An overview of nutrient removal and recovery at WRRFs
By Barry Liner, Ph.D., P.E., is director of the Water Science & Engineering Center at the Water Environment Federation (WEF) Water Science & Engineering Center. Sam Jeyanayagam, Ph.D., P.E., BCEE, is chair of the WEF Nutrient Roadmap publication task force.
Photo: Nutrient removal is an essential part of wastewater treatment to help prevent algal blooms, as shown in this 2011 satellite photo of an especially severe case in Lake Erie.
Credit: MERIS/NASA; processed by NOAA/NOS/NCCOS
In excess, nutrients can be harmful water pollutants. Nutrients are found in agricultural and home fertilizers as well as agricultural operations. Sources include confined animal feeding operations, industrial pretreatment facilities, septic systems, and water resource recovery facilities (WRRFs) as well as municipal and industrial stormwater runoff.
According to the U.S. Environmental Protection Agency (EPA), more than 100,000 mi2 of rivers and streams, close to 2.5 million ac of lakes and ponds, and more than 800 mi2 of bays and estuaries are affected by nitrogen and phosphorus pollution. Excess nutrients can lead to algal blooms, which can produce toxins and result in hypoxic zones. Algal blooms cost the tourism industry some $1 billion annually, according to EPA. These substantial impacts are the reason regulatory nutrient limits are expanding across the country.
Nutrient removal at WRRFs
Nutrient management begins with nutrient removal to meet permit requirements. WRRFs can achieve very low nutrient discharges through a variety of processes, primarily biological nutrient removal (BNR), physical separation, and chemical methods. Most technologies capable of removing both nitrogen and phosphorus utilize BNR, which relies on bacteria to transform nutrients present in wastewater. In BNR, bacteria are exposed to the influent from primary treatment. The selection of a BNR process should be based on influent flow and loadings, such as biochemical oxygen demand (BOD), nutrient concentrations, and other constituents as well as target effluent requirements.
Select species of bacteria can accumulate phosphorus, while others can transform nitrogen, and a few can do both. Achieving significant reductions in both nitrogen and phosphorus requires careful design, analysis, and process control to optimize the environment of nutrient-removing organisms. The uptake of nutrients and growth of microorganisms could be inhibited by a limiting nutrient, available carbon, or other factors, including oxygen levels.
Some nutrient removal systems rely on two separate processes for nitrogen and phosphorus removal. In some cases BNR is used to remove the majority of nitrogen and phosphorus, and then chemical methods are used to further reduce phosphorus concentrations. Mainstream nutrient treatment takes place within the typical plant process flow. However, sidestream treatment refers to liquid resulting from biosolids treatment (anaerobic digestion and dewatering) that is intercepted with an additional treatment goal — to remove nutrients from a concentrated stream and minimize mainstream impacts. Like mainstream nutrient treatment processes, sidestream treatment can also vary from biological to physical and chemical removal methods.
Nitrogen can be removed from wastewater through physiochemical methods, such as air-stripping at high pH, but it is more cost-efficient to use BNR. Conventionally, this method utilizes the natural nitrogen cycle, which relies on ammonia-oxidizing bacteria to transform ammonia into nitrites (NO2–) after which nitrite-oxidizing bacteria form nitrates (NO3–) — a process called nitrification. Other species of bacteria can transform these compounds into nitrogen, a harmless gas (N2) — a process called denitrification. Nitrification can occur in the aeration basin together with BOD oxidation as they both require aerobic conditions. In contrast, denitrification takes place in an anoxic reactor with the nitrate providing the required oxygen. As denitrification occurs, nitrogen gas is produced and released safely into the atmosphere, where nitrogen gas is more abundant than oxygen. Nitrogen gas is inert and does not pollute the atmosphere.
When performing biological nitrogen removal, it is important that the activated sludge has enough available carbon to sustain denitrification. The bacteria that mediate denitrification need carbon to build new cells as they remove nitrogen. This means that utilities must make decisions on how best to use the carbon for the combinations of nutrient removal/recovery, energy generation, and/or recovery of value-added nonnutrient products.
The nitrogen removal rate is also dependent on the amount of time that sludge spends in the reactor (solids retention time), the reactor temperature, dissolved oxygen, pH, and inhibitory compounds. Optimal conditions differ for nitrification and denitrification, but both can be carried out simultaneously in the same unit if anoxic and aerobic zones exist. Some process configurations, such as oxidation ditches and sequencing batch reactors, combine nitrification and denitrification within a single tank while others incorporate two separate stages. Nitrogen removal processes can also be broken down into two categories based on whether bacteria are suspended within the wastestream or fixed to media. Examples include integrated fixed film activated sludge (IFAS) and denitrification filters.
A method of nitrogen removal that has gained favor over the past decade is deammonification, a two-step process that avoids nitrate formation. Aerobic ammonia oxidation to nitrite occurs in the first phase, then nitrogen gas is produced through anaerobic ammonium oxidation (also known as Anammox). Anammox is a biological process carried out by specialized bacteria that oxidize ammonia, and nitrite is used as an electron acceptor (oxygen source) under anaerobic conditions.
