Efficiency of wastewater treatment plants in removal of intestinal parasites: A review approach

Abstract Background: Wastewater treatment plants (WWTPs) in urban areas, are potentially one of the causes of the spread and entry of parasitic pathogens into the environment and water resources. Numerous studies have been conducted on the presence of parasitic agents in the entry and effluent of WWTPs in different countries. This study was conducted to review the parasite prevalence in different WWTPs and assess their removal efficiency. Methods: ScienceDirect, Scopus, Web of Science, and PubMed databases and other sites with the ability to receive a full-text related articles, were reviewed. Various keywords such as parasites, wastewater, effluent, treatment efficiency, protozoan cysts, parasitic eggs, and similar keywords were used to search articles published from 2005 to 2019. After screening the titles and abstracts of articles, 26 articles were reviewed in this study. Results: Giardia spp, Cryptosporidium spp, Entamoeba spp, Blastocystis spp, Toxocara spp, Trichostrongylus spp, A. lumbricoides, and Hymenolepis are the most prevalent parasites. Studies show that parasitic egg hatching rates are up to 5%, in aerated lagoons, up to 99.9%, in activated sludge, and up to 5%, in ponds due to high retention time and downstream artificial reefs. Conclusion: Studies show that due to the current state of treatment plants and adverse operating and maintenance conditions, it is not possible to completely remove parasitic pathogens from these systems, therefore, it should be considered by regulatory authorities and the systems need to be improved for better performance of treatment plants and optimizing them, and training of users should also be improved. This review indicated that the types of natural constructed wetland in Kermanshah (Iran) and activated sludge of wastewater treatment processes in Iran and Brazil are most effective in removal of intestinal parasites.


Introduction
Parasitic infections are among the most important sources of pollution in nature which can be considered as a public health index (1). Traditionally, parasitic infections have been a health problem in raw food and drinking water so that parasitic contamination and its removal have always been one of the concerns of health practitioners and there are many studies conducted to find a method to eliminate it (2). Drinking water is the most important part of food for humans and animals and its contamination is vitally important (3). Drinking water for communities, including potable water, is derived from a variety of sources, including wastewater composed of freshwater higher than 99.9% and impurity less than 0.1%, which can be one of the most important sources of water after treatment (4,5). The reuse of treated domestic wastewater as a valuable source of water for various uses, including agriculture and green space irrigation, is one of the most important purposes of wastewater treatment and conservation of resources, especially in low-water areas (6,7). Wastewater reuse, especially in the agricultural sector, has many benefits, including the benefits from effluent sales, the reduction of dust through spraying, the use of nutrients such as phosphorus and nitrogen in the wastewater (8). The general benefits of wastewater reuse include agricultural irrigation (commercial crops and plants, etc) (9), landscape irrigation (recreation areas, playgrounds, street green spaces, urban green belts, golf courses, cemeteries, and residential areas), water recycling in the industry (cooling water, boiler feed water, industrial consumable water and construction consumable water, groundwater recharge (ground sealing, saline containment, and groundwater recharge), recreational or environmental applications (lakes and wetlands, wetland remediation, increased river water flow, fisheries, etc), non-food applications (fire, air conditioning, toilet flushing, washing the pavements and sidewalks, etc), water required for storage tanks for drinking water supply and water supply network, water supply of heating networks and air conditioning systems, water supply of the secondary water distribution networks (separate from drinking water) and so on (10)(11)(12). Thereby, reducing the use of chemical fertilizers and reducing costs and consumption of freshwater as primary sources are the positive effects of wastewater reuse projects as secondary sources, environmental protection and improvement of its quality and beauty as public resources. The quality of the used wastewater and particularly microbial infections compared with national standards and international guidelines are of great importance in these fields (13)(14)(15). Today, there are many problems related to environmental pollution by various types of effluents and sewage. Pollution of water resources such as drinking water wells by various parasites caused by different sewage and effluents of various inefficient treatment plants, negative effects of effluents on the river water quality, and deterioration of water quality index, are the main problems of communities (16)(17)(18). Poor progress in the construction of wastewater treatment plants (WWTPs) in underdeveloped and developing countries to cover the full establishment of all municipal wastewater and the lack of development of wastewater treatment for small communities and, consequently, the discharge of dispersed wastewater or septages into the environment and other cases, causes the spread of parasitic diseases in society. Poor management and operation of existing treatment plants due to lack of attention to environmental monitoring systems and healthcare providers and environmental health is a contributing factor to this unfavorable environmental condition (18,19). In wastewater reuse, if the microbial quality of the effluent and its health aspects are not addressed, it will pose a serious risk to human health and the environment. This will be even more important when the wastewater is used for irrigation of public lands and food products, including fruits and vegetables (6,20). To remove the pollutants present in the wastewater, including organic matter and pathogens, the wastewater must be treated. There are various purification processes including biological treatment such as activated sludge modifications, stabilization ponds, artificial wetlands, lagoon aeration, and trickling filters (20)(21)(22)(23)(24)(25) and physicochemical treatment (coagulation, oxidation, ozonation, adsorption membrane technology, and other advanced treatment). The mechanism of parasitic egg removal during sewage treatment processes is different. The main mechanisms are sedimentation, filtration, adsorption, entrapment in activated sludge flocs, and inactivation due to adverse environmental conditions (26)(27)(28)(29). In this review, studies conducted on parasitic pollution in WWTPs and the effects of treatment process on parasites and parasitic pollutants removal were investigated.

