Tertiary treatment of [[Glossary_of_sustainability_terms#E|effluent]] provides a final stage to raise the quality of wastewater before it is released into the environment. A treatment plant may opt to use more than one tertiary treatment process, such as combining nutrient removal with mechanical filtration. If disinfection is practiced, it is always the final process. Tertiary treatment is also called advanced treatment, "effluent polishing", or in the case of the Arcata Marsh "enhancement."
Tertiary Treatment. This is a final treatment stage done to purify the effluent, and it is rarely done. It can be a chemical treatment, where reactants are added to the effluent in order to precipitate out certain substances, or radiation treatment.
==Types of Treatment==
;Filtration
:After [[Secondary wastewater treatment|secondary treatment]] there are generally still some suspended solids left in the effluent. The main reason for this is that the secondary process is not perfect at removing microorganisms left over from the biological treatment processes. This "biological floc" can be filtered out mechanically by using a multimedia filter that has progressively smaller and smaller filter grain sizes. This reduces the amount of back washing necessary to clean the filter.
In effect, tertiary treatment “polishes” the effluent. Tertiary-treated effluent is water clean enough to swim in, irrigate with, or discharge into lakes or streams, often exceeding standards for municipal drinking water. Wetlands are perfectly capable of providing all three stages of treatment, but most constructed wetlands in the US are specifically for tertiary treatment only.
;Nanofiltration
:Nanofiltration is a relatively recent membrane process used most often with low total dissolved solids (TDS) waters such as surface water and fresh groundwater, with the purpose of softening (polyvalent cation removal) and removal of disinfection by-product precursors such as natural organic matter and synthetic organic matter<ref>http://en.wikipedia.org/wiki/Nanofiltration (accessed 5/7/2008)
</ref>
The Arcata Marsh uses microscopic screening and biological breakdown and absorption. After the effluent has passed through the primary clarifier and through the secondary treatment Marshes, it is chlorinated on its way out to the tertiary treatment marshes, also called enhancement marshes, where wastes are further broken down, and then the water is de-chlorinated before it is released into Humboldt Bay. In this type of tertiary treatment, the effluent is treated further just as it was in the secondary treatment. There are bacteria in the water eating whatever left over waste there still is and converting it into nutrients that the plants that live on the edges of the ponds can use to grow. Some bacteria need oxygen to digest the particles that they eat and are called aerobic decomposers, while bacteria that don’t use oxygen to digest their food are called anaerobic decomposers. The aerobic decomposers live throughout most of the water in the ponds where there is plenty of oxygen dissolved in the water for them to breathe, and the anaerobic decomposers live in the water and the sediment at the bottom of the ponds where there is not as much oxygen available. The plants that live on the edges of the pond also play an important role. Their roots help filter out some of the particles from the effluent, but they also provide shelter for a variety of small water creatures to live and create a whole ecosystem. The plants also absorb the nutrients that are released into the water by all of the bacteria as they digest their food.
;Ultrafiltration
:(UF) is a variety of membrane filtration in which hydrostatic pressure forces a liquid against a semipermeable membrane (Figure 1).[[Image:Ultrafilt.jpg| thumb| 1000px| right| Figure 1. Ultrafiltration Membrane (Awaiting Permission)]] Suspended solids and solutes of high molecular weight are retained, while water and low molecular weight solutes pass through the membrane<ref>http://en.wikipedia.org/wiki/Ultrafiltration (accessed 5/7/2008)</ref>
In April of 1975 California’s State Resources Control Board (SWRCB) incorporated the Bays and Estuaries Policy which states: “It is the policy of the State Board that the discharge of municipal waters (exclusive to cooling water discharges) to enclosed bays and estuaries…shall be phased out at the earliest practicable date. Exceptions to this provision may be granted by a Regional Board only when the Regional Board finds that the wastewater in question would consistently be treated and discharged in such a manner that it would enhance the quality of receiving waters above that which would occur in the absence of the discharge”. In short it meant that no treated wastewater can be discharged into any bay unless it could be proven that it had been treated above and beyond the quality that it would have been had it never been waste water in the first place, the treatment facility is responsible for the enhancement of the water that is to flow out into the bay.
;Nutrient removal
:Wastewater may contain high levels of [[nutrients]] such as nitrogen and phosphorus. The excessive release of these nutrients to the environment can cause the overgrowth of weeds, algae, and cyanobacteria (blue-green algae). A rapid growth in the population of algae can result in an algal bloom of a magnitude such that the numbers are unsustainable. Eventually most of the algea die. The decomposition of the dead algae by bacteria depletes the dissolved oxygen in the water. As a result most or all of the animals living in the water die. This creates more organic matter for the bacteria to decompose, continuing the cycle. In addition to causing deoxygenation, some algal species produce toxins that contaminate drinking water supplies. This scenario is typical of the Nitrogen cycle. Nitrogen is typically excreted or released by aquatic organisms in the form of amonia. This in turn is converted to ammonium by microorganisms which in turn is a nutrient that plants use for growth. The plants are then in turn eaten by creatures, thus completing the cycle (Figure 3).
