By Yufei Qian, Environmental Horticulture and Urban Forestry, ‘18
“As an international student, English is not my first language. I never felt such confidence writing in academic English before taking Professor Matthew Oliver’s UWP 104E class. This class is about writing in science, and the literature review is the basis of many scientific research projects. To develop future experiments, literature reviews help researchers maintain an up-to-date understanding of the field, identify strengths and limitations in current experimental designs, and get inspirations for their own research. I am interested in ecology and the relationship between humans and environment, so I decided to write about constructed wetlands. Since I am a plant-lover, a focus in my literature review research is plant selections and impacts on E. coli removal efficiency. UWP 104E is a great class that combines my interest with professional writing and helps me overcome language barriers in academic English.”
Human population growth, economic development, and freshwater contamination contribute to pressures on available water systems. Domestic and municipal wastewater contain various pathogens that harm human health (Avelar, de Matos, de Matos, & Borges, 2014). Conventional sewage process systems use chloride widely to eliminate pathogens. Using chloride simply and cost-effectively decreases pathogens, but by-products in the treated water contain carcinogens and toxic substances (Alexandros & Akratos, 2016). People also use UV radiation for safer pathogen elimination, but this method requires high facilities investment and long-term maintenance (Alexandros & Akratos, 2016). Constructed Wetlands (CWs) technology began booming 10-15 years ago, but the primary focus now is on the removal of organic matters, solid waste, heavy metal, nitrogen, and phosphorous (Alexandros & Akratos, 2016). However, recent studies pay more attention to the removal of pathogens by CWs.
CWs have channels that are filled with porous substrates such as coarse sand, crushed stone, or gravel. These bottom and side channels have low permeability and support plant growth (Avelar et al., 2014). Basically, CWs have three categories: 1) free water surface flow, 2) Horizontal Flow (HF), and 3) Vertical Flow (VF). Among these three methods, HF has been studied the most worldwide because it produces less malodor and attracts fewer rats and mosquitoes (Avelar et al., 2014).Although, Alexandros and Akratos suggest that VF bed with 65cm depth achieved a high pathogen removal rate (2016). Therefore, it is necessary to review both methods’ removal efficiencies.
Pathogen removal efficiency relates to different substrate textures, wastewater flow directions in CWs, plants species used in CWs, and mechanisms applied. However, all the experiments are constructed on a small-scale parameter of experimental basis and fail to consider pathogens introduced from the outside environment. For example, in real life application, a CW may attract migratory birds and herbivores around the area. Animal feces can pollute the remediated water again and lower pathogen removal efficiency. In tropical regions, cities with high population are more worthy of study than remote areas. The reason is that there are bigger populations in cities than in remote areas. If waterborne diseases outbreak in population-dense cities, it can be more harmful. CWs in cities should occupy less space because land is a scarce resource in a population-dense area. Overpopulation, high temperature and high humidity all lead to pathogenic outbreak in municipal water systems. Moreover, CW pathogen removal is a developing area, and each factor has some limitations that decrease the removal efficiency.
Two fixed factors in this review: Temperature range and pathogen indicators
Temperature enhances pathogen removal efficiency because pathogens reach maximum activity rate at 37℃ (Alexandros & Akratos, 2016). Pathogens’ metabolism rates are different at the freezing point, tropical temperature, and boiling point. The tropical zone, located near the equator, is characterized by hot climate and high precipitation. Therefore, stable temperature range in a tropical area limits the variables in the research.
It will be time-consuming to test all groups of pathogens in the research, so researchers test the most notorious five groups of waterborne diseases, which are enteric bacteria, protozoa, helminths, fungi, and viruses. E.coli, TC (total coliforms), and FC (fecal coliforms) are common bacterium indicators (Alexandros & Akratos, 2016). In the following review, “pathogens” refers to E.coli, TC, and FC.
