggests that several process variables affecting oxygen transfer are not clearly identifiable based on our current knowledge. For example, αSOTE data collected at Madison over an 800-day period (Figure 3.33) in the first pass of a three pass conventional plug flow system, demonstrate significant variability in SOTE with time. Some of this variability is attributed to wastewater characteristics but does not account for all of the variation.

Multiple linear regression of the data including independent variables of MCRT, F/M, volumetric loading, MLVSS, and airflow rates could only account for up to about 60 to 70 percent of the variability. Similar findings were described by Stenstrom (1994) for the Whittier Narrows treatment plant where 30 to 74 percent of the variability in αSOTE could be accounted for by F/M, airflow rate and time-in-service.

3.4.4 MIXING CHARACTERISTICS

In aeration tanks sufficient mixing is required both to disperse DO throughout the basin and to provide reasonably uniform solids concentrations throughout the liquid.

The former requirement is easier to meet than the latter. Deposition of suspended solids is undesirable in most aeration tanks (aerated facultative lagoons are one exception), and therefore, this requirement most often dictates mixing requirements.

With the exception of the horizontal flow systems, where mixing and aeration are separate functions, the aeration device is expected to deliver adequate oxygen to satisfy FIGURE 3.32 Effect of SRT on diffuser performance.

the oxygen demand and to provide sufficient energy to prevent solids deposition. In activated sludge systems that are completely mixed, oxygen demand typically dictates the aeration energy requirement. However, in plug flow activated sludge systems, mixing energy may dictate aerator design and operation at the effluent end of the process where oxygen demand is low and required airflow (or power input) is also low. This is more likely to be a problem with high efficiency aeration devices and/or with weaker wastewater.

In evaluating mixing requirements, different diffuser configurations exhibit very different mixing characteristics. Unfortunately, only very limited information has been published on minimum mixing requirements. The Aeration-Manual of Practice FD-13 (WPCF, 1988) specifies that for degritted wastewater, a velocity of about 0.15 m/s (0.50 fps) across the tank bottom is required. This is a difficult parameter to measure for many aeration systems. Another mixing parameter often used is the root mean square velocity gradient, G, described by Equation (3.3).

(3.3) Here, G is the velocity gradient, sec^–1, µ is the absolute viscosity, N-sec/m², and W is the power dissipation, W/m³, calculated by the following

(3.4) where E is the power, W, transferred to the fluid, and V is the liquid volume, m³. FIGURE 3.33 Variation in αSOTE in municipal plant. ((From Boyle, W.C. et al. (1994).

Oxygen Transfer Studies at the Madison Metropolitan Sewerage District Facilities, EPA 600/R-94/096, NTIS No. PB94-200847, EPA, Cincinnati, OH.)

G=

(

)

W=E V

The power transferred by a gas to a liquid may be calculated as

(3.5) where P₁ is the absolute pressure at the surface, kP_a, G_s is the airflow rate, m³/h, and P₂ is the absolute pressure at the depth of injection (Fair et al., 1968).

For mixing of biological solids, a recommended value of G ranges from 40 to 80 sec^–1. Combining Equations (3.3), (3.4), and (3.5) yields the following.

(3.6) Most often, rule-of-thumb mixing requirements are used for diffused air systems based on airflow per unit area or volume. For example, one manufacturer recom-mends a minimum mixing intensity of 0.6 to 0.9 m³/h-m³ (10 to 15 cfm/1000 cu ft) for grid systems and 0.9 to 1.5 m³/h-m³ (15 to 25 cfm/1000 cu ft) for a spiral roll system. These recommended values represent calculated values of G ranging from 80 to 125 sec^–1 for a 4.6 m deep (15 ft) aeration tank. Spiral roll systems may also be designed on the basis of airflow per unit length of the header; for example, 16.6 to 38.9 m³/h-m (3 to 7 cfm/ft). For a full floor grid, a minimum mixing requirement of 2.2 m³/h-m² (0.12 cfm/sq. ft) is specified (calculated G value of approximately 70 sec^–1 for a 4.6 m deep (15 ft) tank). The only data for aeration tank mixing reported in the recent literature was for an activated sludge dome grid configuration at Glendale, CA (Yunt, 1980). Measurements revealed no solids settling problems after two weeks of testing at airflow rates as low as 0.9 m³/h-m² (0.05 cfm/sq. ft) (calculated G value of 45 sec^–1). An examination of Tables 3.13 through 3.17 indicates that average airflow rates per unit area are normally higher than the minimum mixing requirements for grid configurations. Presently, there have been no recorded problems with solids separation in aeration basins at these levels of mixing intensity. (It should be noted that upon basin dewatering, operators often notice the accumulation of some solids, usually high-density grit, below the diffuser headers. This is normal and of little real concern unless primary clarifiers or degritting facilities are overloaded. In that case, upstream retrofitting of degritting operations is far more cost effective than efforts to suspend this heavier material in the aeration tanks through the use of greater mixing intensity.) At the present time there is no standard method prescribed for specifying mixing requirements for aeration devices. Over time, operational experience will reveal whether the current rule-of-thumb values are acceptable.

3.4.5 DIFFUSER FOULING

All porous diffusers are susceptible to buildup of biofilms and/or deposition of inorganic precipitates that can alter the operating characteristics of the diffuser element. Porous diffusers are also susceptible to air-side clogging of pores due to particles in the supply air. There is a history of diffuser fouling problems in the U.S.

E=^{0 277.} P G ln P P¹ _s

(

)

G V_s =^{3 61.} G^2µ P ln P P¹

(

)

since the introduction of ceramic plate diffusers in the 1910s (Boyle and Redmon, 1983). Numerous mechanisms have been cited and foulants identified. The list includes the following:

Air Side

• dust and dirt from unfiltered air

• oil from compressors or viscous air filters

• rust and scale from air-pipe corrosion

• construction debris

• wastewater solids intrusion due to power outages or breaks Liquor Side

• fibrous materials attached to sharp edge

• organic solids entering media at low pressures

• oils and greases in wastewater

• precipitated deposits, including iron and carbonates, on and within media

• biological growths on and within media

• inorganic and organic solids entrapped by biomass on or within media The rate of fouling has historically been gauged by the rise in back pressure while in service. Since significant levels of fouling can take place with little or no increase in back pressure but with substantial reductions in OTE, this method provided only a crude and qualitative estimate at best. In fact, by the time back pressures were significantly high enough to observe, fouling may have reached serious proportions within the system. What is often observed is that as one diffuser becomes fouled and less air is distributed to that diffuser, others receive more air and little change is noted in line pressure. Maldistribution of air along the air header exacerbates the problem;

the diffuser with low airflow fouls more rapidly, and grid airflow regimes deteriorate to major turbulence. All of this results in poor OTEs and increased power consump-tion. Better methods of measuring the degree of fouling and the effectiveness of cleaning have been developed (EPA, 1989). These methods include DWP, EFR, off-gas methods to evaluate OTE, and the use of portable diffuser headers that can be removed from the basin and examined for fouling potential. This latter method is recommended where wastewaters may be potentially problematic with respect to liquor-side fouling. DWPs may now be monitored in situ on selected diffusers. Off-gas measurements may be conducted routinely to evaluate changes over time in OTEs.