Unlike nitrogen, phosphorus cannot be removed from wastewater as a gas. Instead, it must be removed in particulate form through chemical, biological, hybrid chemical–biological processes, or nano-processes. Nano methods involve membranes and include reverse-osmosis, nanofiltration, and electrodialysis reversal. Chemical methods (chem-P) typically utilize metal ions, such as alum or ferric chloride. These compounds bind with phosphorus and cause it to precipitate and be removed by sedimentation and filtration. Chemical methods are influenced by a number of factors including the phosphorus species, choice of chemical, chemical-to-phosphorus ratio, the location and number of feed points, mixing, and pH.
Enhanced biological phosphorus removal (EBPR or bio-P) relies on phosphorus-accumulating organisms (PAOs) capable of removing phosphorus in excess of metabolic requirements. While many factors impact the EBPR process, the two most important requirements are availability of a readily biodegradable carbon source (food) and cycling of the PAOs between anaerobic and aerobic conditions. In the anaerobic zone, PAOs take up and store carbon. The energy required for this is obtained by releasing internally stored phosphorus. In the subsequent aerobic zone, the stored carbon is assimilated and the energy is used to uptake excess phosphorus.
Consequently, the design and operation of EBPR systems must consider the availability of a readily biodegradable carbon source (such as volatile fatty acids) and the integrity of the anaerobic zone by eliminating dissolved oxygen and/or nitrate contributions from the influent, return streams, and backflow from the downstream aerobic zone. As with biological nitrogen removal, oxygen levels, solids retention time, and temperature play an important role in EBPR efficiency. It is common practice to add a standby chemical system to account for poor EBPR performance. Many existing biological nitrogen removal processes can be modified to remove phosphorus by adding an anaerobic phase.
However, economic and environmental trade-offs exist, such as greenhouse gas production in the form of nitrous oxide as well as increased energy demands. Nutrient removal techniques can also affect biogas production and dewatering. The dewatering process is negatively affected by bio-P. During anaerobic digestion, flow from the bio-P process can decrease the efficiency of dewatering and require additional polymer as a coagulant, particularly when there are fewer beneficial metal ions, such as iron and aluminum.
From removal to recovery
Beyond simply removing nutrients, WRRFs also can reclaim nutrients. Recovery not only prevents nutrients from entering waterbodies but provides a supply of these essential resources. The most straightforward way of recovering nutrients is through biosolids. EPA estimates that the approximately 16,000 WRRFs in the United States generate about 7 million tons of biosolids. About 60% of these biosolids are beneficially applied to agricultural land, with only 1% of crops actually fertilized with biosolids. However, generating solid fertilizer from biosolids is the most common method of nutrient recovery from wastewater.
Wastewater operations that have adopted the principles of becoming a utility of the future are using the nutrient removal process to produce marketable products beyond simple biosolids, including nutrients, energy, electricity, and vehicle fuels. Phosphorus used for fertilizer is a finite resource, with some estimating that demand will outpace supply within the next century. In a similar vein, ammonia is produced via the Haber-Bosch process, which consumes natural gas (a nonrenewable resource), is an energy-intensive process, and is associated with greenhouse gas emissions. Interest in recovering nutrients from wastewater has increased over the last decade. However, the maturity of nutrient recovery technologies varies, and each has its advantages and disadvantages.
Sidestream treatment of sludge and sludge liquor, where nutrients are more concentrated, is generally the preferable target for nutrient recovery, but resource recovery complexity can vary widely depending on local conditions. In addition to nutrients, there are other types of products that can be recovered, such as metals, heat, and potable or drinking water, which may bring financial rewards and benefits to help offset utility costs.
These are some nutrient-based and other resources that can be recovered at a WRRF:
- Solid fertilizer from biosolids
- Land application of biosolids recycles nitrogen, phosphorus, carbon, and other macronutrients.
- Soil blends and composts are potential phosphorus recovery products.
- Incinerator ash is also a source of phosphorus for recovery.
- Solid fertilizer from the treatment process
- Struvite precipitation and recovery: By this method, both phosphorus and ammonium can be simultaneously recovered, producing a high-quality fertilizer from some sidestream systems.
- Other methods of phosphate precipitation such as brushite are also becoming common.
- Water reuse
- Irrigation with reclaimed water can have some nitrogen and phosphorus benefits.
- Chemical recovery
- Structural materials can be obtained from carbonates and phosphorus compounds.
- Proteins and other chemicals, such as ammonia, hydrogen peroxides, and methanol, can be recovered.
- Solids can be stored for future mining.
A roadmap to nutrient recovery
With the complexity of nutrient removal and recovery alternatives available, utility staff may wonder how to move forward to address current needs or plan for future impacts of nutrient limits. The Water Environment Federation (Alexandria, Va.) has released a Nutrient Roadmap to support the movement toward smarter and sustainable nutrient management in the context of each WRRF’s specific regulatory climate and stakeholder preference. The Roadmap provides a straightforward, high-level framework for planning, implementing, and evaluating different steps of a net-zero nutrient discharge strategy and can be found at www.wef.org/nutrientroadmap.
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