Enhancement as defined by the SWRCB, as it applies to Humboldt Bayinterprets the enhancement provision as: 1) full secondary treatment, with disinfection and dechlorination, of sewage discharges; 2) compliance with any additional NPDES permit requirements issued to protect beneficial uses; and 3) the fuller realization of existing beneficial uses or the creation of new beneficial uses either by or in cojunctin with a wastewater treatment project..
Different treatment processes are required to remove nitrogen and phosphorus. Phosphorous is generally removed using chemicals such as alum, or ferric chloride which precipitate to form a salt and hydronium or hydroxide ions (Figure 2). [[Image:Phosphorous removal.gif| thumb| 1000px| left| Figure 2. Phosphorus Removal (Awaiting Permission)]] These ions can cause the waters pH to fluctuate. Generally lime is used to stabilize the pH of the effluent. These reactions require a settling basin to remove the precipitants.
Existing beneficial uses include: 1) scenic enjoyment 2) Fish and wildlife habitat 3) Water oriented recreation ( in or out of water) 4) commercial fishing 5) Shell fish harvesting (oysters) 6) Navigation 7) Industrial water supply 8) Educational study.
Removing nitrogen can be accomplished using either chemical or biological processes. Nitrogen is generally present in the form of ammonia. The chemical process simply raises the pH of the effluent. [[Image: Nitrogen cycle.gif| thumb| 1000px| right| Figure 3. Nitrogen Cycle (Awaiting Permission)]] This converts the ammonia to ammonium which can be removed from the water simply by passing large quantities of oxygen through it. The biological process uses activated sludge for an extended period of time, typically around 15 days.<ref>Davis, Mackenzie L. and Susan J. Masten. Principles of Environmental Engineering and Science. New York. McGraw Hill. 2004.</ref>
==Disinfection==
The purpose of disinfection in the treatment of wastewater is to reduce the number of pathogenic microorganisms in the water to be discharged back into the environment. The effectiveness of disinfection depends on the quality of the water being treated (e.g., cloudiness, pH, etc.), the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables. [[Turbidity]] will be treated less successfully since solid matter can shield organisms, especially from ultraviolet light or if contact times are short. Generally, short contact times, low doses and high flows all mitigate against effective disinfection. Common methods of disinfection include ozone, chlorine, or ultraviolet light.
;[[Arcata_Marsh_Chlorination|Chlorination]]
Chlorination remains the most common form of wastewater disinfection in North America due to its low cost and long-term history of effectiveness. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful to the environment. Residual chlorine or chloramines may also be capable of chlorinating organic material in the natural aquatic environment. The presence of chlorine gas in the atmosphere can pose immediate and serious hazards to the health of any person breathing the air. Large schale declorination operations are generally conducted indoors. All rooms in which chlorine is to be stored or handled should be adequately ventilated to the outside. Since chlorine is heavier than atmospheric air, fans are generally used to exchange the air. <ref>http://www.usace.army.mil/publications/armytm/tm5-814-3/chap17.pdf (accessed 5/9/2008)</ref>Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be chemically dechlorinated, adding to the complexity and cost of treatment.<ref>
:(UV) light can be used instead of chlorine, iodine, or other chemicals. Because no chemicals are used, the treated water has no adverse residual effect on organisms that later consume it, as may be the case with other methods. UV radiation causes damage to the genetic structure of bacteria, viruses, and other [[pathogens]], making them incapable of reproduction. The key disadvantages of UV disinfection are the need for frequent lamp maintenance and energy use; and the need for a highly treated effluent to ensure that the target microorganisms are not shielded from the UV radiation (i.e., any solids present in the treated effluent may protect microorganisms from the UV light). In the United Kingdom UV is becoming the most common means of disinfection. This is because of concerns about the impacts of chlorine in chlorinating residual organics within the wastewater and in chlorinating organics within the receiving water. Edmonton, Alberta, also uses UV light for its water treatment.<ref>http://www.ozonia.com/tips/wastewater_uv.html</ref>
;Ozone
:Ozone is unstable and reactive and oxidizes most organic material it comes in contact with, thereby destroying many pathogenic microorganisms. Ozone is considered to be safer than chlorine because, unlike chlorine which has to be stored on site (highly poisonous in the event of an accidental release), ozone is generated onsite as needed. Ozonation also produces fewer disinfection by-products than chlorination. A disadvantage of ozone disinfection is the high cost of the ozone generation equipment and the requirements for highly skilled operators.<ref>http://www.ozonia.com/tips/wastewater_uv.html</ref>
Tertiary treatment of effluent provides a final stage to raise the quality of wastewater before it is released into the environment. A treatment plant may opt to use more than one tertiary treatment process, such as combining nutrient removal with mechanical filtration. If disinfection is practiced, it is always the final process. Tertiary treatment is also called advanced treatment, "effluent polishing", or in the case of the Arcata Marsh "enhancement."
