Constructed wetlands for wastewater treatment, Recycling treated wastewater for irrigation, Permissible limits for land application

A constructed wetland (CW)/ reed bed is an artificial wetland to treat municipal or industrial  wastewater, greywater or stormwater runoff. It may also be designed for land reclamation after mining or as a mitigation step for natural areas lost to land development.

It uses natural functions of vegetation, soil, and organisms to treat wastewater. Depending on the type of wastewater the design of the constructed wetland has to be adjusted accordingly.

Similarly to natural wetlands, constructed wetlands also act as a biofilter and/or can remove a range of pollutants (such as organic matter, nutrients, pathogens, heavy metals) from the water. Constructed wetlands are a sanitation technology that have not been designed specifically for pathogen removal, but instead, have been designed to remove other water quality constituents such as suspended solids, organic matter and nutrients (nitrogen and phosphorus). All types of pathogens (i.e., bacteria, viruses, protozoan and helminths) are expected to be removed to some extent in a constructed wetland. Subsurface wetlands provide greater pathogen removal than surface wetlands. A biofilter has some similarities with a constructed wetland, but is usually without plants.

Vegetation in a wetland provides a substrate (roots, stems, and leaves) upon which microorganisms can grow as they break down organic materials. This community of microorganisms is known as the periphyton. The periphyton and natural chemical processes are responsible for approximately 90 percent of pollutant removal and waste breakdown. The plants remove about seven to ten per cent of pollutants, and act as a carbon source for the microbes when they decay. Different species of aquatic plants have different rates of heavy metal uptake, a consideration for plant selection in a constructed wetland used for water treatment. Constructed wetlands are of two basic types: subsurface flow and surface flow wetlands.

Constructed wetlands are one example of nature-based solutions and of phytoremediation. Many regulatory agencies list treatment wetlands as one of their recommended "best management practices" for controlling urban runoff.

Physical, chemical, and biological processes combine in wetlands to remove contaminants from wastewater. Theoretically, wastewater treatment within a constructed wetland occurs as it passes through the wetland medium and the plant rhizosphere. A thin film around each root hair is aerobic due to the leakage of oxygen from the rhizomes, roots, and rootlets. Aerobic and anaerobic micro-organisms facilitate decomposition of organic matter. Organic matter broken was down by micro-organisms by the mechanisms of fermentation, or respiration and used as energy source for wetland or assimilated into biomass. Organic particulate nitrogen (organic matter) in the effluent may settle due to the mechanisms called sedimentation. The settled sediments may deposit in the bottom of constructed wetlands. Nitrogen can be converted into different forms depending on oxidation state of wetland. Microbial nitrification and subsequent denitrification releases nitrogen as gas to the atmosphere. Phosphorus is coprecipitated with iron, aluminium and calcium compounds located in the root-bed medium.  Suspended solids filter out as they settle in the water column in surface flow wetlands or are physically filtered out by the medium within subsurface flow wetlands. Harmful bacteria and viruses are reduced by filtration and adsorption by biofilms on the gravel or sand media in subsurface flow and vertical flow systems.

Constructed wetlands have been used extensively for the removal of dissolved metals and metalloids. Although these contaminants are prevalent in mine drainage, they are also found in stormwater, landfill leachate and other sources (e.g., leachate or FDG washwater at coal-fired power plants), for which treatment wetlands have been constructed for mines.

Typhas and Phragmites are the main species used in constructed wetland due to their effectiveness, even though they can be invasive outside their native range.

In North America, cattails (Typha latifolia) are common in constructed wetlands because of their widespread abundance, ability to grow at different water depths, ease of transport and transplantation, and broad tolerance of water composition (including pH, salinity, dissolved oxygen and contaminant concentrations). Elsewhere, Common Reed (Phragmites australis) are common (both in blackwater treatment but also in greywater treatment systems to purify wastewater).

Plantings of reedbeds are popular in European constructed subsurface flow wetlands, although at least twenty other plant species are usable. Many fast growing timer plants can be used, as well for example as Musa spp., Juncus spp., and sedges.

Plants such as Water Hyacinth (Eichhornia crassipes) and Pontederia spp. are used worldwide.

Locally grown non-predatory fish can be added to surface flow constructed wetlands to eliminate or reduce pests, such as mosquitos.

Stormwater wetlands provide habitat for amphibians but the pollutants they accumulate can affect the survival of larval stages, potentially making them function as "ecological traps".

Case studies

·         The total number of constructed wetlands in Austria is 5,450 in 2015. 

