In a world where life thrives on water, it is important to have methods that will secure the quality of water and provide the nourishment that the environment requires. In this study we observed the effects of pollution on a constructed wetland to determine the wetlands ability to reduce pollution levels. Through chemical tests and biological surveys we were able to determine our wetlands efficiency in pollution reduction. Through chemical testing we found pollution reduction levels of up to twenty percent for phosphate, six percent for nitrate, one hundred percent for iron and ninety-two percent for copper. From the biological surveys taken the bacterial counts and average diversity showed a substantial increase after the introduction of pollutant. The protist population initially showed a substantial drop in numbers followed by a notable increase. Our research has proven that the constructed wetland has the ability for self-purification.
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Water is one of the most abundant molecules on this planet covering approximately 70% of its surface. Oceanic water constitutes 98% of the total planetary water supply. Another 1.6% of the fresh water is locked away in the Earth’s ice cap regions, leaving approximately 0.4% of the fresh water available for human consumption. Mother Nature protects this delicate balance by devising methods for water dispersal and systems for sustaining the quality of water. Throughout the world, in both developed and developing countries, the natural balance that protects our fresh water supply is overwhelmed by human pollution. From industrial waste to simple household runoff, various man-made sources of pollution contribute to the daily depletion of our fresh water. Nature uses wetlands to rejuvenate its water supply as a way to insure a sufficient supply of fresh water.
The ability of wetlands to purify water was first recognized in the 1950’s. Since then the number of wetlands has been decreasing at an alarming rate; as a result it is becoming more apparent that they are a valuable asset for the purification of water. The recognition of wetlands natural ability to purify water has spiked interest in artificial or constructed wetlands (Arthur F. M. Mueleman, 1998; Renee Lorion, 2001). Natural Wetlands are areas of land where the saturation of water is the dominant factor that determines the type of soil development and the types of plant and animal communities living above and below ground. Wetlands are proven to have important filtering capabilities such as retaining excess nutrients and some pollutants, and reducing sediment that would clog waterways and harm fish and amphibian egg development. Provided their ability to filter pollutants, wetlands may provide a cost effective method of treating contaminated waters.
Due to differences in regional climates, hydrology, water chemistry, vegetation, and other factors, there are many different types of wetlands. Generally there are two categories of wetlands: coastal wetlands and inland wetlands.
Coastal wetlands are found along the Atlantic, Pacific, Alaskan, and Gulf coasts in the United States. These wetlands provide a vital gateway connecting fresh water and salt water, setting the stage for the exportation of nutrients and organic materials to the ocean, while providing food and shelter for numerous species of birds and fish. Since coastal wetlands are found in areas where salt water mixes with fresh water, it creates a hazardous environment for most plants. Types of plants that are able to adapt to the high salt condition are certain grasses, grass like plants, and salt-loving shrubs and trees.
Inland wetlands are found along the edges of rivers and streams or anywhere fresh water sufficiently saturates the soil. Inland wetlands include marshes and wet meadows, which contain an abundance of herbaceous plants. Also included in this category are swamps, which are dominated by shrubs, and wooded swamps, which are dominated by trees.
Almost half of the world's population suffer from diseases that are associated with contaminated water; more than 3 million people died in 1995 alone, 80% of them children under age five (Scott D Wallace, 1998). Wetlands may prove to be the answer to this life threatening problem. Acting as the "kidneys" of our planet, wetlands exchange dirty contaminants for clean, pure water and provide wildlife habitat in the process (Scott D Wallace, 1998).
Due to the depletion of natural wetlands we must look for other methods of water remediation. One possible solution is artificial or constructed wetlands.Constructed wetlands are treatment systems that use natural processes involving wetland biota to improve water quality. As water enters the wetland, the water is slowed, and suspended solids are strained by vegetation and settle out. The wetland plants provide the necessary conditions for micro-organisms to thrive. These micro-organisms are able to transform and remove pollutants from the water. For example, microbes can convert inorganic nitrogen into the usable organic forms necessary for plant growth. Constructed wetlands are cost-effective, simple to maintain, and produce consistent quality when mature (Olufemi Oludare Aluko, 2005).
Research has proven that a properly constructed wetland has the ability to reduce, or even eliminate contaminants in livestock wastewater (Kenneth D. Simeral, 1998). Even heavy metals can be reduced either by absorption or by complexing with organic matter (Renee Lorion, 2001). Nitrates can be reduced in wetlands through plant uptake or by de-nitrification into gaseous N2, which is mainly determined by the availability of a biofilm (Volker Lüderitz et al, 2005). Phosphates can be reduced either by precipitation or by absorption into the soil (William F. DeBusk, 1999; Arthur F. M. Mueleman, 1998; Renee Lorion, 2001).