Materials and Methods
The articles reviewed in this study include an original research papers and cross-sectional descriptive studies on the presence of parasites and parasitic pollutants in untreated wastewater and effluent from treatment plants, published in Iranian or foreign journals. All full-text articles that were available and published from 2005 to 2019 were included in this review. Studies that did not meet these criteria were excluded from the study. Therefore, the articles of the congresses and the abstracts of the articles were not examined at all. First, the format of extracting data from articles was determined based on the discussion and agreement between researchers and authors of the articles. After extracting the articles data, they were reviewed several times by three reviewers and the differences were determined after discussion and agreement. The extracted data include the type of WWTPs (industrial, hospital, urban, etc). The study group included a variety of urban and domestic wastewater, hospital, agricultural, and industrial wastewater, and a variety of treatment plants and their performance in eliminating pollution and the presence of parasitic agents in effluents. The parasite species, whether protozoa or worms, were reported to be pathogenic or non-pathogenic. The concentration and density of parasitic pollutants in raw sewage and various types of WWTPs and their prevalence in the studied communities were investigated. Different valid databases such as ScienceDirect, Scopus, Web of Science, PubMed, and other sites with the ability to receive a full-text related articles were reviewed. Various keywords such as parasites wastewater, effluent, treatment efficiency, protozoan cysts, parasitic eggs, and similar keywords were used to search. By searching the above-mentioned key words, 3017 papers were found, and based on title and abstracts, 2939 papers were excluded, and finally, 26 sources were reviewed in this study.

Results
Prevalence of parasitic infections in water treatment system Based on the collected data, nine studies have been conducted to investigate the prevalence of parasitic infections in different treatment plants ( Table 1). The most prevalent infections include protozoan cyst infections such as Giardia spp and Cryptosporidium spp, amoeboid trophozoites such as Entamoeba spp, Blastocystis spp, and helminthic infections such as Toxocara spp, Trichostrongylus spp, Ascaris lumbricoides, and Hymenolepis. The prevalence of G. Lamblia infections were reported 9.09 to 46.0% in Southwest of Iran and the United States, respectively (30,31). The prevalence of Cryptosporidium spp. in raw wastewater was reported 7.24-27% (30)(31)(32). The prevalence of Entamoeba spp, Ascaris, and H. nana was 50, 45.75, and 4.52%, respectively (Table  1) (33,34). The removal efficacy of parasites in WWTPs is presented in Table 2.

