Improving aeration systems in saline water (part II): effect of different salts and diffuser type on oxygen transfer of fine-bubble aeration systems

Today, in biological wastewater treatment plants, mainly fine-bubble aeration systems are used to satisfy the oxygen demand of microorganisms in activated sludge (Wagner & Stenstrom 2014). Air is introduced via diffusers installed at the bottom of the aeration tank. From the rising air bubbles, the oxygen is transferred to the liquid phase. There are different diffuser types available (discs, plates, tubes) made out of various materials. Oxygen transfer from ascending air bubbles to the liquid phase is described by the volumetric mass transfer coefficient (k_La), which represents the product of the liquid-side mass transfer coefficient (k_L) and the liquid/gas interfacial area (a). The k_La mainly depends on water quality parameters (e.g. salt concentration, temperature) and diffuser design. Today, mainly some industrial wastewaters show increased salt concentration (Lefebvre & Moletta 2006; He et al. 2017). Nevertheless, the global trend towards saving fresh water, e.g. through (industrial)-wastewater reuse or by using seawater for toilet flushing (Sander 2018), will raise the volume of saline wastewater that needs to be (biologically) treated. Depending on the origin of the wastewater, the dissolved salt and salt mixture will change. Therefore, a deeper understanding of the effects of increased concentration of different salts and salt mixtures is crucial to enhance the (energy) efficiency of aerobic biological wastewater treatment.

For fine-bubble aeration systems, increased salt concentration (c_Salt) reduces k_L but increases a due to the inhibition of bubble coalescence. The increase is enough to result in a net increase of k_La (Baz-Rodríguez et al. 2014). The increase of k_La in saline water (SW) is described by the f_S value, which is the ratio between k_La in SW to k_La in tap water (TW). In Part I of the present work, we could show in laboratory-scale experiments with different conventional disc diffusers by combining bubble size measurement and oxygen transfer tests, that the length and density of the slits in the diffuser membrane influences f_S (Behnisch et al. 2018). In this case, the smaller the bubbles detaching from the diffuser (primary bubbles) the higher is f_S. In the early 1980s, Zlokarnik (1980) investigated the coalescence behaviour of different aeration devices commonly used at that time. He also concluded that the more the coalescence is inhibited, the more advantageous it is to use aeration devices, which produce small primary bubbles. Sander et al. (2017) carried out oxygen transfer tests at varying sea salt concentrations with disc diffusers in a pilot-scale test tank. They found the simple relationship f_S = k · c_Salt + 1, whereby k mainly depends on the airflow rate per disc diffuser. Nevertheless, f_S does not increase infinitely. At a certain c_Salt, the coalescence is completely inhibited and f_S reaches its maximum (f_S,max). This concentration is called the critical coalescence concentration (CCC) and is specific for each salt (solution) and bubble formation system (Cho & Laskowski 2002; Firouzi et al. 2015). Sovechles & Waters (2015) and Quinn et al. (2014) determined the CCC of varying salts in a lab-scale flotation cell by measuring the bubble size (given as Sauter mean diameter (d₃₂)) via image analyses of the bubble swarm at different c_Salt. Due to the inhibition of bubble coalescence with increasing c_Salt, d₃₂ decreases linearly (Zone 1) up to CCC (Grau et al. 2005). Here (Zone 2), d₃₂ reached its minimum (d_32,min), called the quasi-static-bubble diameter or initial bubble diameter (Marrucci & Nicodemo 1967; Sovechles & Waters 2015). According to Grau et al. (2005), the point at which the fitted lines for Zone 1 and Zone 2 meet defines the CCC. There are other methods to investigate the coalescence of a system than with image analyses. Craig et al. (1993) measured the coalescence by detecting the change of light intensity from an expanded beam of light, which was passed through the bubble swarm produced by a sinter at the base of the transparent test column. Prince & Blanch (1990) used a model-based approach to predict CCC for different salts using test results of Marrucci & Nicodemo (1967) and Lessard & Zieminski (1971). While Marrucci & Nicodemo (1967) also applied image analyses to determine the bubble size in a bubble swarm produced by a bronze porous plate, Lessard & Zieminski (1971) observed bubble pairs detaching at the end of two adjacent capillaries. Similar to CCC, some researchers also use the transition concentration (TC_X) to quantify the grade of decrease of bubble size with increasing c_Salt. The index X indicates the grade at which the bubble size reduces X% from that in pure water to the constant bubble size at high salt concentration. Accordingly, TC₁₀₀ and CCC are synonymous (Sovechles & Waters 2015). Currently, there is no definitive agreement explaining the inhibiting effect of salts on bubble coalescence. Firouzi et al. (2015) gives a detailed review of all proposed theories, which include colloidal forces, gas solubility, Gibbs-Marangoni effect, surface rheology and ion specific effects.

