3.1. Relationship Between Ammonium Concentration and Emerging Contaminants Concentration
In the first step of the study, time profiles of ammonium and CEC concentrations arriving at the WWTP were studied and compared. The concentration of the major CEC (those above 1 ng/L) in the influent of the biological treatment is shown in
Table 2. As has been reported by Luo et al. [
9], most emergent contaminants are typically found between 0.1 and 10 µg/L, and some, such as acetaminophen and caffeine, show much higher levels. On the contrary, concentrations below 1 ng/L were found for the following microcontaminants: atrazine, butylparaben, clofibric acid, genistin, glycitin, imipramine, perfluoro-n-nonanoic acid, potassium perfluoro-1-octanesulfonate, potassium nonafluoro-1-butanesulfonate, simasine, sucralose and sulfadiazine. Therefore, these minor components were not taken into account in the study.
Ammonium concentration levels varied between 27.6 and 55.2 g/m
3 in the different analysis days. As can be seen in
Figure 2a, the ammonium content steadily increased with increasing sampling time, probably because of the increasing levels of human activities throughout the morning.
In the case of CEC (as shown in
Table 2), some of them, such as acetaminophen, acesulfame, telmisartan, and amitriptyline, followed a pattern close to that of ammonium (
Figure 2b—increasing their concentration as noon approached). This was also the case for other CEC, such as bezafibrate, eprosartan mesylate, and valsartan. Nevertheless, other CEC, such as diclofenac and diuron, showed the opposite tendency. However, most of them, such as testosterone and ciprofloxacin, showed a random pattern. Therefore, according to our results, CEC concentration variation in wastewaters cannot be related to ammonium concentration.
3.2. Removal Yields of Emerging Contaminants with the Biological Treatment
Removal rates achieved with the biological treatment for each of the CEC were calculated taking into account the concentration (mean and standard deviation) at the influent and the effluent of the treatment. From all the CEC, 2-hydroxibenzothiazole, ketoprofen, telmisartan, and valsartan have been chosen as representatives of the different behaviors, and the variation of their removal rates for each sampling hour is shown in
Figure 3a. As can be seen, ketoprofen and valsartan showed high removal ratios in all hour samples. However, 2-hydroxibenzothiazole and telmisartan, showed lower values of removal efficiency with higher variability.
As
Figure 3b shows, in general, compounds showing a removal higher than 80% maintained approximately constant values in all hour samples. On the contrary, those having intermediate and low removal percentages displayed higher variability. These results seem to indicate a certain variability in removal rates, which could be explained by taking into account factors such as CEC concentration and biodegradability.
When removal rates are analysed in relation to the concentration values at the influent of the biological treatment (
Table 2), a partial dependence can be observed, probably for kinetic reasons. In fact, solutions with concentrations higher than 1500 ng/L yielded, in all cases, efficiencies above 80.0%, except for the case of sulfamethoxazole in H2. This was the case of acesulfame, acetaminophen, caffeine, methylparaben, and valsartan at all hours, and of eprosartan mesylate, sulfamethoxazole, and trimethoprim only at the hours with a proper concentration. Higher variation of efficiency was observed for most compounds.
Mean removal efficiencies for all CEC under biological treatment are shown in
Figure 3b. As can be seen in this treatment, genistein, methylparaben, progesterone, and testosterone were completely removed, and caffeine and acetaminophen showed removal percentages higher than 99.5%. Contrarily, irbesartan and carbamazepine presented the lowest removal ratios, with maximum values of 15.0% and 22.0%, respectively. Two special cases were perfluorosulfonamide and isoproturon, which, in some cases, were not eliminated at all.
It can also be observed that the efficiency of some CEC is higher depending on the targeted compound, their biodegradation, and adsorption onto activated sludge [
39]. In fact, acesulfame, acetaminophen, caffeine, genistein, methylparaben, progesterone, testosterone, and valsartan showed efficiencies higher than 93.0% at all the hours, and ketoprofen had an efficiency that was higher than 82.0%. Of them, genistein, progesterone, testosterone, and ketoprofen are remarkable, as their concentration was lower than 1500 ng/L. In the rest of the analytes, in general, displayed efficiencies lower than 84.0%, and the variation in the removal rate with the sampling hour was much higher. This higher variability of the rates with lower efficiencies could be due to the higher difficulty for the degradation of compounds with less biodegradability and at lower concentrations.