·         The Arcata Marsh in Arcata, California is a sewage treatment and wildlife protection marsh

·         The Urrbrae Wetland in Australia was constructed for urban flood control and environmental education

·         At the Ranger Uranium Mine, in Australia, ammonia is removed in "enhanced" natural wetlands (rather than fully engineered constructed wetlands), along with manganese, uranium and other metals

 

Recycling treated wastewater for irrigation

The use of treated wastewater in agriculture benefits human health, the environment and the economy. This use represents an alternative practice that is being adopted in different regions confronted with water shortages and growing urban populations with increasing water needs, especially given the decline in surface and groundwater resources caused by climate variability (CV) and climate change (CC). The availability of water resources is also affected by wastewater-sourced pollution, as such water is not always treated before reaching surface channels, and by associated aquifer pollution

One of the most recognized benefits of wastewater use in agriculture is the associated decrease in pressure on freshwater sources. Thus, wastewater serves as an alternative irrigation source, especially for agriculture, the greatest global water user, which consumes 70% of available water. Furthermore, wastewater reuse increases agricultural production in regions experiencing water shortages, thus contributing to food safety. Approximately 805 million people, one-ninth of the global population, suffer from hunger. However, according to FAO’s latest estimations, a decreasing trend in hunger supports the possibility of halving the number of undernourished people. However, to be successful, it is first necessary to adopt a comprehensive approach that includes public and private investment aimed at increasing agricultural productivity, in addition to increasing and improving the availability of water resources and protecting vulnerable groups. Depending on the local situation, another benefit associated with agricultural wastewater reuse could be the avoided cost of extracting groundwater resources. In this regard, it is worth noting that energy required to pump groundwater can represent up to 65% of the costs of irrigation activities

Additionally, the nutrients naturally present in wastewater allow savings on fertilizer expenses to be realized, thus ensuring a closed and environmentally favorable nutrient cycle that avoids the indirect return of macro- (especially nitrogen and phosphorous) and microelements to water bodies. Depending on the nutrients, wastewater may be a potential source of macro- (N, P and K) and micronutrients (Ca, Mg, B, Mg, Fe, Mn or Zn). Indeed, wastewater reuse has been proven to improve crop yield  and result in the reduced use of fertilizers in agriculture. Therefore, eutrophication conditions in water bodies would be reduced, as would the expenses for agrochemicals used by farmers. The prevention of water pollution would be another benefit associated with wastewater reuse in agriculture. A decrease in wastewater discharge helps improve the source quality of receiving water bodies. Moreover, groundwater reservoirs are preserved, as agricultural wastewater reuse recharges these sources with higher-quality water. Additionally, an increased use of wastewater could contribute to the installation and optimization of treatment facilities to produce effluent of a desired quality for irrigation purposes, representing an economic benefit to sanitation projects. In those areas where climatic and geographic characteristics allow, low-cost wastewater treatment systems might also be a viable option, achieved using certain technological options that fulfill the objective of agricultural reuse. Wastewater use in agriculture helps liberate capital resources through the payment of economic instruments by the actors of different countries

Limitations Associated with Agricultural Wastewater Reuse

 The use of treated or untreated wastewater in agriculture is not exempt from adverse effects on the environment, especially on soil. The scientific literature includes evidence of alterations in the physicochemical parameters of soil. Variations have been observed in the structure and magnitude of microbial biomass in soil, as well as an increase in microbial activity caused by agricultural wastewater reuse. Altering physicochemical parameters and soil microbiota can affect fertility and productivity, thus disturbing soil sustainability from inadequate irrigation with wastewater. A review follows on the effects of wastewater reuse in agriculture and the impact on physicochemical parameters such as pH, organic matter, nutrients, salinity and contaminants, as well as on microbial diversity.

Changes in soil pH due to treatment of effluent using constructed wetland technology are correlated with three factors: (i) type of soil cover; (ii) soil texture; and (iii) period of irrigation. The changes in soil pH influence the availability of nutrients and metals, the cation exchange capacity (CEC) and the mineralization of organic matter. Additionally, different researchers consider pH incidence to be a decisive factor in determining the number of species and variety of soil microorganisms, as an increase in free metals is not related to changes in the soil pH, and the concentration and availability of metals have the potential to affect the substrate of the microbial communities

Moreover, organic matter is critical for nutrient storage and soil structure. Through the formation and stabilization of aggregates (sand, lime and clay), the organic matter content contributes to the capacity of the soil to retain water, affecting drainage properties and compaction resistance. Organic matter also constitutes a deposit of important macro- and micronutrients (N, P and S) for plant growth, contributing to the cation exchange capacity (CEC) and, consequently, to soil fertility. Depending on the amount of organic matter contributed, different studies have reported an increase in total organic carbon (TOC) and nitrogen (N) in those soils irrigated with domestic wastewater. This phenomenon also causes the availability of organic matter to increase. As a consequence, the presence of specific bacteria populations may be favored in the soil. Between 40% and 70% of soil bacteria are associated with stable aggregates (clay particles)

The stability of aggregates in the soil and the water retention capacity from the organic matter contributed by wastewater irrigation depend on the concentration levels, the composition of organic matter and soil texture. Thus, sandy-clay soils irrigated with wastewater increase the stability of their aggregates. Conversely, soils with a clayey texture diminish the stability of their aggregates. Additionally, the use of wastewater in prolonged irrigation (more than 20 years) can result in negative changes in soil structure due to the accumulation of sodium in the exchange complex.