Experiments show that bubble formation, as well as bubble size, affect CCC (Firouzi et al. 2015; Sander 2018). Since the bubbles in a flotation cell, as used by Sovechles & Waters (2015), are very small and are formed by cavitation and not by a diffuser, the validity of the results for fine-bubble diffusers is questionable. Furthermore, to investigate the influence of salt on oxygen transfer, previous studies used only disc diffusers (Sander et al. 2017; Behnisch et al. 2018) or porous plates and frits (Marrucci & Nicodemo 1967; Zlokarnik 1980), which are no longer used today for wastewater treatment. Additionally, only the influence of NaCl and sea salt has been investigated so far.

Therefore, we determined for the first time CCC for different salts with conventional fine-bubble diffusers. Because measuring bubble size in a bubble swarm is laborious, we transferred the method of Grau et al. (2005) for determining CCC to our results of oxygen transfer tests. To do so, we took advantage of the fact that f_S behaves similarly to d₃₂ with increasing c_Salt, only with reversed sign. The f_S value increases linearly instead of a linear decrease in d₃₂. In order to investigate the transferability of the results to large scale as well as the influence of the diffuser type and diffuser density on f_S, we also carried out oxygen transfer tests with disc, tube and plate diffusers in a 17,100 L fully glazed steel frame tank at c_Salt between 0 and 20 g/L NaCl. Finally, we show the large improvement of aeration in SW by comparing specific standard oxygen transfer efficiency (SSOTE) and standard aeration efficiency (SAE) with TW conditions, and give some additional design considerations.

Six different conventional membrane diffusers (2 disc diffusers, 2 plate diffusers, 2 tube diffusers) with different slit densities (SD) and slit lengths (d_S) were used (Table 1). SD is defined as the number of slits relative to the active (perforated) membrane area (A_A). In the tube diffusers used here, perforation is only on the side of the diffuser and the top and bottom are not perforated (Figure 1). For plates and discs, A_A is similar to the projected media surface area (A_P). Depending on the percentage of perforated membrane area, for tube diffusers A_A ≫ A_P applies. According to Behnisch et al. (2020), diffuser density (DD) is the total membrane area (including unperforated area) divided by area of the tank floor.

Determining the exact salt concentration during the tests, especially in the large pilot-scale test tank, faced us with some challenges. Large amounts of salt were needed to achieve the desired concentrations (up to 342 kg for a single test). Additionally, the tests for each salt or diffuser type required several days and a regular addition of TW was necessary to compensate for evaporation losses. To ensure constant experimental conditions, an on-line measurement of salinity was necessary. Therefore, we decided to apply a method commonly used in environmental technology and determined c_Salt by measuring the conductivity (EC) on-site standardized at 20 °C water temperature with standard conductivity cell (TetraCon^® 324, WTW, Germany).

The correlation between EC and c_Salt is usually expressed by a simple linear equation: c_Salt = t · EC. However, the relationship between EC and c_Salt is not always linear and depends on the activity of specific dissolved ions, ionic strength (IS) and the average activity of all ions in the liquid (Rusydi 2018). Therefore, before starting oxygen transfer tests, we measured EC of salt solutions with a known concentration in a 500 mL beaker. Our results show that the relationship in the chosen concentration ranges is always linear. This can be seen, for example, in the calibration line for MgCl₂ in Figure 2(a). Therefore, the calibration line is given by the following formula: EC = b₁ · c_Salt + b₀, b₀ being the EC of TW. In all our tests, the EC of TW was 0.7 mS/cm. With the known calibration lines, we were able to calculate the salt concentration by measuring EC on site during oxygen transfer tests. When measuring EC in aerated tanks, care must be taken since air bubbles can lead to incorrect measurements.

To measure CCC for different salts, oxygen transfer tests were performed with Disc 1 in a rectangular tank (0.5 m × 0.5 m) with a water depth of 1.0 m, using the pure oxygen desorption method according to EN 12255-15 (2003). With this method, the oxygen concentration is increased 15–20 mg/L beyond the oxygen saturation concentration by aerating with pure oxygen gas or oxygen-enriched air (Wagner et al. 1998). By switching to aeration with ambient air, the oxygen concentration starts to decrease again until the saturation concentration is reached. From the curve of decreasing oxygen concentration, the k_La for a specific water temperature (T) is calculated (k_La_T) by nonlinear regression. Four electrochemical oxygen probes (Oxymax COS51D, Endress + Hauser, Germany) were installed to record dissolved oxygen concentration. Finally, the k_La was standardized to 20 °C (k_La₂₀) according to the equation: k_La₂₀ = k_La_T·1.024^(20-T).

Tests were performed at different airflow rates (2.0, 3.5 and 4.5 m³/h) and salt concentrations. The airflow rate was measured at standard temperature and pressure (0 °C; 101.3 kPa; 0% humidity) with a thermal flow sensor (TA16, Hoentzsch, Germany). The d_32,min was only measured during tests with NaCl by image analyses (Behnisch et al. 2018). Several experiments show that regardless of salt type, the minimum bubble size is always the same (Lessard & Zieminski 1971; Quinn et al. 2014). The first test was carried out in TW, then salt was added.