These results are generally in agreement with those obtained in the literature. For example, poor removal levels of carbamazepine (23.1%) in combination with high removal of acetaminophen (99.9%), caffeine (99.2%), and ketoprofen (94.2%), medium–high of diclofenac (81.4%) and trimethoprim (69.0%), and medium–low of sulfamethoxazole have been found in different biological-based WWTPs [
40], as in this study. Similarly, in other works, evidence of the poor removal rate of carbamazepine [
3,
41] and the high degradability of acetaminophen [
42,
43] and caffeine or medium–high degradability of trimethoprim has been found [
44].
3.3. Removal Yields of Emerging Contaminants with UF Treatment
After biological treatment, effluent was submitted to c-ultrafiltration.
Figure 4a shows removal rates with this treatment for the same emerging contaminants depicted in
Figure 3a.
Figure 4b shows the average efficiencies obtained with ultrafiltration for all the CEC.
As can be seen, the efficiencies achieved with ultrafiltration treatment were below 50.0% in almost all cases, except for amitriptyline (63.0%), and were systematically lower than those obtained with biological treatment. In addition to this, it is worth noting that the efficiencies were more stable throughout the day, though higher deviations were observed in the replicated measurements, probably due to the low removal rates achieved (high difficulty of the removal).
According to the literature, the predominant driving mechanism in UF is adsorption [
45,
46], and not size-exclusion due to the relatively large pore size. Therefore, the extent of the retention of the different compounds is related to the higher or lower affinity of each compound for the membrane. Nevertheless, it should be emphasized that, in general, ultrafiltration retention efficiency is not very high. For example, according to a reporting study [
45], retention coefficients by different UF membranes were tested—obtaining average values below 50.0%. In addition, in other works, most of the compounds showed retention lower than 30.0% in the UF membrane [
46].
3.4. Removal Yields and Adsorption Phenomena in UF Treatment
Figure 5 shows the removal rates of the 18 CEC detected versus permeate concentration during c-UF in the treated effluent, showing a specific behaviour that could be associated to adsorption phenomena. The concentration variability observed for each contaminant at the ultrafiltration inlet depends not only on the concentration, but also on its biodegradability and the adsorption equilibrium on the surface of the membrane, as will be studied below in detail. Thus, many of those contaminants that are emerging in
Figure 5a are contaminants with high biodegradability; those in
Figure 5c presented high concentration variability in the biological entrance; those in
Figure 5b presented intermediate values of concentration and biodegradability. Examples of CEC with extreme variation in concentration are caffeine, with a low concentration (≈ 50 ng/L), and trimethoprim, with a high concentration (≈ 500 ng/L). Both compounds presented removal rates of about 30.0%, but a sharp decrease of the average yields, beneath 0.1%, was observed at extreme concentration values. This was an extended behavior in many of the emerging contaminants shown in
Figure 5. Thus, one can mention, among those of great variability—ketoprofen, 2-hydroxibenzothiazole, bezafibrate, carbamazepine and phenytoin—and those with medium concentration—valsartan, perfluorosulfonamide, eprosartan mesylate and losartan. Although showing different removal yields at their respective medium–high concentrations, at low concentrations, all these contaminants presented removal rates below 0.1%. On the other hand, in all compounds with high concentration variability—shown in
Figure 5c—removal rates below 0.1% were found both at high and low concentrations (extreme values).
This special behavior, observed in most emerging contaminants analyzed in c-UF, seems to be explained by adsorption phenomena [
47]. Corresponding equilibria of different compounds could be affected by the organic matter present at the entrance of the UF. Nevertheless, the buffering effect of the biological process leads to similar organic matter content concentration and characteristics of organic matter at the outlet. Consequently, the effect on the balance of different CEC is negligible.
