Category Archives: WEF HQ

How Alkalinity Affects Nitrification

WEF HQ banner 1column

Use alkalinity profiling in wastewater operations to control biological activity and optimize process control

By Mary Evans and Gary Sober


 

The Water Environment Federation’s new Operations Challenge laboratory event will determine alkalinity needs to facilitate nitrification. Operators will evaluate alkalinity and ammonia by analyzing a series of samples similar to those observed in water resource recovery facilities.

This event will give operators an understanding of how alkalinity works in the wastewater treatment process to facilitate nitrification, as well as the analytical expertise to perform the tests onsite. This provides the real-time data needed to perform calculations, since these analyses typically are performed in a laboratory that can present a delay in the data.

 

What is alkalinity?

The alkalinity of water is a measure of its capacity to neutralize acids. It also refers to the buffering capacity, or the capacity to resist a change in pH. For wastewater operations, alkalinity is measured and reported in terms of equivalent calcium carbonate (CaCO3). Alkalinity is commonly measured to a certain pH. For wastewater, the measurement is total alkalinity, which is measured to a pH of 4.5 SU. Even though pH and alkalinity are related, there are distinct differences between these two parameters and how they can affect your facility operations.

 

Alkalinity and pH

Alkalinity is often used as an indicator of biological activity. In wastewater operations, there are three forms of oxygen available to bacteria: dissolved oxygen (O2), nitrate ions (NO3), and sulfate ions (SO42-). Aerobic metabolisms use dissolved oxygen to convert food to energy. Certain classes of aerobic bacteria, called nitrifiers, use ammonia (NH3) for food instead of carbon-based organic compounds. This type of aerobic metabolism, which uses dissolved oxygen to convert ammonia to nitrate, is referred to as “nitrification.” Nitrifiers are the dominant bacteria when organic food supplies have been consumed.

Further processes include denitrification, or anoxic metabolism, which occurs when bacteria utilize nitrate as the source of oxygen and the bacteria use nitrate as the oxygen source. In an anoxic environment, the nitrate ion is converted to nitrogen gas while the bacteria converts the food to energy. Finally, anaerobic conditions will occur when dissolved oxygen and nitrate are no longer present and the bacteria will obtain oxygen from sulfate. The sulfate is converted to hydrogen sulfide and other sulfur-related compounds.

Alkalinity is lost in an activated sludge process during nitrification. During nitrification, 7.14 mg of alkalinity as CaCO3 is destroyed for every milligram of ammonium ions oxidized. Lack of carbonate alkalinity will stop nitrification. In addition, nitrification is pH-sensitive and rates of nitrification will decline significantly at pH values below 6.8. Therefore, it is important to maintain an adequate alkalinity in the aeration tank to provide pH stability and also to provide inorganic carbon for nitrifiers. At pH values near 5.8 to 6.0, the rates may be 10% to 20% of the rate at pH 7.0. A pH of 7.0 to 7.2 is normally used to maintain reasonable nitrification rates, and for locations with low-alkalinity waters, alkalinity is added at the water resource recovery facility to maintain acceptable pH values. The amount of alkalinity added depends on the initial alkalinity concentration and amount of NH4-N to be oxidized. After complete nitrification, a residual alkalinity of 70 to 80 mg/L as CaCO3 in the aeration tank is desirable. If this alkalinity is not present, then alkalinity should be added to the aeration tank.

 

Figure 1. pH versus nitrification rates at 68ºF (maximum nitrification rate occurs at 8.0–8.5 pH)

Source: EPA-625/4-73-004a, Revised Nitrification and Denitrification Facilities Wastewater Treatment, U.S. Environmental Protection Agency Technology Transfer Seminar.

Figure 2. Measurement of nitrification activity at a pH of 7.2 and lower

Source: EPA-625/4-73-004a Revised Nitrification and Denitrification Facilities Wastewater Treatment, U.S. Environmental Protection Agency Technology Transfer Seminar.

 

 

Why is alkalinity or buffering important?

