**4. Overview of pollutant pathways and the efficiency of ecosystem technologies**

Nutrients and pollutants in natural systems as well as in ET are subdued to numerous processes that enable pollutant and nutrient transformations, degradation or stabilization. The pathways of pollutants in natural ecosystems and ET depend on the characteristics of the system, namely dissolved oxygen concentration, pH, Eh, mineral composition of the media, bacterial, plant and animal communities; besides this, also external influences like climatic conditions and input loads are of significant importance.

#### **4.1 Nitrogen and phosphorous**

Regarding the nutrients, nitrogen and phosphorous compounds are of major concern. Increased ammonia concentrations in natural water bodies mainly indicate pollution from wastewater discharge, agricultural runoff or organic waste disposal. The presence of

environmental contamination, impacts of point pollution as well as seasonal pollution due to tourism and non-point pollution caused by agriculture. They are also appropriate for protecting sensitive areas and for rational water management in dry areas (Griessler Bulc &

Multi-functionality is an intrinsic feature of ET, which can be considered a flagship

• Water treatment: ET effectively treat a large variety of wastewater (sewage, gray water, agricultural, highway runoff, landfill leachate, wastewater from food processing and textile industry, composting facility runoff, etc.) and increase the self-cleaning capacity

• Water retention in the landscape: ET reduce hydraulic peaks by retaining water in the system and therefore prevent and mitigate floods and droughts. They can contribute to an improved water management, mitigate water abstractions and recharge

• Saving energy: ET can provide their services with very little or no energy input if

• Enhanced biodiversity: ET create a new habitat for wildlife and can contribute to an increased biodiversity in a barren landscape (e.g. spawning ground for frogs and toads,

• Biomass production and nutrient recycling; if designed for this purpose, ET can recycle nutrients from runoff to a large degree and convert them to biomass which can be used

• Recreation: ET can be designed with elements of landscape architecture and can create

• Education: ET are a good and tangible example of a measure aimed to achieve sustainable development. They can be used to present the problems of pollution and its remediation in a natural way to different target groups (e.g. how a waste product can

Nutrients and pollutants in natural systems as well as in ET are subdued to numerous processes that enable pollutant and nutrient transformations, degradation or stabilization. The pathways of pollutants in natural ecosystems and ET depend on the characteristics of the system, namely dissolved oxygen concentration, pH, Eh, mineral composition of the media, bacterial, plant and animal communities; besides this, also external influences like

Regarding the nutrients, nitrogen and phosphorous compounds are of major concern. Increased ammonia concentrations in natural water bodies mainly indicate pollution from wastewater discharge, agricultural runoff or organic waste disposal. The presence of

**4. Overview of pollutant pathways and the efficiency of ecosystem** 

application of good ecological engineering/biotechnology.

as energy or raw material source (*e.g.* thermal insulation).

climatic conditions and input loads are of significant importance.

of natural or revitalized ecosystems.

Šajn-Slak, 2009).

groundwater.

**technologies** 

**4.1 Nitrogen and phosphorous** 

designed accordingly.

breeding sites for birds etc.).

an attractive place for the population.

be transformed into something valuable).

**3.1 Why ET?** 

ammonia can significantly reduce the oxygen level in water body. Due to rapid oxidation to nitrate, nitrite is usually present in negligible concentrations in natural water bodies. In contrast to nitrite, the concentrations of nitrate in water surface and groundwater bodies are elevated in many parts of the world. The areas with intense agriculture are the most vulnerable due to nitrate pollution of drinking water.

Nitrogen and phosphorous are also of big interest because they are needed for the production of human food and have an important role in plant growth which further stimulates the wildlife production. Phosphorus in nature does not appear in elementary form but always in different compounds together with other elements. The extraction of P is a damaging process (strip-mining is most common method) having as result toxic byproducts (arsenic, cadmium, etc.). Phosphorus is highly bondable and is adsorbed to soil molecules and as such it is not available for the plant use. Plants can uptake phosphorous when the soil is saturated and there is free phosphorous available. The result of this fact is high demand on artificial fertilizers to increase nutrient content in less fertile soil.

Nitrogen and phosphorous removal is required in most European countries in order to reduce the input to water bodies or the sea (eutrophication). Removal of nitrogen compounds is also important in order to prevent oxygen depletion and the toxicity on invertebrate and invertebrate species, including humans.

