Phytoremediation and the Wastewater Treatment Process

Introduction 

Presently, the world is facing a crisis on water quality as a result of such factors as increased urbanization, industrialization, increased living standards, change of land use, increased population as well as poor wastewater management practices. Lack or poor management of wastewater has directly impacted the aquatic ecosystem, disrupting the life support systems that support various sectors such as food production, industrialization, and urban growth. For instance, the Millennium Ecosystem Assessment (2005) estimates that at least 60% of the global ecosystem services which support socio-economic activities have been degraded due to poor wastewater treatment practices. For this reason, environmental experts have emphasized the importance adopting proper, effective and ecosystem-based wastewater management practices or strategies to promote sustainable development environmentally, socially and economically. The fourth report of the United Nations World Water Development indicates that only 20% of the total amount of wastewater produced across the world is treated properly (UNESCO, 2012). The report claims that, although the Millennium Development Goal of improved sanitation has focused on improving access to better toilet facilities, there is little progress on collecting and treating waste streams before they are discharged. Environmental scientists have made extensive developments in the knowledge and effective procedures for treating all types of wastewater regardless of the pollutants present. For instance, scientists have developed physicochemical, aerobic and anaerobic processes that could be used in treating effluent discharged from any sector. With a varying complexity, these methods have distinct features concerning affordability, land requirements, sludge production, efficiency, and reliability. Experts have emphasized on developing environmental-friendly (natural) techniques that treat wastewater effectively with use the buffering capacity of plant roots systems instead of chemicals. In fact, most of them argue that natural wastewater treatments like phytoremediation are not only safe but also effective in augmenting the process of wastewater treatment. This paper will demonstrate the idea that using phytoremediation in increasing the effects of wastewater treatment results in a more effective and thorough treatment process. The paper will begin with providing background information on waste water, that is, types, constituents, and issues associated with it. It will then discuss the phytoremediation technique and review existing literature on its successful use in treating wastewater. 

Background information on wastewater 

Waste water is the effluent released from different sectors such as industries, commercial establishments, hospitals and households which originate from cleaners, bathing, toilets, laundry and kitchen sinks. Wastewater also includes urban runoff, aquaculture, and agricultural effluent. This section will outline the various types and constituents of sewage. It will also outline the main issues associated with improperly managed wastewater. 

Types and constituents of waste waster 

The major types of wastewater include yellow water, black water, and gray water. Blackwater refers to the wastewater originating from the household sector and more specifically from food preparation sinks, toilet fixture as well as dishwashers. Constituents of black water may include urine, cleansing water, toilet paper and body cleaning liquids. Blackwater is considered to be very infectious and pathogenic because it has particulate matter and dissolved chemicals that could be harmful to human beings. Gray water is the type of wastewater originating from bathtubs, bathroom sinks, spas and cloth washing machines. Since gray water does not contain any human wastes such as urine, it can be recycled for other purposes like irrigation. Yellow water is the type of wastewater that contains urine which is collected from other channels and has not contaminated with the gray and black sewage. Major constituents of wastewater include microorganisms like pathogenic bacteria, organic materials like fat, cyanide, detergents and heavy metals like lead (Pb), Zinc (Zn), mercury, copper, and cadmium. 

Issues associated with wastewater 

The issues or problems associated with wastewater are linked to the polluting effects of its constituents. The discharge of untreated wastewater into the environment negatively affects ecosystems, human health and the economy as a whole. Out of the many pollutants contained in wastewater, organic matter is considered as the major pollutant as it has adverse effects on human beings and aquatic organisms. 

One of the biggest issues of wastewater is the eutrophication of rivers and lakes. Eutrophication is a nutrient contamination caused by the release of agricultural wastewater containing excess phosphates and nitrates. When disposed into rivers and lakes the nutrients stimulate the growth of algae blooms which release toxins and deplete the amount of oxygen in the water once they decompose. The depletion of oxygen levels not only deteriorates the quality of the water but also reduces the biodiversity of fish and other aquatic organisms as they will die. Studies show that the reduction of water quality due to eutrophication has been the major cause of biodiversity loss in most rivers and lakes in Europe, Southern Africa, and China. According to the Organization for Economic Co-operation and Development, the water quality of surface water bodies in these countries is likely to deteriorate further in the future due to poor treatment and management of agricultural wastewater. Biodiversity loss in the lakes and rivers will worsen the living standards of communities which depend on fish and other aquatic organisms for food and money. 

