Wastewater Characteristics

The presence of surfactants and dissolved solids in wastewater cause changes in bubble shape and size once the bubble begins to rise through the liquid. They also may change the rate of surface renewal at the air-water interface. The mechanisms causing both the changes in bubble geometry and the film surrounding the bubble have been addressed in Chapter 2. The effect on surface renewal rate of the air-water interface is most significant when bubble motion is either spiral or zigzag, charac-teristics most commonly found in fine bubble aeration systems. As a result, the impact of these contaminants is more pronounced in porous diffuser systems than in those producing coarser bubbles. In fact, systems that continuously form fresh air-liquid interfaces through violent mixing are usually not adversely affected by surfactants and may even exhibit alpha values above 1.0 by virtue of the production TABLE 3.17

Process Water Oxygen Transfer — Horizontal Flow

Layout^a CC, ceramic tube 2.06–2.11 5.0 2.1–2.3 0.36–0.38 No Marx & Redmon,

1991 CC, perforated

membrane tube

Stage 1 1.80 4.8 2.0 Yes Marx & Redmon,

1991

CC, ceramic tube 10.9–12.1 4.6 2.3–2.5 0.54–0.56 Yes Groves et al., 1992 CC, ceramic tube 21.5–34.8 4.6 1.6–2.1 0.43–0.58 Yes Groves et al.,

1992 FD, perforated

membrane disc

1.33 2.75 5.5^b 0.62 Yes Gillot et al., 1997

a CC — counter-current aeration; FD — fixed diffusers

bHorizontal velocity = 0.46 m/sec 1 m = 3.28 ft.; 1 m_N³/h = 0.64 scfm

of smaller bubbles (and therefore higher surface area to volume). However, one cannot necessarily assume that coarse bubble diffusers will always produce higher values of alpha than those diffusers producing fine bubbles. Downing and Bayley (1961) demonstrated that both fine and coarse bubbles produced similar values of alpha when rising in a narrow column. Thus, the degree of bulk mixing and the eddy diffusivity of oxygen are important determining factors of the effect of surfactants on alpha. Tables 3.13 through 3.17 illustrate that porous diffusers generally produce lower mean weighted values of alpha than nonporous devices with the exception of jet diffusers that generate a fine bubble. Although the values of alpha presented in these tables depend on several process and design variables for the specific plants tested, it is apparent that the average mean weighted values of alpha are less than 0.5 for porous diffuser systems and perhaps closer to 0.7 for the nonporous systems.

The impacts of process loading and flow regime are described in more detail in later sections.

Alpha in diffused air systems generally decreases with increased concentration of surface-active materials up to a point where further increases in concentration show little additional impact on alpha. The type of surfactant also plays an important role in the degree to which it affects the oxygen transfer coefficient (Figure 3.29).

The removal of these agents by sorption or biodegradation will decrease the impact of the contaminant on oxygen transfer. The wide variation in alpha noted in the tables is likely due to variations in wastewater strength and composition, both in time and space. Examples of this variation are presented in Table 3.18 for several porous diffuser aeration facilities. It should be emphasized that these values are for typical municipal wastewater with only small contributions of industrial wastes. The impact of industrial wastewater on alpha is highly wastewater specific and may or may not have a greater impact on the porous diffuser systems.Attempts to correlate wastewater effects on KLa with organic matter content have not resulted in any generalizations that can be successfully applied from site to site. Masutani and FIGURE 3.29 Effect of surfactant type and concentration on efficiency. (From M. Zlokarnik, Korrespondenz Abwasser, 11, p. 731, 1980. With permission.)

Stenstrom (1991) have demonstrated that dynamic surface tension was a potentially useful tool to determine the impact of wastewater on alpha. Observations of alpha values from different wastewater effluents have shown wide variations in the upper limit on alpha in porous diffuser systems even when quality is very high. It is apparent that very low concentrations of some surfactants may have a significant impact on oxygen transfer in these systems.

Although most effects of wastewater on alpha have been ascribed to surface-active materials, there is good evidence that salts also impact K_La. Hantz (1980) has shown that alpha significantly increases with increased specific conductivity. These laboratory studies were conducted with distilled water and mixtures of distilled water and tap water with a total dissolved solids concentration of about 600 mg/L. Sten-strom (1996) showed similar trends with the addition of sodium chloride to water.

He demonstrated that the high salt concentrations cancelled the effects of surfactants added to the mixture. For many years those that have performed clean water oxygen transfer tests with porous, nonporous, and mechanical aeration systems have noted that additions of sodium sulfite will elevate measured values of K_La (ASCE, 1992).

Attempts to model this effect have been successful for a given device but a rigorous model for all types of aeration systems has not yet been developed. The enhanced mass transfer coefficient occurs because higher salt concentration increases surface tension with concomitant finer bubbles (O’Connor, 1963; Marrucci and Nicodemo, 1967). The salt does not apparently affect surface renewal nor does it block transport at the air-liquid interface. Thus, K_La will increase as the surface area to volume ratio increases. Other salts, including the transition elements such as iron and manganese, may also affect the value of alpha.

The effects of wastewater on oxygen transfer also occur as a result of changes in the steady-state saturation concentration of oxygen as estimated by the factor TABLE 3.18

24-Hr Alpha and Alpha (SOTE) Variations at Selected Porous Diffuser Municipal Treatment Plants (EPA, 1985)

Process Ave. Min. Max. Ave. Min. Max.

CS Step 0.30 0.23 0.44 8.3 6.4 11.2 Influent Pass

C Plug 0.24 0.22 0.29 8.7 7.7 10.4 Inlet End

C Plug 0.46 0.44 0.59 10.7 9.5 13.1 Entire Basin Weighted

C Plug 0.25 0.21 0.27 7.8 6.4 8.7 Influent Grid

C^a Plug 0.26 0.20 0.30 8.7 6.6 9.9 Middle Grid

C Plug 0.45 0.41 0.50 12.2 11.1 13.5 Effluent Grid

C Step 0.23 0.19 0.28 — — — Influent Pass

C Step 0.39 0.33 0.45 — — — Effluent Pass

a Data for 6-hour period

CS — Contact Stabilization; C — Conventional

beta. Dissolved salts and organics tend to lower the saturation concentration of oxygen in wastewater as compared with distilled water. Although the activity of an oxygen-saturated solution of water is by definition independent of the dissolved contaminants, the concentration of oxygen changes as the activity coefficient is altered by the salting-out effect. This fact has important implications in the measure-ment of DO saturation under field conditions. Direct measuremeasure-ment of DO by the Winkler Method (APHA, 1995) is often complicated by oxidizing or reducing compounds in the wastewater. Membrane probes theoretically respond to oxygen activity that depends on the degree of saturation, not the absolute concentration.

Thus, a probe standardized in clean water will not necessarily yield a true reading of DO in contaminated water. As a result, the value of DO saturation in wastewater is usually estimated by means of a total dissolved solids concentration correction (Equation (2.32)). Typically this correction is small in most wastewater, and the error in this estimate will not be significant in estimating αSOTE (αSOTR). It can, however, be an important factor in some industrial wastewaters.