It is expected that a constructed wetland will have the same ability to purify the water as a natural wetland. Nitrate and phosphate levels should reduce after pollution and metals are also expected to be reduced. As for bacteria and protists, it is likely they will only be mildly affected by the pollution.
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Construction of Wetlands
Our study was conducted on a constructed wetland located in the green house on the roof of the Pasadena City College U-building. The constructed wetland consisted of an input container, six flow channels, and an output flow into the drain. The water source was ordinary tap water. The water was allowed to freefall into a tub for dispersal, which will remove chlorine from the water. This is done to remove chlorine from the water. The tub had water control valves which drained into six PVC pipes, three inches wide and ten feet in length. The pipes were cut lengthwise to create channels which held the plants and organisms that would simulate wetland environment. The plants used in the constructed wetland were collected from the L.A River. Mesh filters were placed at the beginning and end of each trough to keep plant and organisms in the wetland, preventing them from entering or leaving the system. Mesh was also used in the water tub to prevent the control valves from clogging. The pollution was introduced to the wetland in a single dispersal. It consisted of detergent based gray water, which was high in phosphate (Drawing 1).
The data was collected in two major stages. These stages were separated by the introduction of pollution to our constructed wetland. In each stage water samples were collected from the input and the output sites in the wetland. The samples were tested for nitrate, phosphate, iron, copper, ammonia ions, dissolved oxygen (DO), and biological oxygen demand (BOD) using the LaMotte AMD–12 test kits. In order to reduce error in the results, samples were taken four days per week. Using this method of collecting consecutive data, we were able to see gradual trends in chemical levels throughout the experiment.
The methods of testing the chemical levels in the wetland are similar in their procedures. They consisted of obtaining a 10ml sample of water from each test site, followed by the insertion of designated indicator tablets according to desired chemical tests (indicators were provided by LaMotte AMD–12 test kit). For greater accuracy, a photo-spectrometer was employed for nitrate and phosphate measurements. Then, approximations of their concentrations were obtained through the use of a Beer’s Law plot. For all other chemicals the results were evaluated using LaMotte testing instructions, which provided the concentrations in parts per million. After the dissolved oxygen test was completed it was then sealed, wrapped in aluminum foil, and placed in the dark for one week. At the end of the week, the sample was then tested for the second reading of dissolved oxygen. The Biological Oxygen Demand was calculated by the difference of the two dissolved oxygen readings (reference LaMotte testing instructions).
A bacterial count and diversity was obtained by means of serial dilutions. This test was conducted before and after the addition of pollution to the wetland. The dilutions were obtained by taking a ten 10ml sample from each test site, pipetting out 1ml of the original 10ml sample, and diluting a new 1ml sample with 9ml of distilled water. This process was repeated three times to obtain serial dilutions of 1:100, 1:10,000, and 1:1,000,000. A 5µl sample of water was removed from each diluted sample and placed in a prepared agar petri dishs. Each culture was allowed to grow for one week, after which they were assessed for bacterial count and diversity (reference p.170 in Biological Investigations seventh edition, Warren D. Dolphin). This information was used to calculate the diversity using the Simpson Diversity Formula along with a calculation of growth rate (r).
The protist count and diversity for the wetland were obtained through the observation of a 0.1ml water sample under a microscope. All observed organisms in the water sample were identified and counted by phylum. Diversity was calculated using the Simpson Diversity formula. Growth rates (r and k) were also calculated. Protist species richness was calculated by taking the sum of protists and dividing by the number of species. This process was repeated for each test date.
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Phosphate, nitrate, and ammonia all increased after the introduction of the pollution but returned to pre-pollution concentrations within 16 days. The phosphate had an initial concentration of 0.18M in the input and 0.2M in the output. These concentrations spiked to 0.35M and 0.37M respectively the day after the pollution was introduced and returned to original concentrations by day 16 (Fig.1). The nitrate levels prior to the pollution event were 0.04M for the input and 0.06M for the output. The concentration of nitrate rose more slowly than the phosphate peaking at day 15 at 0.3M and 0.07M respectively returning to original levels by day 16 (Fig.2.). The initial ammonia levels were 0ppm for the input and 0.2ppm in the output. The input level stayed at 0ppm throughout the study but the output level peaked on day 15 at 0.5ppm and stayed at 0.4ppm (Fig.3.).
