Conventional Sewage Treatment, Living Machines and Constructed Wetlands.
Andrew M. Gascho
November 29, 1999
Thesis: Conventional waste treatment plants, Living Machines and constructed wetlands can all be used for water purification, but only living machines and constructed wetlands will provide the human race with a sustainable future.
Outline:
A. Conventional
Sewage Treatment
B.
Living Machines
C. Constructed
Wetlands
VI Bibliography
Water is the most important resource on the planet. Absolutely no life could
exist with out the presence of water. Humans use water to cook, clean, bathe
and drink. However it seems that humans as a race have extremely little respect
for this resource. Humans continually pollute and damage the health of our aquatic
ecosystems using irresponsible agricultural practices and improper disposal
of our wastes. These issues must be examined because the way humans interact
with the earth have strong moral and ethical implications. As the scarcity of
potable water continues to increase the value of water will continue to rise.
In many places throughout the world clean water sources cause mass immigration
and emigration of people so adequate water resources can be accessed (Homer
73). This causes large-scale political and ethnic upheaval. In the future water
shortages have the potential for invoking war between ethnic groups as well
as nations. It is the best interest of the human race to preserve and restore
the health of our aquatic ecosystems since they have such large implications
for human societies and cultures. One of the major sources of water pollution
throughout the world is human sewage. Understanding how to properly deal with
this waste source will help us come closer to the goal of sustainable water
resources. In this paper I will examine the processes currently used to treat
municipal wastewater as well as alternatives which can also be use to extract
wastes from our water. Conventional waste treatment plants, Living Machines
and constructed wetlands can all be used for water purification, but only living
machines and constructed wetlands will provide us with a sustainable future.
Design and Funciton of Treatment Systems
Conventional Sewage Treatment.
Conventional Sewage Treatment plants are the most common system for wastewater treatment in the United States. These systems are found in all urban communities but only in a few rural communities. This complex system is responsible for processing of the waste of the majority of the people in the United States yet few of them understand the scientific principles involved with treatment. In addition many people are even unaware of where their wastewater goes after it has been flushed down the toilet or disappear through the bath tub drain. I will attempt to describe the biological, chemical and physical process involved with the treatment of wastewater through the use of conventional sewage treatment.
The first step involved with wastewater treatment is the transport of the wastes from houses and industry to the treatment plant. This is done through a complex matrix of pipes and lift stations. All sewage goes to the treatment plant using the laws of gravity. Since the flow of gravity requires the sewage to head continually downward lift stations are necessary. The lift stations are used because they physically raise the sewage several feet to keep the sewage pipes from descending too far into the ground. Important biological processes take place in the sewage pipes as the water travels underground. After the wastewater enters the sewage pipe the environment becomes anaerobic and the microorganisms found in the bio film on the pipes causes organic wastes to experience a preliminary breakdown process.
The preliminary process continues after the sewage arrives at the treatment plant. This part of the process utilizes a bar screen, which is a metal grid the water passes through before entering the rest of the plant. This grid system physically removes grit, sand, floaties and scum. By passing through the screen the water experiences turbulence, causing the noxious gas, hydrogen sulfide, to be released (Umble 5). This gas is a by-product of anaerobic respiration that takes place in the sewage pipes. Next, sewage water experiences chemical separation, causing the process to split into two different strands, a liquid and a solid process. Polymers are added to the water which bind to the organic solids, causing them to fall out of the water.
The next part of the process that the water enters is called primary treatment. This process continues to try to physically separate out the remaining organic particles that are found in the water. The separation is done by passing the water through a series of tanks that allows the water to slow down. As the water begins to slow down it enables the contaminated particles to fall out of solution and settle on the bottom of the tank. Also the slowing down of the system allows grease, oils and other lipid particles, which are hydrophobic, to rise to the surface. This separation makes it convenient for the waste particles to be manually removed, by scrapping off the top and bottom of the water surfaces (Kerezman).
Now that most of the physical particles have been removed the next part of the process confronts the issue of extracting the particles and nutrients that have been dissolved into the wastewater. This process is biological and is called secondary treatment. The water flows into large tanks containing bacterial cultures that work to breakdown and absorb the nutrient load. The chemicals that the bacteria are especially good at breaking down are nitrogen containing ammonia compounds. Although the bacteria are good at breaking down nitrogen they are relatively inefficient at breaking down compounds containing phosphorous (Kerezman). The removal of phosphorous from the system must be done chemically by adding ferrous dioxide. The addition of oxygen is also extremely important for success of the biological removal of wastes. The microorganisms are presented with unlimited amounts of food for consumption; causing oxygen to become the limiting reagent for achieving maximum waste removal. This portion of sewage treatment is the most expensive part of the process in the treatment plant. The addition of oxygen requires massive blowers, which require an equally massive amount of energy to run them. An equation has been formulated to represent the inputs and outputs of this part of secondary treatment:
Organic matter + O2 à CO2 + H2O + New Cells + Byproducts
This equation includes the reproduction of the bacteria cells as part of the outputs of this system (Umble 9).
