Wastewater Treatment

Among various functions and ecosystem services, the wetlands are most valued for their biogeochemical function that results in the improvement of water quality. By virtue of their location, the wetlands receive nutrients and a variety of pollutants with the storm water, agricultural runoff, and other overland or subsurface flows that invariably pass through them from upland areas before entering the open waters. Natural wetlands receive nutrients also from the open waters (rivers, lakes, seas) during periodic flooding. Wetlands retain and/or remove these nutrients

Table 1 Processes in natural wetlands that result in water quality improvement

Category

Processes

Physical Sedimentation of suspended particulate matter Filtration of finer particles by plants, biofilms, and mineral sediments Aggregation of particles followed by sedimentation

Chemical Precipitation

Adsorption onto sediments and detritus Volatilization (e.g., ammonia) Chelation and complexation

Biological Decomposition and mineralization of organic matter

Microbial Microbial transformation (nitrogen fixation) Microbial oxidation (nitrification/anammox) Microbial reduction (denitrification, SO4 reduction) Microbial competition and growth inhibition (hostile environment) Plants Uptake from the water (submerged plants, free floating plants, algae and biofilms) Uptake from the pore water in sediments (benthic algae and rooted plants) Chemical changes caused by plants (oxygen production and diffusion, calcium precipitation during photosynthesis) Habitat support for other organisms, and organic matter production Animals Ingestion of organic matter and microbes (including pathogens) Food chain transfer and accumulation of nutrients and other pollutants and pollutants from the inflowing water through a multitude of processes, thereby improving the quality of the water flowing out of the wetland or infiltrating into the ground water (Table 1).

Wastewaters have also been discharged into natural wetlands such as littoral marshes and floodplains for centuries without recognizing their specific roles. The high nutrient absorbing potential of various aquatic plants was reported in the early 1960s followed by numerous reports on their ability to absorb a variety of trace metals and toxic substances under laboratory and field conditions. Many plants indeed exhibit luxury consumption, that is, excessive uptake when availability of nutrients increases. Around the same time, a German scientist, Kaithe Seidel, also demonstrated the ability of Scirpus lacustris to reduce bacterial/fecal coliform populations from domestic was-tewaters through the production of phenolic metabolites, though other processes are also involved.

During the past two decades, increasingly greater attention has been paid to the wetlands for their potential for improving water quality, and for the development of an energy-efficient inexpensive technology for waste-water treatment based on wetlands. Within the United States, natural wetlands have received much protection because of the recognition of their water-quality improvement function by the Clean Water Act. This important function, which is now being exploited for ecological engineering, is described below in some detail.

Nutrient Transformation Processes

Nutrients and other pollutants in the wastewater passing through the wetland undergo transformations along several pathways involving physical, chemical, and biological processes. The major physical process is the settling of suspended particulate matter. The settling process depends upon, besides the nature and size of the particles, the residence time of inflowing wastewater within the wetland and the physical resistance offered by the vegetation to flow velocity. The reduction in suspended particulates, particularly the organic matter, results in an increase in transparency (lowered turbidity) and a major reduction in the biological oxygen demand (BOD). The chemical processes include adsorption, chelation and precipitation, and reduction and oxidation. Among the biological processes, most important are those mediated by microorganisms which decompose the dissolved and particulate organic matter and also contribute to the oxidation and reduction of C, N, and S depending upon the redox potential. Generally, the reduction reactions dominate under anoxic conditions in the presence of high organic matter load. Interactions between these processes and biota are quite complex and involve mediation by several elements such as Fe, Al, Mn, and Ca.

Among various pollutants, N and P are of greatest importance because they cause eutrophication and are not effectively removed by conventional secondary treatment. The removal of these nutrients from wastewaters has therefore received greater attention in wetland systems.

Mechanisms for phosphorus removal

The total P content in wastewaters comprises inorganic and organic, particulate and dissolved nonreactive forms, such as colloidal-P complexes, polyphosphates, nucleic acids, sugar phosphates, aminophosphonic acids, and organic condensed phosphates. In the wetland substrates, major pools of inorganic P include loosely adsorbed P, and hydrous sesquioxides, amorphous and crystalline aluminum and iron compounds in acidic, noncalcareous soils and calcium compounds in alkaline calcareous substrates. The loosely adsorbed P is important for plant growth and controls the P concentration of the overlying water column. The P associated with oxyhydroxides is readily desorbed under most conditions, but the P associated with crystalline iron and aluminum is desorbed only under prolonged anoxic conditions. The calcium and magnesium forms of P are generally unavailable for biological assimilation under natural conditions and are not common at low pH conditions. However, under anoxic conditions, the sediment pH is mostly neutral to alkaline and calcium and magnesium forms of P are often dominant. Transformations between these various forms occur continuously to maintain equilibrium. The organic P fraction, primarily comprising phospholipids, inositols, fulvic acids, and humic acids, is generally biologically reactive and can be hydrolyzed to bioavailable forms. Organic P is mineralized by alternate wetting and drying cycles, changes in substrate pH, and increased microbial activity.

Phosphorus removal from wastewaters entering a wetland occurs through several pathways, namely (1) from water column to the sediment; (2) adsorption on to the fflM—

Anoxic

Figure 2 Major processes and pathways of phosphorus transformation in wetlands. DIP, dissolved inorganic phosphorus; DOM, dissolved organic matter; DOP, dissolved organic phosphorus; FC-Tr, food chain transfer; Phytopl, Phytoplankton; POM, particulate organic matter, POP, particulate in organic phosphorus; Precip., precipitation; Sed., sedimentation; SMac, submerged macrophytes. Two larger arrows represent adsorption and desorption under changing redox conditions.

