3.1 Characterization of Wastewater Samples
The average concentrations of physico-chemical parameters analyzed from the Kombolcha Institute of Technology Wastewater Treatment Plant were presented in Table 1. The mean, skewness and standard deviation were shown for all studied parameters. Values exceeding the standards are in bold type. As shown in Table 1, parameters namely phosphate (PO43−), nitrate (NO32−), nitrite (NO2−), pH and sulfate (SO42−) have a smaller standard deviation value compared to the remaining parameters. This implies that they show a small deviation from the mean or average value. This is because most of these compounds are stable and there was no significant variation in their values during the measurement of sampled wastewater. The values of these organic anion pollutants were very smaller compared to previous research done by Wyasu (2020). This difference may be due to the type of wastewater used for analysis and sampling technique differences during the experiment.
COD (mg/l), alkalinity (mg/l) and TSS (mg/l) values of the sampled wastewater show huge deviations from their mean values. This was due to the difference in water usage in the institution, most of the time there was great consumption of water for cleaning and cooking purposes and an enormous amount of urine and feces was generated, which directly affect the COD (mg/l), alkalinity (mg/l), conductivity (µs/cm) and TSS (mg/l) values of wastewater. The values of COD (mg/l) and TSS (mg/l) of this study agree with the study done by Wyasu (2020). The value of alkalinity again shows good agreement with the study done by Saurabh et al (2020), the value of alkalinity was higher during weekends due to the high consumption of soap and other related detergents, and became lower during the first two days of the week.
Table 1
Physico-chemical properties of raw wastewater (January to June 2022).
Parameters (Unit) | Average Value | Standard Deviation (SD) | Skewness | EPA (US) | WHO | ES |
NO3− (mg/L) | 3.58 | 0.298 | -0.838 | 50 | 40 | 20 |
NO2− (mg/L) | 7.4 | 0.238 | 0.630 | 10 | 10 | 0.1 |
COD (mg/L) | 563.33 | 118.97 | -1.431 | 250 | 150 | 250 |
pH | 8.33 | 0.47 | 0.935 | 6–9 | 6.5–9.2 | 6–9 |
Conductivity (µS/cm) | 1244.67 | 42.25 | -0.558 | 750 | NA | 1000 |
Turbidity (NTU) | 863.5 | 32.843 | -0.266 | 75 | 25 | NA |
TSS (mg/L) | 208 | 32.78 | 0.366 | 50 | 5 | 100 |
Temperature (℃) | 25.33 | 2.51 | 0.585 | \(30\text{℃}\) | NA | 40 |
PO43− (mg/L) | 4.483 | 0.291 | -0.687 | 2 | 5 | 10 |
SO42− (mg/L) | 9.83 | 1.34 | 0.744 | 20 | 200 | 1000 |
Alkalinity (mg/L) | 281.33 | 31.21 | -0.441 | < 20 | 250 | NA |
Values exceeding the standards are in bold type.
NA: not available
3.2 Seasonal Variation of Physico-chemical Parameters
pH All analysis results for pH from January to June are revealed in Fig. 2. Significant variations of pH were observed for sampled wastewater and some values were relatively similar. The average maximum and minimum pH value of wastewater was 9.3 and 8.6 in April and June, respectively. This increment in pH may be due to the high amount of detergent and soap used in the institute for cleaning purposes. The standard values of pH for discharged wastewater are 6–9, 6.5–9.2, and 6–9 respectively as per the standard of the United State Environmental Protection Agency (USEPA 2023), World Health Organization (WHO 2011) and Ethiopian Standards (ES 2001). However, the overall average effluent pH was within USEPA, WHO, and ES acceptable limits. When compared to different other studies on the analysis of wastewater and sewage water, the pH results from this study demonstrate an increase in pH. According to the Velusamy et al (2016) study, the pH ranged from 7.62 to 7.96. This finding demonstrates that the pH in the wastewater from the Kombolcha Institute of Technology is more basic than the sewage water from sewage treatment plants examined by Velusamy et al (2016). A pH of 5 or above indicates the presence of industrial wastes and is incompatible with biological treatment, while a pH of less than 5 or greater than 10 indicates septic conditions. A biologically viable environment normally requires a pH range of 6 to 9, which is relatively limited. Low pH levels can be affected by the action of the methanogen bacteria (Wijaya & Soedjono 2018).