Biological Oxygen Demand (BOD) input reduced after the pollution was introduced then increased and stabilized by day 16, whereas, the output increased after the introduction of the pollution and also stabilized by day 16. The initial input and output BOD was 4ppm. The input level BOD stabilizes at 7ppm by day 16 after reaching its lowest at 3ppm on day 8 (Fig.4.). The Dissolved Oxygen input levels remained fairly stable between its initial levels of 7ppm to a minimum of 6ppm and a maximum of 8ppm throughout the experiment. The output increased after the pollution was introduced then stabilized at its peak and remained stable. The output initial level was 8ppm and peaked at 10ppm on day 7 where it remained constant for the remaining 9 days (Fig.5.).
The metal ions, iron and copper, had lower output than input concentrations after the pollution onwards. The initial input and output levels for both metal is 0ppm. The iron input level increases to 1ppm after pollution, however output remains at 0ppm (Fig.6). The copper ion input concentration increases to 1.2ppm after pollution and remains constant. The output decreases from 2ppm on day 5 to 0.2ppm by day 16 (Fig.7).
The pH levels remain fairly stable throughout the experiment with very minor fluctuations. The initial pH is 8 for the input and 8.5 for output and returned to the original concentrations by day 16. The output pH increased to 10 and stabilized here by day 16 (Fig.8).
Before pollution was added, the count of protist was 2861 then dropped by more than 50 % after the pollution event, returning to 1534 3 weeks later. (Fig 9)
The diversity showed an over all ‘S’ shape, similar to the logistical growth model. Before pollution was added, the diversity of protist was 1.43 then increased to 2.97 after 3 weeks (Fig 10).
The graph shows an increase after pollution from 119 to 194, before pollution on 3-Oct. It then dropped dramatically to 21 on Day 24, but picked up to 38 on November 3rd (Fig 11).
The bacteria count before the addition of pollution was around 125, lower than the count after the addition of pollution (255) (Fig 12)
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The pollution added to the wetland consisted mainly of phosphates. On day 9, directly after the addition of pollution, input levels for phosphate increased approximately 0.05moles/liter (Fig. 1). The wetland exhibited its ability to remove phosphates on Day 10, when the input level was 0.52moles/liter and the output level was 0.4moles/liter, a reduction of 20% (Fig.1). Explanations for phosphorus removal are the physical and chemical mechanisms of absorption, sedimentation, and precipitation for long-term phosphorus retention in constructed wetlands (B. C. Anderson, 2004). Another mechanism for phosphorus removal is plant absorption, which can be a short and long term solution to nutrient removal. Plant absorption mechanisms can be supported by the increase of dissolved oxygen levels at this time (Fig. 5). Plants take up dissolved nutrients and other pollutants from water. The nutrients and pollutants then move through the plant body to underground storage organs; where after death they are deposited into the sediments as litter and peat (M Sundaravadivel, 2001).
By examination of chemical data for heavy metals, it can be concluded that the pollution also contained iron and copper (Fig. 6 and Fig. 7). This is proven by the fact that both copper and iron input levels increased after the addition of pollution. The wetland also demonstrates it is capable of removing copper and iron from the water. The wetland iron removal on all dates after the addition of pollution had an input level of 1ppm and an output level of 0ppm, a reduction of 100%. The wetland copper removal displayed its highest efficiency on Day 9 and Day 10 where the input level was 1.2ppm and the output level was 0.1, a reduction of 92%. This can be explained by chemical reactions between substances, leading to their precipitation from the water column as insoluble compounds (M Sundaravadivel, 2001). Another explanation for the change in levels would be the process of plant absorption, again supported by the high level of dissolved oxygen at this time (Fig. 5).
The removal of macronutrients, such as nitrogen and phosphorus by use of constructed wetlands, is a complex cyclical process which is believed to involve a number of ‘conceptual compartments’ which include the water column, sediments, plant roots, plant stems, and leaves (M Sundaravadivel, 2001). In a stabilized constructed wetland, the proportions of the total wetland nutrient load retained in these compartments are: soils, sediment and litter/peat 80%, water column 15 to 20%, plants and other biota 5% (M Sundaravadivel, 2001).
After analyzing ammonium levels on Day 9, there is a large increase of ammonia from 0ppm to 0.4ppm, directly after the addition of pollution (Fig. 3). It is important to understand that the ammonia was not supplied by the pollution or water source; instead it was created in the wetland. This can be proven by the fact that there is 0ppm of ammonia input observed for all test points following the pollution (Day 9 to Day 17), yet there are high output ammonia levels (Fig. 3). An explanation for the source of ammonia increase on Day 9 can be described by the decrease of protist levels (Fig. 9), due to an increase in dissolved oxygen levels and pollution.