The discharge of untreated wastewater into underground water sources such boreholes poses threats to human health. For example, contamination of ground water by wastewater containing bacteria can result in such diseases as cholera, typhoid, gastroenteritis, abdominal pain and gastric cancer among others. Diseases and infections may also occur due to exposure to recreational water and food contained by untreated wastewater. Such food may include meat and dairy products obtained from animals infected with various bacteria or viruses contained in wastewater. The negative effects of untreated wastewater are evident through reduced or delayed yield due to unsuitable chemicals that do not support plant growth. 

Understanding the general wastewater treatment process 

Ideally, wastewater treatment aims at reducing the level and concentration of the various pollutants present in wastewater before it is used or disposed into the environment. The two major types of wastewater treatment include physical (chemical) treatment and the biological treatment. While the biological treatment uses biological matter to break down the organic matter contained in the wastewater, the physical treatment involves the use of chemicals and physical processes. The biological treatment procedures are commonly used to treat wastewater from businesses and households. Physical treatment, on the other hand, is commonly used in treating effluent from manufacturing companies and industries. The rationale for using this type of treatment for industrial wastewater is that it is effective in removing the excess toxins and chemicals contained in the effluent released from industries and factories. 

The process of wastewater treatment begins with the collection of wastewater at the collection systems installed by business owners, or municipal administration. After the effluent is collected at a central point, it is then directed to the treatment plant through underground drainage systems. The handlers at the central point should maintain good hygiene while transporting the wastewater as it could easily infect them if they are exposed to it. More specifically, they should put on protective clothing and should ensure that the pipes are leak proof. The first step involved when the wastewater is in the treatment plant is to control its odor which is caused by the foul smell of the dirty substances in it. Odor control is instrumental and important in preventing the spread of foul smells in the surrounding. Odor control in the treatment plant is achieved by using chemical substances to neutralize the smell. 

After odor control, the next step in the wastewater treatment process is screening: removal of such objects like cotton buds, diapers, sanitary items, broken bottles, bottle tops and plastics among others. The importance of screening is to remove large objects that would cause mechanical problems of the machines used in the treatment plant. Screening also involves the removal of grit by the use of equipment which is designed specifically for that purpose. After the objects are removed, they are transported to the landfills for disposal. The next step is the primary treatment whereby the wastewater is separated from the macrobiotic solid matter. The wastewater is then poured into big tanks with the aim of settling the solid matter at the tanks’ surface. The settled solid waste and sludge is then removed using scrappers and is directed to the cylindrical tanks. The mixture of sludge and solid wastes is then subjected to further treatment after it’s pumped out of the cylindrical tanks. The water that remains in the tanks is then taken for secondary treatment (activated sludge process). At this stage, the wastewater is further broken down by adding seed sludge to it. Air is then pumped through the aeration tanks to mix with the mixture of seed sludge and wastewater. There are two main reasons for pumping air into the mixture of seed sludge and wastewater: to allow bacteria growth that helps in consuming the oxygen in the wastewater stimulating the growth of other microorganisms that help in eliminating or consuming the organic matter in the wastewater. As a result of this process, large particles are formed and settle on the bottom surface of the tanks. For the secondary treatment process to be effective, the wastewater is allowed to stay in the tanks for approximately three to six hours. 

The next step in the process of wastewater treatment is the bio-solids handling whereby the solid matter that settles in the previous stages is directed to the digesters. These solid wastes undergo anaerobic digestion for about four weeks, a process that produces methane gas and bio-solids which are recycled. The methane gas produced during the anaerobic digestion is mainly used as an energy source for producing electricity in the various engines used in the treatment plant. The methane gas is also used to generate heart in the boilers. The next stage is the tertiary treatment which involves the elimination of impurities contained in the wastewater. This process is achieved by disinfecting the wastewater for at least twenty minutes in tanks containing sodium hypochlorite and chlorine. The disinfection of the wastewater is important because it removes the impurities and other substances that would be harmful to humans and animals. The treated wastewater is then released through different waster ways and is ready for use. The sludge collected in the primary and secondary treatment is put in thickening tanks for about 24 hours where it settles down and separates from water. The remaining water is then returned to the aeration tanks to undergo further treatment. The sludge, on the other hand, is further treated and released into the environment and can be used for other purposes like watering plants. 

