The collected samples were successfully analyzed for physical and chemical parameters. A total of 17 parameters (pH, turbidity, color, TSS, free chlorine, total chlorine, nitrites, nitrates, phosphates, ammonium, total iron, aluminum, COD, BOD, chromium, nickel, manganese) were investigated.
Turbidity, TSS, Color, BOD, and COD
Table 4 presents the results of the physicochemical water quality parameters analyzed at the ENU Environmental and Water Management lab. Before treatment; the minimum, maximum, and average concentration values for turbidity were 68.70, 647.00, and 259.62 FAU, respectively. However, after the EC treatment process, 1.08, 114.00, and 70.32 FAU values for minimum, maximum and average concentrations were achieved, respectively, which is equivalent to 98.43%, 82.38%, and 72.91% change of the minimum, maximum, and average concentrations, respectively. According to the WHO drinking water quality standards [
34], the turbidity should be kept below 5 FAU. In this case, from the average turbidity concentration, the IMF treatment system is of high potential to further reduce the concentration of turbidity to acceptable levels.
TSS removal in feed water prior to a membrane filtration treatment process is important to improve membrane flux [
35]; especially for highly polluted wastewater such as the one generated from the poultry slaughterhouse activities. In the raw wastewater, minimum, maximum, and average recorded TSS concentrations were 116.00, 1068.00, and 452.67 mg/L, respectively. After the EC treatment process, 13.00, 198.00, and 128.25 mg/L were achieved as minimum, maximum, and average concentrations, respectively; achieving an average removal efficiency of more than 71%.
High BOD concentrations were also observed in the raw wastewater; with 1220, 1409, and 1321.33 mg/L being minimum, maximum, and average concentrations, respectively. Also, the recorded COD concentrations from the raw wastewater ranged from 1274 to 7401 mg/L, with 3527.17 mg/L being the average concentration. The high concentrations of BOD and COD can be linked to the fact that the poultry slaughterhouse activities are associated with a high generation of blood and complex mixture of fats, proteins, and fibers, which contribute to the increasing the organic matter [
36]. After the EC treatment, the average removal efficiencies of more than 85% and 78% were achieved for BOD and COD, respectively. In the literature, BOD and COD removal efficiencies from different wastewater sources were observed to be ranging from 73 to 85% using EC treatment methods [
37,
38,
39].
The concentrations of free and total chlorine were observed to be low in the raw wastewater; 0, 0.9, and 0.23 mg/L were recorded as the minimum, maximum, and average concentration values in the raw wastewater for free chlorine, respectively. While 0.03, 1.04, and 0.28 mg/L were recorded as a minimum, maximum, and average concentrations for total chlorine, respectively. The low chlorine concentration in the raw wastewater is due to the fact that the poultry farm intend to achieve a chlorine-free production system to improve the general quality of the products as well as environmental conservation in general. After the EC treatment process, 0, 0.22, and 0.15 mg/L were recorded as the minimum, maximum, and average concentrations of free chlorine, respectively; with an average removal efficiency of 34.78%. Also, 0, 0.24, and 0.18 mg/L were recorded as minimum, maximum, and average concentration of total chlorine after the EC treatment; with an average removal efficiency of 35.71%.
Nitrogen and phosphorus are referred to as nutrients; nitrogen, in the form of nitrate, nitrite, as well as ammonium, is an essential nutrient needed for plant growth. But, when the water bodies receive a relatively high amount of nutrients, they are likely to be polluted by excessive growth of algae endangering fish and other aquatic life. Moreover, according to studies, some forms of algae such as blue-green may produce toxins with possibilities of being harmful if ingested by humans and animals [
40]. The EC pre-treatment system faced a significant challenge in nutrients removal. Average removal efficiencies of 33.01%, 12.5%, 34%, as well as 4.19% were achieved for nitrate, nitrite, ammonium, and phosphates, respectively. In the literature, it has been observed that the efficiency of ammonium removal in wastewater can be highly affected by the type of electrode material used, HRT, pH, and the current density applied [
41,
42]. In this study, electrode material, HRT, pH, and current density were not adjusted to investigate their potential influence on ammonium nitrogen removal. However, Umesh et al. [
43] investigated the removal of ammonium nitrogen using the EC method with platinum-coated titanium; during the seven-hour experiment, as low as 10% removal efficiency was achieved following some adjustments to the current density. Also, Hanspeter et al. [
44] studied direct electrochemical oxidation of ammonia on graphite and observed that a pH value greater than 9 is required for effective removal of ammonia. However, in this study, the pH values in the raw wastewater ranged from 6.80 to 7.40 with 7.13 being an average value. Similar factors are highly linked to the challenges in the removal of phosphates, nitrates as well as nitrites. Therefore, the selection and application of an EC pre-treatment method for nutrients removal have to properly observe all these parameters.
Chromium, nickel, and manganese are among the potentially toxic elements (PTEs) released from poultry production activities [
45]. The Canadian water quality guideline for chromium in drinking water is 0.1 mg/L [
46] and the protection of freshwater life is 0.005 mg/L [
47]. A water quality guideline for hexavalent chromium of 0.008 mg/L in irrigation water is recommended for the protection of agricultural crop species [
48]. The average chromium concentration in the raw wastewater is about 11.9 times higher than the recommended guideline for drinking water, 238 times higher for protection of aquatic life, as well as 148.75 times higher than the guideline for protection of agricultural crop species. After the EC treatment process, an average concentration of 0.72 mg/L was achieved, which is equivalent to 39.5% removal efficiency.
