DMP levels in sewer and outlet stream
The DMP levels in our study area showed independent variability in both space and time from one sample point to the other as shown in table 1. The level of DMP in the rainy season was 0.0278 ± 0.324 µg/mL (mean± standard deviation, at 95% confidence interval), this is far higher than that obtained in a study in a China study, in which the DMP level found in the water was 0.05 ± 0.11 µg/L (median± interquartile range) (Liu et al., 2014). Also, this study obtained a 100 percent detection ratio as compared to 88.9% obtained in the China study, which seems to suggest a higher level of leaching of the phthalate esters to the sewer in our study than that in China study. The DMP levels in the sewer in the dry season in our study was 0.0079 ± 0.038 µg/mL, this is far higher than that obtained in the Guangzhou China study which had a mean concentration of 0.018 µg/L, and however, the Guangzhou study recorded 100% detection frequency just like this study (Zeng et al., 2008). The level of DMP in the sewer in the dry season was lower by a factor of three, as compared to the rainy season. This could be basically because the rainy season encourages the wash down of plasticizers from the immediate environment into the nearest sewer which might not be possible during the dry season.
Also, the DMP level in the outlet stream in the rainy season was 0.0070 ±0.036 µg/mL, this had the lowest value for all the phthalate esters and all the samples examined in this study. Nonetheless, this is far higher than that obtained in a Taihu, China study, which had a mean DMP concentration of 0.05 µg/L in the wet season (Luo et al., 2021). Besides, the detection frequency in the China study was 94%, which is lower than that in this study which was 100%. The DMP level in the outlet stream in the dry season was 0.0248 ±0.050 µg/mL, this is far higher than that obtained in the Taihu, China study, in which a mean DMP concentration in the dry season was 0.06 µg/L (Luo et al., 2021). The detection frequency in the China study was 95%, which is lower than that obtained in this study which was 100%. The level of DMP in the wastewater in the dry season in this study was higher by a factor of three more than in the rainy season. This could be because the dry season is hotter and encourages the leaching of the phthalates esters from the polymer materials in the immediate wet environment.
DEP levels in sewer and outlet stream
The mean DEP in the sewer in the rainy season in our study was 0.0526 ±0.496 µg/mL, this was the highest recorded for all seasons and sample points for DEP in the wastewater examined in this study. The level of DEP in the rainy season in our study is far higher than that obtained in a Saudi Arabia study in which the mean DEP obtained in Wadi Hanifah wastewater was 0.304 ± 0.155 µg/L (mean ± std. dev.) (Al-Saleh et al., 2016). This suggests a higher level of DEP leaching in our study wastewater as compared with that in the Saudi Arabia study. The level of DEP in the sewer in the dry season in this study was 0.0367 ±0.204 µg/mL, this is far higher than that obtained in a China study lake which was 0.165 µg/L (Li et al., 2020). The China lake also had a 100% detection frequency for DEP just like in this study. The DEP level in the dry season in this study was lower by a factor of 1.4 than that in the rainy season, which suggests higher wash-down in the rainy season.
DEP in the outlet stream in the rainy season in this study was 0.0469 ±0.294 µg/mL, this is far higher than that obtained in the rainy season in Taihu lake water in China which had a mean DEP concentration of 0.06 µg/L (Luo et al., 2021). The Taihu lake water had a detection frequency of 71% for DEP in their samples examined, which is far less than that obtained in this study which had a detection frequency of 100%. The DEP in the outlet stream in the dry season in this study was 0.0462 ± 0.087 µg/mL, this is far higher than that obtained in the dry season in Taihu lake in China study which had a mean of 0.09 µg/L (Luo et al., 2021). The China study had a detection frequency of 84% for DEP in the samples examined, whereas, the detection frequency for this study was 100%, which suggests a higher level of leaching in this study. There was no significant difference between the mean DEP in the wet and dry seasons, which suggests persistence in contamination in both seasons examined.
