Microbial processes in contructed wetlands for wastewater treatments
1. Introduction
2. Characteristics of water before and after the treatment
Where does the water going into the treatment come from?
Some specific substances are found in high concentrations...
-Nitrogen and phosphorus: human activities in excessive dumping leads to eutrophication, reduction in sunlight, lower dissolved oxygen leves, changing rates of plant growth, reproduction patterns and overall deterioration of water quality.
-TDS (Total Dissolved Solids): inorganic salts and a small part of organic matter. It is increased with human activity such as agricultural and rural runoff.
-Hydrogen sulfide (H2S) formed during the process of organic matter decomposition.
Which are the most significant components of municipal wastewater
Difference between incoming and outgoing water:
Where does this water go after the treatment?
Pollutant removal in wetlands take place due to the combination of physical, chemical and microbial processes. The processes involved in pollutant removal are sedimentation, sorption, precipitation, evapotranspiration, volatilization, photodegradation, diffusion, plant uptake and microbial degradation, among other (De la Varga et al., 2017)
One of the most important mechanisms for pollutant removal in wetlands is done by biological means. One of those is done by plant root uptake. The process, called phytodegradation, takes place when plants directly uptake contaminants into their root structure.
Wastewater treatment wetlands have the ability to remove pathogens like helminthes, protozoans, fungi, bacteria and viruses. Another main process that takes place is sedimentation, in which sludges accumulate coliforms and bacteria that precipitate to the ground. Another way to filter out pathogens is by root's plants absorption.
Apart from these important processes, it is necessary to pay especial attention in pollutant removal performed by microorganisms. They are capable of transforming many substances that van be hazardous, and it must be taken as beneficous.
Microbial biochemical processes are essential for wastewater treatments in order to reduce nutrients concentration in it. Thanks to their metabolic diversity, they are capable of transforming many organic and inorganic compounds that can be hazardous for humans. The main processes in which microbes are involved are especified bellow.
It is necessaryto eliminate compounds as ammonia and ammonium from water because it is potencially toxic in concentrations above 0.02 mg/L.
Some heavy metals present in wastewater are very toxic and must be removed. Some bacteria have a primary role in the removal of these toxic compounds through different processes.
One of the most important processes is the mercury demethylation. Methyl mercury is an extremly toxic compound that can be easily acumulated in fatty tissue. The process of demethylation consist in the disintegration of the methyl form the metallic mercury through methane production.
Its is also important the cadmium biosorption, among other metals, a process in which biomass can naturally bind contaminants onto its cellular structure. It has been proved that low levels of biomass of Spirulina can remove up to a 98% of Cadmium by biosorption processes (Solisio, Lodi, Soletto, Converti et al., 2008).
By last, many bacteria are capable to perform redox reactions of many metal ions, such as iron, manganese, arsenic, antimon, vanadium, selenium or palladium, changing their oxidizing state in order to make them capable to bond with other elements which may help them precipitate and be removed form water.
ORGANIC MATTER
Organic matter dissolved in water is a limiting factor in order to discharge it to streams due to microbial blooms phenomena, which is promoted in highly organic matter environments.
Organic matter can be removed through aerobic and anaerobic metabolisms, but in wetlands it would be optimal to promote anaerobic organisms processes to degrade organic matter from wastewater.
The main reason is the oxygen demand by aerobic microorganisms in order to consume organic matter. In cases of high organic matter inputs, the enormous quantity of oxygen demanded would cause anoxic environments undesirables for other living species in wetlands systems.
In order to avoid this scenario, it is convinient to promote anaerobic processes, which can only happen in deeper water, where there is low oxygen concentration,and warm environments, which are absolutely possible in wetlands, due to its physico-chemical characteristics.
The overall reaction consist in degrading organic matter to form methane and carbon dioxide by methanogenic bacteria or methanogenic archea.
4. Which microorganisms are involved in such processes?
5. Types of constructed wetlands
-Can be integrated better in landscapes.
-More secondary benefits (e.g., wildlife habitat, but this can be a disadvantage if excessive contamination exposure results).
-Cheaper to build and their construction, operation, and maintenance are straightforward.
-Shorter development period to reach full performance.
-Good to treat mine drainage and agricultural runoff.