Aerobic wastewater operations are net-acid producing. Processes influencing acid formation include, but are not limited to

  • biological nitrification in aeration tanks, trickling filters and rotating biological contactors;
  • the acid formation stage in anaerobic digestion;
  • biological nitrification in aerobic digesters;
  • gas chlorination for effluent disinfection; and
  • chemical addition of aluminum or iron salts.

In wastewater treatment, it is critical to maintain pH in a range that is favorable for biological activity. These optimum conditions include a near-neutral pH value between 7.0 and 7.4. Effective and efficient operation of a biological process depends on steady-state conditions. The best operations require conditions without sudden changes in any of the operating variables. If kept in a steady state, good flocculating types of microorganisms will be more numerous. Alkalinity is the key to steady-state operations. The more stable the environment for the microorganisms, the more effectively they will be able to work. In other words, a sufficient amount of alkalinity can provide for improved performance and expanded treatment capacity.

 

How much alkalinity is needed?

To nitrify, alkalinity levels should be at least eight times the concentration of ammonia in wastewater. This value may be higher for untreated wastewater with higher-than-usual influent ammonia concentrations. The theoretical reaction shows that approximately 7.14 mg of alkalinity (as CaCO3) is consumed for every milligram of ammonia oxidized. A rule of thumb is an 8-to-1 ratio of alkalinity to ammonia. Inadequate alkalinity could result in incomplete nitrification and depressed pH values in the facility. Plants with the ability to denitrify can add back valuable alkalinity to the process, and those values should be taken into consideration when doing mass balancing. (For Operations Challenge event, the decision has been made to not incorporate the denitrification step in process profiling.) To determine alkalinity requirements for plant operations, it is critical to know the following parameters:

  • influent ammonia, in mg/L,
  • influent total alkalinity, in mg/L, and
  • effluent total alkalinity, in mg/L.

For every mg/L of converted ammonia, alkalinity decreases by 7.14 mg/L. Therefore, to calculate theoretical ammonia removal, multiply the influent (raw) ammonia by 7.14 to determine the minimum amount of alkalinity needed for ammonia removal through nitrification.

 

For example:

 

Influent ammonia = 36 mg/L

36 mg/L ammonia ´ 7.14 mg/L alkalinity to nitrify = 257 mg/L alkalinity requirements

257 mg/L is the minimum amount of alkalinity needed to nitrify 36 mg/L of influent ammonia.

 

Once you have calculated the minimum amount of alkalinity needed to nitrify ammonia in wastewater, compare this value against your measured available influent alkalinity to determine if enough is present for complete ammonia removal, and how much (if any) additional alkalinity is needed to complete nitrification.

 

For example:

 

Influent ammonia alkalinity needs for nitrification = 257 mg/L

Actual measured influent alkalinity = 124 mg/L

257 – 124  = 133 mg/L deficiency

 

In this example, alkalinity is insufficient to completely nitrify influent ammonia, and supplementation through denitrification or chemical addition is required. Remember that this is a minimum — you still need some for acid buffering in downstream processes, such as disinfection.

 

Bioavailable alkalinity

Most experts recommend an alkalinity residual (effluent residual) of 75 to 150 mg/L. As previously identified, total alkalinity is measured to a pH endpoint of 4.5. For typical wastewater treatment applications, operational pH never dips that low. When measuring total alkalinity, the endpoint reflects how much alkalinity would be available at a pH of 4.5. At higher pH values of 7.0 to 7.4 SU, where wastewater operations are typically conducted, not all alkalinity measured to a pH of 4.5 is available for use. This is a critical distinction for the bioavailability of alkalinity. Therefore, in addition to the alkalinity required for nitrification, additional alkalinity must be available to maintain the 7.0 to 7.4 pH. Typically, the amount of residual alkalinity required to maintain pH near neutral is between 70 and 80 mg/L as CaCO3.