Anoxic

Figure 2 Major processes and pathways of phosphorus transformation in wetlands. DIP, dissolved inorganic phosphorus; DOM, dissolved organic matter; DOP, dissolved organic phosphorus; FC-Tr, food chain transfer; Phytopl, Phytoplankton; POM, particulate organic matter, POP, particulate in organic phosphorus; Precip., precipitation; Sed., sedimentation; SMac, submerged macrophytes. Two larger arrows represent adsorption and desorption under changing redox conditions.

nutrient removal. Increased phosphorus loading enhances biomass production and hence peat accretion which in turn provides effective storage of P for extended periods. In alkaline waters, P is co-precipitated with calcium carbonate during photosynthesis by submerged plants and algae, and settles on the sediments.

The direction of P flux across the substrate-water interface is regulated by the P concentration gradient, pH of the water column, sorption/precipitation reactions, plant uptake, the physico-chemical properties of the substrate, and the incidence of any bioturbation at the interface. The substrate-water interface layer is usually oxidized and its thickness depends on oxygen diffusion potential and oxygen demand within the zone. This zone can therefore potentially function as a P sink by immobilizing P into insoluble ferric or calcium phosphate, as well as uptake and storage of P into the bacterial biomass. It is often assumed that aerobic conditions completely prevent P release from the substrate but mass balance studies show that substantial P is released from sediments to well-aerated waters that are weakly buffered and have a low pH and low P concentration waters.

Table 2 Range of tissue concentrations (% dry weight) of major nutrients in some wetland plants

Specie s

Location

N

P

K

Ca

Mg

All wetland plants

0.09-

4.23

0.01-

0.82

0.04-

4.95

0.02-

8.03

0.03-1.05

Acorus calamus - shoots

Fishponds, CSSR

1.26-

2.92

0.20-

0.35

1.85-

3.67

0.34-

0.85

0.14-0.21

Carex acutiformis

L. Balaton, Hungary

1.41-

1.87

0.05-

0.10

0.44-

1.03

0.29-

0.44

0.42-0.89

Carex gracilis - shoots

L. Balaton, Hungary

1.13-

1.90

0.05-

0.19

0.85-

1.67

0.10-

0.45

0.22-0.44

Czech Fishponds

1.15-

2.46

0.18-

0.32

1.07-

1.90

0.20-

0.25

0.07-0.09

Cyperus papyrus - whole plant

Kenya

0.78-

1.75

0.02-

0.1

1.55-

2.84

0.14-

0.23

0.03-0.11

Eichhornia crassipes - whole plant

Florida

2.64

0.43

4.25

1.00

1.05

Glyceria maxima - shoots

England

0.40-

1.57

0.06-

0.19

0.30-

1.80

Hydrocotyle umbellate

2.56

0.18

1.73

1.85

Juncus effuses

1.24

0.27

0.89

0.38

Phragmites communis

- Shoots

Fishponds, CSSR

1.00-

2.77

0.17-

0.48

0.55-

2.76

0.14-

0.29

0.08-0.14

- Rhizomes

Fishponds, CSSR

1.05-

1.60

0.11-

0.19

1.13-

1.45

0.02-

0.06

0.06-0.07

- Shoots

Netherland

1.11-

1.96

0.07-

0.19

0.37-

1.65

0.13-

0.29

0.06-0.15

Pontederia cordata

1.40

0.24

2.58

0.96

Potamogeton pectinatus - whole plant

CSSR

4.23

0.82

2.39

0.46

0.20

Schoenoplectus lacustris - shoot

Fishponds, CSSR

0.84-

2.44

0.24-

0.41

0.69-

2.90

0.13-

0.25

0.09-0.13

Scirpus americanus

1.22

0.18

2.83

0.50

Taxodium distichum

USA

- Whole tree

0.14

0.01

0.06

0.22

- Foliage

1.37

0.13

0.65

0.78

- Branches

0.34

0.04

0.14

0.69

- Stem bark

0.46

0.03

0.13

1.42

- Stem wood

0.09

0.01

0.04

0.10

Typha angustifolia - shoots

England

1.00-

2.2

0.10-

0.35

1.60-

3.60

0.32-

0.60

0.12-0.18

Typha latifolia - shoots

New Jersey, USA

2.21-

2.86

0.37-

0.46

1.65-

4.95

0.80-

1.80

0.14-0.40

S. Carolina, USA

0.51-

2.40

0.09-

0.31

1.60-

3.46

0.53-

0.92

0.10-0.21

Calluna

- Green shoots

Bog, UK

1.35

0.13

0.57

0.32

0.19

- Live wood

Bog, UK

0.59

0.05

0.27

0.14

0.07

Eriophorum vaginatum - leaves

Bog, UK

1.83

0.17

0.64

0.15

0.16

Rubus chamaemorus

Bog, UK

2.43

0.16

0.93

0.84

0.71

Sphagnum papillosum

Bog, UK

0.86

0.04

0.35

0.18

0.08

organic or mineral sediments; (3) co-precipitation with carbonates during photosynthesis (generally at Ca concentrations of >100 mgl~ and pH > 8.0); and (4) uptake by macrophytes (from the water column or the substrate), algae and epiphytes, and incorporation by microorganisms (Figure 2).

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