A study on sugarcane irrigated with treated wastewater for 12 months found an increase in the content of organic matter in the soil that, according to the authors, favored the reuse of wastewater in the areas under study. Different research studies have noted an increase in the different forms of nitrogen (N-NO₃, NH₄-N or Total N) after irrigation with wastewater for periods ranging from one to 20 years. However, despite existing benefits in agricultural production and a reduction in chemical agents (fertilizers) from the increase in N and P contributed by wastewater, soil microbial communities can be affected, particularly the activities associated with the cycle of these elements

More than ninety percent of the soil’s nitrogen is in organic form. Ammonium and nitrate are the main forms of absorption by plants, in addition to some organic nitrogen compounds. It is generally believed that nitrite is an intermediate product in the conversion of Ammonium to Nitrate in the soil, where the conversion of Nitrite to Nitrate is important, since relatively small amounts may have toxic effects on plant growth. These intermediate products of complex organic substances of nitrogen can be absorbed by the plants. Organic nitrogen nutrition can affect the quality of the plant product and the metabolism of the plan. Similarly, under excessive application of nitrogen (by fertilizer, sewage, or other source), vegetables can accumulate high levels of nitrate and, when consumed by living things, can pose serious health hazards.

Another effect is the accumulation of inorganic N in the soil that can affect the biodegradation of carbon compounds. Additionally, the excessive supply of nutrients in the soil may have adverse effects. Nutrients such as phosphorus and nitrate can be included in the runoff or can be leached towards groundwater, thus causing the eutrophication or toxicity of other habitats. Irrigated wastewater can promote soil salinization (an increase in the concentration of soluble salts) or sodification (an excess of interchangeable sodium in relation to other cations).

Salinity problems occur when the soluble salts are concentrated in the root zone, thus causing osmotic stress that limits the capacity of plants to absorb water and nutrients. Sodicity therefore negatively affects the stability of aggregates and soil structure, as high interchangeable sodium content causes a decrease in permeability. Sodicity is caused by expansive and dispersive processes on clays as a consequence of the destruction of aggregates due to high Na+ concentrations. Different research studies noted that changes in sodicity generate an increase in soil compaction and reduce the infiltration rate of water. As a result, soil microbiota is affected by variations in soil salinity or sodicity.

The effects on microbial communities are primarily related to changes in soil structure and decreases in osmotic potential. Another study assessed the effects of salinity on the structure, activity and community of soil microorganisms. Their results suggest that higher salinity content metabolically stresses soil microbiota. Additionally, the Carbon Nitrogen relation of the biomass tends to be lower in higher salinity soils, which reflects the predominance of bacteria in the microbial biomass of saline soils. Furthermore, soil degradation increases due to the disposal of pollutants (metals and pharmaceutical compounds) through different media such as wastewater, which accumulate in the soil as a result of irrigation.

Typically, metal concentrations in soils not subjected to anthropogenic activities depend primarily on the parental material (stone) and can be present in the soil at non-toxic levels for living beings. However, population growth and industrialization have resulted in an increase in the presence of such polluting agents in wastewater and, consequently, in irrigated soils. Metals such as Fe, Cr, Zn, Pb, Ni, Cd and Cu, which are abundant in wastewater, lead the list of possible polluting agents that have accumulated in soil as a result of wastewater irrigation. The presence of these elements in the soil can limit fertility and/or modify soil microbial communities; they also affect a soil’s phytotoxicity potential with consequent effects on plant growth and pollution.

Other ecosystem functions affected due to metal pollution include organic matter mineralization, changes in soil enzyme activity, litter decomposition, microbial biomass reduction and changes in microbial structure. Additionally, the metals accumulated in a soil can interact with pharmaceutical products or other ECs, exacerbating the potential effects on the soil. Several studies have also noted strong co-occurrence patterns between the metals in a soil and a resistance to antibiotics in certain environmental conditions. The fate and effect of these compounds (emerging metals and/or polluting agents) depend on several factors such as the chemical properties of the pollutant type, the species and age of the vegetation cover, the composition of the rhizosphere microorganisms and soil characteristics (temperature, pH of the nutritional environment, soil texture and structure).