-Requires a larger land area tan other systems.
-Less or not attractive to wildlife.
-Best suited to wastewaters with relatively low solids concentrations and under relatively uniform flow conditions.
-Used frequently to reduce 5-day biochemical oxygen demand (BOD5) from domestic wastewaters.
-Greater cold tolerance.
-Reduces pest and odor problems as well as insects.
-Greater assimilation potential per unit of land area than in surface flow systems.
-Less land requirement.
6. Conclusions
All in all, constructed wetlands for wastewater treatment work thanks to the metabolism of specific bacteria and seem to be an intelligent alternative to convencional wastewater treatment plants because of:
On the other hand, there are some other issues which seem to be disadvantages:
7. References
-De la Varga, David & Oirschot, Dion & Soto, Manuel & Kilian, Rene & Arias, Carlos & Pascual, Ana & Alvarez, Juan. (2017). Constructed Wetlands for Industrial Wastewater Treatment and Removal of Nutrients. DOI: 10.4018/978-1-5225-1037-6.ch008. [20/10/2018]
-https://microbewiki.kenyon.edu/index.php/Wetlands [01/11/2018]
-Geradi, M. (2006). Wastewater bacteria. New Jersey, John Wiley & Sons, Inc., Hoboken [24/11/2018]
-Ayanda, O., Akinsoji, O. (2011). Biological wastewater treatment: Microbiology, Chemistry and Diversity measurament of ammonium oxidizing bacteria. African Journal of Microbiology Research Vol. 5(32), pp. 5831-5840, 30 December, 2011 from: http://www.academicjournals.org/AJMR [24/11/2018]
-Vymazal, Jan. (2010). Constructed Wetlands for Wastewater Treatment. Water. 25. DOI: 10.1016/j.ecoleng.2005.07.002.
-Springer Science + Business Media B.V. (2008) Types of Constructed Wetlands for Wastewater Treatment. In: Wastewater Treatment in Constructed Wetlands with Horizontal Sub-Surface Flow. Environmental Pollution, vol 14. Springer, Dordrecht. DOI: 10.1007/978-1-4020-8580-2_4
-Ghermandi, Andrea & Bixio, D & Thoeye, C. (2007). The Role of Free Water Surface Constructed Wetlands as Polishing Step in Municipal Wastewater Reclamation and Reuse. The Science of the total environment. 380. 247-58. DOI: 10.1016/j.scitotenv.2006.12.038. [09/12/2018]
Wetlands are among the most important ecosystems on Earth because of
their unique hydrologic conditions and their role as ecotones between
terrestrial and aquatic systems (Mitsch and Gosselink, 1993). Historically
wetlands have been undervalued and they have been drained, turned into
agricultural land and commercial and residential developments.
At the beginning of the 20th century, a few cities and industries began
to recognize that the discharge of sewage directly into the streams caused
degradation of natural water ecosystems and this led to the construction of
sewage-treatment facilities. However, during the first half of the 20th
century, few municipalities and industries provided wastewater treatment, due to
the social and economic problems as well as no perception on environmental
issues.
It was during the second half of 20th century when the multiple
functions and values of wetlands were recognized not only by the scientists but
also by the public. The first experiments on the use of wetland plants to treat
wastewaters were carried out in the early 1950s in Germany and the first full
scale systems were put in operation during the late 1960s (Vymazal, 2008).
Since then constructed wetland systems have been spreading throughout the world
and they are a reliable wastewater treatment technology for various types of
wastewater.
2. Characteristics of water before and after the treatment
Where does the water going into the treatment come from?
| Figure 1. Wastewater procedence. Own elaboration |
Figure 2. Waste water characteristics.Own elaboration |
-Nitrogen and phosphorus: human activities in excessive dumping leads to eutrophication, reduction in sunlight, lower dissolved oxygen leves, changing rates of plant growth, reproduction patterns and overall deterioration of water quality.
-TDS (Total Dissolved Solids): inorganic salts and a small part of organic matter. It is increased with human activity such as agricultural and rural runoff.
-Hydrogen sulfide (H2S) formed during the process of organic matter decomposition.
Which are the most significant components of municipal wastewater
Difference between incoming and outgoing water:
| Figure 3. Water characteristics through the treatment. Own elaboration |
Where does this water go after the treatment?