 

Proper alkalinity levels for treatment

Alkalinity is a major chemical requirement for nitrification and can be a useful and beneficial tool for use in process control. Several things to keep in mind:

  • Alkalinity provides an optimal environment for microscopic organisms whose primary function is to reduce waste.
  • In activated sludge, the desirable microorganisms are those that have the capability, under the right conditions, to clump and form a gelatinous floc that is heavy enough to settle. The formed floc or sludge can be then be characterized as having a sludge volume index.
  • The optimum pH range is between 7.0 and 7.4. Although growth can occur at pH values of 6 to 9, it does so at much reduced rates (see Figures 1 and 2). It is also quite likely that undesirable forms of organisms will form at these ranges and cause bulking problems. The optimal pH for nitrification is 8.0, with nitrification limited below pH 6.0.
  • Oxygen uptake is optimal at a 7.0 to 7.4 pH. Biochemical oxygen demand removal efficiency also decreases as pH moves outside this optimum range.

 

Mary Evans is a regional account manager for Premier Magnesia (Flint, Texas). She is a past president of the Water Environment Association of Texas and is the laboratory event coordinator of the WEF Operations Challenge Committee. Gary Sober is the vice president of technology for Byo-Gon Inc. (Chandler, Texas).

 

Please Note: The information provided in this article is designed to be educational.  It is not intended to provide any type of professional advice including without limitation legal, accounting, or engineering. Your use of the information provided here is voluntary and should be based on your own evaluation and analysis of its accuracy, appropriateness for your use, and any potential risks of using the information.  The Water Environment Federation (WEF), author and the publisher of this article assume no liability of any kind with respect to the accuracy or completeness of the contents and specifically disclaim any implied warranties of merchantability or fitness of use for a particular purpose. Any references included are provided for informational purposes only and do not constitute endorsement of any sources.

U.S. EPA Finalizes the Clean Water Rule

WEF HQ banner 1column

By Claudio Ternieden, Kristina Twigg, and Seth Brown

On May 27, the U.S. Environmental Protection Agency (EPA) and the U.S. Army Corps of Engineers (USACE) finalized the Clean Water Rule (http://www2.epa.gov/cleanwaterrule), which EPA and the USACE believe, ensures that waters protected under the Clean Water Act are more precisely defined and predictably determined, making permitting less costly, easier, and faster for businesses and industry,

The rule, published in the Federal Register on June 29, is expected to become effective August 28, depending on the outcome of some lawsuits filed by a number of states seeking to stop the rule from going into effect. The rule is grounded in law and the latest science, according to an EPA fact sheet (http://www2.epa.gov/cleanwaterrule/documents-related-clean-water-rule#Fact), and it received substantial  public input from more than 400 stakeholder meetings and more than 1 million public comments. EPA and USACE also maintain that the rule creates no new permitting requirements for agriculture and maintains all previous exemptions and exclusions, including dredged or fill requirements.

Stormwater controls not affected

In general, the Clean Water Rule clarifies which bodies of water are classified as “waters of the United States,” thereby requiring federal pollution controls. The Rule maintains the current status of municipal separate storm sewer systems (MS4s) and encourages the use of green infrastructure to protect water quality. Specifically, the final rule states:

 

(2) The following are not “waters of the United States” even where they otherwise meet the terms of paragraphs (1)(iv) through (viii) of this section.

 

(vi) Stormwater control features constructed to convey, treat, or store stormwater that are created in dry land.

 

By using the terms “constructed” “in dry land,” the new rule allows EPA to assert jurisdictional authority over the natural lakes, ponds, wetlands, rivers, and streams while not impacting MS4 elements. This section should help to exclude urban stormwater control measures in most cases, as the rule also stresses in the preamble: “This exclusion responds to numerous commenters who raised concerns that the proposed rule would adversely affect municipalities’ ability to operate and maintain their stormwater systems, and also to address confusion about the state of practice regarding jurisdiction of these features at the time the rule was proposed.”

Existing jurisdictional determinations and permits are valid until they expire. By promoting more consistent and effective implementation of Clean Water Act regulatory programs, the rule sets the stage for permit streamlining during implementation.

