What Is Nutrient Reduction?

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What is Nutrient Reduction?

Nutrient reduction is a process or series of processes used to reduce the mass of nutrients in sewage. This fact sheet will focus on the reduction of nitrogen and phosphorus in effluent. Depending on how (or where) treated effluent is discharged back into the environment, these two nutrients could produce a detrimental effect. Some wastewater experts consider nutrient removal to be a tertiary treatment – a third level. Tertiary treatment can also include disinfection. The fact sheets in this series separate nutrient removal and disinfection to prevent any confusion.

Nutrients are considered excessive if the receiving environment cannot assimilate them without causing excessive growth of aquatic plants or other organisms. Excessive nutrients in water bodies can potentially cause eutrophication (extreme productivity in a water body) and hypoxia (a low concentration of dissolved oxygen). Nitrogen (N) and phosphorus(P) are the two most common limiting nutrients that cause eutrophication of aquatic systems. Nitrogen is typically more of a problem in saltwater environments like bays and estuaries while phosphorus is more problematic in freshwater environments such as lakes, streams and rivers. Excess nutrients stimulate excessive plant growth (algae, and nuisance plants weeds), which results in reduced sunlight penetrating the water and a loss of habitat for aquatic animals and plants. As these excess organisms die, they can cause a decreased amount of dissolved oxygen in the water.These conditions reduce the diversity of species and the overall health of the ecosystem. In many cases hypoxic waters do not have enough oxygen to support fish and other aquatic animals. In situations where the hypoxic conditions develop abruptly, massive fish kills can occur. In other situations where it happens gradually, it causes a demographic shift in populations. Game fish, such as trout, may need as much as 4 mg/L oxygen to thrive. Less desirable species of fish, such as carp, may thrive on oxygen levels of less then 2 mg/L.

In some jurisdictions, there is also concern over nitrogen as a human health issue. Nitrogen in the nitrate form can cause blue baby syndrome (methemoglobinemia)by affecting the blood’s ability to carry nitrogen. High nitrate levels could potentially be carcinogenic, but this has not yet been conclusively demonstrated. Some studies have also implicated high nitrate levels in increased risk of birth defects.

Nitrogen

Wastewater can contain several nitrogen species: nitrate, nitrite, ammonia, and organic nitrogen. These nitrogen compounds result from the biological decomposition of proteins and from urea, which are discharged as human waste. Primary treatment can remove 10 to 20% of the total wastewater nitrogen through solids separation. The nitrogen that leaves primary treatment is primarily in the form of ammonia nitrogen.

In soil-based systems that receive septic tank effluent, nitrogen will undergo several transformations within and below subsurface soil dispersal components. The ammonia nitrogen may be taken up by plants or volatilize to ammonia gas under high pH conditions in alkaline soils. Ammonia nitrogen may also be biologically converted to the nitrate form. The process of converting ammonia into nitrate is called nitrification.

Like ammonia, nitrate is plant available; however, it is also very water soluble and will tend to downward to the groundwater and into nearby surface water. Denitrification is the process of converting nitrate into a nitrogenous gas, which is released to the atmosphere. This nitrogen reducing process can occur in the soil if there is sufficient carbon or sulfur present and if low oxygen conditions exist. Under these circumstances, microorganisms can convert nitrate to nitrogen gas.

In order for community-scale treatment system to provide predictable nitrogen reduction, the system has to be carefully managed. From the perspective of biological processes, there are two limiting factors. The first is that the microorganism that convert nitrate to nitrogen gas must not have available dissolved oxygen. Secondly, these microorganism need a source of organic carbon. Creating these denitrifying conditions (commonly termed “anoxic”) is problematic. The problem is that most of the organic carbon was removed during aerobic treatment, which was needed to create the nitrate. Further, the dissolved oxygen now needs to be removed. A successful solution to this problem is to recirculate a portion of the nitrate-rich water back through primary treatment. This process places nitrate in an anaerobic environment.

Passive treatment components like lagoons and constructed wetlandsprovide conditions conducive to denitrification by having aerobic zones close to the air-water interface and anaerobic zones near the bottom. In cases where organic carbon continues to be a limiting factor, a media made of bioavailable organic carbon may be placed in anaerobic zone.

Phosphorus

Phosphorus is comprised of orthophosphate, polyphosphate, and organic phosphate. Organically bound phosphorus originates from human waste and food scrapes. Upon biological decomposition, organically bound phosphorus is releasedas orthophosphate. Detergents are another source of phosphorus. Polyphosphates are used in synthetic detergents and often contribute up to one-half the phosphorus in wastewater. In raw sewage, the concentration of phosphorusis usually between 5 to 15 mg/L as P. Acceptable levels in sensitive natural water systems vary from 0 to 3 mg/L.