- Once water quality has improved to an acceptable standard, it is removed from the wetland and is sent to a polishing treatment plant for disinfection (such as chlorination) to kill any remaining microorganisms allowing water to be drinkable and reused.
- After this process, it is pumped through a pipeline and distributed where it is nnecessary.
3. Processes that take place in constructed wetlands
Pollutant removal in wetlands take place due to the combination of physical, chemical and microbial processes. The processes involved in pollutant removal are sedimentation, sorption, precipitation, evapotranspiration, volatilization, photodegradation, diffusion, plant uptake and microbial degradation, among other (De la Varga et al., 2017)
One of the most important mechanisms for pollutant removal in wetlands is done by biological means. One of those is done by plant root uptake. The process, called phytodegradation, takes place when plants directly uptake contaminants into their root structure.
Wastewater treatment wetlands have the ability to remove pathogens like helminthes, protozoans, fungi, bacteria and viruses. Another main process that takes place is sedimentation, in which sludges accumulate coliforms and bacteria that precipitate to the ground. Another way to filter out pathogens is by root's plants absorption.
Apart from these important processes, it is necessary to pay especial attention in pollutant removal performed by microorganisms. They are capable of transforming many substances that van be hazardous, and it must be taken as beneficous.
Microbial biochemical processes are essential for wastewater treatments in order to reduce nutrients concentration in it. Thanks to their metabolic diversity, they are capable of transforming many organic and inorganic compounds that can be hazardous for humans. The main processes in which microbes are involved are especified bellow.
NITROGEN COMPOUNDS
It is necessaryto eliminate compounds as ammonia and ammonium from water because it is potencially toxic in concentrations above 0.02 mg/L.
AOB and NOB bacteria, which performs the nitrification metabolic processes, are participants in the nitrogen cycle. This reaction consist in the reduction of ammonia to nitrite and nitrate, but those compounds turn out to be even more toxic.
Even though nitrate is highly toxic, it is essential to produce it because it is beneficial for plants thanks to PGPR mechanism, so in wetlands is necessary to form this compound, allthough it is necessry to remove nitrite from water in order to reduce its toxicity.
The following metabolic process in nitrogen cycle is denitrification and ANAMMOX, in order to reduce nitrite to nitrogen gas, which is inocuos and will elevate to atmosphere.
It is essential to avoid DNRA and partial-denitrification processes, which may result in an inefficient treatment of wastewaterr and pollution. If DNRA occurs, ammonia would be formed again, so water toxicity would increase again and the process would be inefficient. It is necessary to also avoid the partial-denitrificatio process, which happens when the bacteria find themselves under reduced oxygen conditions and metabolic route stops at half, producing nitrous oxide or nitric oxide. Those two nitrogenous compounds are really hazardous.
 |
| Figure 4. Nitrogen cycle. Retrieved from: https://www.semanticscholar.org/paper/Biological-sources-and-sinks-of-nitrous-oxide-and-Thomson-Giannopoulos/2b2681af82925f0967a78bbf7b50aa59be55acf2 |
PHOSPHATE
Phosphate ion is a compound found in wastewater that must be removes in order to prevent eutrophication of surface waters. The mechanism used in wetlands for wastewaters treatment consist in using phosphate solubilizing bacteria (PSB) present in plants rhizosphere, so they can execute PGPR mechanisms. This process consists in the transformation of phosphate into phosphoric acis in order to make it soluble, so plants can uptake it for they growth through their roots.
 |
| Figure 5. Phosphate solubilization. Own elaboration |
In the other hand, if water contains iron ion, it can react with phosphate or phosphoric acid and create ferric phosphate or ferrous phosphate. These compounds are used as legal pesticides, but they may have negative effects.
Iron phosphate can react with chelating agents, but in this case resulst are negative because it could leach heavy metals from soil into water streams, what may increase water toxicity, making the treatment inefficient.
SULFATE
Sulfate is the most oxidized state of sulfur, so the process that takes place is sulfate reduce, but it can be observed that there are two types of metabolisms that can be performed by bacteria, which may have different results.
