(Perma)Culture and Sanity
Suspended organic matter carried in wastewater feeds vast numbers of bacteria and other microbes in the course of decomposition, and these microbes are able to—and do—take much or all of the available dissolved oxygen present in the water. This demand for dissolved oxygen by decomposition organisms is called biological oxygen demand (BOD).
The higher the organic matter or nutrient loads in wastewater, the higher the BOD, and a high BOD creates very low oxygen conditions that can result in suffocation of fish and other aquatic organisms. Furthermore, the nutrients constituting BOD cause a bloom of algae and aquatic plants, which further lower dissolved oxygen levels during night time and early morning hours by consuming oxygen when (oxygen-producing) photosynthesis stops but (oxygen-consuming) respiration keeps going.
A primary function of any wastewater treamtment system is the removal of BOD from the water. In constructed wetlands suspended solids (typically accounting for the majority of BOD, the rest being dissolved nutrients) are largely removed by settling in septic tanks or settling ponds. This initial settling treatment stage is called 'primary treatment.'
After primary treatment or settling, remaining BOD (as dissolved nutrients) is removed from the water by mechanical filtration and nutrient absorption in a combination aerobic and anaerobic setting ('secondary treatment') of sand, plants' roots, etc. as it flows through the canals. Microbes living on the sand and roots consume very small suspended particles and dissolved nutrients. Plants then absorb basic nutrients from the microbes when these die. When these plants are removed, the nutrients absorbed from the wastewater stream are removed with them.
As blackwater moves slowly through a septic tank, suspended solids settle out of the water stream and sink to the bottom of the tank to form an anaerobic, or oxygen-starved, sludge layer (this sludge layer is slowly broken down and made water-soluble by anaerobic bacteria, releasing methane in the process).
The sludge layer needs continual contact with new, rich wastes entering the tank in order to decompose fully—if the sludge layer can't be stirred occasionally to keep it 'activated' by fresh nutrients and continually breaking down, it will accumulate and need to be periodically cleaned out of the tank and composted elsewhere. Since a primary function of a septic tank is settling solids out of the water stream, agitation is somewhat counter to its purpose, and periodic cleaning is accepted.
Settling can also be achieved by a methane digester, which returns a yield—methane—from the settling process. In a methane digester, organic solids are quickly and efficiently broken down through an anaerobic process into methane (instead of being settled out for slower and less-complete conversion to methane and sludge as in the septic tank).
Whereas methane produced in a septic tank is released to the atmosphere (where it contributes to greenhouse warming), methane produced in a digester is collected and burned for heat, for cooking or for running a motor to stir the digester's sludge or compress the methane for storage. Since the methane produced by a family and their livestock will just serve to cook their food, storage capacity does not need to be large if the methane is used for cooking as it is produced.
As solids settle out of the blackwater and begin to break down in the anaerobic septic tank, gravity and incoming wastewater push liquid effluent (relatively free of suspended solids) from the opposite end of the tank. The effluent at this stage (after primary treatment) has a heavy load of nitrogen, phosphorous and potassium as well as many very fine suspended particles of organic matter and large pathogen populations—all of which must be removed for the water to be safe for release or reuse.
Secondary treatment systems combine the features of aerobic and anaerobic marshes using various forms of 'sand filter.' Secondary treatment areas in constructed wetlands are periodically flooded to alternately create aerobic and anaerobic conditions (as opposed to permanently flooed, which would create anaerobic conditions). Secondary treatment converts organic pollutants to harmless or useful forms by filtering, chemical transformation, adsorption (in the case of phosphates in the presence of filtration materials containing iron oxides) and consumption by microbes and plants' roots.
Sand filters for secondary treatment can be mounded, boxed or in canals—the former are drier and therefore more aerobic, the latter stay moister and allow anaerobic conditions to exist or even predominate in their wetter extremes. Combinations of these components are most effective.
A sample secondary treatment system might begin by admitting effluent into the top of a mound of sand (capped with soil for hygiene and to minimize odors) for aerobic filtering. This first, aerobic or 'high marsh' portion of the filter can be maintained in an aerobic condition by mounding the sand, by allowing the sand to dry out between dosings of septic tank effluent, or both. At this first part of the secondary treatment stage both simple filtration and aerobic decomposition processes act to begin to quickly break down suspended particles and dissolved nutrients.
