Effectiveness of wastewater treatment systems in removing microbial agents: a systematic review - Globalization and Health

07 Dec.,2022


containerized wastewater treatment plants

Examination of microbial removal rates in the study of Ghoreishi et al., 2016 [38] that none of the Total Coliforms removal was US-EPA standard although both extended aeration activated sludge and conventional activated sludge systems were used to remove Total coliforms. The US-EPA standard for Total coliforms removal is 1000 MPN/100 mL, and wastewater showing this amount of Total coliforms is capable of being discharged into the receiving waters [26, 27]. A study by Paiva et al., 2015 on domestic wastewater in tropical Brazil also showed that removal of Total coliforms through the use of activated sludge was not a desirable remediation method [42]. The reason for the poor performance of activated sludge to remove Total coliforms can be attributed to factors such as management problems and operation of the activated sludge system, which results in the production of bulk waste and sludge. This problem is one of the most important disadvantages of activated sludge systems and should be addressed once a month by experienced staff and monitoring experts to correct it. Overall, different activated sludge systems are the best choice for this type of wastewater due to the amount of municipal wastewater pollutants because of high purification efficiency to reduce biochemical oxygen demand (BOD5) [43, 44].

Removal of Cysts and Parasitic eggs in the study of Derayat et al., (2011), which used stabilization pond systems, was reported as being in accordance with US-EPA standards [30]. A study by Amahmid et al. (2002) aimed at the treatment of municipal wastewater with a stabilized pond system in Morocco showing that Cyst and Parasitic egg removal efficiency was 100% and that the pond system showed a proper performance [45]. A large number of stabilized pond systems were been constructed and used in countries such as the United States, New Zealand, India, Pakistan, Jordan and Thailand [3]. In Iran, a number of these systems were constructed for the treatment of wastewater in Arak, Gilan West and Isfahan [46]. Stabilization ponds have a high acceptability due to their simplicity of operation, and lack of mechanical and electrical equipment compared to other sewage treatment systems, their high efficiency in removing pathogenic organisms [47]. A major drawback for stabilization ponds is the need for extensive land, the low quality of effluents due to the presence of algae, and odor production that limits the use of this type of treatment system near habitated areas. To improve the quality of resulting effluents, chemical compounds need to be consolidated, such as by coagulation and the application of microstrainers, stabilization ponds and rock filters [47, 48].

As for wetlands by Karimi et al. (2014) on Fecal coliforms, Escherichia coli and Fecal streptococci show that wetlands did not perform well to remove microbial agents (removal rate for Fecal coliforms 1.13 × 1014 MPN/100 mL and Escherichia coli 5.03 × 1012 MPN/100 mL) [34]. In a study by Decamp et al. (2000), the mean removal of Escherichia coli through the wetland was 41 to 72% at the in situ scale and 96.6 to 98.9% at the experimental scale [49]. In the study of Evanson et al. (2006), Fecal coliforms removal rate was 82.7 to 95.99% [50]. Removal of Total coliforms and Fecal coliforms in the wetlands is done by various biological factors such as nematodes, protozoa, bacterial activity, bacteriophage production, chemical factors, oxidation reactions, bacterial uptake and toxicity [51] and the interference in each of these (microbial communities) will affect the rate of removal of Total coliforms and other microbial agents. Removal of pathogens such as Escherichia coli and Cryptosporidium was also performed in wetlands but is often not in compliance with environmental standards [52]. In addition, although wetlands are economical and widely used in wastewater treatment systems because of easy to operate, maintain, and operate at a low price [53,54,55], but they don’t seem to be a good option for removing all of the microbial agents.

In a study by Hashemi, et.al. (2010) on UV disinfection system included low pressure (LP) and UV disinfection system including medium pressure (MP) to remove Total coliforms, Fecal coliforms and Fecal streptococci. All investigated microbial agents were completely eliminated [28]. However, it was reported that the direct disinfection of secondary effluents with LP and MP systems and even their integration due to high concentrations of suspended solids was not possible. Therefore, disinfection of wastewater with UV irradiation requires higher effluent quality through improved system utilization or application of an advanced treatment plant prior to disinfection [28]. In 1988, about 300 and in 2004 about 4300 sewage treatment plants in the United States, (that are more than 20% of filtration plants) used a UV system for wastewater disinfection. The number of wastewater treatment plants having UV systems has increased in the US, Europe and East Asia. This trend is expected to expand further in the coming decades. Although the use of UV radiation for wastewater disinfection has many potential advantages, it also has disadvantages in terms of cost, lamp deposition, and the possible reactivation of targeted pathogenic microorganisms after treatment [56]. Wastewater treatment professionals should therefore be aware of new replacement processes and perform pilot scale assessments prior to changing treatment processes.

One of the strengths of this study is addressing the efficacy of wastewater treatment systems by comparing the removal efficiency of various microbial agents that have received little attention as yet. In most studies, only one type of system for removing different physical, chemical and microbial contaminants in a single type of wastewater was investigated and it was not possible to compare the removal efficiency of microbial agents. One of the limitations of this study was the lack of reviewing published articles on wastewater treatment systems in other than the 5 Iranian journals. This limitation, however, might be negligible because the research on wastewater treatment was done by environmental health professionals. Therefore, most studies in this area are published in specialized environmental health journals.