Effectiveness of wastewater treatment systems in removing microbial agents: a systematic review

Due to hazardous impacts of municipal, industrial and hospital wastewater on water, soil, air and agricultural products, wastewater treatment and the proper disposal of the sludge produced are indispensable from an environmental safety point of view [1, 2]. Economically, effective wastewater treatment has important effects on saving water, and preventing unnecessary water losses [3]. In arid and semiarid countries such as Iran, the water demand has increased and annual rainfall is low also in regions of North Africa, Southern Europe, and in large countries such as Australia and the United States. Consequently, reuse of sewage is the most sustainable and long-term solution to the problem of water scarcity [4, 5]. In the next 30 years, the world’s population will increase by more than double. Due to population growth, the amount of water available in 1960 was reduced to 3300 cubic meters and in 1995 it was reduced to 1250 cubic meters. This trend is projected to decrease to 650 cubic meters worldwide by 2025 [6]. Due to this water shortage crisis, water from wastewater treatment need to be reused increasingly in the near future [6]. Wastewater reuse requires treatment and application of appropriate wastewater treatment systems [7]. In recent years, increased research has been done on wastewater treatment using simple, low-cost, easy-to-use methods in developing countries [8, 9]. Systems and processes such as activated sludge, aerated lagoons, stabilization ponds, natural and synthetic wetlands, trickling filters, rotating biological contactors (RBCs) have been used for wastewater treatment and removal of physical, chemical and biological contaminants [10, 11]. Among different contaminants of wastewater, microbial agents becoming increasingly important and their removal efficiency should be reported in different wastewater treatment systems [12, 13]. Biological contaminants in wastewater are different types of bacteria (Fecal coliforms and Escherichia coli, Salmonella, Shigella, Vibrio cholerae), diverse Parasite cysts and eggs, viruses and fungi. All of them can be hazardous to environmental and human health depending on the type and amount [14, 15]. For example, bacteria in wastewater cause cholera, typhoid fever, and tuberculosis, viruses can cause hepatitis, and protozoa can cause dysentery [16, 17]. Many microbial agents attached to suspended solids in wastewater if inadequately treated and wastewater discharge into the environment, such as river water, green space, and crops, put humans and aquatic organisms at risk [18, 19]. Therefore, utilization of appropriate wastewater treatment systems tailored to a variety of microbial agents is essential to achieve as complete as possible elimination of biological agents. For example, in the study of Sharafi et al., (2015) with the aim of determining the removal efficiency of parasites from wastewater using a wetland system, the removal rates of protozoan cysts and Parasite eggs were 99.7 and 100%, respectively [20]. Okoh, et.al. (2010) reported that activated sludge processes, oxidation pools, activated carbon filtration, lime and chlorination coagulation eliminated removed 50–90% of wastewater viruses [21]. Wastewater from wastewater treatment plants, is used in Iran without restrictions and controls like in many other countries. Therefore, it is necessary to employ proper sewage treatment systems, before water can be publicly used such as for irrigation. This study is focusing on the efficacy of different wastewater treatment systems in removing microbial agents.

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 [53‐55], 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.

Different types of activated sludge systems have better efficacy to remove microbial agents and are more effective than other systems in removing the main indicators of sewage contamination including Total coliforms and Fecal coliforms. However, inappropriate operation, maintenance and inadequate handling of activated sludge can also reduce the efficiency of microbial agent removal, which has been reported in some studies. Therefore, it is recommended to conduct research on how to increase the operation, maintenance and proper management of activated sludge systems and provide the results to utility personnel responsible to work with this system in order to correct the system quality output in a timely manner. In future research, it is recommended that employed treatment methods integrate two or more purification systems, which then could more effectively remove microbial agents. Additionally, the reports of removal efficiency should include each of the indicated microbes so that health and environmental professionals can make better decisions about using the systems or prevent future eventualities.