Discussion
In this study, the studies conducted on the status of parasitic contamination in the wastewater of treatment plants, as well as the effect of treatment plants on removal of these factors, were reviewed. Due to climate changes, drought, and urban expansion, as well as the increasing need for water for drinking, irrigation, agricultural and industrial uses, water reuse is one of the most important subjects in the world (7,14,50,66). For recycling wastewater, examining sewage pollution and the role of water treatment plants in removing these pollutants is of great importance. Parasitic infections are one of the most important causes of water contamination and even drinking water contamination which can have irreparable risks to human health if not eliminated. This will be even more important when the wastewater is used for irrigation of public green space and food products including poultry and vegetables (35,67,68). In order to remove pollutants present in wastewater, including organic matter and pathogens, wastewater should be treated. There are various purification processes including activated sludge, stabilization ponds, artificial wetlands, aerated lagoon, and drainage filters (7,38,42,48,(69)(70)(71). Different mechanisms are used for removal of parasite egg during sewage treatment processes. The main mechanisms are sedimentation, filtration, plant root uptake, entrapment in activated sludge flocs, and inactivation due to adverse environmental conditions (7,34,37,44,46,62,70,(72)(73)(74). In recent years, many studies have been conducted to investigate parasitic contamination in water resources and the results have shown that many of these sources are contaminated with parasitic infections. Protozoan such as G. lamblia, Cryptosporidium spp, Entamoeba spp, Blastocystis, various species of free-living amoebae and worms such as Trichostrongylus spp, A. lumbricoides, Enterobius vermicularis, Trichuris trichiura, and Hymenolepis spp. are the most commonly reported parasites (4,32,40,52,57). Javanmard et al examined the contamination of WWTPs in southwestern Iran and found that 41.7% of the municipal WWTPs were contaminated with parasites such as Toxocara, Trichuris, Trichostrongylus, and various species of amoeba (35). In another study, Sharafi et al examined the treatment plants in different cities of Kermanshah province and showed that all municipal WWTPs in this province were infected with parasitic infections (34,37). In Brazil and South Africa, 30.43% and 31.65% of wastewater was respectively infected with parasitic infections (36,44,75,76). One criterion for checking the quality of the WWTPs is the number of parasitic eggs per liter of wastewater, which varies across countries. Different rates have been reported in developing countries such as Brazil (206-166), Morocco (840-214), Jordan (300), Pakistan (144), Russia (≥2000), and Ukraine (60). In advanced countries such as USA, France, and Germany, it was reported 1-8, 9-10, and less than 40 per liter, respectively (40,41,43,70,75). Based on the results of the studies, it can be concluded that the sewage systems of large and industrial cities are less polluted than those of small cities, which could be due to the production of industrial wastewater (without parasitic pollution) in these cities. Another factor that is very important is the use of more advanced systems in larger cities, as well as more and more strict control in larger cities. The results of these studies show that parasitic infection is one of the inevitable factors of urban and rural wastewater that its removal is necessary for reuse of these resources.  (14,29,43,44,51,53,(55)(56)(57)60,61,72). Overall, helminth infections, including parasitic eggs and larvae, are more easily removed by the treatment systems compared to the protozoans. The main protozoa species include Giardia, Cryptosporidium, and Acanthamoeba, which are not completely (100%) eliminated (42,56,77,78). Since retreated wastewater will be reused and given the potential exposure of humans to these sources, it can be an important factor in causing infections in humans. Diarrhea, abdominal pain, bloating, and irritable bowel syndrome are clinical signs of G. lamblia, and diarrhea, nausea, vomiting, abdominal cramps, and fever are symptoms of Cryptosporidium that may even cause death in people with immunodeficiency. Acanthamoeba develops granulomatosis, amoebic encephalitis, and ocular keratitis in humans. Due to the complications of parasitic diseases, inadequate treatment and reuse of these resources can have irreparable consequences for society. Studies show that parasitic egg hatching rates are up to 5%, in aerated lagoons, up to 99.9%, in actived sludge, and up to 5%, in ponds due to high retention time and downstream artificial reefs. The access level of 5% in each of these processes is a function of the characteristics of the incoming wastewater and the design criteria of the WWTP, which can be very unstable. Also, the present review show that the type of the natural constructed wetland in Kermanshah (Iran) and activated sludge processes in Iran and Brazil had the highest efficiency in removal of intestinal parasites. Also, units of aeration basin and secondary clarifier in municipal treatment plants have more roles in removal of intestinal parasites.

Conclusion
According to the results of the studies reviewed, treatment plants are not capable of complete removal of parasitic contaminants using existing systems and need to be addressed by health authorities. The present review showed that the natural constructed wetland in Kermanshah (Iran) and activated sludge of wastewater treatment processes in Iran and Brazil had the highest efficiency in removal of intestinal parasites. It is suggested that other treatment plants also need to be examined for removal of parasitic and other microbial and viral infections. It is also essential to investigate the efficiency of wastewater treatment systems in removal of parasitic infections in different countries using uniform tests and identify the best process to use in the future.