The Greywater Guerrillas

WETLANDS

(This page gets into the science of wetlands)

In a wetland's complex web, water, muck, plants, aquatic creatures, and microorganisms all conspire to settle silt and consume organic matter. Aquatic plants take up nutrients and use them to grow. They absorb oxygen from the air, which they secrete out their roots. As water percolates through wetland soil, bacteria and microorganisms in the oxygenated root zone filter impurities and break down pollutants into water, carbon dioxide, and insoluble substances that no longer pollute. using a constructed wetland.
Because of wetlands' ability to remove nutrients, chemical pollutants, and heavy metals, people have begun building constructed wetland ecosystems as alternatives to leach fields and sewage treatment plants, like the Arcata Marsh. Constructed wetlands can be any size, from a bathtub in an urban backyard to acres of land treating the water from an entire city. Constructed wetlands are divided into subsurface-flow, or reedbed, systems and surface-flow, or pond, systems. The Greywater Guerrillas usually construct our urban reedbeds in bathtubs filled with gravel that we plant with cattails, bulrushes, and other wetland plants. Pond systems are usually planted with water hyacinth; duckweed often comes in on its own.

How to construct a greywater wetland -- online plans and instructions.

Treatment wetland physical characteristics and biogeochemical cycling

Constructed wetlands can be divided into two basic categories: surface flow wetlands, which mimic pond habitats, and subsurface flow wetlands, which incorporate a substrate and are structurally similar to marshes. Constructed wetlands are further characterized by flow direction (horizontal or vertical) and loading regime (pulsing versus continuous).

Surface flow constructed wetlands are generally created by flooding an upland or former wetland area and loading it with wastewater or stormwater. Over a period of months or years, native soils equilibriate to the nutrient loading regime. The soil’s water holding capacity and organic content increases as biosolids and mineral flocs settle and form complex organic molecules with high binding strengths. If nutrient loading reaches equilibrium with internal cycling and export, treatment wetlands become largely self-regulating ecosystems dependent on the new soils, hydrology, and biotic communities. (Kadlec and Knight, 1995) Subsurface flow wetland construction usually involves addition of a porous substrate. If this substrate is sourced from natural sand, gravel, or substrate, an adjustment period similar to that of surface flow wetlands will be required for optimum BOD and nitrogen removal. (Kadlec and Knight, 1995)

While the large land area needed for a horizontal surface flow wetlands has limited their application to small communities, a new generation of biological treatment systems, known as “living machines,” significant advantages in certain urban settings. Todd has designed small “package” treatment systems for schools and other large buildings. Todd’s systems are complex ecologies that feature fish, snails, and a variety of emergent and floating wetland plants housed in a compact series of interlinked cells. They are housed in greenhouses and aerated to provide a high and consistent level of treatment (Todd et al., 2003). Todd, an innovator in the field of ecological design, conceives of buildings as organisms. He is currently looking up beyond wastewater treatment facilities to integrated waste treatment and food production system that will convert a range of organic byproducts into edible mushrooms, fish, vegetables, and soil amendments (Todd et al., 2003).

In constructed wetlands, the nutrients in wastewater provide energy to bacteria, protists, fungi, aquatic macroinvertebrates, and wetland plants. Nutrient and pathogenic microbe removal occurs in the rhizosphere—the root zone, 12” to 24” deep—where plants take up nutrients and water and release oxygen. Plants and plant litter provide carbon (an energy source) and microbe habitat, and aid in filtration and sedimentation (Kadlec and Knight, 1995). Constructed wetland designers choose plants based on what pollutants occur in the wastewater they are trying to treat. Some plants remove more nitrogen, while others take up heavy metals more effectively. For general wastewater treatment, cattails, rushes, sedges, and reeds (Scirpus, Typhus, Carex, and Juncus) are used (Kadlec and Knight, 1995, Towler et a;. 2004, Stein et al, 2004). Plant species that remove heavy metals or other toxics are harvested and removed from the wetland and then may be processed to extract and recycle the heavy metals (Ottoson, 2003).