High levels of nickel were observed in the raw wastewater; with 1.96, 21.96, 7.53 mg/L as the minimum, maximum, and average concentrations, respectively. The potential sources of nickel in the poultry wastewater can be leaching from metals, chicken feeds, as well as offal products especially the liver [
49]. The EC pre-treatment unit achieved nickel removal efficiency of approximately 52.06%. Nickel is considered to be toxic when exceeding 0.2 mg/L, leading to dermatitis, nausea, chronic asthma, coughing as well as a human carcinogen [
50].
Manganese concentrations in the raw wastewater ranged from 0.29 to 0.95 mg/L with 0.72 mg/L being the average concentration. According to the Canadian guidelines, the maximum acceptable concentration (MAC) for total manganese in drinking water is 0.1 mg/L, while the aesthetic objective (AO) for total manganese in drinking water is 0.02 mg/L [
51]. The average manganese concentration in the raw wastewater is approximately 7.2 times (720%) MAC according to the Canadian guidelines. After the EC treatment process, an average concentration of 0.21 mg/L was achieved for manganese; which is still 2.1 times (210%) higher than MAC according to the Canadian guidelines. However, as a pre-treatment unit, the EC process achieved a 70.8% average removal of manganese.
Moreover, lowering the concentrations of iron and aluminum before a membrane filtration treatment process is crucial because of their potential of reacting with antiscalant components to form colloidal foulants [
52,
53]. The average concentration of total iron in the raw wastewater was 0.90 mg/L; after the EC treatment process, the average concentration of 0.17 mg/L was achieved, which is equivalent to 81.11% removal efficiency. Also, the average concentration of aluminum in raw wastewater was 0.78 mg/L; however, after the EC treatment, an average concentration of 0.22 mg/L was achieved, which is equivalent to 71.8% removal efficiency. The high removal efficiency of the EC pre-treatment unit on total iron and aluminum plays an important role in the reduction of scaling of mineral salts onto the membrane surface as well as maintaining the design flux.
Apart from the samples collected after the EC pre-treatment processes, also samples were collected from the IMF treated effluent. From
Table 5, it can be observed that after the IMF treatment process, 0 mg/L was achieved as the minimum recorded concentrations for turbidity, TSS, free and total chlorine, nitrites, ammonium, chromium, and nickel. From the concentrations of treated effluent, 0 mg/L is equivalent to 100% removal efficiency.
Based on the SD results, it can be observed that the treatment plant was able to maintain high removal consistency over time.
From the average concentrations, 70.32 FAU of turbidity was achieved as the EC treatment process effluent, while, 0.32 FAU of turbidity was achieved as average concentration after the IMF treatment, which is equivalent to a 99.5% change from the EC effluent to the IMF effluent. Following a similar approach, 98.9% change was achieved from color as well as 99%, 98.9%, 98% from TSS, COD, and BOD, respectively.
The lowest percent change is observed from nickel and manganese with 61.8% and 42.9% respectively. The phenomenon is also reflected in the total removal efficiency of nickel and manganese, presenting the lowest removal efficiency among the studied water quality parameters.
Table 6 reveals further that, the EC pre-treatment processes had an impressive performance on the removal of turbidity, color TSS, total iron, aluminum COD, BOD, as well as manganese. However, EC low removal efficiency can be observed for parameters such as free and total chlorine, nitrites, nitrates, phosphates and total phosphorus, and ammonium nitrogen, with removal efficiency ranging from 4 to 45%.
In this study, seven parameters (turbidity, color, TSS, nitrates, phosphates, ammonium, and COD) were selected as case studies for correlation analysis. From
Table S4 in Supplementary Materials, it can be seen that the correlation coefficient between turbidity and color is 0.999834, which falls under the “very strong” correlation category. In literature, turbidity and color are observed to be linearly correlated [
54], of which turbidity can be created by the particulate matter resulting from particles too large to be in true solution and yet small enough to remain suspended against the force of gravity, imparting color to water [
55]. A very strong correlation can also be observed between COD and other parameters such as turbidity, color TSS, nitrate, and phosphates. COD is a critical parameter of determining water quality status which can define the degree of contamination in water. Therefore the strong correlation can be linked to the fact that COD covers the total amount of oxygen consumed in water to chemically oxidize organic contaminants to inorganic end products [
56].
Figure 2 shows the general performance of the integrated treatment system by comparing the analysis results before treatment (BT), after electrolysis (EC), and after the IMF treatment processes. The error bars illustrate the range of the studied data, the lower quartile, median, upper quartile, and outliers are also presented. It can be observed that the purification performance increases as more treatment units are applied in the system. The concentrations of BOD in raw wastewater ranged from 1200 to 1400 mg/L with high consistency (no outliers) (
Figure 2d). The highest purification performance can be seen from the IMF effluent achieving almost 0 mg/L of turbidity, BOD, COD, and TSS as well as 0 degrees of color.
Figure 2f reveals further that, the pretreatment processes had a lower influence on nitrate removal which is also reflected in the final effluent. The use of the graphite electrode as a cathode has also shown to be a challenge for nitrate removal in some other studies; the phenomenon is highly linked to the low removal rate and nitrogen selectivity [
57,
58,
59].
In general, from the Box and Whisker plots, it can be seen that the concentration of the released pollutants from the poultry slaughterhouse activities is of high fluctuation, which means a pre-treatment unit prior to the membrane filtration treatment process has to be carefully selected to withstand the highly fluctuating wastewater characteristics. The errors can also be highly linked to the relatively high fluctuation of the wastewater characteristics affected by the general complex production procedures in the poultry farm.