1. Introduction
The process of slaughtering chickens is associated with the consumption of large quantities of water from the meat processing activities, cleaning of the processing environment, disinfection as well as transportation of the slaughter by-products. The processes also generate large quantities of highly polluted wastewater with organic matter including biological oxygen demand (BOD), chemical oxygen demand (COD), and suspended particles [
1] as well as pathogenic microorganisms [
2], characterized by a high content of proteins, fats, carbohydrates from meat, blood, skin, and feathers. As the population increases around the world, the demand for poultry products has also increased, which in turn affects the general water demand as well as increases the generation of highly polluted wastewater [
3]. The rising cost of discharging untreated process water into the local sewage systems, as well as the presence of more restrictive requirements for discharging process water onto surface water [
4], has made many poultry farms in the world think of in situ treatment of the wastewater generated from slaughterhouses. Microbes are among the contaminants of significant concern generated from poultry slaughterhouses [
5].
Biological contaminants are of different types including different types of bacteria such as fecal coliforms and Escherichia coli [
6], Salmonella, Shigella, Vibrio cholerae as well as Pseudomonas aeruginosa [
7]. Moreover, some other biological contaminants, such as viruses, fungi as well as diverse parasite cysts and eggs, can be found in the wastewater. The degree to which the biological contaminants may pose threats to environmental and human health is dependent on the type and concentration [
8].
There are many health risks associated with exposure to biological contaminants in water including diseases, such as typhoid, cholera, and tuberculosis, caused by bacteria [
9], hepatitis caused by viruses [
10], as well as dysentery caused by protozoa [
11]. Therefore, it is of great importance to ensure that the wastewater from poultry slaughterhouses has been adequately treated to achieve as complete as possible elimination of biological contaminants before either discharge or any other utilization. In general, there are many technologies used for microbial elimination [
12,
13,
14]; conventional technologies are the most widely used processes for disinfection of water. Chlorine, chlorine dioxide, ozone, and peracetic acid are a few examples of the chemical-based microbial elimination approaches. In addition to chemical disinfectants, ultraviolet (UV) radiation has also been used for many years as a water disinfection technology in the field of wastewater treatment [
15]. Also, some other advanced approaches, such as ozonation and membrane filtration [
16] as well as electrochemical (EC) methods [
17], have been applied to poultry slaughterhouse wastewater treatment. But, the performance of the treatment technologies has also been observed to be affected by the scale and characteristics of wastewater subjected to the treatment process [
18]. With the fact that each treatment system is characterized by its advantages and disadvantages in terms of strengths and weaknesses, it is preferable to combine several treatment technologies to form an integrated treatment to achieve higher treatment efficiency for poultry slaughterhouse wastewater [
19]. Previous studies have observed that a combination of different technologies has the potential to improve the general performance of a treatment system as pollutants that were not removed by one unit can be removed by the other subsequent units [
20]. Electrochemical, ultrafiltration (UF), and UV are among the treatment technologies used in poultry slaughterhouse wastewater treatment. However, the information about their technical feasibility on microbial elimination from poultry slaughterhouse wastewater is still scarce especially when integrated together.
Electrochemical treatment systems have been considered cost-effective and highly efficient wastewater treatment technologies. The EC treatment systems have been observed to be highly efficient in the removal of pollutants from poultry slaughterhouse wastewater [
21,
22]. The electrolysis process produces oxidants, such as dissolved oxygen, hydrogen peroxide, hydroxyl, and ozone, that are responsible for the degradation of the outer membrane of the bacterial cell and the destruction of proteins in the cytoplasm, and, finally, the microbial cell death [
23]. Despite the EC treatment technologies being extensively studied for recalcitrant organics removal, its application potential towards microbial elimination from wastewater, such as that generated from poultry slaughterhouse processes, is still not well known [
24].
Generally, UF is a pressure-driven membrane separation mechanism that removes suspended particulate matter and some dissolved compounds with high molecular weight, including organics and colloids from wastewater [
25]. Ultrafiltration is also regarded to be effective in removing bacteria and most viruses present in wastewater [
26]. The efficiency of UF systems in filtering out microorganisms from wastewater makes the technology ideal for the treatment of poultry slaughterhouse wastewater. Moreover, the UF treatment systems are also useful when applied as pre-treatment units before reverse osmosis, UV, and ozone treatment systems, as disinfection requirements are greatly reduced due to the reduction in suspended solids [
27].