When comparing the ammonium and nitrate levels in Fig. 2 and Fig. 3, we are able to observe the process of nitrification. On Day 9, we observe normal nitrate levels; this allows us to conclude that nitrate was not affected by the addition of pollution (Fig. 2). On Day 10, nitrate levels increased approximately 0.04moles/liter, while ammonia output levels stabilized on Day 10 from its previous increase, 0ppm to 0.4ppm on Day 9; this stabilization of ammonia and increase of nitrate can be explained by the process of nitrification. Some environmental factors that can impede nitrification are the availability of dissolved oxygen and the temperature and pH of the wastewater (M Sundaravadivel, 2001). Regarding dissolved oxygen, if it is not available oxidation can not occur, inhibiting nitrification. For all test dates, the dissolved oxygen levels were 10ppm, which allowed a sufficient supply of dissolved oxygen for oxidation. Nitrification cannot occur under thermophilic conditions; it is only achievable at temperatures as high as 44°C in wastewater. Even at this temperature, the rate of nitrification is decreased significantly (N D Berge, 2005). PH levels must be high for nitrification to occur because nitrification results in the consumption of alkalinity as nitrous acid is formed (N D Berge, 2005). This is supported by the decrease of the output pH level on Day 10 (Fig. 8).
Denitrification is a microbial process in which nitrate is reduced to nitrogen gas when dissolved oxygen is less than 0.5ppm (Julie Ambler, 2001). Levels of dissolved oxygen were at 10ppm for all output test dates after pollution was added. This high level of dissolved oxygen helped in the process of nitrification but also rendered it impossible for denitrification to occur.
In Fig. 2, Nitrate levels on Day 16 show that the wetland can remove nitrate from the water. This can be explained by the fact that excessive nitrates stimulate growth of algae and other plants. Growth is followed by death, which causes decay and finally increased levels of biochemical oxygen demand (BOD) from decomposition. This is supported by the results in Fig. 4 where the increase of BOD on Day 16 from 7ppm to 8ppm can be observed. By using the bacteria statistics in Fig.12, it can be observed that both count and diversity increased after the addition of pollution, which then suggests that the number of decomposers increased. This is further supported by the increase in ammonia output levels also on Day 16 from 0.4ppm to 0.5ppm (Fig. 3); which suggests decomposition.
The protist diversity shows the S-shaped curve generated by the logistics model. From the data, the rate of reproduction was found to be 0.4 and the carrying capacity was around 3.
Colin R. Jackson (2003) states that the reason for the first jump in species richness among biofilm is the result of the initial colonization by many bacterial species, and the decline afterwards is an effect of competition for broad-scaled resources, such as space. A similar concept can be used for the protists. Because the protist data was tested three weeks later, the peak of species richness can not be said to be an initial colony of protists, but perhaps a colony re-emerging after a decline. This is not certain given that there is no available data for the two previous weeks. However, it is interesting to observe the peak of species richness while the sum of protists continues to decline. This is probably the result of competition within species for resources that may still be depleted from pollution. The species richness then began to decline and hit its minimum on Day 24 as did the protist sum. This suggests possible competition among species for scarce resources still returning to normal after pollution.
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The constructed wetlands performed as expected with regards to the metals and chemicals. Nitrates and phosphates went up after pollution, but were returned to original concentrations. Ammonia outputs remained high throughout, possibly due to the high bacterial presence in the input regions leading to high decomposition rates. Iron and copper outputs remained low. The average bacterial count grew after pollution explaining the reduction in some chemicals. The protists, however, experienced some decrease in their sums and species richness after pollution, but eventually showed that they were returning to normal.
Comparing the data from the constructed wetland to data collected from a natural wetland (Arthur F. M. Mueleman, 1998; Volker Lüderitz et al, 2005; Renee Lorion, 2001), it was found that the constructed wetland purified the water as efficiently as a natural wetland. Therefore, the use of constructed wetlands should be considered for the purification of polluted water.
Possible areas of further research include analyzing competition within and among different protist species and their trophic level. In regards to the bacteria, more precise measurements can be taken by testing over a period of days rather than only before and after pollution. The data for protists should be recorded immediately after pollution in order to better observe the patterns that emerge. Also, experimenting with the construction of the wetlands may provide better means for purification (i.e. inclined troughs, more sediment, etc.).
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