Some areas may be challenged

However, questions remain about the rule’s definition of tributaries and when that definition applies to ephemeral or intermittent streams — which would make them jurisdictional. According to EPA, 60% of U.S. stream miles flow only seasonally or after rain, and 1 in 3 Americans rely on these sources for drinking water.  According to some environmental attorneys, the tributary definition is the part of the rule most likely to be challenged. Based on the rule, a tributary must possess the physical characteristics of a bed, bank, and an ordinary high water mark as well as evidence of the frequency, duration, and volume of flow characteristic of a tributary. Further, to be considered jurisdictional, tributaries must significantly affect the health of downstream waters. Based on these definitions, tributaries primarily include headwater streams. Erosional features and ditches with intermittent flow specifically are excluded along with ditches draining into wetlands.

The final rule also further defines adjacent open waters and wetlands as jurisdictional if they are within 100 feet of the ordinary high water mark of a jurisdictional water or within the 100-year floodplain and within 1500 feet of the ordinary high water mark of covered waters. Certain isolated waters also could fall under the scope of the Clean Water Act based on both their connectivity and proximity to traditional navigable waters, interstate waters, and territorial seas. A significant nexus determination is based on the isolated water’s effects on the physical, biological, or chemical integrity of jurisdictional waters, such as through an exchange of pollutants, flow, or organisms. Additionally, scientific analyses assessing connectivity will consider how isolated waters affect the nearest jurisdictional water as a group rather than individually.

That analysis will be informed by EPA’s final report published last January, Connectivity of Streams and Wetlands to Downstream Waters: A Review and Synthesis of the Scientific Evidence (http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=296414), where it summarized current understanding about the connectivity and mechanisms by which streams and wetlands affect the physical, chemical, and biological integrity of downstream waters. The report serves as the technical backbone of the final Clean Water Rule.  Even if an excluded ditch falls within the defined limits of adjacent waters, an exclusion will trump an inclusion, said Ken Kopocis, deputy assistant administrator of EPA’s Office of Water, in a recent webcast about the rule.

Other clarifications

In addition to the issue of defining tributaries, EPA and USACE say the Clean Water Rule:

  • Protects prairie potholes, Carolina and Delmarva bays, pocosins, western vernal pools in California, and Texas coastal prairie wetlands when they affect downstream waters.
  • Focuses on streams, not ditches. The rule limits protection to ditches that are constructed out of streams or function like streams and can carry pollution downstream. Ditches not constructed in streams and that flow only when it rains are not covered.
  • Significantly limits the use of case-specific analysis by creating clarity and certainty on protected waters and limiting the number of similarly situated water features. Previously, almost any water could be put through a lengthy case-specific analysis, even if it would not be subject to the Clean Water Act.
  • Only protects the types of waters that have historically been covered under the Clean Water Act. It does not regulate most ditches and does not regulate groundwater, shallow subsurface flows, or tile drains. It does not make changes to current policies on irrigation, water transfers, or erosion in a field. The Clean Water Rule addresses the pollution and destruction of waterways, not land use or private property rights.

The Water Environment Federation will continue to follow the developments related to this rule and provide analysis and information.

Claudio Ternieden is the director of government affairs and Kristina Twigg is the associate editor of World Water: Stormwater Management at the Water Environment Federation (WEF; Alexandria, Virginia). Seth Brown, P.E., is a WEF senior adviser on stormwater issues and is the principal/founder of Storm and Stream Solutions, LLC (Springfield, Virginia).

 

“The information provided in this article is designed to be educational.  It is not intended to provide any type of professional advice including without limitation legal, accounting, or engineering. Your use of the information provided here is voluntary and should be based on your own evaluation and analysis of its accuracy, appropriateness for your use, and any potential risks of using the information.  The Water Environment Federation (WEF), author and the publisher of this article assume no liability of any kind with respect to the accuracy or completeness of the contents and specifically disclaim any implied warranties of merchantability or fitness of use for a particular purpose. Any references included are provided for informational purposes only and do not constitute endorsement of any sources.”