Unlike the chemical approaches to water disinfection, UV light has been observed to offer a more rapid and effective inactivation of microorganisms through a physical process [
28,
29]. The UV rays must strike the cell to inactivate microorganisms in water. Ultrafiltration energy breaches the outer cell membrane of the microorganism, of which in the process the DNA is disturbed which, in turn, prevents reproduction [
30]. Among the crucial benefits of water treatment with UV is the fact that the treatment process does not alter water chemically as no additional chemicals are added except energy [
31]. However, unlike the membrane filtration treatment systems, the sterilized microorganisms with UV rays are not removed from the water. It should also be noted that generally, the UV disinfection process is not primarily designed to remove dissolved organics, inorganics, or any other sort of particles in the water. When the water is subjected to UV rays, the microorganisms exposed to the germicidal wavelengths of UV light are rendered incapable of reproducing and infecting making them harmless [
32]. In this study, an integrated lab-scale treatment plant with EC, UF, and UV was studied for its potential application towards microbial elimination from poultry slaughterhouse wastewater.
3. Results and Discussion
The wastewater samples treated using the combination of EC, UF, and UV were analyzed.
Table 5 and
Table 6 provide a summary of the microbial analysis results presented in terms of minimum values (Min), maximum (Max), arithmetic mean (AM), median, standard deviation (SD), as well as log reduction value (LRV).
Figure 2 shows the trend of microbial concentrations in the raw wastewater for the case of TMN and TCB from the defeathering and cooling wastewater samples. From
Figure 2a, it can be observed that the TMN concentration in the defeathering wastewater had a more symmetric distribution with the median observed to be closer to the middle. That means, from the list of experiments, most of the concentration values from the counted microbes were around 1400 CFU/100 mL. The boxplot from the cooling section wastewater samples shows the median closer to the upper or top quartile; in that matter, the distribution of microbial concentration in the studied wastewater samples is considered to be “negatively skewed”, with concentration values ranging from 500 to 1500 CFU/100 mL.
From the
Figure 2b defeathering boxplot, the median was observed to be closer to the lower quartile with an indication that the TCB concentration data from the studied wastewater samples constitute a higher frequency of more high concentration values than the low concentration values (“positively skewed”), with most of the concentration values being below 800 CFU/100 mL. While that of the cooling section observed to be symmetric, with concentration values ranging from 40 to 1200 CFU/100 mL. Moreover, the boxplots show a significant variation in terms of microbe concentrations in the raw wastewater. Therefore, a relatively stable treatment system is of significant necessity to achieve a high-quality effluent from such a fluctuating flow in terms of pollution loading.
In general,
Figure 2 reveals further that the wastewater from the defeathering section is more contaminated with microbes than the wastewater from the cooling section of the poultry slaughterhouse. The phenomenon can be highly linked to the nature of production activities between the two sections, of which the defeathering process is regarded to be generating wastewater with a higher organic load than the cooling wastewater [
35].
Both TMN and TCB were the main microbial parameters investigated when the wastewater samples from defeathering and cooling sections of the slaughterhouse were treated separately. In general, the integrated treatment plant achieved more than 99.83% of microbial removal efficiency from both defeathering and cooling wastewater samples. The difference in terms of removal efficiency for TMN and TCB from both defeathering and cooling sources is literary small (defeathering: 99.86% removal efficiency from TMN and 99.97% from TCB, cooling: 100% removal efficiency from TMN and 99.83% from TCB). The results indicate that the treatment approach can be highly effective even when subjected to wastewater with fluctuating pollution load. The impressive performance can be highly linked to the fact that each unit within the integrated plant has some degree of microbial elimination capacity. For instance, during the electrolysis process, microbes are killed by a variety of oxidants that are produced within the process [
36], as well as the UF treatment process can retain some of the microorganisms depending on the filter pore size [
37]. While, the UV disinfection technology was used in this study specifically for the elimination of microbes in the pre-treated water.
3.1. EC Effluent Quality
From
Table 5, it can be observed that in some cases the EC treatment unit was able to eliminate all the microbes in wastewater with a zero (0) microbial count being achieved as the minimum concentration value for TCB, TTCB,
pathogenic flora, including
salmonella,
coli phages,
Staphylococcus aureus as well as
Enterococcus. In general, the lowest average concentration value from the EC effluent was observed from the
coli phages achieving 1 CFU/1000 mL. However, it should also be noted that the concentration of
coli phages was observed to be generally low in raw wastewater. Moreover, the EC treatment unit faced a significant challenge in the removal of
pathogenic flora, including
salmonella. The average concentration of
pathogenic flora, including
salmonella, was 78 CFU/100 mL in the raw wastewater, while the EC treatment unit was able to reduce the concentration to 28 CFU/100 mL, which can be termed as low performance in comparison to the other studied microbial parameters. The EC current and electrode (anode) potential are among the important parameters that have the most influence on the production rate of strong oxidants responsible for microbe elimination [
38]. In that matter, adjustments to the EC current may significantly improve the performance of the treatment unit in terms of microbial elimination. Also, based on the LRVs (see
Table 4), the EC treatment unit achieved two as the maximum LRV (from TCB,
Pseudomonas aeruginosa,
Staphylococcus aureus, and
Enterococcus) which is equivalent to approximately 99% removal efficiency, with 0 LRV observed from
pathogenic flora, including
salmonella, indicating less than 90% removal efficiency.