Figure 2(b) shows how CCC was determined using the example of test results with MgCl₂. According to the method we adapted from Grau et al. (2005), the CCC results from intersection of the fitted lines of Zone 1 (linear increase of f_S) and Zone 2 (constant f_S at f_S,max). Accordingly, CCC for MgCl₂ (CCC_MgCl2) is 0.065 mol/L or 6.2 g/L. To verify the reproducibility of the measurement, we repeated the experiment with a new disc diffuser of the same design. In the second run, we measured a CCC of 0.061 mol/L or 5.8 g/L, which represents a deviation of 6%. Therefore, the reproducibility of the results was good.

The pilot-scale test tank is a fully glazed steel frame tank (L/W/H = 3.0 m/1.5 m/4.0 m) with a water level of 3.8 m, yielding a water volume of 17.1 m³. Ten test series with different diffuser types (Figure 1) and diffuser densities were carried out in the test tank. Therefore, 32 or 6 disc diffusers, 10 or 6 tube diffusers and 4 plate diffusers were installed in the glass tank, respectively. Diffuser depth of submergence was on average 3.65 m and varied negligibly between the individual diffusers.

The oxygen transfer was measured as before in the lab-scale experiments by using the desorption method according to EN 12255-15 2003. As before during lab-scale experiments, dissolved oxygen in water is measured using four electrochemical oxygen probes (Oxymax COS51D, Endress + Hauser, Germany) installed at different positions in the test tank. The compressed air is produced by means of a positive displacement blower (GMA 10.0, Aerzener Machine Factory, Germany). To measure the airflow rate precisely, a calibrated rotary-piston gas meter (Aerzener G 65, type ZB 039.0, Aerzener Machine Factory, Germany) with manometer and thermometer was installed downstream of the blower. The airflow rate (Q_A) was normalized at standard temperature and pressure conditions (0 °C; 101.3 kPa; 0% humidity). Q_A is indicated per aerated tank volume (Q_A,VAT = Q_A · (tank volume)⁻¹), per disc diffuser (q_A,Disc = Q_A · (number of disc diffusers)⁻¹), per plate diffuser (q_A,plate = Q_A · (number of plate diffusers)⁻¹), per length of tube diffuser (q_A,tube = Q_A · (number of tube diffusers · length of single tube diffuser)⁻¹) or per slit of diffuser membrane (q_A,slit = Q_A · (number of diffusers · slits per diffuser)⁻¹).

Tests were performed at different airflow rates and NaCl-concentration (c_NaCl). The airflow rate set during the tests depends on operational range of diffuser specified by the manufacturer and the capacity of the blower. As described before, c_NaCl was calculated by measuring EC. Also, the standard oxygen transfer rate per aerated tank volume (SOTR_VAT = SOTR · (aerated tank volume)⁻¹) was calculated, SOTR being the standard oxygen transfer rate normalized to 20 °C water temperature and atmospheric pressure of 101.3 kPa (EN 12255-15 2003). Here it must be noted, that the solubility of oxygen (c_S) decreases with increasing c_Salt. The oxygen saturation concentration for different temperatures and salt concentrations can be taken from tables, to be found e.g. in ISO 5814 2012. According to ASCE/EWRI 18-18 2018, c_S in SW (c_S,SW) can also be estimated with known c_S in TW (c_S,TW) and c_Salt (in g/L) as follows: c_S,SW = (1.0–0.01 · c_Salt) · c_S,TW. However, when comparing c_S,SW calculated according to the given formula and the tabulated values, we found an increasing deviation with increasing c_Salt (see Annex 1). Therefore, we have adjusted the formula to calculate c_S,SW as follows: c_S,SW = (1.0–0.0059 · c_Salt) · c_S,TW.

In Table 2 determined dependencies between EC and salt concentration are listed. The coefficients of determination (R²) close to 1.0 prove the linear dependence between EC and c_Salt within the chosen concentration range up to maximum selected salt concentration (c_max). As previously mentioned, the relationship is influenced by many factors, and therefore the equations shown here are only valid in TW for the specified concentration ranges. To calculate the salt concentration in solutions with other background contamination (i.e. dissolved substances other than the salt to be measured), the equation may have to be adapted.

Results in Table 3 summarize the determined CCC for different salts together with values given in literature supplemented by information of the observed d_32,min, test setup and measurement method of coalescence. Regardless of salt type, f_S,max reached during CCC measurement varied marginally and was on average 2.2. This confirms several experimental studies (Lessard & Zieminski 1971; Quinn et al. 2014), which show that in coalescence inhibited systems, k_La and bubble size reach a fixed value regardless of salt type. During the tests of Sander et al. (2017), the bubble size was not measured and is therefore not available (n.a.). However, since similar fine-bubble diffusers were used, we can assume that similar bubble sizes were achieved as in our tests.

Already, Sovechles & Waters (2015) showed that CCC decreases for salts containing multivalent ions. While 1:1 (cation:anion charge) salts had the highest CCC; 1:2 and 2:1 salts had intermediate CCC; and 2:2, 3:1 and 3:2 salts had the smallest CCC. Therefore, they try to describe the inhibition of coalescence by salt (and multicomponent salt solutions) by calculating IS. In analogy to CCC, they postulated that coalescence is completely inhibited in their flotation cell when IS = 0.28, which they called Critical-Coalescence Ionic Strength (CCIS). To verify this, all results in Table 3 were supplemented by the IS we calculated. IS-values of salt solution by Prince & Blanch (1990) varies between 0.13 and 0.62. Also for our test results, IS fluctuates between 0.18 and 0.27. This variation is very large, which is why we cannot confirm the approach postulated by Sovechles & Waters (2015) to describe coalescence inhibition by determining the IS. Indeed, the CCC must be determined for each new (bubble formation) aeration system individually.