In one hand, there is the assimilatory sulfate reduction, which fully incorporates the sulfur atoms thanks to endoenzymes and produce carbon dioxide and water. This process is very simple and beneficious for wastewater treatments due to the fact that resulting products are not toxificants or negative in any way.
 |
| Figure 6. Assimilatory sulfate reduction. Retrieved from: Geradi, M. (2006). Wastewater bacteria. New Jersey, John Wiley & Sons, Inc., Hoboken, |
In the other hand, there is the dissimilatory sulfate reducing process, in which sulfate is not assimilated by bacteria through the endoenzymes and it must be rejected to the exterior of the cell, but in this case the rsulting product is not pale sulfur atom.
The main problem with this metabolism is the resulting products, which is mainly hydrogen sulfide, and although it is not toxic, it may produce malodor effects that are undesirable.
 | |
Figure 7. Dissimilatory sulfate reduction.
|
By return, hydrogen sulfide is also found in wastewater due to organic matter decomposition and it must be removed to avoid malodor problems. The metabolic process that take place is the hydrogen sulfide oxidation, performed by sulfur oxidizing bacteria, to finally produce sulfate, which would be reintroduced in the initial process.
HEAVY METALS
Some heavy metals present in wastewater are very toxic and must be removed. Some bacteria have a primary role in the removal of these toxic compounds through different processes.
One of the most important processes is the mercury demethylation. Methyl mercury is an extremly toxic compound that can be easily acumulated in fatty tissue. The process of demethylation consist in the disintegration of the methyl form the metallic mercury through methane production.
 |
| Figure 8. Mercury demethylation. Own elaboration. |
Its is also important the cadmium biosorption, among other metals, a process in which biomass can naturally bind contaminants onto its cellular structure. It has been proved that low levels of biomass of Spirulina can remove up to a 98% of Cadmium by biosorption processes (Solisio, Lodi, Soletto, Converti et al., 2008).
| Figure 9. Cadmium biosorption. Own elaboration |
By last, many bacteria are capable to perform redox reactions of many metal ions, such as iron, manganese, arsenic, antimon, vanadium, selenium or palladium, changing their oxidizing state in order to make them capable to bond with other elements which may help them precipitate and be removed form water.
ORGANIC MATTER
Organic matter dissolved in water is a limiting factor in order to discharge it to streams due to microbial blooms phenomena, which is promoted in highly organic matter environments.
Organic matter can be removed through aerobic and anaerobic metabolisms, but in wetlands it would be optimal to promote anaerobic organisms processes to degrade organic matter from wastewater.
The main reason is the oxygen demand by aerobic microorganisms in order to consume organic matter. In cases of high organic matter inputs, the enormous quantity of oxygen demanded would cause anoxic environments undesirables for other living species in wetlands systems.
In order to avoid this scenario, it is convinient to promote anaerobic processes, which can only happen in deeper water, where there is low oxygen concentration,and warm environments, which are absolutely possible in wetlands, due to its physico-chemical characteristics.
The overall reaction consist in degrading organic matter to form methane and carbon dioxide by methanogenic bacteria or methanogenic archea.
 |
| Figure 10. Anaerobic methanogenesi. Own elaboration |
4. Which microorganisms are involved in such processes?
 |
Table 1. Main microbiological processes in wastewater treatment wetlands. Own elaboration |
5. Types of constructed wetlands
The classification of constructed wetlands
is based on: the vegetation type (emergent, submerged, floating leaved,
free-floating); hydrology (free water surface and subsurface flow); and
subsurface flow wetlands can be further classified according to the flow
direction (vertical or horizontal). For improving
the performance and the removal of pollutants and nutrients, a combination of
these systems can be used, known as hybrid systems. The hybrid systems can
combine several features in only one or in several sequential steps (De la Varga et al., 2017).
 |
| Figure 11. Classification of
constructed wetlands. Springer
Science + Business Media B.V. (2008). Retrieved from: https://link.springer.com/chapter/10.1007/978-1-4020-8580-2_4 |
Surface flow
wetland
Water level is above the
ground surface. Vegetation is rooted and emergent above the water surface and
water flow is primarily above ground. Vegetation may be planted or allowed to
colonize voluntary. The
nearsurface layer is aerobic while the deeper waters and substrate are usually
anaerobic.
-Can be integrated better in landscapes.
