Living Machines - Hydroponics and Aquaculture

Hydroponics and Aquaculture

Some ecological wastewater treatment systems, including first-generation Living Machine systems, employed hydroponics and even aquaculture. However, these processes are not part of today's Tidal Flow Wetland Living Machine systems.

• The first step of the process is an anaerobic settling tank. This closed anaerobic tank serves as a pre-treatment to allow solids to fall out of suspension and precipitate to the bottom of the reactor to reduce the turbidity of the water. A variety of anaerobic bacteria are present in this tank; they generate acids and ferment methane. This step may be unnecessary if the influent has low levels of solids.

• Next, the sewage flows through a biofilter of bark and humic materials. This gives the influent its first filtration and reduces the odors prevalent in anaerobic conditions.

• The mixture then moves into a series of aerobic tanks. The first tank is a dark, closed-top aerobic reactor that serves as a transitional step. The next tank is an open-top, aerobic reactor that contains photosynthetic algae that fix oxygen back into the formerly anoxic, turbid water. This provides oxygen and organic food (dead algae) for biological metabolism and respiration. Microbial communities proliferate, and eventually must consume all of the photosynthetic algae so that the algae do not choke out macrophytes in later steps.

• Many types of bacteria immobilize pollutant minerals, but certain species of bacteria are crucial to nutrient conversion. Specifically, Nitrosomonas and Nitrobacter work in steps to nitrify ammonia, making it into nitrates, which are available for plant and microbial uptake. These bacteria need calcium carbonate to catalyze this reaction, so managers must maintain sufficient calcium levels in the water. Denitrifying bacteria such as Pseudomonas fluorescens convert nitrates into gaseous nitrogen, which is volatilized in these open aerobic tanks. Denitrification is the most desirable sink for nitrogen in living machines. Protozoa have been shown to be capable of coliform and pathogen suppression. Microbial breakdown is the primary biological treatment of both the conventional activated sludge process as well as these aquatic ecosystem sludge reactors.

• Higher plants are grown hydroponically in the aerobic tanks and provide multiple services. The most common plant used is water hyacinth (Eicchornia crassipies), which has filamentous aquatic roots with a high specific area. These feather-like roots provide a stable habitat for microbes, and over time a bacterial biofilm builds up around the roots. Water hyacinth, bulrush and other macrophytes sequester heavy metals. The bodies of these plants can be harvested and burned, and the heavy metals can be chemically isolated to take them out of the environment. Brassica juncea growing in waste streams has been found to contain up to 10% of its dry weight in lead.

• Plankton carries out multiple functions in the system with varying efficacy. Zooplankton feed on extremely small (<25 µm) particles. In juvenile stages they feed on particles smaller than 1 µm. Conventional waste treatment cannot process these fine suspended solids. Although zooplankton do consume these fine particles, which are difficult for conventional treatment systems to process, the placement of plankton in the system is more valuable as a trophic link. Plankton can eat microbes, which are abundant in the system, and the plankton is an ideal food for filter feeding fish and mollusks. This food chain transfers biomass to higher trophic levels and increases the diversity and complexity of the ecosystem. John Todd thinks that “Since zooplankton can exchange the volume of a natural body of water several times per day it is difficult to overstate their importance in ecological engineering.”

• According to Björn Guterstam, another one of the most well-published and experienced ecological engineers, this theoretical role has not been as successful in practice. He concedes that phytoplankton populations have been limited by toxic and somewhat deoxidized water at the bottom of tanks, as well as light limitations. Phytoplankton are primary producers, which provide food for larger zooplankton species, so the zooplankton population drops with its photosynthetic counterpart. Because these principles have been implemented only on a small scale, these systems have a lowered buffering capacity due to issues of scale and separation from the macroecosystem, even though genetic and functional diversity is encouraged.

• Aquaculture can take place in more dilute tanks downstream after the eutrophication-causing contaminants have been ameliorated. Snails slide along the tank walls and graze on slime and sludge buildup, cleaning the tank. This self-regulation improves light penetration, which stimulates photosynthetic forms of algae, bacteria and plankton. Filter feeders sift through large volumes of water each day and consume the bacteria and plankton that are small enough to pass through. Mollusks such as mussels and snails, as well as some fish, are filter feeders. Detritus-feeding fish consume larger particles of suspended biosolids. Herbivorous fish are excluded from tanks where macrophytes carry out useful functions (such as biofilm hosting), but when plants are eventually harvested from the system, this plant tissue can be fed to a tank of herbivorous fish for aquaculture production.

• A single Anodonta freshwater clam can filter as much as 40 litres/day of water, absorbing colloidal materials and other suspended solids at a removal rate of 99.5%. Many freshwater clams are in danger of extinction, in part because some have gills that perform poorly in polluted environments. Since some of these clams can sequester colloids from streams or lakes, this provides an ecosystem service by slowing the erosion of soil colloids. Humans can strike up a symbiotic relationship with the clam genera Unlo and Anodonta by providing a clean habitat (when the water reaches the clam tank it is cleaner than some of their wild habitats). In exchange for a good home, the clams could aid humans by filtering colloids and suspended solids out of our wastewater. It is yet to be determined if the clams break up these colloids at all or if it is feasible to recycle clam compost back into field (which increases cation exchange capacity—-an agricultural benefit). Ecological engineering supports symbiotic relations between different species to serve the needs of humans as well as promoting the health of the ecosystem.