From the overview of the wastewater treatment process, it is clear that treating wastewater is important as it helps in preventing water pollution, an outbreak of water-borne diseases like typhoid and keeps the environment clean. Now let’s look at how the technique of Phytoremediation can be used to augment the process of wastewater treatment. 

Phytoremediation and the wastewater treatment process 

Scientists have developed many conventional methods that could be used to eliminate the contamination of heavy metals and other pollutants in, amongst others, surface water, underground water, and wastewater. However, experts have raised concerns regarding the safety and implementation cost of these conventional treatment methods. A majority of them argue that most conventional methods are costly and their safety is not assured. For this reason, scientists concerned about the environment proposed phytoremediation as a safer and less expensive remediation technology for eliminating pollutants in wastewater. This section will discuss the phytoremediation and demonstrate how it is effective in upgrading the treatment of wastewater giving examples of studies done on the effectiveness of phytoremediation in the treatment of sewage. The section will also outline the various water plants used in phytoremediation and discuss the merits and demerits of the technique. 

History of Phytoremediation technology 

Although phytoremediation technologies have been occurring naturally for many years, scientists began testing and using living green plants to remove heavy metals and other contaminants in soil and wetlands in the 1970s. The use of phytoremediation became widespread across different countries and non-governmental organizations in the 1980s. The United States Environmental Protection Agency EPA initiated Phytoremediation in the year 1991 and was used in technical literature for the first time in the year 1993. Towards the end of the 1990s, scientists discovered news used for phytoremediation technology and listed it among the most innovative scientific technologies. The technology was derived from different disciplines including microbiology, agronomy and agricultural engineering among others. Since then, phytoremediation has rapidly developed and is now being used by most environmental scientists to remove contaminants not only in wastewater but also in groundwater and surface water. It can be used along with or as a replacement for other conventional technologies that require high inputs of energy, labor, and capital (Farraji, 2014). 

Overview of how phytoremediation works 

Phytoremediation is a “green revolution” technology that uses living plants to metabolize, reduce and degrade pollutants like heavy metals, pesticides as well as hydrocarbons through the biological and physical processes of the plants (Farraji, 2014). The plants clean up the contaminated sites through several ways or techniques. To remove contaminants from contaminated water or the soil, the plants acts as filters to stabilize or degrade the organic pollutants (Dordio & Carvallo, 2013). The plants take up the contaminants through their root system which provides surface area for optimal absorption and accumulation of water, growth nutrients and non-essential contaminants (Starkl et al. 2013b). Environmental researchers argue the that using trees instead of small plants to absorb the contaminants is effective because the tree roots penetrate deeper into the ground as compared to the roots of smaller plants. Another way which plants remediate contaminated sites is by releasing root exudates which affect the aggregation of soil particles, availability of contaminants as well as microorganisms’ activity. These root exudates can enhance or hinder the availability of pollutants in the plant’s rhizosphere due to several factors such as increased microbial activity, availability of organic compounds and changes in the composition of the soil (Dordio & Carvallo, 2013). 

According to Dordio and Carvallo (2013), plants commonly used in phytoremediation are macrophytes (aquatic plants) as they have unique components of wetlands. Macrophytes play a critical role in promoting the biochemical processes that occur in the treatment of wastewater. More specifically, they enhance sludge stabilization and provide favorable conditions for microorganism growth and water filtering (Dordio & Carvallo, 2013). The aquatic plants used in phytoremediation also have unique characteristics that make them ideal for remediation purposes. For example, they have aerenchyma and thin outer tissues that are specialized to allow air distribution to different parts of the plant beneath the wastewater. Macrophytes have the ability to accumulate the contaminants in their cells, absorb the xenobiotics and adapt to the harsh conditions of contaminated sites. The biggest advantage of macrophytes is they are readily available as they grow naturally (Mielcarek & Krzemieniewski, 2013). 