Help for States Considering Direct Potable Reuse

WEF HQ banner 1column

Framework provides guidance on program development and costs, federal regulations, and public outreach

By Justin Mattingly


 

As interest in direct potable reuse (DPR) has grown, so has the need to ensure water quality and safety. State regulators and local government and water utility decision-makers must make water supply decisions, but without specific criteria or guidelines, excessive treatment redundancies may result that impede or slow down projects, causing high costs, project delays, and public distrust.

As a result, a framework is being developed that outlines the most important issues that states will need to address as they develop DPR guidelines. This framework, which will be released in the fall, will focus on two forms of DPR, the first of which is defined as introducing highly treated wastewater effluent — with or without an engineered storage buffer — into the intake water supply upstream of a drinking water treatment facility. Another form introduces highly treated wastewater effluent directly into a drinking water distribution system. The framework is the culmination of a DPR framework project being developed by the WateReuse Research Foundation (Alexandria, Virginia) in coordination with the Water Environment Federation (WEF; Alexandria, Virginia) and the American Water Works Association (Denver).

 

What the framework will include

DPR projects are not necessarily new; Windhoek, Namibia, has been operating one since 1967 that introduces water directly into the drinking water distribution system. In the United States, permitted operational DPR projects add highly treated wastewater ahead of a water treatment facility. Currently, only Wichita Falls and Big Spring in Texas have operational DPR facilities, but DPR is currently under consideration in California, New Mexico, and several other states.

The framework will summarize Texas’ experience in implementing DPR and California’s creation of regulations for groundwater recharge indirect potable reuse projects. This framework also will cover a broad spectrum of issues in DPR implementation, including

  • a background on DPR as well as the cost of implementing a DPR program compared to other water resource options;
  • public health protection and how DPR may be affected by existing federal statutes such as the Clean Water Act and Safe Drinking Water Act;
  • source water control programs, wastewater treatment, advanced wastewater treatment, residuals management, and monitoring and control strategies; and
  • system operation to ensure that utilities have sufficient staff training and resources to properly operate these systems, which in many cases are more advanced than traditional wastewater or drinking water treatment facilities.

 

Managing public perception

Water treatment technology and operations are an ever-evolving process, and technology and regulatory needs for DPR may require future development. In addition, public perception is important in any statewide or local discussion of implementing a DPR program. Community organizations need to be engaged early to ensure that the public understands the DPR concept to dispel fears about using recycled water as a source of drinking water. The framework will include information on public outreach including the key factors that should be included in a communication plan, communication tools, as well as examples of successful DPR outreach programs.

The framework effort is part of a WateReuse Research Foundation Direct Potable Reuse initiative that has already allocated $5.8 million to fund 34 research projects. The National Water Research Institute (Fountain Valley, California) expert panel developing this framework is chaired by George Tchobanoglous of the University of California-Davis along with Joseph Cotruvo of Joseph Cotruvo & Associates, Jim Crook, Ellen McDonald of Alan Plummer Associates, Adam Olivieri of EOA Inc., Andrew Salveson of Carollo Engineers, and R. Shane Trussell of Trussell Technologies Inc.

The framework directly addresses a key theme of found in EPA’s Water Technology Innovation Blueprint. The Blueprint outlines the business case for investment in new tools in the 10 most promising market opportunities in the water quality sector, one of which is “Conserving and Eventually Reusing Water.”  Intended to be released by WEFTEC 2015 (September 26–30, 2015) in Chicago, the framework will be featured at the WEFTEC Innovation Pavilion discussion “Overcoming Barriers to Water Reuse.”

Justin Mattingly

Justin Mattingly is a research manager at the WateReuse Research Foundation. He can be reached at

jmattingly@watereuse.org.

“The information provided in this article is designed to be educational.  It is not intended to provide any type of professional advice including without limitation legal, accounting, or engineering. Your use of the information provided here is voluntary and should be based on your own evaluation and analysis of its accuracy, appropriateness for your use, and any potential risks of using the information.  The Water Environment Federation (WEF), author and the publisher of this article assume no liability of any kind with respect to the accuracy or completeness of the contents and specifically disclaim any implied warranties of merchantability or fitness of use for a particular purpose. Any references included are provided for informational purposes only and do not constitute endorsement of any sources.”

