constructed wetlands- living machines
LIVING MACHINES
The Living Machine is a series of wastewater treatment cells initially designed by John Todd Ecological Design, Inc. Living machines are a series of various cells that treat wastewater in several stages.
The system is generally defined by the following biologic treatment phases:
1. Anaerobic settling tank (underground). Suspended solids precipitate to the bottom of the tank reducing turbidity. Anaerobic bacteria begin treatment process.
2. Series of aerobic tanks. Series of closed or open tanks house various organisms- aerobic bacteria, fungi, plants, snails, clams, or fish- to break down effluent.
a. Photosynthetic algae begin process by fixing oxygen in anoxic water thus providing oxygen and food (dead algae) for subsequent bacteria.
b. Bacterial communities continue to immobilize contaminants and convert nutrients (nitrogen is key). Nitrosomonas (for example) converts ammonia to nitrites, which other bacteria and plants can then process. Denitrifying bacteria convert these nitrites into nitrates which is volatized in open tank.
c. Higher plant material provides filamentous root system for microbes to live in. Water hyacinth, bulrush and other macrophytes sequester heavy metals.
d. Zooplankton consume suspended solids too small for conventional treatment and act as key trophic link since they are food source for filter-feeding fish and mollusks.
e. Aquaculture can occur after eutrophication-causing contaminants have been removed. Detritus feeding fish consume larger particles of the suspended biosolids. Herbivorous fish can be placed in separate cell to avoid consuming useful macrophytes.
f. Mollusks process large amounts of water and consume more suspended biosolids. For example, one Anodonta freshwater clam can filter as much as 40 litres/day of water and remove 99.5% of the suspended solids.
That was perhaps a bit more detailed than necessary for our design purposes, but I think it is interesting (and hopefully useful) to understand the various biologic elements and what functions they provide.
This system combines two wetland technologies- horizontal and tidal flow wetlands. Horizontal subsurface flow wetlands are a simple, long-established system where water flows horizontally across an aggregate-filled cell. Tidal flow wetlands utilize a fill-and-drain technology with special media to increase treatment efficacy and final effluent quality. The hybrid system combines the two systems with a hydraulic control/pump structure to enhance BOD (biological oxygen demand→ chemical procedure that determines how fast biological organisms use up O2 in water), TSS (Total Suspended Solids → organic & inorganic particulate matter in water), and nitrogen removal.
Benefits of this hybrid system are:
- 50% smaller footprint than traditional subsurface flow wetlands
- no clarifier cell and thus no residual sludge (waste bacterial material) to remove
- energy cost is 20% of equivalent activated sludge system
Leave a Reply