So even if the absolute values of the determined CCC do not match those from the literature, which is due to the different bubble size, it can still be said that they are in a specific order relative to each other, independent of the aeration system and the measurement method. The validity of the determined CCC and thus of the new measurement method is therefore given.

In the following, first we will present the results of the tests in the pilot-scale test tank for each individual diffuser type. Then we try to consider all the results together and discuss the different effects on f_S more in detail. At the end, we will discuss other parameters that have to be taken into account in design of aeration systems in SW in addition to k_La (or f_S).

In Figure 3,f_S-values from test series with disc diffusers are plotted against c_NaCL for different airflow rates together with the results from lab-scale experiments described in Part I of the present paper (Behnisch et al. 2018). For Disc 1 f_S increases up to 10 g/L NaCl, which corresponds to the CCC of NaCl (CCC_NaCl) previously determined in the lab-scale experiments. Then f_S reaches its peak value (f_S,max). The same is true for Disc 2, although the transition point from Zone 1 (linear increase) to Zone 2 (constant value) is not as obvious as with Disc 1. The pilot-scale experiments show, that for both disc diffusers, the level of f_S,max is strongly dependent on q_A,Disc. The highest f_S values could be reached in the middle of the operation range (3.0–4.0 Nm³/h per diffuser) given by the diffuser manufacturer (1.5–8.0 Nm³/h per diffuser) which corresponds to the results of Sander et al. (2017). In case of Disc 2 at very low q_A,Disc of 0.2 m³/h/disc (Figure 3(d)), f_S is only about 1.0. This means that no salt effect can be observed at such low q_A,Disc values. The reason for this will discussed below in the present paper, when discussing the influence of airflow rate on f_S.

Comparing the results of both disc diffusers shows that higher f_s,max values were reached with Disc 1 than with Disc 2. Considering the results when the full range of airflow rate was tested (Figure 3(a) and 3(b)), mean f_S,max of Disc 1 (2.0) is 10% higher than of Disc 2 (1.8). In comparison, k_La₂₀-values in TW are in a comparable range for both diffusers (Annex 2). This confirms the results of previous lab-scale experiments with similar disc diffusers (Behnisch et al. 2018). Accordingly, higher f_S values could be reached with smaller slit lengths (Disc 1 superior to Disc 2). Nevertheless, the effect of airflow rate on f_S was not as obvious as in the experiments conducted in the pilot-scale test tank, because of the limited blower capacity and reactor volume during the lab-scale experiments. Consequently, a scale effect has to be taken into account in order to obtain adequate results during measurement of oxygen transfer in SW. A sufficient reactor volume as well as blower capacity is necessary to ensure that the full range of airflow rate of diffusers can be set during the tests. If only two diffusers have to be compared relative to each other, a small reactor is sufficient, whereby the test conditions must be identical.

If both the f_S value and the k_La value rises, so does the SOTR_VAT. While in TW there is a linear relationship between SOTR_VAT and Q_A,VAT (Behnisch et al. 2020), this is not the case in SW due to the dependence of f_S (or k_La) on q_A,Disc. At a salt concentration of 12 g/L NaCl and a Q_A,VAT = 1 Nm³/m³/h, almost similar SOTR_VAT of 123 g/m³/h and 132 g/m³/h could be achieved with 6 and 32 Disc 1 diffusers respectively. This represents an increase of 24% (99 g/m³/h) and 45% (91 g/m³/h) compared to 6 and 32 Disc 2 diffusers respectively. Compared to SOTR_VAT in TW it is an increase of 105% (6 diffusers: 60 g/m³/h) or 60% (32 diffusers: 84 g/m³/h). The results show that due to the influence of q_A,Disc on f_S, it is possible that in SW with the same Q_A,VAT but a different number of diffusers, due to the higher f_S-value (or k_La), similar or even more oxygen can be transferred at low than at high diffuser density.

In Figure 4, f_S-values from test series with plate diffusers are plotted against c_NaCl for different airflow rates. Unexpectedly, there are big differences compared to the results of the disc diffusers. For Plate 1 (Figure 4(a)), f_S increases up to a concentration of 5 g/L NaCl, which is quite lower than the measured CCC_NaCl in lab-scale experiments. Furthermore, the measured f_S,max for Plate 1 is lower than for Plate 2 (Figure 4(b)), even though Plate 1 has smaller slits. In comparison, k_La₂₀-values in TW are in a comparable range for both diffusers (Annex 3). In addition, the clear dependency between airflow rate and f_S observed for the disc diffusers cannot be observed for Plate 1, in contrast to Plate 2 (see Figure 4(b)). Here, the f_S-value increases with increasing airflow rate without reaching a noticeable peak, which is consistent with the results of the tests with 32 disc diffusers. However, as before with the disc diffusers, the operational range for Plate 2 (25–35 m³/h/plate) specified by manufacturer was not fully reached due to the limited blower capacity. Therefore, it can be expected, similar to the tests with the 32 disc diffusers, that with increasing airflow rate f_S would first increase further, reach a maximum and then decrease again. If the f_S value is plotted against the airflow rate, this is confirmed, as we will show below.