The aerobic, high marsh part of the secondary treatment process mechanically filters BOD from the water in the sand and—if iron-containing crushed brick or rocks are used—removes (by surface adsorption) mostly-insoluble phosphorous as well. Removal of BOD and phosphorous are both quick processes that work best in aerobic environments. Soil must dry for phosphorous to bind to iron; later flows of water will flush some of this collected phosphorous from the brick to make it available to microbes and plants' roots for removal and uptake. Oxygen-rich high marsh environments are also very effective at beginning the process of removing nitrogen from the waste stream.
After percolating through the mostly-aerobic sand mound, effluent would fall by gravity into a mostly-anaerobic 'marsh' of sand over gravel, topped by soil (the high marsh mound can be stacked directly on top of the low marsh canal to save space and simplify plumbing). Sand in the marsh filters the effluent, the soil covers the sand to prevent odors escaping and to provide a medium in which plants can grow, and the gravel allows the effluent to drain out of the system once it has travelled through the sand. The water level in the anaerobic marsh can be adjusted to come to just below the soil's surface or it can be kept lower down to provide another area of aerobic filtering, just under the sand mound but on top of the marsh.
The 'low marsh' portion of the wetland (i.e., subsurface or horizontal flow) works to anaerobically remove nitrogen from the waste stream—gravel, sand, microbes and plants' roots work together to form the filtration system. This process takes several days, beginning at the start of the low marsh and continuing through the length of the canal (also a low marsh).
A layer of crushed brick or coarse gravel separating high marsh and low marsh areas of the wetland (if these are in contact) keeps water from wicking from the low marsh into high marsh areas, keeping the latter from becoming waterlogged and anaerobic and also preventing surface evaporation of water that could concentrate salts at the soil's surface.
The largely-cleaned effluent from the initial low marsh is then led into a canal (lined with clay or plastic to prevent nutrients from leaching into groundwater) filled with gravel and topped with a layer of soil and aquatic or other plants (depending on drainage). Functionally this canal also qualifies as low marsh. The canal bottom should be well below frost line (so that the wetland will continue to function in winter), about 60 feet long (to provide enough treatment time as water moves through the system) and wide enough to accomodate expected flows (3 to 4 feet wide is probably adequate for most home systems).
This third or 'tertiary' stage of treatment gives wastewater additional time for microbes and plants' roots to further remove remaining nutrients carried in the water. During this stage of treatment plants covering the wetland will be seen to shrink in size and health as the canal progresses and nutrients remaining become insufficient to fertilize plantings adequately. Microbes in this final section of the canal operate under 'starvation' conditions and are capable of removing dissolved nutrients and even many chemical pollutants to extremely low concentrations.
Water emerging from a properly-constructed wetland will be cleaner than World Health Organization standards for drinking. Nonetheless this water can be further cleaned or 'polished' with exposure to UV light in a shallow pond (i.e., 1½ to 3 feet deep) on the edges of which are planted 'emergent' aquatic plants (such as cattail and reeds). Aquatic plants are very efficient at removing both organic and inorganic pollutants (most of which will already have been removed earlier in the process).
A variety of plants are useful in or around polishing ponds, or planted on filter marshes and canals. Plants beneficial in constructed wetlands include aquatic plants, riparian edge plants and upland plants, each of which will serve different functions in the system. For best operation, plants must remain actively growing throughout the year—use native or adapted plants that are evergreen, or use greenhouse growing beds for final polishing of water with actively-growing plants (lacking actively-growing plants to absorb nitrogen, this valuable nutrient is lost to the air or released to waterways).
Aquatic plants most useful for polishing water include the classes of 'submerged' and 'emergent' plants (emergent plants grow in shallow water but their tops emerge from the water to grow in the air—e.g., cattail, reeds, rushes). The significance of emergent plants is that though their roots remove nutrients directly from the water, their tops are in the air and easy to remove—therefore, they have effectively removed the nutrients from the water itself.
Floating or submerged aquatic plants achieve the same result only if they are physically removed from the water or eaten by herbivorous aquatic animals such as grass carp, tilapia, nutria, etc. Submerged plants are useful for yield-producing water polishing in that many are preferred foods for herbivorous and omnivorous fish, or poultry. Emergent plants are capable of removing large amounts of organic nutrients from water, and are also able to remove metals and many biotoxins (they can actually metabolize many common biotoxins, rendering them harmlessly into their constituent elements).