Phytoremediation techniques 

This section will provide a literature review of the various phytoremediation techniques that have successfully been used in the treatment of wastewater. The various phytoremediation techniques presented in this section include phytoextraction, phytodegradation, phytovolatilization, Phyto filtration, and phytostabilization. 

Phytoextraction involves the removal of excessively toxic substance fro wastewater using macrophytes that have a high capacity and ability to accumulate high amounts of toxins. Therefore, plants used in the extraction process should be tolerant of heavy metals and organic compounds should have a fast growth rate and should have a high capacity of producing biomass. The process of phytoextraction is categorized into two: induced and continuous phytoextraction. While continuous Phytoremediation involves the use of plants with a high capacity of accumulating contaminants that are excessively toxic, induced Phytoremediation uses chelators with enhancing the plant’s ability to accumulated toxins from the wastewater. An example of an aquatic plant that has successfully been using to extract heavy metals from wastewater is Potamogeton pusillus (commonly known as small pondweed). Studies show that this aquatic plant is effective in removing high toxic heavy metals like copper from wastewater as their leaves and roots have a high capacity of accumulating toxic substances (Monferran, Pignata & Wunderlin, 2012). 

Phytodegradation is a phytoremediation technique that involves the use of aquatic plants that produce enzymes for enhancing or speeding up degradation reactions of the xenobiotics. The process of phytodegradation can either occur inside or outside the plant and is effective in treating not only wastewater but also rivers, soil and contaminated groundwater. In one experiment, Zazouli, Mahdavi, Bazrafshan and Balarak (2014) used Azolla filiculoides to test its ability to remove or degrade the organic synthetic compound, bisphenol A from aqueous solution. Azolla filiculoides is a water fern that mostly grows in the tropical regions of Australia, America, and Asia. Although the water fern has a fast growth rate, its tolerance of very cold temperatures is low. In the experiment, the researchers cultured the plant in a solution containing different concentrations of the bisphenol (BPA). The removal of the BPA was dependent on its concentration and the biomass of the plant. After the experiment, the researchers found out that the degradation of the BPA was more efficient when the plant’s biomass was 0.9g while the BPA concentration as 90%. After a thorough investigation of the action of water fern in removing BPA, the researchers concluded that the plant removes the BPA through two ways: breakdown of the toxic contaminants by its metabolic processes or by removal of surrounding toxic substances by the plant’s enzymes (Zazouli et al. 2014). 

Phytovolatilization technique involves the use of aquatic plants that have the ability o absorb the contaminants in the wastewater, metabolize and release them into the atmosphere in a form which is less toxic. This method is commonly used to purify or treat wastewater with such contaminants as mercury, selenium and toxic organic substances like nitrobenzene and trichloroethylene. A good example of a plant used to eliminate contaminants in wastewater through phytovolatilization is Pteris vittatta , commonly referred to as Chinese brake fern. Evidence shows that the fern is effective in removing contaminants like Arsenic in the presence of phosphorous. More specifically, it is effective in reducing the concentration of Arsenic (As) from As (V) to As (III) and synthesizing organo-sulfur compounds (thiols), resulting in the hyper-accumulation of the As contaminant. The phytovolatilization method has proven to be effective in treating wastewater contaminated with metals that easily form methyl and volatile hydride compounds. Recently, researchers have developed transgenic plant species with a high capacity of treating wastewater which contaminated with such contaminants as mercury and Selenium. 