Secondary treatment continues as the water moves out of the tanks charged with bacteria and into clarifying tanks. Here the ferrous dioxide settles out of the water as it moves from the center of circular tanks to its outer rim. The tank has an arm that rotates around the circumference of the tank collecting the grease that remains in the water. As the water leaves the clarifying tank it receives a treatment of chlorine.
The addition of the chlorine begins the tertiary treatment process. Tertiary treatment uses chemical and physical properties to remove the final wastes from the water (Umble 10). One hazard left in the water after the first two treatment processes are pathogenic microorganisms. These microorganisms include fecal coliforms, Escherichia coli., and Aremonas. Chlorine is added at the end of the secondary treatment and as the water travels through the pipes to the tertiary contact tank the chlorine mixes with the water. Once the water enters the contact tank it slows down so all the microorganisms are killed by coming in contact with the chlorine. The chlorine then needs to be removed before the water is released from the treatment plant. Often a bisulfite compound is added to the water, such as sodium bisulfite. The final stage of this process is oxygenating the water before being released into a river or other water source, since the water has been severely depleted of its oxygen from the microbial activity.
To complete the cycle of conventional waste treatment we must return to the beginning to earlier stages of the process where the sewage sludge was removed. The sludge is placed into a biosolid digester where the thick organic soup is broken down by microorganisms. This process requires heat because the microorganisms that work most efficiently are thermophilic bacteria. A byproduct of the process of decomposition is the production of methane gas. This gas can be collected and used as a fuel agent. Often it can be recycled to heat the biosolid reactor. The sludge is required by law to stay in the reactor for at least twenty days so adequate digestion can take place (Kerezman). The final product is a rich organic substance which is either taken to a sanitary landfill or given to farmers for fertilizer for their fields.
Living Machines are an alternative form of sewage treatment. This system of waste treatment was developed by John Todd of Living Technologies. A living machine models a machine because it has many parts that are interconnected and function together to do work. Also, the system is called living because it models natural ecosystems and is made up of living organisms (Todd 167). The basic design of the living machine is to use the principles of a food chain to process nutrient rich municipal waste. Similar to natural ecosystems whose only energy input is solar radiation, living machines main source of energy comes from the sun. High light sources are in fact directly responsible for the success of the process (Todd 168). One of the main sources of input into the system is oxygen, which works to support a large community of microorganisms. Bacteria and algae are mostly responsible for the breakdown of the aquatic effluent. Aquatic macrophytes also play a role in the reduction of waste by absorbing nutrients and are also responsible for slowing down the particles that are suspended in the water, causing them to drop out. Organisms present in living machines, such as zooplankton, snails, and fish, are responsible for controlling the populations of microorganisms, (Lerner 49). These primary consumers are responsible for helping maintain stable ecosystems. Balance is critical in ecosystems so the proper assortment of organisms is an essential to the design of the system. Organisms may need to be added or removed to find the proper balance to achieve the highest rate of waste removal.
John Todd presents nine principles that are used when designing a living machine. These principles are also helpful for understanding the process that is essential for the success of a living machine (Todd 170).
The wastewater, after being retained for several days in the bioreactor, move into the living machine. The living machine is enclosed inside a greenhouse to maximize the potential solar radiation for the system. Contained inside the greenhouse is a series of tanks (30-50) which the water flows consecutively from tank to tank. This system is also classified using the terms primary, secondary, and tertiary treatment. The water enters the primary treatment, which consists of tanks that contain primitive organisms such as cyanobacteria and water hyacinths, that can process the sewage in its harshest form (Lerner 50). The water then passes through a marsh to continue the process of solid wastes.
The water has now had enough solids physically and biologically removed
to begin the secondary stages of treatment. As the water progresses through
the system the toxicity of the water decreases, thus allowing for an increase
complexity of the organisms used for extracting the remaining wastes (Lerner
50). The water is then passed through tanks that contain rock biofilters.
These tanks contain porous rocks which provide increased surface area for
the bacteria to live and substrate for plants to anchor roots. The water
leaving these tanks is of tertiary quality and in some advanced treatment
systems this final water is filtered through constructed wetlands to ensure
the purest possible water before being released into a receiving stream.
Wetlands are an extremely essential part of the natural ecosystems on our planet. The functional values of wetlands are life support, hydrological modification, water purification and erosion protection (Hammer 69). The function of wetlands that individuals who are interested in sewage treatment are most concerned about is its ability to purify water. Wetland construction is done to model the abilities of natural wetlands to remove the impurities, toxins and pathogens from wastewater. These systems work to replicate and intensify natures process of organic recycling. Although wetlands are quintessential purifiers, wastes should not be placed in natural wetlands because they are not able to process unnaturally large amounts of extra organic wastes. However wetlands can be constructed with a collage of plants that are hardy and tolerant of nutrient overload. Three types of wetland systems have been developed to process waste: aquaculture, wetland, and root zone treatment (Lyle 235). All of these systems have two common characteristics, bacteria are the most important at nutrients breakdown and that plant roots are critical for slowing the water down and absorbing nutrients (Hammer 70).