Researchers have noted that low-mobility compounds accumulate in soils with an irrigation period ranging from one to 100 years, in contrast with high-mobility compounds. Additionally, researchers worldwide have highlighted the risks posed by high-mobility compounds, given the possible leaching that may pollute groundwater sources. For example, in some amoxicillin-degradation products, it was observed that high-mobility compounds polluted the groundwater of wastewater-irrigated agricultural fields. Another study concluded, after discovering low retention rates for ibuprofen in soils, that this compound has a high potential to percolate through soil and pollute groundwater sources

Specific ion toxicity

Toxicity due to a specific ion occurs when that ion is taken up by the plant and accumulates in the plant in amounts that result in damage or reduced yield. The ions of most concern in treated wastewater are sodium, chloride, and boron. The source of boron is usually household detergents or discharges from industrial plants. Chloride and sodium also increase during domestic usage, especially where water softeners are used. For sensitive crops, toxicity is difficult to correct without changing the crop or the water supply. The problem is usually accentuated by severe (hot) climatic conditions Soil permeability In addition to their effects on the plant, sodium in irrigation water may affect soil structure and reduce the rate at which water moves into the soil as well as reduce soil aeration. If the infiltration rate is greatly reduced, it may be impossible to supply the crop or landscape plant with enough water for good growth. A permeability problem usually occurs in the surface few centimeters of the soil and is mainly related to a relatively high sodium or very low calcium content in this zone or in the applied water. At a given SAR, the infiltration rate increases as salinity increases or decreases as salinity decreases. Therefore, SAR and ECw should be used in combination to evaluate the potential permeability problem. Sometimes, treated wastewaters are relatively high in sodium and the resulting high SAR is a major concern in planning wastewater reuse projects. Chemical or biological amendments are needed over time to prevent soil structural degradation when irrigating exclusively with sodic water. On calcareous soils that contain appreciable amounts of precipitated or native calcite (CaCO₃), the dissolution of calcite in the root zone is enhanced by adding acid formers and by the actions of plant roots that increase the levels of carbon dioxide, thereby providing soluble calcium to offset sodium effect

Clogging problems with sprinkler and drip irrigation systems have been reported when treated municipal wastewater is used. The most frequent clogging problems occur with drip irrigation systems. In drip irrigation, vortex emitters were more sensitive to clogging than labyrinth emitters and no significant difference was observed between the same kind of emitter placed on soil or sub-soil; in filters, gravel media and disk filters assured better performance than screen filters. Another possible problem of the wastewater reuse is the excessive residual chlorine in treated effluent. Residual chlorine causes plant damage when sprinklers are used if the high chlorine residual exists at the time the effluent is sprinkled on plant foliage. Residual chlorine less than 1 mg/l should not affect plant foliage, but when chlorine residual is in excess of 5 mg/l, severe plant damage can occur.

Possible solutions of problems associated with the sewage and industrial effluents

 To exploit the sewage waters as a potential source of irrigation and maintain environment the sewage waters must be diluted either with canal or underground water to a avoid the excessive accumulation of soluble salts in the soils. It will help in maintaining the productivity of agricultural crop without any harmful effect on soil properties.

 Entry of heavy metals into food chain can be reduced by adopting soil and crop management practices, which immobilize these metals in soils and reduce their uptake by plants.

Heavy phosphate application and also the application of kaolin / zeolite to soils can reduce the availability of heavy metals.

 Application of organic manures can mitigate the adverse effect of the toxic metals on crops. Thus in the soil contaminated with high amount of toxic metals, application of organic manures is recommended to boost the yield potentials as well as decrease the metal availability to plants.

 Raising hyper accumulator plants (mustard/trees) in toxic metals contaminated soils is recommended to avoid the entry of toxic metal in the food chain.

Permissible limits for land application

S.No.	Parameters	Maximum permissible limit
1	Color and odor	-
2	Suspended Solids, mg/L	200
3	Particle size of Suspended solid	-
4	Dissolved solids (inorganic) mg/L	2100
5	pH value	5-9
6	Temperature	-
7	Oil & Grease, mg/L	10
8	Biochemical Oxygen Demand (3 days at 27 0C), mg/L	100
9	Chemical Oxygen Demand, mg/L	-
10	Arsenic (as As), mg/L	0.2
11	Mercury (as Hg), mg/L	0.01
12	Chlorides (mg L-1)	600
13	Sulphates (mg L-1)	1000
14	Total Cr (mg L-1)	-
15	Cr (VI) (mg L-1)	-
16	Fluoride (mg L-1)	-
17	Faecal coliforms	-