Duckweed, pennywort, cattail, bulrush and water hyacinth are all particularly well-suited to taking up and storing nutrients, and can be harvested and used as natural fertilizers or animal feed (fish, poultry, pigs) if metals are not present. Aquatic systems are extremely productive—at warm temperatures, duckweed can more-than-double its biomass in two days, pennywort in three (Gotu Kola, Hydrocotyl asciatica, is an economically-important pennywort). Pennywort is tolerant of cool temperatures (to USDA Zone 6), but all these plants would need some protection to remain vegetative during winters in colder areas.
An added benefit of floating aquatic plants such as duckweed (Lemna spp.), Azolla spp. and pennywort (Hydrocotyl spp.) in arid areas is that their leaves, in covering the water's surface, significantly lower evaporation.
Plants which naturally grow along edges of creeks, rivers or ponds are useful in constructed wetlands to produce yields from edges of the treatment process. These plants include blueberries or cranberries (Vaccinium spp., 'bog' or wet-mud plants), wax myrtle (Myrica cerifera, which produces berries edible to birds and is used to make aromatic candles or for seasoning foods), wild rice, bamboos, figs (which evolved in desert oases where water tables reach the surface or very near) and numerous other useful or edible plants.
Given enough soil on top of canals or soil filters for adequate drainage, any conventional plants can be grown in these systems and benefit from access to deeper soil moisture within the wetlands. These plants can include fruit trees, ornamental plants or even vegetables (and no—infectious microbes and 'poop' molecules do not travel through plants' vascular systems to infect unsuspecting humans).
Conventional, riparian and aquatic plants can all combine not only to create pure, sparkling water from waste streams, but also to convert wastes into creative, useful or nourishing products instead of pollutants and toxins.
Greenhouses have several useful features for constructed wetland operation. One is the warmer environment which not only allows chemical processes to take place more readily during cold weather, but also keeps plants in green and growing condition year-round (they would otherwise not be removing nutrients from the water during dormant periods, and will even release nutrients back into the stream if winter-killed).
By running water through greenhouse planter boxes for final cleaning, or even by enclosing the canal or polishing portion(s) of the wetland in the greenhouse, polishing can take place year-round and provide year-round productivity from the waste stream even in cold areas. Growing boxes should be shallow and wide, to maximize exposure of the contents to oxygen (but since this also maximizes evaporation which concentrates salts, growing media should be flushed with rainwater on a regular basis). Growing media can be fine porous gravels, sand, light soil, pumice, rock wool, etc.
Aquatic systems are the Earth's most productive systems ("4 to 20 times the productivity of adjoining land," Bill Mollison), and wastewater streams are particularly nutrient-rich aquatic systems. In Asia, in fact, fish have long been grown with the aid of fresh, uncomposted human and animal manures (not a recommended practice as fresh feces are a disease vector for such diseases as Cholera, still a problem in underdeveloped countries) and there are in fact safe, simple ways to make wastewater streams produce yields.
Effluent which has passed through initial anaerobic digestion in septic tank, methane digester or anaerobic (i.e., deep) ponds can be used to fertilize algae or submerged vegetation production systems for feeding fish or poultry—Israel, for instance, uses its septic waters to grow algae for poultry feed. Duckweed-Azolla systems to feed ducks, chickens and/or tilapia are becoming common in Asia because of their high productivity, with very low inputs beyond wastewater (duckweed is a very-nutritious floating plant which in warm polluted waters can grow an additional 50% of its total biomass every day... Azolla is a nitrogen-fixing floating plant).
Nitrogen-fixing Azolla is being integrated into rice production in Asia (fertilized with wastewater), as are ducks, and both additions increase rice yields. Such ponds—in fact fish ponds in general—are profitably fertilized with phosphorous and calcium in the form of rock phosphate (calcium phosphate) and lime (calcium carbonate), and pH adjusted to around 7.5 - 8.5. Wastewater can replace a large part of these inputs.
Septic tank effluent can be introduced directly into natural ponds as fertilizer (i.e., ponds vegetated with submerged, emergent and edge plants) where shallow and deep sections are segregated (as by narrow barriers of emergent vegetation such as cattail). Effluent introduced into shallow pond areas feeds phytoplanktons (algae, bacteria) which in turn feed zooplanktons (water fleas, etc) and insect larvae, these forming the basis of a natural pond food web. Zooplanktons and insect larvae feed minnows, which then feed bluegills and trout, etc.
How natural and constructed wetland systems work to clean polluted or waste water; pros and cons of constructed wetlands as compared with conventional systems.
Issues surrounding how to clean polluted water, and how to keep it clean in the first place!
Removing metals and other persistent pollutants from wastewater streams using constructed wetlands.
Reprinted from (Perma) Culture and Sanity Website