Types of Treatment
Filtration
After secondary treatment there are generally still some suspended solids left in the effluent. The main reason for this is that the secondary process is not perfect at removing microorganisms left over from the biological treatment processes. This "biological floc" can be filtered out mechanically by using a multimedia filter that has progressively smaller and smaller filter grain sizes. This reduces the amount of back washing necessary to clean the filter.
Nanofiltration
Nanofiltration is a relatively recent membrane process used most often with low total dissolved solids (TDS) waters such as surface water and fresh groundwater, with the purpose of softening (polyvalent cation removal) and removal of disinfection by-product precursors such as natural organic matter and synthetic organic matter[1]
Ultrafiltration
(UF) is a variety of membrane filtration in which hydrostatic pressure forces a liquid against a semipermeable membrane (Figure 1).Figure 1. Ultrafiltration Membrane (Awaiting Permission) Suspended solids and solutes of high molecular weight are retained, while water and low molecular weight solutes pass through the membrane[2]
Nutrient removal
Wastewater may contain high levels of nutrients such as nitrogen and phosphorus. The excessive release of these nutrients to the environment can cause the overgrowth of weeds, algae, and cyanobacteria (blue-green algae). A rapid growth in the population of algae can result in an algal bloom of a magnitude such that the numbers are unsustainable. Eventually most of the algea die. The decomposition of the dead algae by bacteria depletes the dissolved oxygen in the water. As a result most or all of the animals living in the water die. This creates more organic matter for the bacteria to decompose, continuing the cycle. In addition to causing deoxygenation, some algal species produce toxins that contaminate drinking water supplies. This scenario is typical of the Nitrogen cycle. Nitrogen is typically excreted or released by aquatic organisms in the form of amonia. This in turn is converted to ammonium by microorganisms which in turn is a nutrient that plants use for growth. The plants are then in turn eaten by creatures, thus completing the cycle (Figure 3).
Different treatment processes are required to remove nitrogen and phosphorus. Phosphorous is generally removed using chemicals such as alum, or ferric chloride which precipitate to form a salt and hydronium or hydroxide ions (Figure 2).
These ions can cause the waters pH to fluctuate. Generally lime is used to stabilize the pH of the effluent. These reactions require a settling basin to remove the precipitants.
Removing nitrogen can be accomplished using either chemical or biological processes. Nitrogen is generally present in the form of ammonia. The chemical process simply raises the pH of the effluent.
Figure 3. Nitrogen Cycle (Awaiting Permission)
This converts the ammonia to ammonium which can be removed from the water simply by passing large quantities of oxygen through it. The biological process uses activated sludge for an extended period of time, typically around 15 days.[3]
Disinfection
The purpose of disinfection in the treatment of wastewater is to reduce the number of pathogenic microorganisms in the water to be discharged back into the environment. The effectiveness of disinfection depends on the quality of the water being treated (e.g., cloudiness, pH, etc.), the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables. Turbidity will be treated less successfully since solid matter can shield organisms, especially from ultraviolet light or if contact times are short. Generally, short contact times, low doses and high flows all mitigate against effective disinfection. Common methods of disinfection include ozone, chlorine, or ultraviolet light.
Chlorination remains the most common form of wastewater disinfection in North America due to its low cost and long-term history of effectiveness. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful to the environment. Residual chlorine or chloramines may also be capable of chlorinating organic material in the natural aquatic environment. The presence of chlorine gas in the atmosphere can pose immediate and serious hazards to the health of any person breathing the air. Large schale declorination operations are generally conducted indoors. All rooms in which chlorine is to be stored or handled should be adequately ventilated to the outside. Since chlorine is heavier than atmospheric air, fans are generally used to exchange the air. [4]Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be chemically dechlorinated, adding to the complexity and cost of treatment.[5]
(UV) light can be used instead of chlorine, iodine, or other chemicals. Because no chemicals are used, the treated water has no adverse residual effect on organisms that later consume it, as may be the case with other methods. UV radiation causes damage to the genetic structure of bacteria, viruses, and other pathogens, making them incapable of reproduction. The key disadvantages of UV disinfection are the need for frequent lamp maintenance and energy use; and the need for a highly treated effluent to ensure that the target microorganisms are not shielded from the UV radiation (i.e., any solids present in the treated effluent may protect microorganisms from the UV light). In the United Kingdom UV is becoming the most common means of disinfection. This is because of concerns about the impacts of chlorine in chlorinating residual organics within the wastewater and in chlorinating organics within the receiving water. Edmonton, Alberta, also uses UV light for its water treatment.[6]
Ozone
Ozone is unstable and reactive and oxidizes most organic material it comes in contact with, thereby destroying many pathogenic microorganisms. Ozone is considered to be safer than chlorine because, unlike chlorine which has to be stored on site (highly poisonous in the event of an accidental release), ozone is generated onsite as needed. Ozonation also produces fewer disinfection by-products than chlorination. A disadvantage of ozone disinfection is the high cost of the ozone generation equipment and the requirements for highly skilled operators.[7]