 

 

Flushables-Toilet-Trash FI

The State of the Flush!

WEF HQ banner 1column
The State of the Flush!
Better product guidelines, marketing standards for pipe-clogging “flushables” are on the way

By Brianne Nakamura, Program Manager in the Water Science & Engineering Center at the Water Environment Federation (Alexandria, Va.).

Flushable wipes: To flush or not to flush?

While the average consumer might wash their hands of the matter without a thought, for those in the wastewater industry, the nightmares of clogged pumps and sanitary sewer overflows (SSOs) come to mind. Recently, the topic of “flushable” wipes has become front and center within the wastewater industry, as more consumers are turning to a wet wipe rather than the common dispersible toilet paper.

While flushable wipes have been on the market for years, the question of their degradability has been garnering more attention in the media and prompted state-level responses, such as the recently proposed bill in Maine requiring that products labeled “flushable” live up to their claim.

Advertising versus reality

According to the current Association of Nonwoven Fabrics Industry (INDA; Cary, N.C.) guidelines (GD3, June 2013), a “flushable” is “any product that is marketed as ‘flushable’ [that] can be flushed into the wastewater system without adversely impacting plumbing or wastewater infrastructure and operations.” Under voluntary INDA guidelines, a product must pass seven assessment tests or be clearly labeled with the “Do Not Flush” logo.

These tests include a toilet and drain-line clearance test, disintegration “slosh box” test, household pump test, settling column test, aerobic test, anaerobic test, and municipal pump test. According to INDA guidelines, if a product passes all seven tests, it should not “under normal circumstances” block toilets, drainage pipes, water conveyance, and treatment systems or become an aesthetic nuisance in surface waters. But testing and real life can have different outcomes, especially under “normal circumstances.” The U.S. Federal Trade Commission (FTC) recently announced its tentative agreement with wipe manufacturer Nice-Pak Products Inc. (Orangeburg, N.Y.), that might further define some of these issues.

Problems can’t be wiped away

For wastewater utilities, these “nondispersibles,” or anything other than human waste and toilet paper flushed down the toilet, are problematic throughout the treatment process. They cause ragging in pipes and lift stations and get caught in screens, pumps, and settling basins.

Nondispersibles wreak havoc in rainy and dry climates alike. They clog collection systems during storms and cause SSOs or, in a drought-ridden area (we’re looking at you, California), the lack of water velocity in collection systems prevents wipes from breaking down. In extreme and highly publicized cases, the accumulation of wipes and other nondispersibles can cause the formation of “fatbergs,” such as those weighing as much as 15 tons in London sewers.

Industry response to the flushables flood

Although recent media attention has increased awareness of the consequences of convenient-yet-clog-causing wipes (and other nonflushable materials), wastewater utilities throughout the country have responded with their own public education campaigns, such as “What2Flush” in California and “Don’t Flush Baby Wipes” in Maine. These initiatives, as well as the wastewater industry’s “Three P’s (Pee, Poop, and “Toilet” Paper) standard, have been informing homeowners and renters about what’s OK to flush and to not use toilets as trash cans.

The Water Environment Federation (WEF; Alexandria, Va.) has also been involved in the initiative to improve flushability requirements and educate the public. In 2010, the WEF Collection Systems Committee formed a Flushables Task Force in response to the growing concern about wipes-related problems. The WEF House of Delegates (HOD) followed suit in 2012 to involve Member Associations with the formation of the HOD Non-Dispersible Work Group.

To create a singular message, the WEF Flushable Task Group, formed in 2014 and currently chaired by Scott Trotter, has worked on several initiatives including a 2013 billing stuffer campaign with the tagline, “It’s a Toilet, Not a Trashcan!” The group also advocated for collaborative studies conducted by the Water Environment Research Foundation (Alexandria, Va.).