After analyzing the removal yields, it is deduced that the maximum removal rate, once filtered, lies within the middle values, within the variability of each contaminant. In such cases, a good recovery of the filter material after washing is deduced. The retention capacity for a contaminant can be defined by the corresponding equilibrium adsorbed amount,
q∞ (ng/g f.m.), which depends on the contaminant concentration in contact with the filter material, according to the Freundlich adsorption isotherm:
where
Ce (ng/L) is the equilibrium concentration and
KF (ng/g) (L/ng)
nF is the Freundlich constant corresponding to a given contaminant and adsorbent material, when
nF = 1. For many compounds at low concentrations, as in the emerging compounds, the heterogeneity factor,
nF, is one [
48].
Moreover, the duration of the filtration stage in the tests reported here was 47 min. For these times, it can be assumed that the retained load,
qtf, at the end of the filtration period, is in equilibrium with the contaminant concentration at the c-UF output (permeate). Consequently,
qtf ≈
q∞, and is, therefore, in equilibrium with the output concentration,
Cp =
Ce. In this way, the
KF constant can be derived from:
Equation (2) relates the removal yield (η) to qtf, with MF (g) being the ultrafiltration membrane mass used to treat a flow Q (L/min), in which C0 (ng/L) is the input concentration for a certain CEC. Under certain conditions, also serves to estimate the retention capacity in equilibrium, q∞. In other words, Equation (3) is assumable, as long as the adsorption capacity is maintained at the maximum value; that is, q∞ does not decrease and the adsorption kinetics, kads (g(ng min)−1), are sufficiently fast. In principle, these circumstances would occur for the highest removal rate observed in each contaminant. In this case, the corresponding qtf is assimilable to q∞.
An estimation of the adsorption constant can be made by considering a pseudo-second order kinetics:
Equation (5) enables the amount of contaminant retained during filtration to be determined. Solving for a time
tf = 47 min (filtration period), the
qtf value is obtained that should coincide with the experimental one from Equation (2). In the case of a low removal yield, the experimental
qtf values can be explained through Equation (5), because of the
q∞ reduction to a lower effective value,
q∞* (= q
∞ −
qirr), depending on the filtration conditions. This value tends to be zero at the extreme concentration values, within the variation range of each CEC.
Table 3 shows the values of the adsorption parameters estimated from the retention observed for each compound. Diuron and caffeine were selected among the low-concentration CEC, telmisartan and losartan among those of medium concentration, and sulfamethoxazole and trimethoprim among those of high concentration. The
qirr value represents the amount irreversibly retained in the filtration membrane and not removed during washing, causing fouling [
25]. In general,
qirr approaches
q∞ at extreme values (high and/or low), leading to a low removal yield.
A fouling factor,
Fq, was defined for each component by the ratio of adsorbed contaminant, not eliminable or irreversible,
qirr, to the original adsorption capacity,
q∞, according to Equation (6):
In
Table 3, the variability of the removal yields with the concentration for some selected compounds are shown.
The highest removal yields correspond to situations with a fouling factor,
Fq, close to zero. Thus, for each compound, the greatest removal yields correspond to situations, with a fouling factor,
Fq, close to zero. In
Figure 6, the retention capacity in equilibrium has been represented, according to Equation (3), for three representative compounds, of low (amitriptyline), medium (losartan), and high concentration (sulfamethoxazole). As explained above,
q∞ is composed of a reversible part
q∞* (white area below
q∞ profile in
Figure 6) which is removed by washing, after each filtration period, and another irreversible part or fouling,
qirr (shaded area). As
qirr approaches
q∞, it will be more difficult to recover the adsorption capacity after each cycle.
Consequently, highest removal yields will occur as
q∞* approaches
q∞. These favourable situations correspond to intermediate concentrations represented in
Figure 6 by a large white area beneath the
q∞ line, whereas
qirr (shaded area) is negligible. This concentration range of high removal yields can be more or less centred depending on the compound. Thus, losartan and sulfamethoxazole present similar situations, whereas in the case of amitriptyline, the highest elimination yields correspond to the lowest concentrations.