Phosphorus and Wastewater Treatment: What You Need to Know
Paul Dombrowski, PE, PLS, DEE 
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What impacts do excessive levels of the nutrients nitrogen and phosphorus have on water bodies?
Depending on the characteristics of the particular water body, either nitrogen, phosphorus or a combination can act as a stimulus for aquatic plant growth. Excessive plant growth can cause negative impacts in a number of ways, most significantly by causing a condition known as hypoxia – the depression of dissolved oxygen in the lower levels of the water body due the oxygen demand associated with the decay of the dead plant matter. In addition to hypoxia, nitrogen in the form of ammonia utilizes dissolved oxygen and can result in effluent toxicity problems. Nitrate can contaminate groundwater drinking water sources.
How does the implementation of nutrient limits impact existing wastewater treatment facilities?
The magnitude of adding nutrient limits to an existing secondary treatment plant is dependent on many site-specific factors including which nutrient is involved, the level of treatment required, the process configuration employed, and the influent wastewater characteristics.
Regardless of any of these factors, nutrient removal systems generally require consistent removal of conventional pollutants; Biochemical Oxygen Demand (BOD5) and Total Suspended Solids (TSS). This is particularly important when attempting to achieve stringent nutrient limits because effluent TSS contains nutrients in particulate form. In many cases, some form of effluent filtration is necessary to consistently achieve low effluent nutrient levels.
Phosphorus can be removed either biologically or via chemical precipitation. Under either approach, dissolved phosphorus compounds are converted into particulate form and then removed via solids separation processes such as sedimentation and/or filtration. Generally, neither mechanism for removing phosphorus requires construction of significant tank volume but may require significant quantities of chemicals added with resultant increases in sludge production. In situations where stringent effluent phosphorus levels are required, tertiary systems such as filtration are generally required.
The removal of nitrogen is generally accomplished via the biological processes of nitrification and denitrification. Consistent nitrification is a requirement for any nitrogen removal system and converting an existing plant from secondary treatment to nitrification usually requires significantly more biomass in the system, especially at colder temperatures. This can be accomplished by either increasing the concentration of biomass in the system or increasing the volume of the biological reactors. Increasing the concentration of solids in an activated sludge system reduces the hydraulic capacity of the secondary clarifiers and requires that a balance be achieved between the two components of the system. Alternatively, other approaches such as using Integrated Fixed-Film Activated Sludge (IFAS) or membrane bioreactor (MBR) configurations can allow use of much higher biomass concentrations in a given reactor volume.
Optimized denitrification is the key to achieving stringent effluent total nitrogen limits. Anoxic conditions are required and any significant dissolved oxygen inputs to these portions of the process can significantly hinder performance. For a given configuration, any combination of three factors may limit performance: the amount of biomass in the anoxic reactors, the amount of nitrate available to denitrify, and both the amount and characteristics of the organic carbon (which serves as the food source) available to the biomass. For the most part, denitrification usually requires significantly less tank volume than nitrification but may require tertiary “add-on” units, more complex processes and chemical feed systems if particularly stringent levels are required.
Based on the preceding discussion, it quickly becomes apparent that the operators of these facilities are the key to successful performance. Operators of nutrient removal facilities must clearly understand the different process control parameters and the sometimes complex interrelationships between the various system components. This would be difficult enough if the system could operate under constant “steady-state” conditions but many of the system inputs and operating variables change dramatically over time. Wastewater flows and pollutant loadings are dynamically changing as are conditions within the plant. The biomass that is the “engine” of the system is an ecosystem onto itself – a compilation of living, breathing organisms that do not always respond in a predictable or controllable manner. Successful operation requires plant staff who understand the balance required among all of the various processes and who regularly monitor both quantitative and qualitative parameters to achieve consistent performance.