-More secondary benefits (e.g., wildlife habitat, but this can be a disadvantage if excessive contamination exposure results).
-Cheaper to build and their construction, operation, and maintenance are straightforward.
-Shorter development period to reach full performance.
-Good to treat mine drainage and agricultural runoff.
-Requires a larger land area tan other systems.
Subsurface flow wetland
Water level is below ground and water flow is through porous substrate
of rock or gravel. Wetland plants are generally common reed, bulrush, or cattail.
-Less or not attractive to wildlife.
-Best suited to wastewaters with relatively low solids concentrations and under relatively uniform flow conditions.
-Used frequently to reduce 5-day biochemical oxygen demand (BOD5) from domestic wastewaters.
-Greater cold tolerance.
-Reduces pest and odor problems as well as insects.
-Greater assimilation potential per unit of land area than in surface flow systems.
-Less land requirement.
The most widespread constructed wetlands are the
surface flow systems (FWS), the horizontal subsurface flow systems (HF) and the
vertical subsurface flow systems (VF):
 |
| Figure 12. Different
types of constructed wetlands, classified on the basis of the water flow type. A. Ghermandi (2007). Retrieved from: https://www.researchgate.net/publication/6515984_The_Role_of_Free_Water_Surface_ Constructed_Wetlands_as_Polishing_Step_in_Municipal_Wastewater_Reclamation_and_Reuse |
Hybrid systems
The hybrid systems were developed in the 1960s, but their use increased only during the late 1990s and the 2000s, mostly because of more tringent limits for nitrogen and also more complex watsewaters treated in constructed wetlands (Vymazal, 2013). Single stage systems require that all of the removal processes occur in the same space. In hybrid or multistage systems. different cells are designed for different types of reactions. Most of them combine vertical filter and horitzonal filter stages.
 |
| Figure 13. Longitudinal scheme of a hybrid constructed wetland. Józwiakoski et al. (2013). Retrieved from: http://br.wszia.edu.pl/zeszyty/pdfs/br38_09jozwiakowski.pdf |
The most commonly used hybrid system is a VF-HF constructed wetland (figure 13) which has been used for treatment of both sewage and industrial wastewaters (and it is the most efficient hybrid system in ammonia removal).
Ont he other hand, constructed wetlands with FWS units substantially more total nitrogen as compared to other types of hybrid constructed wetlands. However, all types of hybrid constructed wetlands are more efficient in total nitrogen removal than single HF or VF constructed wetlands. some of hybrid system's disadvantages are:
- Very space consuming
- Expert design, construction and supervision needed
- Requieres a pre-treatment to prevent clogging
-Not very tolerant to cold climates
6. Conclusions
All in all, constructed wetlands for wastewater treatment work thanks to the metabolism of specific bacteria and seem to be an intelligent alternative to convencional wastewater treatment plants because of:
On the other hand, there are some other issues which seem to be disadvantages:
7. References
-https://en.wikipedia.org/wiki/Wastewater#Pollutants [18/10/2018]
-Skrzypiecbcef, K., & Gajewskaad, M. H. (2017). The use of constructed wetlands for the treatment of industrial wastewater, Journal of Water and Land Development, 34(1), 233-240. DOI: 10.1515/jwld-2017-0058. [20/10/2018]-De la Varga, David & Oirschot, Dion & Soto, Manuel & Kilian, Rene & Arias, Carlos & Pascual, Ana & Alvarez, Juan. (2017). Constructed Wetlands for Industrial Wastewater Treatment and Removal of Nutrients. DOI: 10.4018/978-1-5225-1037-6.ch008. [20/10/2018]
-https://microbewiki.kenyon.edu/index.php/Wetlands [01/11/2018]
-Geradi, M. (2006). Wastewater bacteria. New Jersey, John Wiley & Sons, Inc., Hoboken [24/11/2018]
-Ayanda, O., Akinsoji, O. (2011). Biological wastewater treatment: Microbiology, Chemistry and Diversity measurament of ammonium oxidizing bacteria. African Journal of Microbiology Research Vol. 5(32), pp. 5831-5840, 30 December, 2011 from: http://www.academicjournals.org/AJMR [24/11/2018]
wastewaters[06/12/2018]
Anchor10[06/12/2018]
-https://www.youtube.com/watch?v=wxxleTPZbGQ[06/12/2018]
-https://www.lenntech.com/phosphorous-removal.htm[06/12/2018]
-https://pubs.acs.org/doi/10.1021/acs.est.6b00401[06/12/2018]
-https://en.wikipedia.org/wiki/Biosorption[06/12/2018]
-Vymazal, J (2005) Horizontal sub-surface flow and hybrid constructed wetlands systems for wastewater
treatment. Sciencedirect. (478–490) DOI: 10.1016/j.ecoleng.2005.07.010 [07/12/2018]
treatment. Sciencedirect. (478–490) DOI: 10.1016/j.ecoleng.2005.07.010 [07/12/2018]
-Technical and Regulatory Guidance Document for Constructed Treatment Wetlands (2003). Washington, D.C.: Interstate Technology & Regulatory Council. Retrieved from: https://www.itrcweb.org/GuidanceDocuments/WTLND-1.pdf [07/12/2018]
-Brix,
Hans. (1994). Use of Constructed Wetlands in Water-Pollution Control -
Historical Development, Present Status, and Future Perspectives. Water Science and Technology. 30. 209-223.