Because phosphorus is a component in many organic solids, liquid/solid separation provides a significant phosphorus reduction. In soil-based dispersal systems, phosphorus is adsorbed by calcium, aluminum, and iron compounds. Clayey soils have a significant capacity to adsorb phosphorus. From a biological unit process perspective, phosphorus removal occurs when biomass is wasted from suspended growth systems (i.e., when solids are pumped from an ATU or activated sludge treatment component). This is called biological nutrient removal (BNR). A well operated BNR system will reduce phosphorus to 3-5 mg/L, but requires significant expertise and attention to be successful. In reality, the principal means of phosphorusreduction in large scale waste water treatment is chemical precipitation (forming of settleable solids) using calcium, aluminum or iron. A chemical feed system is used to add these metals to the system. BNR and chemical precipitation are not necessary if the effluent is applied to the soil. In small, surface discharging system, if the phosphorus reduction is mandated, a chemical treatment technology is usually the best choice.

Nutrient reduction is theoretically feasible in systems of all sizes. From a practical perspective, it is relatively straightforward to incorporate nitrogen and phosphorus reduction into small scale systems. However, performance is highly dependent upon diligent operation and maintenance (O&M). Successful nutrient reduction requires an extremely knowledgeable operational staff, and frequent operator intervention.

Compatibility of Nutrient Reduction with Community Vision

The issue that is frequently discussed relevant to nutrient control is the value of natural waters to the community. If a community economy relies on tourism, then nutrient control may be a critical issue. Another issue is where treated effluent is discharged or dispersed relative to the community drinking water source. It there is no possible connection between the two, then nitrogen removal may not be a critical issue for the community. Likewise, water reuse can play a major role in local water planning. If the treated wastewater is to be used for irrigation, the nutrients become valuable and should not be removed.

Land Area Requirements for Nutrient Reduction

For soil-based effluent application, the land area requirements are usually based on hydraulic loading. If nutrient reduction becomes a determining factor, then land area requirement may dramatically increase. More land may be needed if the soil is low in aluminum and iron needed to bind phosphorus. If unit processes are needed for nutrient removal, the system footprint may increase. Communities that have the luxury of excess land availability can simply make everything larger and let nature remove the phosphorus.

Construction and Installation of Nutrient Reduction Mechanisms

Nutrient removal must be designed into the treatment process. Aeration systems must be built that allow a portion of the nitrified effluent to recirculate back to an anoxic zone for denitrification. InBNR systems, conditions must be kept conducive to the luxurious uptake of phosphorus by microorganisms. And, when the phosphorus-rich microbes are removed, an appropriate management practice must be in place to handle the biosolids.

Operation and Maintenance of Nutrient Reduction Processes

Because this is a class of treatment and not a specific technology, it is difficult to make specific statement with regard to O&M requirements. However, the recirculating nitrogen reduction systems should not require much increase in OM. Separate denitrification systems are simple in design, so the O&M increase should also be minimal. If phosphorus reduction over and above the soil adsorption capacity is required, some form of adsorption system such as a sodium aluminate column would also represent a small increase in system O&M needs.

Systems that treat large volumeswill all have a rigorous mechanical component because of the inclusion of recycle pumps which must be maintained on a regular basis. Addition of chemicals, such as calcium, iron or aluminum coagulants, or lime on a continuous basis will require a person knowledgeable in setting chemical dosage, purchase and handling of the chemicals, and reduction of the solids generated from the system. These systems will require the highest level of maintenance of any system described in this series of Fact Sheets.

Costs for Nutrient Reduction

The incremental cost of adding a nitrogen reduction system a septic tank system for a 3-bedroom home will range from $5,000 to $20,000 (assuming 20 mg/L of N in effluent). The technology selected will be the driving force for cost. If phosphorus is removed by chemical precipitation, then the purchase of replacement chemicals will be an ongoing cost. Energy consumption will increase with the addition of pumps to attain the necessary energy for recirculation. Most nutrient reduction processes will not require blowers or aerators. Costs vary from system to system and are essentially tied to the components used.

References

1. Crites, R. and G. Tchobangolous. 1998. Small and Decentralized Wastewater Management Systems. WCB/McGraw Hill Company, Boston, MA.
2. Oakley, S. 2005. Onsite Nitrogen Removal Module. in (M.A. Gross and N.E. Deal, eds.) University Curriculum Development for Decentralized Wastewater Management. National Decentralized Water Resources Capacity Development Project. University of Arkansas, Fayetteville, AR.
3. Onsite Sewage Treatment Program, University of Minnesota. 2009. Manual for Septic System Professionals in Minnesota. St. Paul, MN.