In VF CWs, finer texture substrate, such as sand, has a higher pathogen removal rate than coarser substrate, gravel (Bohórquez, Paredes, & Arias, 2016). According to figure 1, raw wastewater is pre-treated in a sedimentation tank, attached by pump 1 and 2. There are eight smaller experimental units with different treatments, respectively. Letter “S” stands for sand and letter “G” stands for gravel. Units 2,3,6, and 7 have finer sand and the other four units have coarser gravel. Pathogen removal results show a significant difference between sand and gravel. The removal of TC and E. Coli in the gravel beds is very low: 0.00–0.08 log units, while 1.2–2.7 and 1.5–3.5 log units of TC and E. Coli are removed from the sand beds (Bohórquez et al., 2016). Removal rates of suspension solids and pathogens (TC and E. Coli) are very low or null in gravel beds.
However, pre-treated raw wastewater in the sedimentation tank is a limitation of this experiment. Particle sizes influence the sedimentation speed, which means the bottom layer of the sedimentation tank acts as a pathogen sink (Alexandros & Akratos, 2016). In this experiment, it is more plausible to say that the unsettled pathogens, which have smaller particle sizes, are removed more efficiently by sand than gravel.
- HF Combination
Contrary to substrates in VF CWs, a combination of HF CW with slow sand filtration (SSF) cascade becomes an optimal pathogen removal approach (Seeger, Braeckevelt, Reiche, Müller, & Kästner, 2016). Because HF CWs require the least technology to construct, wastewater sometimes cannot meet the European Irrigation Water Standards through this process (Seeger et al., 2016). Using a weekly rotational cascade filter with a static cascade filter as a secondary treatment increases the removal efficiency (Seeger et al., 2016). Although, the investment in CWs and a series of SSF for secondary treatments should not be overlooked. Europeans tend to use HF for less odor, but SSF might not work in a tropical region, where water has high salinity. The HF combination system in Germany includes heating fans to prevent freezing during winter (Seeger et al., 2016). Freezing is not a problem in tropical regions, but water salinity can be. EC, measuring ions and electrical conductivity, determines water salinity. As water moves through fine texture sand layers, salt accumulates and the scale builds up, gradually. Scale deposit comes from the dissolved calcium and magnesium salts in the municipal water. Accumulation of scale can clog the whole SSF system, because the finer the sand, the slower the percolation rate. Changing the filtration layers frequently can help. More research in scale-breaking will be interesting.
Plant species influence
Vegetation contributes to HF and VF CWs in the following aspects. According to Fountoulakis et al., root systems increase filtration and sedimentation, accumulate oxygen and microbial activity in the root zone, and increase plant uptake of organic matter and heavy metals (2017). Halophytes, a group of plants, only grow under high salinity conditions. Research focused on halophytes in CWs started growing five years ago, but research focused on salt remediation began more recently (Fountoulakis, Sabathianakis, Kritsotakis, Kabourakis, & Manios, 2017). In this study, researchers compare the pathogen removal ability between halophytes and reeds. Two identical VF CW tanks are filled with the same materials. One tank has three species of halophytes, and the other tank has reeds. Then, wastewater flows into both tanks and is distributed by valves onto the substrate surface (Fountoulakis et al., 2017).
After nine months, TC and E.coli have a decreasing trend in the outlet planted with halophytes. In the presence of halophytes, TC decreases from 1.2 log units to 0.7 log units (Fountoulakis et al., 2017). On average, TC concentration in the inlet is 6.2 log units, while in the outlet of the VF CW planted with halophytes is 5.0 log units. The mean value of E. coli’s concentration in the outlet is also statistically important (p < 0.05) (Fountoulakis et al., 2017). An explanation of this decreasing trend (Fig 6) is that halophytes’ exudate contains methanol or hexane, and both substances act as antibacterial agents against E. Coli (Fountoulakis et al., 2017).
This experiment took place in Greece, known for its Mediterranean climate. Halophytes that survive in Greece might not be able to deal with the high temperature and moisture in the tropical area. However, salinity is the most essential requirement for halophytes’ survival (Fountoulakis et al., 2017). Tropical areas tend to be near coastal regions, and the municipal water supply there has high salinity. Therefore, this salt tolerant plant may favor tropical areas. Also, in constructed wetland research, Halophytes are commonly used to treat salt accumulation more than to treat pathogens (Fountoulakis et al., 2017). Moreover, tidal surge and tidal backup, which bring brackish water into coastal cities, increase Halophytes’ adaptability. Halophytes, growing in a densely populated city, help remove coliforms in the wastewater, which is beneficial to the public health.