DOI: 10.2166/wst.1994.0413. [07/12/2018]-Vymazal, Jan. (2010). Constructed Wetlands for Wastewater Treatment. Water. 25. DOI: 10.1016/j.ecoleng.2005.07.002.
-Springer Science + Business Media B.V. (2008) Types of Constructed Wetlands for Wastewater Treatment. In: Wastewater Treatment in Constructed Wetlands with Horizontal Sub-Surface Flow. Environmental Pollution, vol 14. Springer, Dordrecht. DOI: 10.1007/978-1-4020-8580-2_4
-Ghermandi, Andrea & Bixio, D & Thoeye, C. (2007). The Role of Free Water Surface Constructed Wetlands as Polishing Step in Municipal Wastewater Reclamation and Reuse. The Science of the total environment. 380. 247-58. DOI: 10.1016/j.scitotenv.2006.12.038. [09/12/2018]
-Vymazal, Jan.
(2013). The use of hybrid constructed wetlands for wastewater treatment with
special attention to nitrogen removal: A review of a recent development. Water Research, vol 47 (14), 4795-4811.
DOI: 10.1016/j.watres.2013.05.029. [09/12/2018]
El treball està ben plantejat, tot i que cal definir millor alguns apartats i precisar en alguns continguts. Per exemple, a part dels processos que es descriuen, no queda clar si tots aquests processos microbians són positius per l’eliminació de nutrients o bé hi ha processos que no són desitjables. Caldria precisar millor quin o quins nutrients es volen eliminar i quins metabolismes ho afavoreixen. No cal que parleu dels processos que hi ha en una estació de tractament d’aigües residuals ja que el treball està centrat en sistemes d’aiguamolls construïts. La Introducció és molt simple. Recordeu que les imatges s’han de poder visualitzar bé i que han de ser informatives. Per exemple, la primera imatge que aporteu és molt poc informativa, la podeu eliminar. Totes les imatges han de portar un peu de figura. No queda clar d’on prové l’aigua que entra en els sistemes d’aiguamolls contruïts. Quines característiques té?. Porta metalls?. Concentracions elevades de determinats nutrients?. Quina diferència hi ha en la composició de l’aigua d’entrada i sortida d’aquests sistemes?. On s’aboca aquesta aigua?..... Cal aprofundir més en els continguts que s’exposen, per exemple, per què existeixen diferents sistemes d’aiguamolls construïts?. Tenen aplicacions diferents?.
ResponderEliminarIn the article, when you talk about heavy metals, which are the main processes to remove those?
ResponderEliminarIn this are only especifide microbial processes for the removal of heavy metal removal, which can be done through different processes.
EliminarOne of the most important processes for heavy metal removal is biotransformation, like redox reactions in which their oxidizing state change and can be removed, such as iron reducing, which precipitates and it is removed from water.
Another main process for heavy metal removal is biosorption or bioacumulation, in which microbes incorporate metal ions into their cell or into their cell wall. This process is really important due to the fact that they are able to remove this metals from water.