Nutrient Removal - What is Available for Municipal Utilities?
Nutrient removal is a growing concern for many municipalities. The nutrient problem is mainly associated with excess discharges of nitrogen and/or phosphorus.
By John Dyson
Nutrient removal is a growing concern for many municipalities. The nutrient problem is mainly associated with excess discharges of nitrogen and/or phosphorus. It is mostly a regional issue (Chesapeake Bay, Long Island Sound, etc.) right now or specific to water bodies that need to be protected. The reality is that all our water bodies need protection because they are current as well as future drinking water sources, whether they are freshwater or seawater sources. We must also protect these bodies for our children to enjoy for recreational use and to provide a source of food to our country. Many of us enjoy the seafood, but if we don’t protect the estuaries, then the seafood we enjoy will be lost or limited.
The good news is that we have the technologies available to provide the solution(s) that a municipality needs to protect water bodies. What is the right solution for each municipality? This is a difficult question and it depends on many factors. These factors include effluent water quality requirement(s), size of the wastewater plant, site constraints, capital and operating costs, operation skill level and staff availability, etc.
In many cases, phosphorus is needed so the biological nitrogen removal can occur, but there may also be a requirement for very low phosphorus in the treated water. Having both a nitrogen and phosphorus effluent limit typically requires a balancing act of providing enough phosphorus for the biological process, but then reducing it to a low enough level that can be easily and economically removed in the last stage of treatment by a physical chemical process.
Nitrogen removal is achieved biologically by nitrification and denitrification occurring in different stages. Nitrification occurs in an aerobic stage of treatment and denitrification in an anoxic stage of treatment.
There are many possible solutions, and the process or technology to be used is dependent on the factors listed above. In many cases these processes and technologies may be combined together to provide a complete solution. Below is a list of the processes and technologies used to perform nitrogen removal:
• Oxidation Ditch Process – This is a loop process which creates zones of aeration and anoxic conditions. The process can be operated in a cyclical manner to achieve nitrification and denitrification.
• Cyclically Aerated Activated Sludge – The aeration system is programmed to turn off periodically, allowing denitrification and nitrification to occur in the same tank. This can be achieved in existing cyclical plants with little or no capital expenditures if there is sufficient SRT.
• Modified Ludzack-Ettinger Process (MLE) - This process consists of the modification of a conventional activated sludge process where an anoxic zone is created or added upstream of the aerobic zone. The process uses an internal recycle that carries nitrates created in the nitrification process in the aerobic zone along with the mix liquor to be mixed in the influent to the anoxic zone. The amount of nitrates potentially removed in the anoxic zone depends on the recycle flow and availability of influent BOD.
• Four-Stage Bardenpho Process – This process is similar to the MLE process but has a second anoxic zone after the aerobic zone where an external carbon source is typically added to aid in the denitrification process. This process is suitable for achieving very low total nitrogen values.
• Biological Active Filters (BAF) – These are typically upflow filters that can be used for BOD removal, nitrification and denitrification. BAFs use granular or plastic media with a large surface area to allow for attached growth to occur, resulting in a compact footprint. Nitrifying BAFs operate with aeration and denitrifying BAFs operate with the addition of an external carbon source. Unlike other processes, BAFs do not require any downstream units such as secondary clarifiers for liquid-solid separation.
• Downflow Denitrification Filters – These are downflow filters that are only used for denitrification. These granular media filters are applied after secondary clarifiers and are used to remove any nitrates not removed in the conventional activated sludge process or other secondary biological process upstream.
• Moving Bed Biofilm Reactor (MBBR) – This is a fluidized fixed-film process using a small plastic media (carriers) in anoxic or aerobic zones that allows the attached growth to occur. The MBBR process operates without the presence of suspended phase and is typically retrofitted into an existing basin when other conventional processes, such as MLE or Bardenpho, will not fit. This process can be used for nitrification and denitrification, in addition to BOD removal. The reactor is more compact than a conventional activated sludge process. These reactors can be used before the secondary treatment process, with the main process train, or after the secondary treatment process. The versatile nature of this process affords a lot of flexibility with the reactor configurations.
• Integrated Fixed-Film Activated Sludge System (IFAS) – This is a fixed-film process which combines the benefits of an MBBR process with an activated sludge process. They can be arranged in many configurations, just like many of the conventional activated sludge processes. This system is more compact and has smaller footprint than the conventional process configurations since it combines the suspended growth and attached growth phases. Because of the presence of the attached growth phase, IFAS process is also more resilient to hydraulic and/or pollutant load variations. This advantage is shared by the MBBR process as well.
• Membrane Biological Reactors (MBR) – This is a process which can use many configurations with the biological portion of the process and typically includes anoxic and aerobic zones followed by a membrane that acts as a filter to remove the solids from the mixed liquor and therefore eliminates the need for a secondary clarifier. This process typically provides high MLSS concentrations to achieve the nitrogen removal.
• There are additional processes such as Step-Feed Activated Sludge, Biodenitro (oxidation variation), Sequencing Batch Reactor, etc.
• Other solutions include processes for side-stream treatment (high strength ammonia reduction) to reduce the nitrogen load to the main treatment train.
These various nitrogen removal solutions, many times with a combination of options, must be analyzed to determine which process is best for each municipality.
Phosphorous removal processes basically involve two methods: 1) biological and 2) physical/chemical treatment. The biological removal of phosphorus is achieved with the following processes:
• Fermentation - in an anaerobic condition with VFAs providing the carbon sources for the microorganisms or
• Anaerobic/Oxic (A/O) Process
In most cases when low levels of phosphorus are required, physical/chemical processes are utilized. These technologies include the following:
• Media Filters with coagulation in upstream processes
• Gravity Filters
• Moving-Bed Filters
• Pulsed-Bed Filters
• Traveling Bridge Filters/Automatic Backwash Filters
• Cloth Filters
• Membrane filters
• Tertiary Clarification with and without filtration. Tertiary clarifiers include the following technologies:
• Sludge Ballasted/Sand Ballasted Clarifiers
• Dissolved Air Flotation
• Other conventional clarification processes
In all cases with physical/chemical processes, chemicals like ferric, alum, and PACL are utilized to achieve the phosphorus removal. The lower the effluent limits for phosphorus, the higher the coagulant dosage. It is important to note that the coagulant demand is not linear with the more stringent low phosphorus effluent limits.
The processes and technologies outlined above show that there are many possible solutions available to achieve the desired nutrient removal requirements. The selection of the appropriate process is dependent on the specific project conditions. There is no perfect process or technology for every project. A municipality must evaluate all the key factors before selecting a process, including capital cost, operability, and O&M costs, to determine the best solution. In many cases this can be achieved through a pre-selection process where all the evaluated costs are considered to determine the best solution.
About the Author: John Dyson is Vice President, Municipal Equipment, for Infilco Degremont Inc. He currently serves on the WWEMA Board of Directors.
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