- Native Tropical Species
Even though new species can potentially become invasive, they are necessary because local species are ineffective towards pathogen removal. A study shows that the presence or absence of a tropical plant (Heliconia psittacorum) does not influence pathogen removal efficiency (Bohórquez et al., 2016). Bohórquez (2016) discovers that non-native species such as Phragmites, Typha, and Cyperus are widely used in tropical CWs worldwide. He tests a tropical plant specie Heliconia psittacorum, but the presence of this plant does not affect the removal of TC and E.coli (Bohórquez et al., 2016). More tropical species are worth testing.
- Plant Coverage & Root System
Plant species may have different abilities of removing pathogen indicators, but research shows that CWs covered with plants get benefits from several pathogen removal mechanisms. At a same water flow rate, TC decreases more in a planted wetland than an unplanted wetland (Weerakoon et al., 2016). Similar results come from research in southeastern Brazil, which has a tropical climate. E. coli removal efficiency reaches 2.8 log units in planted CWs compared to 2.3 log units in unplanted CWs (Avelar et al., 2014).
A possible mechanism is that plant roots grow in water, and pathogens attached to the biotic roots have better removal rates (Avelar et al., 2014). Plant coverage is an advantage in tropical areas. Stable temperature range allows long growing seasons for plants. Adequate precipitation and sunlight foster fast-growing plants. Fibrous plant roots intertwine in the water and construct a huge pathogen filtration system. Many plants are proven to have this biofilm function along their root structures (VanKempen-Fryling & Camper, 2017). If pathogens prefer attaching to biotic plant roots, it is easier to capture and analyze them compared with free-flowing pathogens in the waterway. A research study shows that E. Coli strain O157:H7 has a significantly higher affinity to biotic plant roots than the abiotic nylon control group (VanKempen-Fryling & Camper, 2017).
Another mechanism of effective pathogen removal is the aeration and oxygen level around plant roots. Higher levels of dissolved oxygen (DO) around plant roots facilitate microorganism, such as E. coli, removal(Avelar et al., 2014). Fibrous roots act as a supportive system which enables a good aeration condition. TC removal efficiency reaches 1.37 log units and is higher in planted CWs than unplanted control CWs (Sharma & Brighu, 2016). In this experiment, researchers study plant species Phragmitis australis and Canna indica. In the VF CW design, C. indica has a higher pathogen removal efficiency than P. australis, which are 1.87 log unit and 1.01 log unit, respectively (Sharma & Brighu, 2016). An explanation is that P. australis removes pathogens more efficiently in HF CWs than VF CWs (Sharma & Brighu, 2016).
C.indica has a higher DO level than P. australis and the unplanted control group (Sharma & Brighu, 2016). Fibrous root systems increase the aeration capacity of C. indica, and pathogens do not favor the aerobic environment (Sharma & Brighu, 2016). Therefore, improved aeration in the root zone provides additional removal efficiency; oxidizing pathogens on the root surface decreases the total pathogen load (Sharma & Brighu, 2016). VF CWs should have an aerobic environment in the water and an anaerobic environment at the bottom. In water, pathogens have a predilection to biotic medium, which is the fibrous root system. Roots respire all the time and provide an aerobic environment. More DO around the root zone reduces the pathogenic activity, and increases the removal efficiency. This research takes place in Rajasthan, India, where the climate is semi-arid. However, C. indica can survive in tropical areas without any problems. C. indica is a frost-sensitive plant, which means it cannot survive in cold areas. It has been found in South America, Mexico, southeastern United States, Africa, and Asia. This research potentially refutes the idea that tropical plants cannot remove pathogens (Bohórquez et al., 2016), because C. indica grows worldwide in tropical areas.
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