Biogeochemical Cycling

Nutrients which are not extracted from wastewater become pollutants when released unchecked into the environment. Natural processes that are promoted in constructed wetlands can remove pollutants.

Biochemical oxygen demand (BOD) is a phenomena caused by the bacterial metabolism of organic molecules, such as those found in solid human or agricultural wastes, and other substances that require large amounts of oxygen in order to be broken down. When BOD is released into a natural environment its decomposition can rob oxygen, causing zones of hypoxia or even anoxia that can kill aquatic plants, fish, and other aquatic animals. A constructed wetland can mechanically filter out most solid BOD as it passes through sand, soil, crushed rock or brick. Microbes colonize on the surfaces of the fill substrate and plant roots where oxygen is available breakdown the solid organics. The posses of BOD reduction requires an aerobic environment supplied either by vertical flow designed wetlands, or the addition of supplemental aeration.

Effluents like dairy manure, agricultural runoff, or human wastes is burdened with bio-available nutrients such as Nitrogen and Phosphorous. When nutrient-rich wastewaters are allowed to enter ponds and streams they supplement the limiting nutrients and create blooms of blue-green algae that is really a cyanobacterium spp. that utilizes the anoxygenic photo system 1 form photosynthesis depleting waters of oxygen needed by plant and animal species much like BOD. Blooms of cyanobacterium may also produce toxic alleochemicals such as microcystin and anatoxin.

Nitrogen treatment requires a two-step nitrification-denitrification process. The transformation of nitrogen is made through the metabolism of microbes as they consume carbon, phosphorus, and nitrogen. The first step, nitrification, is an aerobic process performed by chemolithotrophic bacteria as ammonia (NH₄) is oxidized to form nitrates (NO₃). harmless hydrogen gas (H₂) and ATP for the cell. The second step of denitrification is anaerobic and is performed by heterotrophic bacteria. In this step, organic carbon (C) is used as the electron donor and the oxidized nitrogen compounds from step 1 are reduced to form ATP, the harmless gases nitrogen (N₂) and carbon dioxide (CO₂). If denitrification is incomplete then harmful nitrates or poisonous nitrous oxide (NO₂) may be released. The nitrogen cycle and microbial nitrification / denitrification is covered in much grater detail.

Phosphorous is taken up by growing organisms for production of ATP and other metabolic and catabolic activities. Biological uptake of phosphorous is a limited sequestration of the nutrient as it is released back in to the system when the organism dies. Phosphorous may also chemically sorbed to the fill substrate of the wetland The capacity for phosphorus adsorption is limited by the quantity of material to which it can bond. Flooding with fresh water may change the redox potential of the wetland and can release phosphorus. Efficient removal of phosphorous prior to discharge of effluent from the constructed wetland is important due to the strongly limiting effect on primary producers that it imparts in fresh water aquatic systems, and overloading can lead to sever eutrophication of receiving waters.

Many metals are readily circulating in our environment and food, and water supplies. Indeed we, and all life need some of the metals for metabolic cofactors. Over abundance of needed trace metals (such as copper), and the occurrence of toxic heavy metals (such as lead) in effluent is extremely detrimental to the health of the ecosystem that is contaminated. The major sources of metal contamination are from industrial waste. Metals and other toxic substances found in water can accumulate in aquatic organisms, mammals, birds, reptiles and humans. Constructed wetlands often work better to remove such substances from wastewater than conventional wastewater treatment systems.