3.2. Final Effluent Quality
From
Table 6, it can be observed that after the combined treatment, the treatment plant was able to eliminate all the microbes in the wastewater for all the studied microbial parameters, except
Pseudomonas aeruginosa. For
Pseudomonas aeruginosa, the treatment plant did not achieve 0 CFU/100 mL during some experimental sessions, and the microbial parameter was generally observed to be the most resistant group among the studied microbial parameters. On average, a microbial count of 5 CFU/100 mL for
Pseudomonas aeruginosa was recorded in the final effluent after the combined treatment. The maximum recorded
Pseudomonas aeruginosa microbial count was 13 CFU/100 mL. However, in some experiments, the treatment approach achieved 0 CFU/100 mL for
Pseudomonas aeruginosa as observed in
Table 6.
In general, despite the challenge with Pseudomonas aeruginosa, the treatment approach was able to achieve 0 CFU/100 mL as the minimum recorded concentration value for all the studied microbial parameters after 40 min of retention and an average of 5.5 A from the EC treatment unit as well as a combination of 60 mJ/cm2 radiation dose and 24 min retention time from UV unit. That means, in the list of the experiments at least one experiment observed the treatment plant inactivating all the microbes in the wastewater. The phenomenon indicates that depending on the characteristics of the wastewater, the treatment approach can be able to eliminate the microbes to 0 CFU/100 mL.
3.3. General Removal Efficiency
From
Figure 3, it can be observed that in terms of the removal efficiency, the EC treatment unit was able to remove the majority of the microorganisms with an efficiency ranging from 64.1% to 99.83%. In the raw wastewater, an average of 1780 CFU/mL of TMN was recorded, after the EC treatment, an average of 62 CFU/mL was recorded which is equivalent to 96.52% removal efficiency. For the TCB, an average of 1991CFU/100 mL was recorded in the raw wastewater, with 20 CFU/100 mL recorded as an average count after the EC treatment which is equivalent to 99% removal efficiency. Moreover, an average of 793 CFU/100 mL of TTCB was recorded in the raw wastewater, while after the EC treatment an average count of 29 CFU/100 mL was recorded, equivalent to 96.34% removal efficiency. An average of 78 CFU/100 mL of
pathogenic flora, including
salmonella was recorded from the raw wastewater, while an average of 28 CFU/100 mL of
pathogenic flora, including
salmonella, was recorded after the EC treatment, equivalent to a 64.1% removal efficiency.
From the combined treatment, the treatment plant achieved almost a 100% removal efficiency for all the studied microbial parameters, except for
Pseudomonas aeruginosa of which an average of 99.84% removal efficiency was achieved. According to the literature [
39,
40,
41], the high removal resistance observed from the
Pseudomonas aeruginosa can be highly linked to the ability of the bacteria to form a biofilm as observed from the UV lamp fouling, which is a consortium of bacteria embedded in a self-produced polymer matrix composed of protein, polysaccharide, and DNA. In general, bacteria associated with biofilms are much more difficult to kill and remove [
42].
Pseudomonas aeruginosa is highly resistant to disinfectants as well as antibiotics and is considered as one of the most problematic bacteria in healthcare facilities and is responsible for approximately 10–20% of hospital-associated infections [
43]. The bacterium is naturally resistant to many antibiotics and disinfectants as a result of the permeability barrier from its Gram-negative outer membrane. Once the biofilms are formed, they are difficult to remove because the extracellular polymeric substance (EPS) is firmly attached to the surface and can block access of antimicrobial agents to individual cells, leaving behind a source for recontamination [
44]. In general,
Pseudomonas aeruginosa is referred to as one of the most problematic bacteria [
43]. Chronic infections are among the significant concerns of bacterial biofilms, as they are characterized by high tolerance to antibiotics and disinfectants as well as resisting phagocytosis and other components of the human body’s defense system [
45].
Although the general lowest removal efficiency (99.84%) from the final effluent was observed from the Pseudomonas aeruginosa microbial count, the EC treatment unit was able to lower the count from an average of 3197 to 31 CFU/100 mL, which is equivalent to 99% removal efficiency. This means the combination of UF and UV radiation did not seem to be that effective for the elimination of Pseudomonas aeruginosa.