The processes of volatilization, plant uptake, nitrification and denitrification enable efficient removal of nitrogen from wastewaters in ET (Brix, 1993; Šajn-Slak et al., 2005). However also numerous other processes affect nitrogen elimination and retention in ET, i.e. ammonia volatilization, anaerobic ammonia oxidation (ANAMOX), fixation of nitrogen from the atmosphere, incorporation into plant and microbial biomass, fragmentation, burial, degradation of organic nitrogen compounds (mineralization), reduction of nitrate to ammonia, sorption and desorption from different media, filtration and leaching. Only certain pathways enable N elimination, whereas other processes only transform N from one form to another (Vymazal, 2007). A crucial condition for N elimination in an ET is an exchange of oxic and anoxic areas in micro level, which enable a co-existence of nitrifying and denitrifying bacteria. First step in N elimination is degradation of organic N compounds (ammonification) where ammonia is generated. NH4-N is then oxidized to nitrite and further to nitrate, but in contrast to organic material, NH4-N is more demanding for oxidation in TW as nitrifying microbes are autotrophic bacteria which have slow respiration and need a significant amount of oxygen to function. Besides oxygen conditions plants can also be an important factor in nitrogen elimination. Plants preferably take up the reduced form of nitrogen (ammonia), but can also take up nitrate. Nitrogen is integrated in plant tissues and can be eliminated from the ET by mowing and removing the plant biomass. This can present a significant nutrient removal in systems that receive low nutrient loads (Langergraber, 2005); according to Vymazal (2006) plant harvesting can significantly contribute to nitrogen removal in wetlands that receive less than 100-200 g N m-2 year-1. However, in other free water ET denitrification is usually the main mechanism for nitrate removal), which enables reduction of nitrite and nitrate to gas nitrogen. N2 is released to the atmosphere and thus presents ultimate elimination of N from the ET. Unlike nitrogen removal, the capacity of ET to eliminate phosphorus is of a concern and has not been reasonably resolved. Phosphorous in TW systems is mainly accumulated in the sediment/media and/or accumulated in the plant biomass. Elimination of P from a wetland

Ecosystem Technologies and Ecoremediation for Water Protection, Treatment and Reuse 201

The removal of metals from water in wetlands can result in accumulation in the sediment, which might be harmful for the organisms that live or feed on these sediments. To avoid this problem pretreatment to reduce inflow metal concentrations, installation of deep water systems or subsurface wetlands can be considered. In deep water systems with free-floating plants the sediment is deposited at great depths and is thus not available to the top feeders and subsurface wetlands minimize the opportunity for ingestion of metals (Kadlec & Wallace, 2009). Depositing sediments have the ability to adsorb significant quantities of trace metals. Especially organic matter, iron and manganese oxyhydroxides act as metal adsorbents in aerated systems. Under anaerobic conditions iron and manganese oxyhydrohydes dissolves and thus release metals into the aqueous phase. This may lead to a repartitioning of metals into the sulphide or carbonate precipitates. In the conditions where metal's concentrations are in excess of sulphides, metals may complex with organic matter. Organic matter can appear as surface coatings or as particulates and significantly affects metal speciation and bioavailability (Kadlec & Wallace, 2009). Besides, at oxic conditions, coprecipitation of heavy metals with iron, manganese, and aluminum hydroxides also relies on considerable supplies of secondary metals in the system, which might not be present. According to this retention of metals in the sediment can be modified by changes in substrate chemistry and redox potential, which is affected by wetland water depth and biological processes. Besides redox potential, also pH affects sorption/desorption of heavy

Metals are also accumulated in plant tissues. Most of the metals found in plants are stored in the roots and rhizomes, only small amounts may find their way to stems and leaves as plant physiological mechanisms prevent the transport of heavy metals from the underground tissues to the aboveground tissues, where the toxic heavy metals could damage the photosynthetic tissues. Consequently, harvesting aboveground parts does not enable effective removal of metals from the wetland. However, with the root's death some fraction

Numerous studies investigate the elimination of organic micropollutants such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), phthalates, linear alkylbenzene sulphonates (LAS), nonylphenols etc. Besides listed, there is also a long list of emerging pollutants which are gaining lot of attention from the scientist in recent years. Mainly they focus on personal care products and pharmaceuticals (Hijosa-Valsero et

The pathways of micro and emerging pollutants in ET systems are not clearly known and are still under research. As aromatic compounds, PAHs have low water solubility/high hydrophobicity and thus they tend to absorb to solid particles and are thus removed by sedimentation and filtration. Higher organic carbon content in soils and sediments increases sorption of PAHs. It is known that PAHs are decomposed relatively rapidly in many vertebrates, but more slowly and in a different way in certain other life forms; nevertheless, the degradation products of PAHs can be more harmful than the original compound. The PAHs with four or less aromatic rings are subdued to biodegradation by microbes or are metabolized by multicellular organisms. The heavier PAHs (four, five and six rings) are more insistent compared to the lighter (two and three rings) and tend to have greater carcinogenic and other chronic impacts (Mangas et al., 1998). Also many emerging

of the metal may be permanently buried (Kadlec & Wallace, 2009).

metals at/from the sediment.