Temperature Measurements of temperature are crucial for determining the degree of density, viscosity, vapor pressure, and surface tension that is present in water. The temperature of water affects the saturation value of the solids and gases that are dissolved in it, BOD standards, and the behavior of organic compounds. Water temperature fluctuates according to the season (Dey et al. 2021). As shown in Fig. 3, the recorded values of temperature results from January to June highly fluctuated. The average highest and lowest temperature value of wastewater was 31℃ and 23℃ in January and March, respectively. The temperatures of all the wastewater samples are within the standard limit set by the USEPA and ES. Upon conducting this particular study, it was found that the temperature results achieved were slightly lower compared to the study conducted by Dey et al (2021). This suggests that there may be minor inconsistencies in the results, which could be attributed to a variety of reasons. These may include variations in the methodology, the equipment used, or even the environmental conditions under which the study was conducted.
Turbidity (NTU) Fig. 4 illustrates the extreme variability of the measured turbidity findings from January to June. In March and February, respectively, wastewater had average maximum and minimum values of 896 NTU and 820 NTU. This fluctuation of turbidity may be due to on and off construction work taking place on the campus, which is washed off by rain and mixed with the sewer system. In addition to that starting in June more students/residents joined the campus and every activity was affected by them, which in turn changed the amount of sewer discharged from the campus. An ineffective biological treatment, sedimentation, or final filtration process may be responsible for the decrease in monthly turbidity removal efficiency (Nasier & Abdulrazzaq 2022). The study conducted to examine the turbidity levels in various sampling points has yielded results that are significantly higher than the values recorded in previous research conducted by Wyasu (2020). This suggests a significant increase in water turbidity levels in the investigated sites. High turbidity levels can potentially impact the quality of water and make it unsuitable for use for various purposes.
Chemical Oxygen Demand (COD) (mg/l) Characterized COD (mg/l) comprises the oxidation of reductive inorganic and organic matter, which is demonstrated in Fig. 5. All observed values showed higher variability and significant deviation as seen in the figure for all months (Fig. 5). The average highest and lowest values observed for COD (mg/l) in the sampled wastewater were 622 mg/l and 580 mg/l, respectively. The high value of COD) might be caused by urine, faces and organic food wastes generated from the student dormitory and cafe. The COD values of sampled wastewater surpassed the permissible limit set by USEPA, WHO and ES, in all months and thus imply gross pollution. The results of this study's COD analysis were compared to those obtained from another study conducted by Karmoker et al (2018). It was found that the COD (mg/l) values for the wastewater samples in this study were significantly higher than those obtained by Karmoker et al (2018) even though both studies were conducted over several months. This discrepancy in findings could be attributed to a number of factors, such as differences in the sampling methods, the physical and chemical characteristics of the wastewater samples, or variations in the treatment processes and technologies used by the respective facilities. As Ai et al. (2020) state this higher COD value could be a sign that microorganisms might not be able to decompose organic matter as effectively as before due to a lack of enough oxygen, which would result in a rise in COD. The removal of debris and solids that contribute to the increase in COD during primary treatment may also result in a decrease in COD.
Alkalinity The alkalinity level of water plays a crucial role in determining the overall quality of soil and affects agriculture. In the context of wastewater treatment systems, alkalinity is an essential parameter that guides the selection of the right anaerobic system. The tremendous diversity of the measured Alkalinity results for all six months is shown in Fig. 6. The greatest and minimum alkalinity values for wastewater samples were 290 mg/l and 241 mg/l in March and April, respectively. This fluctuation of alkalinity value may be due to the variability usage of soaps and detergents for showering and washing in the student dormitory. A higher alkalinity level in treated wastewater suggests that a wide range of incoming influent pH variation was intended to be accommodated by the wastewater treatment process. The more acidic wastewater can be adjusted and treated and a wider range of pH values may be handled while operating at the higher alkalinity. Furthermore, this greater degree of alkalinity may be caused by the aeration unit's ineffective oxygen supply, which would otherwise increase dissolved oxygen and decrease wastewater's alkalinity (Hollas et al. 2019). The alkalinity values of sampled wastewater surpassed the permissible limit set by USEPA and WHO. Though, the measured alkalinity of the study was closer to the limit value of WHO. A study conducted on wastewater samples by Baharvand & Mansouri (2019) demonstrated that alkalinity levels were high and surpassed the threshold limit of USEPA and WHO. By contrast, the results of the current study's alkalinity analysis presented lower levels, indicating a lesser risk of environmental harm.