In Figure 5, f_S-values from test series with tube diffusers are plotted against c_NaCl for different airflow rates. For both tube diffusers, f_S increases up to 10 g/L NaCl as with the disc diffusers until f_S,max is reached. Therefore, the results confirm the CCC_NaCl determined in the lab-scale experiments once again. The present data show that the average f_S,max of Tube 1 (2.1) is 10% higher than that of Tube 2 (1.9), regardless of the number of installed diffusers. No effect of the number of installed diffusers (or diffuser density) on f_S, in THE case of tube diffusers, was detected. In comparison, k_La₂₀-values in TW are comparable for both diffusers (Annex 4). Thus, the higher f_S values could be reached with the diffusers with smaller slits, which corresponds to the results of disc diffusers.

At a salt concentration of 13 g/L NaCl and a Q_A,VAT = 1 Nm³/m³/h, a SOTR_VAT of 156 and 182 g/m³/h could be achieved with 6 and 10 Tube 1 diffusers respectively. This corresponds to an increase of 7% (146 g/m³/h) and 9% (167 g/m³/h) using Tube 2 diffusers. Compared to SOTR_VAT in TW it is an increase of 79% (6 tube diffusers: 87 g/m³/h) and 84% (10 tube diffusers: 99 g/m³/h). Therefore, for tube diffusers, SOTR_VAT can always be increased by a higher number of installed diffusers independent of q_A,tube. However, as in TW, it is expected that the increase of SOTR_VAT is minimal when DD exceeds a specific value. Behnisch et al. (2020) observed in TW only minor improvement in oxygen transfer for tube diffusers, when DD exceeds 35%.

To illustrate the dependency of f_S,max and airflow rate for the different diffuser types, f_S as a function of the airflow rate is plotted in Figure 6. For a simplified presentation, only results of measurements are shown, where c_NaCl is higher than CCC_NaCl and f_S is constant. The airflow rate is given as the airflow rate per slit and slit length (q_A,dS = q_A,Slit · (d_S)⁻¹). This enables a comparison between the different diffusers, which have different membrane design properties (i.e. d_S, SD, number of slits). It should be noted, however, that this does not take into account the elasticity of the slits, which expand depending on different material properties of the membrane (e.g. deflection, flexibility, thickness) with increasing q_A,Slit (Loubière & Hébrard 2003). However, the expansion is very small despite high airflow rates as shown in experiments with multi-orifice diffuser membranes (Painmanakul et al. 2004). Therefore, the expansion of the slits with rising airflow rate is ignored here.

Figure 6 illustrates the difference between the individual diffusers. While for tube diffusers q_A,dS is very low (0.1–0.5 dm³/h/mm) and for disc diffusers and Plate 2 moderate (up to 2 dm³/h/mm), q_A,dS for Plate 1 is very high (1.7–9 dm³/h/mm). The reason for this is the combination of the very low number of slits of Plate 1 (low SD) with relatively high q_A,Plate values. Therefore, for a better illustration of the results of the tube and disc diffusers as well as Plate 2 diffuser, in Figure 6 the range q_A,dS from 0 to 2 dm³/h/mm is shown enlarged.

As mentioned before, Figure 6 shows that there is no dependency between f_S and airflow rate for tube diffusers within the considered range of airflow rate. A slight effect on f_S can be seen for Plate 1. In contrast, the effect is very strong for disc diffusers and Plate 2. For both disc diffusers as well as for Plate 2, the highest f_S values were reached at ∼1 dm³/h/mm. If the airflow rate changes, f_S decreases again. Sander et al. (2017) attribute the dependency between f_S-value and airflow rate to the fact that at low airflow rates, coalescence also occurs less frequently in TW because of the isolated bubble rise behaviour. Baz-Rodríguez et al. (2014) also observed no dependence between k_La and c_Salt when gas hold-up or airflow rate is very low and the bubbles are relatively far from each other. The decrease of f_S at very high airflow rates is explained by an increase in initial bubble size (Sander et al. 2017). Several experiments show that the initial bubble size increases with rising airflow rate (Loubière et al. 2003; Painmanakul et al. 2004; Hasanen et al. 2006). Since the bubble size remains constant during the entire ascent in a coalescence-inhibited system (Baz-Rodríguez et al. 2014; Behnisch et al. 2018), the oxygen transfer and thus f_S decreases.

