Advantages And Disadvantages Of Wetlands

3.7. Constructed wetland treatment mechanisms

The major pollutant removal means in constructed wetlands includes physico-chemical processes such as sedimentation, adsorption and precipitation at the water-sediment, root-sediment and plant-water interfaces uptake as well as biological processes such as microbial metabolic activity and plant. Wetlands have been found to be effective in treating BOD, TSS, N and P as well as for reducing metals, organic pollutants and pathogens (Reddy and Smith, 1987).

BOD Removal

In wetland BOD removal take places through settling and by entrapment in the submerged plant parts and litter in the media of surface-flow (SF) or subsurface-flow (SSF) wetlands systems. The trapped material and soluble BOD are oxidized

The inorganic forms of nitrogen present in wastewater are ammonium (NH4 +), nitrite (NO2 -) and nitrate (NO3 -). When plants uptake at low loading rate, all the inorganic form of nitrogen are converted into organic compounds that serve as building blocks for cells and tissues. High concentration of nitrogen in the domestic and industrial wastewater causes a very serious problem of eutrophication in wastewater receiving bodies. Nitrogen may be removed from wastewaters by several processes in CWs like adsorption, volatilization, plant adsorption & uptake, ammonification and nitrification-denitrification are the most important removal pathways around the root zone. The major process responsible for nitrogen removal during wastewater treatment in wetlands is nitrification, denitrification complex mediated by the microorganisms.

Nitrification is the oxidation of ammonium to nitrate by nitrifying bacteria. It is only operational under aerobic conditions; however, the environment in a wetland is not generally aerobic, except near the water surface and on submerged plant and root surfaces.

〖NH〗_4^++1.5O_2 →〖NO〗_2^- +H_2 O+2H^+ ------------Equation

Phosphorus is removed from wastewater through plant uptake, adsorption, complexation, and precipitation. Most of the phosphate is removed from wastewater through sediment retention. Uptake of Phosphorus by plants is usually highest during the beginning of the growing before maximum growth rate is attained.

Long-term phosphorus sequestration in wetlands is controlled by Soil adsorption. Adsorption refers to movement of soluble inorganic P from soil pore water to soil mineral surfaces, where it accumulates without penetrating the soil surface. Precipitation can refer to the reaction of phosphate ions with metallic cations such as Fe, Al, Ca or Mg, forming amorphous or sometimes poorly crystalline solids (Reddy and D'angelo, 1997). Available data indicate that 30 to 50% phosphorus removal is a reasonable expectation for wetland systems with residence times of less than 10 days (Filipe et al.,