**4.3 Organic micropollutants** 

al., 2010a, 2010b).

treatment system is possible only by harvesting of the plant biomass and removing the saturated sediment.

The investigations on TW show that wetlands have a limited capacity to remove phosphorus and are greatly dependent on the characteristics of the media integrated into TW, namely as the media gets saturated with phosphorus, phosphorus removal efficiency decreases. Phosphorous removal mechanisms can be affected also by other factors, such as biofilm growth on the media, which limits the contact between the media and the treated water. In open water ET P is mainly accumulated in the bottom sediment. A crucial condition for trapping P in the sediment is a consistent and high redox potential in the surface sediment layer. P is leached from the sediments during anoxic conditions, however also microbial activity can cause its release. In neutral and acid conditions, microorganisms can use Fe3+ as electron acceptor thus releasing bound P. In basic conditions, microorganisms can dissolve insoluble P by increasing the ion exchange of OH- and PO43-, which arise from Fe-P or Al-P (Huang et al., 2008).

As for nitrogen also for elimination of P with plant harvesting is efficient only in the systems with low P loads. Inflow P concentrations depend on the type of water treated, while P content in the plant tissues remains in the same range, i.e. 1-5 g P m-2. Primary treated municipal wastewater usually presents between 100 in 200 (800) g P m-2 year-1, which means low P elimination with plant harvesting. However when treating secondary treated wastewater with less than 20 g P m-2 year-1, harvesting plant biomass can contribute to P elimination from the system for more than 20 % yearly. Despite this, plant harvesting demands specific attention in terms of timing the harvest, as plant harvesting in growth season can severely damage the canopy. Removal of plant biomass can also present a problem in temperate and cold climates due to dead plant material acts as an isolation layer during winter months. In tropical climates with long vegetation period plant harvesting can significantly contribute to nutrient elimination due to several harvests per year are feasible (Vymazal, 2004).

To improve the removal of P in ET numerous solutions have been proposed and examined: e.g. a use of chemically enhanced material with high sorption capacity for P (e.g. Filtralite) for construction of the ET system; an integration of a pre-treatment step for chemical precipitation of P; an elimination of P in separate filters with specific media, etc. The tests of finding appropriate media for P elimination suggest the selection of materials, which must at the same time have good hydraulic features and consistent and continual elimination of P from treated waters.

#### **4.2 Heavy metals**

Unlike nutrients, metals in natural ecosystems and ET are not subdued to degradation, but can only change the ionic form. During water treatment with an ET metals cannot be eliminated but are accumulated in the systems' sediment, soil or plant tissues. Many heavy metals are micronutrients for animals and plants and are essential in low concentrations (e.g. Zn and Cu). Other heavy metals (e.g. Cd) as well as high concentrations of micronutrients can be toxic to biota. In water, metals take forms of free metal ions or can be bound to or adsorbed onto organic and inorganic particulate matter and complexes. The most bioavailable form is the soluble form, especially when the metal is present as a free ion or is weakly complexed, and can cause bioaccumulation in the food chain.

The removal of metals from water in wetlands can result in accumulation in the sediment, which might be harmful for the organisms that live or feed on these sediments. To avoid this problem pretreatment to reduce inflow metal concentrations, installation of deep water systems or subsurface wetlands can be considered. In deep water systems with free-floating plants the sediment is deposited at great depths and is thus not available to the top feeders and subsurface wetlands minimize the opportunity for ingestion of metals (Kadlec & Wallace, 2009). Depositing sediments have the ability to adsorb significant quantities of trace metals. Especially organic matter, iron and manganese oxyhydroxides act as metal adsorbents in aerated systems. Under anaerobic conditions iron and manganese oxyhydrohydes dissolves and thus release metals into the aqueous phase. This may lead to a repartitioning of metals into the sulphide or carbonate precipitates. In the conditions where metal's concentrations are in excess of sulphides, metals may complex with organic matter. Organic matter can appear as surface coatings or as particulates and significantly affects metal speciation and bioavailability (Kadlec & Wallace, 2009). Besides, at oxic conditions, coprecipitation of heavy metals with iron, manganese, and aluminum hydroxides also relies on considerable supplies of secondary metals in the system, which might not be present. According to this retention of metals in the sediment can be modified by changes in substrate chemistry and redox potential, which is affected by wetland water depth and biological processes. Besides redox potential, also pH affects sorption/desorption of heavy metals at/from the sediment.