Conductivity The electro-conductivity value ranged between 1143 and 1244 µS/cm which was above the allowed limit value of USEPA (< 750 µS/cm) and ES (1000 µS/cm) in all months. The electro-conductivity of characterized wastewater showed a general decrease pattern from January to June except for the unpredicted rise in April. This unexpected rise in conductivity in April may be due to considerable effluents generated by engineering laboratories. Since all laboratories in the institute are connected to the sewer system, they affect most wastewater parameters. The findings of the current study on wastewater conductivity demonstrate a significant level of variability when compared to the study conducted by Velusamy et al (2016). The measurements taken during each month of the study consistently showed differences in conductivity values, indicating that fluctuations in wastewater conductivity are ongoing and unpredictable. The study results also reveal that the conductivity level of wastewater tends to be higher during the first four months of the year, which suggests that seasonal factors may play a significant role in determining conductivity levels. The inadequate activated sludge treatment and final filtration process employed during wastewater treatment are the main causes of the recorded higher value of conductivity (Levlin & Levlin, 2007).
TSS The average values of TSS (mg/l) for all six months were demonstrated in Fig. 8. The TSS values of sampled wastewater for all specified months were above the recommended limit set by USEPA (50 mg/L), WHO (5 mg/L), and ES (100 mg/l). All observed values showed gradual increments as seen in Fig. 8 for all months except for the unexpected fall in May. The average highest and lowest detected TSS values of sampled wastewater were 226 mg/l and 196 mg/l, respectively. Again, the construction work on the campus and increased rainfall may be the reason for this substantial rise in TSS (mg/l). Furthermore, seasonal variation and human activities may be to blame for such variations. When compared to Dey et al's study from 2021 on the analysis of water samples in a certain pond, this study's TSS (mg/l) results were lower. The TSS (mg/l) ranged from 1126 to 870 mg/l, according to the study by Dey et al (2021). This finding indicates that the TSS (mg/l) at the wastewater treatment facility at the Kombolcha Institute of Technology was in better shape than it was during the investigation. TSS (mg/l) can induce sedimentation in water bodies and reduce oxygen levels. For the treatment process to be controlled and for laws governing effluent to be followed, TSS is crucial (Wijaya & Soedjono 2018). Higher-value TSS effluents are created by grit chambers and main sedimentation tanks. The removal rate in the primary sedimentation tank was higher. Thus, inorganic elements were added to the biochemical tanks in large amounts, where they later accumulated in the mixes and at the bottom of the tanks and were left untreated (He et al. 2019).
Organic anions The phosphate (PO43-), sulfate (SO42-), nitrite (NO2-) and nitrate (NO3-) values of characterized wastewater for various seasons are denoted in Fig. 9. As shown from the figure all observed values for phosphate revealed an overall increment except for April. Furthermore, the phosphate (mg/l) of wastewater showed a smaller deviation compared to other parameters. Whereas, the amount of sulfate in wastewater showed great fluctuation. The highest values for phosphate and sulfate were 4.8 mg/l and 11 mg/l, respectively. On the other hand, the minimum values for phosphate (mg/l) and sulfate (mg/l) were 4.1 mg/l and 8.3 mg/l, respectively. Therefore, phosphate values of characterized wastewater were beyond the standard limit value.
In the comparison between this study and the Dey et al (2021) study on water sample analysis, some noteworthy differences were found. One of the most noticeable differences was in the levels of nitrite (mg/l) and phosphorus (mg/l), which were found to be higher in this study than Dey et al (2021) study. This could be indicative of higher levels of pollution in the water source being studied, possibly due to nearby laboratory and agricultural activities. On the other hand, this study also had lower levels of nitrate (mg/l) compared to the Dey et al (2021) study, which could be an indication of different environmental conditions affecting the water sample. A high concentration of phosphate is detrimental to water bodies as it causes eutrophication, and hence leads to the extermination of aquatic life. The eutrophication of surface water is due to an increase in the growth of algae and aquatic weeds, with a subsequent oxygen shortage. Although nitrogen and carbon are also essential to the growth of aquatic biota, most attention has been focused on phosphorus inputs. Ingestion of high levels of phosphorus interferes with the metabolism of calcium and results in bone loss in both humans and animals.
As shown in Fig. 9, all observed values for nitrate (mg/l) showed an overall increment except for May. While the amount of nitrite showed high variation from month to month as can be seen in Fig. 9. The averaged peak values for nitrite and nitrate were 7.8 mg/l and 3.9 mg/l respectively. However, the lowest values for nitrite and nitrate were 7.1 mg/l and 3.1 mg/l respectively. Nitrate contamination deteriorates water quality and causes eutrophication and algal blooms. A high concentration of nitrate in water can cause diseases in humans and animals such as spontaneous abortions, diabetes, methemoglobinemia, thyroid problems and cancer (Anjana & Iqbal 2007)