Aquaculture sewage treatment is a process in which wastewater flows through a series of ponds. The ponds contain floating plants that work to remove the waste (Lyle 235). The common plants in this system are the water hyacinth and duckweed. Most of the particles settle out into the sediments on the bottom of the pond where bacteria break up the waste compounds. The plants absorb the nutrients found in the water. This process is quite slow because the water needs to be contained in the ponds for an extended period of time to insure that an acceptable amount of waste has been removed.
Wetland treatment uses endemic species to remove the nutrients from wastewater. Before the wastewater enters the wetland it passes through a bar screen to remove the larger suspended particles. The water often spends a long period of time in oxidation ponds to further the break down of nutrients (Lyle243). In the final part of the process the water is sent into the wetland for the final purification process. The most common plants in wetlands are the cattails, reeds, bulrushes, rushes and sedges. One of the key functions of these plants is the process of transporting oxygen down to the roots and it is these locations that the microbial communities thrive because of a constant source of oxygen from the plant. Constructed wetlands are typically created only two feet deep to maximize the amount of plants that are able to grow, thus increased oxygen of the microbial community.
Root zone systems are made up of plants in a porous material such as gravel. The plants are placed directly in the gravel medium and soon develop a matrix of intertwined roots that work to slow the flow of water, absorb nutrients and provide a microhabitat for microorganisms. These systems work best at a smaller size, handling a small input of wastewater (Lyle 245).
The analysis of conventional sewage treatment plants displays the pros and cons of this system. The treatment of the sewage in a relatively small and confined space allows for appropriate monitoring of the effluent water. This causes the output of pollutants into the receiving source to be regulated and mandated. Typically treatment plants do an excellent job of removing many of the wastes from the water, such as phosphorous and nitrogen. However the process of removing the nutrients is chemically intensive. This traditional system is extremely dependent on large amounts of fossil fuels for the success of the system.
Living machines also have the advantage of being easy to monitor. All of the tanks can be regularly tested to check the breakdown of the wastes as they progress through the system. Living machines have been proven to be efficient at decreasing the Biological Oxygen Demand and the Chemical Oxygen Demand (Lerner 55). Some problems the living machine has is decreasing the amount of phosphorous and nitrogen in the water. Plants and algae are only able to consume a certain maximum amount of these chemicals and turn them into biomass. All the remaining phosphorous and nitrogen passes through the system without being absorbed. This remaining waste is not an outrageous amount above the legal releasable quantity, but it must be considered. The systems can work to remove heavy metals from the water. Willow and eucalyptus trees have been proven to extract cadmium, copper and lead. One of the greatest benefits of a living machine is that its main source of energy comes from the sun and is not entirely dependent on fossil fuels. The byproducts of this system which the organisms harvested from the tanks, are fuel, fish and fruits and vegetables can be used to the benefit to human societies.
Several issues have thwarted the foreword movement of constructed wetlands being used for sewage treatment. One issue is that wetlands require large amounts of space and that natural processes can take a longer period of time to adequately treat waste water. Because the waste demands a large area it makes it almost impossible to monitor the progress and treatment of the sewage. Just as the living machine, constructed wetlands have a difficult time removing the standard amount of phosphorous and nitrogen from the water because of the limitations imposed by plant metabolism. The wetlands have the capability to remove metals because of plant absorption and decomposition by microbes in the sediments. This system however requires extremely low energy input. All the power is solar. By implementing constructed wetlands we are replacing wetlands that have been drained, diked and dredged. Wetlands also provide habitat for a diverse community of wildlife.
Is it important how we decide to treat our sewage? This issue is of utmost importance, as I indicated in the introduction, because water is our most important resource and if we do not learn to use it sustainably the future generations will have to live in a world of polluted and contaminated water. The way in which we relate to our aquatic resources is more than just a societal issue, it is a spiritual issue. God is the immanent source of life that sustains the whole planetary community (Chapple 164). The way in which we relate to the natural world is more than just a senseless act, it is a way in which we relate to God. Since God is directly responsible and concerned for the well being of the earth our every interaction with nature is a reflection of our relationship and the respect we have for God.
Through the use of conventional sewage treatment plants we continue to add chemicals to the environment which add to the contamination of our water and air. Also, because of the systems dependence on fossil fuels it should not be viewed as a sustainable system. We must seriously consider alternatives for waste water treatment, especially systems modeled after natural ecosystems. Living machines and constructed wetlands are made in the image of ecological processes and because of this they do not require intense chemical treatment and fossil fuel use. These organic systems will be able to provide a sustainable way to interact with our precious water resources. It is our responsibility to choose a system for treating water that will give clean water to future generations and provides a way to maintain a healthy relationship with God and the natural world.
"It is our hope that by studying human waste recycling in a beautiful, ecologically diverse and dynamic Living Machine, (humans) will begin to comprehend the meaning of natural systems in their lives. Equally important, it may allow them to engage with the natural world in sustaining the communities of tomorrow." Dr. John Todd,
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