Phytofiltration, also known as rhizofiltration, is a phytoremediation used to remove contaminants from agricultural and industrial wastewater. The method is effective in eliminating or removing radioactive substances and heavy metals like lead. Due to the high toxicity of agricultural and industrial wastewater, plants used in this technique should have a high tolerance of toxic compounds and high resistance of low concentrations of oxygen. The roots of plants used in Phyto filtration should be extensive and should have the ability to produce biomass (Srivastava, Sounderajan, Udas & Suprasanna, 2014). Examples of plant species that have been tested and proven to be effective in reducing the concentration of contaminants like Uranium in wastewater include Potamogeton pectinatus , Callitriche stagnalis and Potamogeton natons . In one experiment, for example, Pratas, Paulo, Favas, and Venkatachalam (2014) used Callitriche stagnalis (common water-starwort) plant species to determine its ability to remove uranium from uranium-contaminated wastewater. From the experiment, they found out that the plant absorbed approximately 1567mg.kg-1 of uranium from the wastewater. The researchers, therefore, concluded that common water-starwort is effective treating uranium-contaminated sewage (Pratas et al. 2014). 

Phytostabilization is a remediation technique that uses plant roots to accumulate the contaminants within the root zone, preventing off-site contamination through soil erosion, leaching or water erosion. In this case, plants are used to cover the surface of the contaminated site to avoid their exposure to human beings, animals, the wind, and water. Minimizing leaching and reducing the solubility of the contaminants through phytostabilization can also be achieved by adding soil amendments like phosphates, biosolids and alkalizing agents among others. Since phytostabilization heavily relies on the transformation of the metals and accumulation in the soil particles, plants chosen in this method should have genotypic and phenotypic features suited for the remediation process. Critical factors to consider when choosing plants for this remediation method include density and morphology of the plant roots as well as depth to which they can penetrate underneath the ground. Plants used in phytostabilization should have a root system with a high ability to absorb, adsorbing and accumulating contaminants in its tissues and convert them to compounds that are less soluble. The roots should also have a large surface area for enhancing the stabilization of the soil surrounding the wastewater site and improving the volatilization of metals within the root zone (rhizosphere). The plants selected for this method should have fibrous roots to facilitate phytoextraction (absorption of the contaminants) and phytovolatilization (interaction of the plants and microbes). Deep-rooted plants are ideal for phytostabilization and phytoextraction as they have higher rates of transpiration and that they have a higher capacity of accumulating heavy metals like zinc and cadmium. The plants should also have a low capacity of accumulating contaminants in its parts above the ground. Also, the plant's tolerance to soil moisture, varying PH and salinity should be high (Segura & Ramos, 2013). Examples of plants that are tolerant and can accumulate metals like zinc are referred to as the hyperaccumulators. Hyperaccumulators are the plants that have the capacity of accumulating metals whose concentrations is above 10, 000 mg.kg-1 for zinc and above 1000 mg.kg-1 for arsenic, selenium, and copper. The first plant species to be identified as hyperaccumulators were from the families of Brassicaceae and Fabaceae . Presently, more than 300 plant species have been identified, tested and proven to be effective hyperaccumulators that could be used for phytostabilization. 

Several studies have been conducted to investigate the effectiveness of plants in removing heavy metals from wastewater through phytostabilization. An example of such studies was carried out by Plechonska and Klink in the year 2014. The two researchers used Phalaris arundinance (commonly referred to as red canary grass) to test its abilities in removing heavy metals like copper, lead, iron, and zinc. After carrying out an experiment, the duo found out that different concentration of the heavy metals had accumulated in various regions of the plant species. The concentration of the contaminants was high in the root system and low in the leaves. The duo concluded that the absorption abilities of the plant make it ideal for remediation of wastewater contaminated with such heavy metals as cadmium (Cd) and Cobalt (Plechonska & Klink, 2014). It is worth noting that treating wastewater contaminated with organic matter using the phytostabilization method would be more effective with the involvement of phytovolatilization process. 

Merits and demerits of phytoremediation techniques 

One of the main reasons as to why phytoremediation techniques have widely been accepted is that they reclaim a polluted or contaminated environment in situ. Using plants to remove contaminants in the wastewater is effective and safe than using traditional clean-up methods based on the extraction of xenobiotics. Unlike conventional methods, biological methods of treating wastewater cause less secondary pollution. Another advantage of phytoremediation relates to the fact the technology uses plants with extensive and well-developed roots. These well-developed root systems increase soil productivity, soil aeration and prevent soil erosion. The plant roots also provide habitat for microorganism growth and cause absorption of nutrients that are important for growth. Another merit of using the phytoremediation technique than the conventional technologies is that it depends on the natural processes of the plants and is, therefore, a cost-effective method concerning labor and equipment required. 