DBP in sewer and outlet stream
The mean DBP in the sewer in the rainy season in this study was 0.1472 ±1.667 µg/mL, this was the highest value obtained in this study for all the phthalate esters and all the sample points examined in this study. The level of DBP obtained in this study is far higher than that obtained in a China study which had DBP a concentration of 0.19 ± 0.63 µg/L (median ± interquartile range) (Liu et al., 2014). The frequency of detection for DBP in the China study was 98%, which is slightly lower than that in this study which was 100%. DBP in the sewer in the dry season in this study was 0.0644 ±0.589 µg/mL; this is far higher than that obtained in a China study, which had a DBP median concentration of 6.420 µg/L (Li et al., 2020). The China study had a frequency of detection of 100% similar to that in this study. DBP in the sewer in the rainy season in this study was higher by a factor of 2.3 than that in the dry season, which suggests that the rain encouraged wash-down in our study.
DBP in outlet streams in the rainy season in this study was 0.0483 ± 0.290 µg/mL, this is far higher than that obtained in a China study, which had a rainy season mean of 1.31 µg/L (Luo et al., 2021). The China study had a similar detection frequency as this study which was 100% for DBP. The mean DBP in outlet stream in the dry season in this study was 0.0406 ±0.241 µg/mL, this is far higher than that obtained in a China study which had a mean DBP of 2.03 µg/L for Guangzhou lake water (Zeng et al., 2008). The China study had an exact frequency of detection in DBP with that in this study which was 100%. There is no significant difference between DBP level in the rainy season and the dry season even though the rainy season was higher, which may suggest wash-down in the rainy season.
Heavy metals in the sewer and outlet stream
The mean concentrations, limits of detection, and limits of quantifications of heavy metals in wastewater in both rainy and dry seasons examined in this study are as shown in table 2. The limits of detection were 0.0015, 0.0015, 0.0005, 0.0005 and 0.0005 µg/mL for Pb, Cr, Cd, As, and Hg respectively. And the limits of quantification were 0.0045, 0.0045, 0.0015, 0.0015, and 0.0015 µg/mL for Pb, Cr, Cd, As and Hg respectively.
Lead in sewer and outlet stream
The mean concentration of lead in the sewer in the rainy season in this study was 0.139 ± 0.081 µg/mL (mean ± standard deviation, at 95% confidence interval), as shown in table 2, this is higher than that obtained in hospital effluent in a Mexico study which had a mean lead concentration of 0.123 ± 0.001 mg/L (Perez-Alvarez et al., 2018). This suggests that lead leaches into wastewater from hospitals in most parts of the world. The mean concentration of lead in the sewer in the dry season in this study was 0.117 ± 0.024 µg/mL, this indicates that there was no significant difference between the level of lead in the wastewater in the rainy and dry seasons. It possibly suggests that lead is a persistent waste from the hospital environment in both rainy and dry seasons. The level of lead in obtained in the dry season in this study was higher than that obtained in the Ethiopia study in which the mean lead concentration in sample site 1 was 0.031 ± 0.008 mg/L (Mekuria et al., 2021), which suggests that lead leaches into the wastewater more in our study than that in the Ethiopia study.
The outlet stream in the rainy season had a mean lead concentration of 0.137±0.106 µg/mL, this is lower than that obtained in a Pakistan study, which had a mean lead concentration in seawater of 16.03±0.13 mg/mL (Chan et al., 2021). The seawater is a receiver of much more sources of a pollutant than, the outlet stream considered in this study. Again there was no significant difference between the level of lead in the sewer and the outlet stream in the rainy season in this study. This could suggest that the waste from the hospital wastewater could be the main source of lead in the outlet stream. The outlet stream in the dry season had a mean lead concentration of 0.157 ±0.048 µg/mL, this is lower than that obtained in the Dhaka Bangladesh study which had a mean lead concentration in textile wastewater of 0.22 ± 0.02 mg/L (Islam et al., 2016), the Dhaka study examined textile industrial wastewater which could necessitate the high value observed as against hospital wastewater in this study. There was no significant difference between the lead concentration in the outlet stream in the rainy season and the dry season in this study. The concentration of lead in the sewer and outlet stream examined in this study was in all cases higher than the WHO recommended value in wastewater which was 0.1 mg/L. This suggests there is a need to review the purification process in the sewer examined in this study.