  Though wetlands have the potential to transform or immobilize N, P, and other potential pollutants, the long-term sustainability of nutrient removal depends on ecosystem structure, hydrology, and the length and concentration of nutrient loading. Natural wetlands are coupled to both upstream and downstream ecosystems and nutrient cycles, and productivity varies based on nutrient availability and climate. In constructed wetlands, productivity is further influenced by strength and composition of the influent, management practices such as periodic draining, aeration, and climate control. Wetlands take up nutrients during the growing season and release them during senescence. (Mitch and Gosselink) N and P uptake increases with plant growth, while nitrification depends on dissolved oxygen, which increases at low temperatures (Hill and Payton, 1998, Stein and Hook, 2005). Plants vary greatly in their ability to absorb nutrients at cold temperatures (Stein et al., 2006).
Treatment goals and mechanisms in constructed wetlands

Constructed wetlands have been used to treat a wide range of waters, including municipal wastewater, storm runoff, agricultural runoff, feedlot lagoons, leachate from landfills and peat mining operations, acid mine drainage, oil refinery wastewater, and effluent from chocolate factories (Kadlec and Knight, 1995, Todd et al, 2003). In most agricultural, stormwater, and municipal wetlands, which treat wastewater from a variety of sources, goals include removal of total suspended solids (TSS), biological oxygen demand (BOD), N, P, and toxins including pesticides and heavy metals.

BOD is a measure of how much oxygen aerobic decomposer organisms require to remove the organic matter and nutrients in a wastewater stream that can otherwise deplete oxygen in the water body into which the treated effluent flows. In natural systems, this oxygen is replenished by wetland plants, wind, and the fall of water over rapids. Oxygen depletion stresses and harms fish and other aquatic life and in extreme cases produces eutrophic “dead zones” in coastal areas. An effluent’s dissolved oxygen concentration significantly affects redox potential within the wetland and determines nitrogen and carbon removal and transformation pathways. In continuous horizontal subsurface flow wetlands, methanogenesis accounts for a significant percentage of carbon decomposition.

The first constructed wetlands were designed primarily to remove TSS, BOD, and pathogenic microbes. Passive, subsurface flow wetlands have proven remarkably effective at BOD removal (Green et al., 1998). BOD removal results primarily through oxidation of carbon during heterotrophic respiration. TSS is removed through settling.

Pathogens are removed through predation, adsorption onto substrates, and natural die-off. Constructed wetlands with a 3-5 day retention time can remove upwards of 99 percent of fecal indicator organisms, especially E. coli and fecal Streptococci (Kadlec and Knight, 1995, Erickson and Andersson, 2002). The long-term Swedish study (Erickson and Andersson, 2002) found less efficient removal of resistant spores of sulfate reducing Clostridia and somatic coliphage viruses in the surface flow wetland.

However, another phase of the same study examined pigs and cows enclosed in an area with the wetland as their sole drinking water source, and deer killed nearby. Feces and autopsies showed the animals to be good health. The livestock showed no health disturbances during or after pasture seasons. Deer showed normal parasite levels and no salmonella. The researchers concluded that while recreational contact with the wetlands posed little risk to humans, zoning based on hygienic criteria was a good harm reduction strategy. In response to study, primary sedimentation pond was moved to a fenced area.

More recently, constructed wetlands have been designed specifically for removal of N and P. Nitrogen and phosphorous removal can be significant in constructed wetlands, particularly soon after they are constructed. Since N and P removal mechanisms differ and nutrient removal in passive subsurface flow wetlands tends to decline over time, new constructed wetland designs have incorporated hybrid, multi stage systems. Systems might incorporate vertical flow through a phosphorus-adsorbing substrate followed by a pulled flow through a horizontal subsurface flow reed bed to maximize nitrification, which is often the limiting step in nitrogen removal