Metals are also accumulated in plant tissues. Most of the metals found in plants are stored in the roots and rhizomes, only small amounts may find their way to stems and leaves as plant physiological mechanisms prevent the transport of heavy metals from the underground tissues to the aboveground tissues, where the toxic heavy metals could damage the photosynthetic tissues. Consequently, harvesting aboveground parts does not enable effective removal of metals from the wetland. However, with the root's death some fraction of the metal may be permanently buried (Kadlec & Wallace, 2009).

## **4.3 Organic micropollutants**

200 Studies on Water Management Issues

treatment system is possible only by harvesting of the plant biomass and removing the

The investigations on TW show that wetlands have a limited capacity to remove phosphorus and are greatly dependent on the characteristics of the media integrated into TW, namely as the media gets saturated with phosphorus, phosphorus removal efficiency decreases. Phosphorous removal mechanisms can be affected also by other factors, such as biofilm growth on the media, which limits the contact between the media and the treated water. In open water ET P is mainly accumulated in the bottom sediment. A crucial condition for trapping P in the sediment is a consistent and high redox potential in the surface sediment layer. P is leached from the sediments during anoxic conditions, however also microbial activity can cause its release. In neutral and acid conditions, microorganisms can use Fe3+ as electron acceptor thus releasing bound P. In basic conditions, microorganisms can dissolve insoluble P by increasing the ion exchange of OH- and PO4

As for nitrogen also for elimination of P with plant harvesting is efficient only in the systems with low P loads. Inflow P concentrations depend on the type of water treated, while P content in the plant tissues remains in the same range, i.e. 1-5 g P m-2. Primary treated municipal wastewater usually presents between 100 in 200 (800) g P m-2 year-1, which means low P elimination with plant harvesting. However when treating secondary treated wastewater with less than 20 g P m-2 year-1, harvesting plant biomass can contribute to P elimination from the system for more than 20 % yearly. Despite this, plant harvesting demands specific attention in terms of timing the harvest, as plant harvesting in growth season can severely damage the canopy. Removal of plant biomass can also present a problem in temperate and cold climates due to dead plant material acts as an isolation layer during winter months. In tropical climates with long vegetation period plant harvesting can significantly contribute to nutrient elimination due to several harvests per year are feasible

To improve the removal of P in ET numerous solutions have been proposed and examined: e.g. a use of chemically enhanced material with high sorption capacity for P (e.g. Filtralite) for construction of the ET system; an integration of a pre-treatment step for chemical precipitation of P; an elimination of P in separate filters with specific media, etc. The tests of finding appropriate media for P elimination suggest the selection of materials, which must at the same time have good hydraulic features and consistent and continual elimination of P

Unlike nutrients, metals in natural ecosystems and ET are not subdued to degradation, but can only change the ionic form. During water treatment with an ET metals cannot be eliminated but are accumulated in the systems' sediment, soil or plant tissues. Many heavy metals are micronutrients for animals and plants and are essential in low concentrations (e.g. Zn and Cu). Other heavy metals (e.g. Cd) as well as high concentrations of micronutrients can be toxic to biota. In water, metals take forms of free metal ions or can be bound to or adsorbed onto organic and inorganic particulate matter and complexes. The most bioavailable form is the soluble form, especially when the metal is present as a free ion

or is weakly complexed, and can cause bioaccumulation in the food chain.

3-,

saturated sediment.

(Vymazal, 2004).

from treated waters.

**4.2 Heavy metals** 

which arise from Fe-P or Al-P (Huang et al., 2008).

Numerous studies investigate the elimination of organic micropollutants such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), phthalates, linear alkylbenzene sulphonates (LAS), nonylphenols etc. Besides listed, there is also a long list of emerging pollutants which are gaining lot of attention from the scientist in recent years. Mainly they focus on personal care products and pharmaceuticals (Hijosa-Valsero et al., 2010a, 2010b).