Chromium in sewer and outlet stream
The mean chromium concentration in the sewer in the rainy season in this study was nd, this was the lowest level obtained in this study in all the heavy metals in the wastewater in this study. This is lower than that obtained in a Mexico study which had a mean chromium concentration of 0.051 ± 0.001 mg/L in hospital effluent (Perez-Alvarez et al., 2018). The chromium level obtained in the sewer in the dry season was 0.076 ± 0.029 µg/mL, this is lower than that obtained in a study in Dhaka Bangladesh, in which the chromium level was found in textile industrial wastewater was 4.9 ± 0.92 mg/L (Islam et al., 2016). This could be expected in textile industrial wastewater since pieces of machinery are involved in most of the textile processes. It could however be observed that the chromium level in the rainy season in this study was higher than that in the dry season.
Chromium in the outlet stream in the rainy season in this study was 0.319±0.245 µg/mL, this is lower than that obtained in a study in Pakistan in which the chromium level in seawater was 40.06±0.21 mg/mL (Chan et al., 2021). The seawater considered in the Pakistan study had much pollutants as a result of the influence of the river Lyari. The chromium in the outlet stream in the dry season in this study was 0.317±0.079 µg/mL, this is higher than that obtained in a recent study in Saudi Arabia in which the chromium found in site 1 of the gulf was 0.34 ± 0.006 µg/L (Mahboob et al., 2022), the level obtained in the Saudi Arabia seawater was relatively lower, even though there was the influence of industries in their study area. There was no significant difference between the level of chromium in rainy and dry seasons in this study. All the wastewater examined in this study had chromium concentration below the WHO recommended level which was 2 mg/L in wastewater (Islam et al., 2016).
Cadmium in sewer and outlet stream
The cadmium concentration in the sewer in the rainy season in our study was 0.005 ± 0.003 µg/mL, this is lower than that obtained in a Mexico study in which the effluent received from the hospital had 0.039 ±0.001 mg/L (Perez-Alvarez et al., 2018). The effluent in our study was treated before the discharge into the receiving stream whereas the effluent in the Mexico study was not treated before being discharged into the receiving stream, this could have necessitated the relatively high level obtained in the Mexican study. The cadmium concentration in the dry season in sewer obtained in this study was 0.063±0.021 µg/mL, this is higher than that obtained in an Ethiopian study in which the concentration of cadmium in Akaki river water was 0.017 ± 0.007 mg/L (Mekuria et al., 2021). The level of cadmium in this study was far higher than all the range of cadmium in the Ethiopian study as the range in the ten samples analyzed was < 0.014 ± 0.0007 to 0.02 ± 0.001mg/L. This suggests that the wastewater in this study is a higher source of contamination to the immediate environment than that examined in the Ethiopian study.
The cadmium concentration in the outlet stream in the rainy season in our study was 0.004 ± 0.002 µg/mL, this is lower than that obtained in the Bangladesh study in which the mean concentration obtained in textile wastewater was 0.08 ± 0.07 mg/L (Islam et al., 2016). The equipment and machine used in the textile industry in Bangladesh could have accounted for the high value obtained in the study. However, the level obtained in this study was lower than the WHO recommended value in inland surface water which was 0.1 mg/L (Islam et al., 2016). The cadmium concentration in the outlet stream in the dry season in this study was 0.062 ± 0.014 µg/mL, this was far higher than that obtained in sample 4 water obtained from the Gulf of Saudi Arabia which had a concentration of 0.003 ± 0.000 µg/L (Mahboob et al., 2022). This could suggest that the wastewater outlet stream examined in this study was a higher source of contamination to the immediate environment than that in the Saudi Arabian study.