The biggest disadvantage or setback of phytoremediation is that it is slow and may last for several years before the clean-up is fully complete. Another disadvantage is that plants used in the various phytoremediation techniques are highly vulnerable to reduced temperatures during the winter period. Very cold temperatures may stop or slow down the growth of the macrophytes used in the remediation. 

Conclusion 

The world, indeed, is facing a crisis of water quality due to contamination with untreated wastewater as a result of population increase, urbanization, industrialization and agricultural practices among others. Contamination of surface and underground water with untreated sewage is a public health issue especially in most developing nations across the world. Environmental scientists have emphasized the need to treat wastewater before it is released into the environment as a way of improving the overall health of the people and reduced health care costs incurred by citizens and government. From the paper, it is clear that scientists have developed different conventional ways of treating wastewater released from different sectors including industrial sector and agricultural sector. However, due to increased concerns about environment protection, experts in the field of the environment have developed “green technologies” to replace the conventional clean-up technologies. 

Phytoremediation technology is a “green technology” that is effective and economical in augmenting the process of wastewater treatment. From the paper, it is clear that phytoremediation has extensively been used in the extraction and elimination of heavy metals and other toxic contaminants from wastewater. The technique involves several processes including extraction, stabilization, volatilization, degradation and filtration. It would be a demise to ignore the fact that many experiments and studies have been conducted to verify or prove the effectiveness of phytoremediation in treating wastewater. Despite this, the application of this method is still low in many countries. Therefore, there is need to carry out extensive research on the technology and incorporate biotechnological interventions that could be useful in developing plant species to be used in phytoremediation. 

References 

Dordio, A.V., A.J.P. Carvalho. 2013. Organic xenobiotics removal in constructed wetlands, with emphasis on the importance of the support matrix. Journal of Hazardous Materials 252– 253: 272–292. 

Farraji, H. (2014). Wastewater Treatment by Phytoremediation Methods.    Wastewater Engineering: Advanced Wastewater Treatment Systems , 194. 

Mielcarek, A., M. Krzemieniewski. 2013. Research on the use of selected macrophytes in the process of methane fermentation . Rocznik Ochrona Ârodowiska 15: 2611-2624 (in Polish).7 

Millennium Ecosystem Assessment (2005) Millennium Ecosystem Assessment, Concepts of Ecosystem Value and Valuation Approaches . Island Press, Washington DC 

Monferrán, M.V., M.L. Pignata, D.A. Wunderlin. 2012. Enhanced phytoextraction of chromium by the aquatic macrophyte Potamogeton pusillus in presence of copper. Environmental Pollution 161: 15-22 

Plechoƒska, L., A. Klink. 2014. Trace metal bioindication and phytoremediation potentialities of Phalaris arundinacea L. (red canary grass). Journal of Geochemical Exploration 146 : 27- 33. 

Segura, A., J.L. Ramos. 2013. Plant–bacteria interactions in the removal of pollutants. Current Opinion in Biotechnology 24 : 467-473 

Srivastava, S., S. Sounderajan, A. Udas, P. Suprasanna. 2014. Effect of combinations of aquatic plants (Hydrilla, Ceratophyllum, Eichornia, Lemna and Wolffia) on arsenic removal in field conditions. Ecological Engineering 73 : 297- 301. 

Starkl M, Amerasinghe P, Essl L, Jampani M, Kumar D, Asolekar SR (2013b) Potential of natural treatment technologies for wastewater management in India. Journal of Water, Sanitation and Hygiene for Development 3 , 500-511. 

UNESCO (2012). Managing water under uncertainty and risk. The United Nations World Water Development Report 4 . United Nations Educational, Scientific and Cultural Organization, Paris .

Zazouli, A.M., Y. Mahdavi, E. Bazrafshan, D. Balarak. 2014. Phytodegradation potential of bisphenol A from aqueous solution by Azolla filiculoides. Journal of Environmental Health Science and Engineering 12: 1-5.