When looking at the results of Plate 1, it has to be taken into account that the SD of Plate 1 is very low (0.88 slits per cm²) compared to Plate 2 (5.5 slits per cm²) and Disc 1 and Disc 2 (15.5 and 10 slits per cm²). This results in distances between the slits (P_R/d_S and P_S/d_S; see Table 1) being up to nine times larger than for the other diffuser types. To prevent coalescence close to the diffuser (in TW), Painmanakul et al. (2004) recommend a distance between the slits in relation to the bubble size of 1.0 (i.e. P_R/d_32,min and P_S/d_32,min > 1.0). The d_32,min value of Plate 1 was not measured. If we assume a d_32,min between 1 and 2 mm, this ratio will be exceeded by more than four to nine times in the case of Plate 1. Therefore, no or less coalescence occurs also in TW. In addition, q_A,dS increases very rapidly with increasing q_A,Plate and so does the initial bubble size. Both effects lead to the fact that the coalescence behaviour as well as the bubble size distribution might differ fundamentally from the other diffusers. However, the slight influence of airflow rate on f_S for Plate 1 and the lower CCC cannot be explained finally on the basis of the present results. Therefore, further experiments are necessary.

For the missing dependency of f_S on airflow rate for tube diffusers, there are in our opinion three possible explanations, which we will discuss in more detail below.

• The material: Unlike the plate and disc diffusers made of EPDM, the tube diffusers are made of silicon. However, the material properties only influence the initial bubble size (Loubière & Hébrard 2003) but should not affect the coalescence behaviour of the rising bubble. Hence, the missing dependency of f_S values on airflow rate cannot be explained by the membrane material.

• Low q_A,dS values: The number of slits per tube diffuser is much higher than for the other two diffuser types. Therefore, maximum q_A,dS for tube diffusers (0.49 dm³/h/mm) is very low compared to disc diffusers (1.9 dm³/h/mm) and Plate 2 diffuser (1.6 dm³/h/mm). Initial bubble size should therefore change only slightly. The decrease of f_S observed for disc diffusers and Plate 2 with increasing airflow rate is therefore missing. In contrast to the maximum q_A,dS, the achieved minimum q_A,dS compares well with values for disc and tube diffusers (∼0.14 dm³/h/mm). The missing decrease of f_S with decreasing airflow rate can therefore not be explained by different q_A,dS values.

• The coalescence behaviour: as described previously, for disc diffusers f_S decreases at low airflow rates because coalescence also occurs less frequently in TW due to the isolated bubble rise behaviour. The slits of tube diffusers are on the side of the diffuser and therefore the bubble formation occurs tangentially to the diffuser membrane and not vertically, as for disc and plate diffusers. Furthermore, this arrangement of the slits makes the cross-section of the assumed flow channel along the rising bubble swarm much thinner than with disc and plate diffusers (Figure 1). While the cross section of the flow channel for discs and plates roughly corresponds to the perforated membrane area, this is not the case for tubes. Here, the cross section of the flow channel corresponds to the projected surface area of the tube diffuser (A_P = 665 cm² per diffuser), which is much smaller than the active (perforated) membrane area (A_A = 1,450 cm² per diffuser). If the number of slits is related to A_P of the tube diffuser, the resulting quasi-slit-densities (26 and 33 slits/cm²) are more than twice as high as the SD of disc and plate diffusers. Therefore, it is obvious that for tube diffusers, even at relatively low airflow rates, the bubbles are not ascent isolated and coalesce continuously in TW. This results in high f_S values even at low airflow rates.

After discussing the effect of airflow rate on f_S, we checked if there was any influence of the number of installed diffusers (or DD) on f_S. According to results from Figure 5(a) and 5(c) as well as Figure 5(b) and 5(d), there is no effect of the number of installed diffusers on f_S in the case of tube diffusers. Regardless of the number of installed diffusers, the same f_S,max was achieved as CCC_NaCl was reached. In the case of disc diffusers, a comparison between the different diffuser densities is more difficult, because of the overlapping influence of the airflow rate. Considering individual results of the disc diffusers in Figure 6, the same f_S could be achieved with both 6 and 32 disc diffusers for the same disc diffuser and the same q_A,dS (or q_A,Disc = (q_A,dS) · d_S · slits per diffuser). Hence, for 32 disc diffusers, the increase of f_S with increasing q_A,dS is identical to that for 6 disc diffusers. Due to the high number of diffusers and limited blower capacity, the range of tested airflow rates with 32 disc diffusers is significantly below the tests with 6 disc diffusers or the given specified operating range from manufacturers. If q_A,dS and q_A,Disc continue to increase, a similar dependency between f_S and q_A,dS (or q_A,Disc) will most likely result as was shown for the tests with 6 disc diffusers. Therefore, no influence of diffuser density on f_S for disc diffusers as well as for tube diffusers was detectable. For plate diffusers only one diffuser density was tested. A comparison is therefore not possible. The result that diffuser density has no effect on f_S promotes the notion that bubbles only coalesce with bubbles from the same diffuser or adjacent slits and not with bubbles from other diffusers. This confirms the observations of Hasanen et al. (2006) and Behnisch et al. (2018), that coalescence occurs mainly close to the diffuser.