Arsenic in sewer and outlet stream
The arsenic concentration obtained in the sewer in the rainy season in this study was 0.002 ± 0.001 µg/mL, this is lower than that obtained in a Mexican study in which the level of arsenic was obtained in the hospital effluent was 0.014 ± 0.001 mg/L (Perez-Alvarez et al., 2018). The effluent in this study was treated before the discharge into the receiving sewer but the effluent in the Mexico study was not treated before discharge into the receiving stream. However, the level of arsenic obtained in this study is lower than the WHO recommended limit in wastewater which was 0.2 mg/L (Islam et al., 2016). The level of arsenic in the sewer in the dry season in this study was 0.021 ± 0.006 µg/mL, this is similar to that obtained in the Saudi Arabian study in which the arsenic concentration in water from the gulf of Saudi Arabia in sample site 1 was 21.07 ± 1.19 µg/L (Mahboob et al., 2022). This suggests that the wastewater from this study had a similar level of contamination to the gulf water of Saudi Arabia that was said to be polluted from a large industrial effluent. The arsenic concentration in the sewer in this study showed ten (10) folds increase in the dry season as compared to the rainy season.
The arsenic concentration in the outlet stream in the rainy season in this study was 0.003 ± 0.001 µg/mL, this is lower than that obtained in a Mexico study in which the concentration of arsenic in wastewater used as drinking water for cow was 0.01 ± 0.004 mg/L (Numa Pompilio et al., 2021), this is expected as the wastewater in the Mexican study was said to be polluted by textile, refreshment and chemical industries in addition to volcanic sources. The concentration of arsenic in the outlet stream water in the dry season in this study was 0.021±0.005 µg/mL, this is lower than that obtained in a study in Dhaka in Bangladesh in which the mean arsenic concentration obtained in textile wastewater was 4.5 ± 0.75 mg/L (Islam et al., 2016). This suggests that the industrial waste had impacted a high level of arsenic in the Dhaka wastewater analysed in the Bangladesh study. There was about a seven (7) folds increase in the concentration of arsenic in the dry season as compared with the rainy season in this study.
Mercury in sewer and outlet stream
The concentration of mercury obtained in the sewer wastewater in the rainy season in this study was 0.007 ± 0.001 µg/mL, this is three (3) folds lower than that obtained in Mexico study in which the mercury concentration in hospital effluent was 0.021 ± 0.001 mg/L, (Perez-Alvarez et al., 2018). This could suggest that the treatment of the effluent in this study had taken care of the mercury concentration in the wastewater as compared to that in the Mexico study. The concentration of mercury obtained in the sewer in the wastewater in the dry season in this study was 0.003±0.001 µg/mL, this is higher than that obtained in a Paris study in France in which the median mercury concentration obtained in wastewater in the dry season was 0.12 µg/L and the range was 0.07 – 0.29 µg/L (Gasperi et al., 2008). This could suggest that the Paris wastewater treatment system had better efficiency than that examined in this study. There was a reduction in concentration to less than one-half of mercury concentration in the dry season in this study as compared to the rainy season.
The mean concentration of mercury in the outlet stream obtained in this study in the rainy season was 0.004 ± 0.03 µg/mL, this is far higher than that obtained in an Italian study in which the mercury obtained in the study was 144.7 ng/L in the chlor-alkali channel (Covelli et al., 2009). This suggests that the outlet stream in this study is a more potent source of mercury contamination as compared to that examined in the Italy study. The mean concentration of mercury in the outlet stream in the dry season in this study was nd, this is far lower than that obtained in Arabian Gulf in which the mercury concentration was 0.84 ± 0.15 µg/L (Mahboob et al., 2022). The treatment in the sewer in this study area could have eliminated mercury in the dry season which necessitates the low level obtained in this study as compared to that in the Arabian Gulf. There was no significant difference between the mercury concentrations in the outlet streams obtained in the rainy season and dry season in this study.