The pathways of micro and emerging pollutants in ET systems are not clearly known and are still under research. As aromatic compounds, PAHs have low water solubility/high hydrophobicity and thus they tend to absorb to solid particles and are thus removed by sedimentation and filtration. Higher organic carbon content in soils and sediments increases sorption of PAHs. It is known that PAHs are decomposed relatively rapidly in many vertebrates, but more slowly and in a different way in certain other life forms; nevertheless, the degradation products of PAHs can be more harmful than the original compound. The PAHs with four or less aromatic rings are subdued to biodegradation by microbes or are metabolized by multicellular organisms. The heavier PAHs (four, five and six rings) are more insistent compared to the lighter (two and three rings) and tend to have greater carcinogenic and other chronic impacts (Mangas et al., 1998). Also many emerging

Ecosystem Technologies and Ecoremediation for Water Protection, Treatment and Reuse 203

they help to reduce surface water retention and low water tables for optimum plant production and therefore representing integral components for sustaining the economic development. Nonetheless, drainage networks affect several hundred thousand hectares of land in Western and Eastern Europe, leading to reduced water's self purification and retention capacity and the loss of biodiversity. Although the amount of new drainage networks declined significantly in Europe during the 90's, the existing drainage systems continue to pose negative impacts on the environment. Therefore, management of VDD to optimize sorption, complexation and sedimentation processes of pollutants is an important issue in drainage pollution control, which complies with AEM at the point of conservation

WSP are simple man-made basins for primary, secondary and tertiary treatment of variety of wastewaters. They are used worldwide, alone or in combination with other treatment processes. Anaerobic, facultative and maturation ponds are constructed in one or several series. Anaerobic ponds are designed for primary treatment. They remove suspended solids and some of the soluble element of organic matter (BOD). Most of the remaining BOD is removed in facultative pond (secondary stage) by algae and heterotrophic bacteria. Tertiary treatment takes place in maturation pond where pathogens and nutrients (especially nitrogen) are removed. WSP are low cost treatment technology with simple operation and

Detention ponds are open water bodies for the retention of stormwater runoff from urban, agricultural and other areas. The stormwater treatment facility must be flexible to manage high flow rates of the runoff followed by dry periods, and high pollutant concentrations in the first flush followed by diluted concentrations in the main flow. Stormwater detention ponds are diverse biological system with a high buffering capacity that enable water detention, minimize the hydraulic peaks and reduce the pollutant input in downstream facilities and/or receiving waters (Hvitved-Jacobsen et al., 1994). Detained and treated water can be used for different purposes or discharged to the environment. Different plant species can appear in detention ponds (usually colonized by natural way): at the shallower marginal areas emergent and in deeper parts floating and submerged species. The treatment processes in wet detention ponds are similar to those occurring in natural smaller lakes and pools: e.g. contaminant accretion in the bottom deposits via sedimentation, adsorption to colloidal and particulate matter, conversions and degradation of biodegradable compounds by microorganisms and uptake of contaminants by plants. Among those, the key mechanism for pollutants removal in detention ponds is sedimentation. Since the main removal mechanism in wet detention ponds is sedimentation, the wet detention ponds generally have high efficiency in particulate matter removal (Terzakis et al., 2008). Organic matter is subdued to microbial and macroinvertebrate decomposition and final transformation to inorganic matter in the sediment, where it is stored. Sediment accretion at the bottom of wet ponds might vary greatly according to inflow and catchment characteristics. Hvitved-Jacobsen et al. (1994) estimated that excavation and removal of the sediments from wet ponds would be needed every 25 years of the operational period.

of habitats and their associated biodiversity.

**5.2 Waste stabilization ponds (WSP)** 

maintenance (Ramadan & Ponce, n.d.).

**5.3 Stormwater detention ponds (SDP)** 

pollutants are rather lipophilic in character, and are consequently likely to bind with the particulate matter. For this reason mechanical treatment can present an important step in elimination of these substances, but nevertheless, the absence of any biological step may have substantial effect on the total removal of more biodegradable compounds (Vogelsang et al., 2006).

Many treatment processes in ET (e.g. nitrification, aerobic degradation of organic matter and P trapping) are oxygen-limited. Studies have shown that the amount of oxygen transferred through the plants is very small compared to the oxygen demand utilized by the wastewater under usual loading rates. Consequently, many recent studies entirely neglect oxygen transfer by plants and include plants mostly as a microorganism carrier and biodiversity factor. The inadequate oxygen transfer of typical subsurface flow wetlands resulted in the progress of improved treatment systems, which are able to assure adequate oxygen levels for nitrification, degradation of organic matter, and prevention of P leaching. The systems include introduction of oxygen to the wetland by means of regular water level oscillations, passive air pumps or powered mechanical aeration of the reed bed (Nivala et al., 2007). Yet, the elimination of nitrogen in ET has been enhanced by the different flows, cascades, open areas, and with the application of recirculation of the treated outflow back to the inflow (Griessler Bulc & Šajn-Slak, 2009; Griessler Bulc et al., 2011).