The results shown above correspond to the results from Part I of this paper: Both c_Salt as well as the design of the diffuser membrane (see Table 1) have an influence on k_La or f_S in SW (Behnisch et al. 2018). Hence, at high c_Salt diffusers with smaller detaching bubbles show higher f_S and k_La values. Additionally, we could show that the type of the diffuser also influences the oxygen transfer in SW. For tube diffusers, f_S only depends on c_Salt, while for disc and plate diffusers f_S also depends on q_A,Disc and q_A,Plate respectively. Except Plate 1, for all diffusers f_S,max was reached when c_Salt ≥ CCC. Furthermore, diffuser density does not seem to affect f_S.

Other factors also have to be taken into account for design of aeration systems in SW. An important parameter is c_S, which decreases with increasing c_Salt (Annex 1). Due to the lower concentration gradient between gas and liquid phase and reduced molecular diffusivity, k_L is expected to decrease, resulting in a lower f_S and k_La. When looking at the results of plate and tube diffusers (Figures 4 and 5), we did not observe a significant decrease in f_S after reaching its peak value in the considered concentration range. Because of the lower range of tested salt concentration, a clear conclusion is not possible for disc diffusers. Nevertheless, we do not expect a significant decrease of f_S for disc diffusers with further increasing c_Salt. This is confirmed by the results of Baz-Rodríguez et al. (2014), who found no dependence of k_La, a and k_L on c_Salt when coalescence is completely inhibited. They show that the reduction of the molecular diffusivity with increasing c_Salt and the effect on k_L is marginal. According to these authors, the main effect on k_L is the slip velocity, which is substantially influenced by the bubble size (i.e. d₃₂ or a). However, the reduced c_S also decrease SOTR_VAT and SSOTE, as shown in Figure 7(a). Similar to f_S, SSOTE increases with increasing c_NaCl until a maximum is reached. SSOTE of up to 15%/m could be reached. Diffusers with the smaller slits (Tube 1, Disc 1) always exhibit higher values than diffusers with larger slits (Tube 2, Disc 2). In TW, SSOTE increases with increasing diffuser density (Behnisch et al. 2020). When comparing the results of 6 and 10 Tube 1 diffusers, the same can be observed in SW. When reaching the peak value, SSOTE slightly decreases as a result of the reduced c_S. The decrease averages about 0.7%/m for c_NaCl = 20 g/L compared to the peak value at c_NaCl = CCC_NaCl. Nevertheless, SW SSOTE still outperforms SSOTE values that can be achieved in TW with conventional fine-bubble diffusers (8.5–9.8%/m), even at very high diffuser densities (Behnisch et al. 2020).

Another factor that must be considered in the design of aeration systems is the total air supply pressure (p_T), which includes the hydrostatic pressure resulting from depth of submergence (p_h), pressure drop of pipes and valves (p_s) and pressure drop of diffusers (p_d) (Krampe 2011). The p_h increases with increasing c_Salt due to the increment of water density. Nevertheless, in practice this increase can be neglected. The density of water rises marginally by 1.6% with an increase in c_NaCl from 0 g/L (997 kg/L at 20 °C) to 20 g/L (1.013 kg/L at 20 °C). The p_S is not affected by c_Salt. Also no dependence against c_Salt was found for p_d (Annex 5). Nevertheless, when improving the oxygen transfer in SW by installing diffusers with smaller slits, p_d increases. In our tests, for tube and disc diffusers the difference in p_d between the diffuser with the smaller slits and those with the larger slits was on average 2.2 kPa (at the same q_A, see Annex 5). Thus, in SW, a higher f_S tends to go hand in hand with a higher p_d and therefore p_T. The increased p_T results in a higher power requirement (P) for the blowers.

When calculating P, we must differentiate between isochoric compression and adiabatic compression. Positive displacement blowers (as used here) are widely used in WWTP. Since they compress a fixed volume of air in an enclosed space to a higher pressure, they operate using the isochoric compression principle (Mueller et al. 2002). P can be calculated according to the following formula: P = Q_A · p_T/η; η being the overall efficiency of the blower. For our calculation, we assumed a typical η of 0.60 (Bell & Abel 2011). Then we calculated the standard aeration efficiency (SAE = SOTR/P).

With SAE, we are able to illustrate the interaction between increased energy demand due to increased p_T through the smaller slits and the improved oxygen transfer. The average SAE is shown in Figure 7(b) as a function of c_NaCl. Since identical Q_A were set for each diffuser type, its influence on SAE is eliminated and the average values are sufficient for a comparison. Figure 7(b) shows that SAE in TW ranges between 3.0 and 4.4 kg/kWh and thus within an expected range for such a kind of operation conditions (Behnisch et al. 2020). With increasing c_Salt SAE increases and reaches its peak value, similar to SSOTE. SAE for the diffusers with smaller slits (Tube 1, Disc 1) are comparable or higher than for the diffusers with larger slits (Tube 2, Disc 2). Hence, with Tube 1 (smaller slits) SAE of up to 8.4 kg/kWh could be achieved.

Contrary to positive displacement blowers, with turbo blowers internal air compression takes place (= adiabatic compression). Turbo blowers are generally designed for large airflow rates and are therefore used especially on large WWTP. P depends upon the inlet pressure (p₀), air density (ρ) and inlet temperature (T₀) (Mueller et al. 2002). Depending on design and operation conditions, overall efficiencies for turbo blowers of up to 0.85 are possible (Bell & Abel 2011). In order to consider these types of blowers as well, we calculated SAE again using the adiabatic power formula (EPA 1989). All results and the formula with the assumed operating conditions are shown in Figure 8. Due to the higher overall efficiency (using a constant η of 0.80) and the chosen operation conditions (T₀ = 20 °C; p₀ = 101.3 kPa; ρ = 1.2 kg/m³), on average 52% higher SAE values of up to 12.8 kg/kWh were achieved compared to positive displacement blowers. However, it has to be taken into account that the presented power requirement values are only based on theoretical calculations. Since overall efficiency and the power requirement will change under varying operation conditions, lower SAE values are likely to be achieved in practice. Apart from that, the previous conclusions remain valid. Higher SAE values were achieved with diffusers with smaller slits.

In summary, the SAE results show that, independent of the blower type used and despite the higher pressure drop, in SW higher efficiency of the aeration system is achieved using the diffusers with smaller slits. However, it is questionable whether this can be applied to all diffusers. Whether a diffuser with smaller slits in SW ensures a more efficient aeration depends largely on the difference in oxygen transfer as well as on pressure drop compared to the diffuser with larger slits. Additionally, further measurements need to show whether a higher SAE value can also be achieved in activated sludge. The activated sludge could have an influence on the coalescence behaviour in the aerated tank. Diffusers with smaller slits could also lead to increased fouling. Therefore, further experiments are already planned to answer these questions.

In the present work, the experiments presented in Part I of this paper (Behnisch et al. 2018) were continued to investigate the oxygen transfer of fine-bubble aeration systems and their influencing factors in saline water. In Part I, data from bubble size distribution were combined with results from oxygen transfer tests to investigate the impact of the design of the diffuser membrane. Within Part II, the effect of different salts as well as the effects of diffuser type and density on oxygen transfer of fine-bubble aeration systems in saline water is presented.

First, we determined the critical coalescence concentration (CCC) for various salts for the first time using conventional fine-bubble diffusers, for which we developed a new analytical approach. When CCC is reached, coalescence is completely inhibited and k_La₂₀ and f_S remain constant. Regardless of salt type, the f_S,max values reached were the same. With the new analytical approach, the CCC is determined by evaluating the oxygen transfer rate at different salt concentrations. The new method is much faster and easier to use than bubble size measurement and provides valid results. In the future, this will make it possible to investigate the coalescence behaviour for any aeration system and (mixed) salt solution quickly and easily. Furthermore, since salt and salt mixture in wastewater change and depend on the origin of the used water, we recommend that in the future not only the total salt concentration should be considered in wastewater analytics but also its composition from different salts (e.g. by ion analysis).

Second, we carried out oxygen transfer tests with three different types of diffusers at different diffuser densities in a pilot-scale test tank at a water depth of 3.8 m and at different NaCl concentrations. The effect of the design of the diffuser membrane observed in Part I was confirmed. Except for Plate 1, diffuser membranes with smaller detaching bubbles show higher f_S values. Furthermore, our results show that there is a clear influence of the diffuser type. While f_S for tube diffusers only depends on the salt concentration, the other diffuser types show a clear influence of the airflow rate per slit and slit length (q_A,dS). For disc diffusers and Plate 2, the highest f_S values were achieved at q_A,dS ∼ 1.0 dm³/h/mm. If q_A,dS changes, f_S and k_La will decrease again. This complicates the operation of an aeration system. When using tube diffusers, this problem can be avoided. To achieve a high oxygen transfer in saline water, the slit density of the diffuser membrane can be further increased. In tap water, the slit density is usually up to 15 slits per cm² to prevent coalescence close to the diffuser. Under saline conditions (c_Salt > CCC), the coalescence is inhibited and the slit density can be increased further. This also prevents high q_A,dS and thus an increase in initial bubble size.

Third, despite the reduced oxygen saturation concentration in saline water, extremely high SSOTE values compared to tap water of up to 15%/m were achieved during the tests in the pilot-scale test tank. SSOTE are much higher than is possible with conventional fine-bubble diffusers in tap water. Using isochoric power formula (valid for positive displacement blowers), calculated SAE values of up to 8.4 kg/kWh show that by using diffusers with smaller slits, the energy efficiency of the aeration system will improve in saline water despite the increased pressure drop. If even more efficient blowers (e.g. turbo blowers) were used, theoretically (using adiabatic power formula) even higher SAE of up to 12.8 kg/kWh values could be achieved. However, it is not yet known whether the use of diffusers with smaller slits also improves aeration in activated sludge tanks. The coalescence behaviour in the aerated tank as well as the fouling of the diffusers could be influenced by the activated sludge. Therefore, further investigations are already planned.

We thank the German Federal Ministry of Education and Research (BMBF) for funding the research project WaReIp ‘Water-Reuse in Industrial Parks’ (Grant No. 02WAV1409A).

All relevant data are included in the paper or its Supplementary Information.