More recently, the Task Group, as a representative of WEF, is collaborating with four other associations representing the water sector and the nonwoven fabrics industry: INDA, the National Association of Clean Water Agencies (Washington, D.C.), the American Public Works Association (Kansas City, Mo.), and the Canadian Water & Wastewater Association (Ottawa, Ontario). The goal is to develop a new, fourth edition of guidelines (GD4) that will influence product design and support the marketing of nonwoven products as “flushable.” The guidelines are scheduled to be released in July 2016.

In addition, the collaborative effort is behind the Product Stewardship Initiative to increase public and consumer awareness about the proper disposal of wipes. The initiative seeks to improve the labelling of both flushable and nonflushable products, as well as increase the industry’s responsibility over the downstream impacts of flushable products.

WEF has been heavily involved in both GD4 and the Product Stewardship Initiative. As the awareness of the problems of flushable wipes continue to increase, both in the media and within the wastewater industry, WEF continues to support the initiatives of the Flushables Task Force. While we can’t stop consumers from flushing things down their toilets, we can stem the tide with better education and incentives for corporate responsibility.

Brianne Nakamura is a Program Manager in the Water Science & Engineering Center at the Water Environment Federation (Alexandria, Va.). She is the staff liaison for the Collection System Committee and can be contacted at bnakamura@wef.org.

BNakamura Flushables-Toilet-Trash-Bill-Stuffer.jpg

Photo caption: The WEF Flushable Task Group, formed in 2014 and currently chaired by Scott Trotter, has worked on several initiatives for better public awareness about nondispersibles, including this 2013 billing stuffer campaign.

Nutrient article FI

Turning a Pollutant into a Resource

WEF HQ banner 1column

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.

Nutrient article photo of bloom

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 removal

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.

Phosphorus removal

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.

Barry Liner Sam Jeyanayagam

Note: The information provided in this article is designed to be educational.  It is not intended to provide any type of professional advice including without limitation legal, accounting, or engineering. Your use of the information provided here is voluntary and should be based on your own evaluation and analysis of its accuracy, appropriateness for your use, and any potential risks of using the information.  The Water Environment Federation (WEF), author and the publisher of this article assume no liability of any kind with respect to the accuracy or completeness of the contents and specifically disclaim any implied warranties of merchantability or fitness of use for a particular purpose. Any references included are provided for informational purposes only and do not constitute endorsement of any sources.

 

VillageCreek_Photo3

From Problem to Profit

WEF HQ banner 1column
A Fort Worth water resource recovery facility turns industrial waste challenges into energy opportunities

By Kristina Twigg and Peter V. Cavagnaro. Kristina Twigg is the Associate Editor, World Water: Stormwater Management at the Water Environment Federation (Alexandria, Virginia.). Peter V. Cavagnaro is a project development consultant at Johnson Controls Inc. (Milwaukee, Wisconsin).

The Village Creek Water Reclamation Facility in Fort Worth, Texas, lies on Trinity River’s west fork. Every day, the facility treats more than 378,541 m3 (100 million gal) of wastewater. With about 6437 km (4,000 miles) of sewers, the wastewater, carried largely by gravity, can take 8 to 12 hours to travel to the facility. Within this time, flows can become septic, and high-strength industrial wastes can be problematic for local industries to dispose of.

However, the Village Creek plant has turned the problem into an energy solution: Now the facility generates 75% of its electricity onsite.

“The plant’s co-digestion program has shifted the industrial wastes to a point in the plant where their energy can be harnessed,” said Madelene Rafalko, a senior professional engineer at the Fort Worth Water Department. “By injecting these concentrated wastes directly into the digester, the plant has decreased the amount of energy needed for aeration treatment.”

Wastes boost methane production
With the addition of co-digestion waste, the facility has doubled its gas production. However, facility staff are very selective about the wastes they bring in. “We are looking for wastes with high COD [chemical oxygen demand], which are more easily converted to methane,” said Jerry Pressley, water systems superintendent. The plant looks for wastes that produce a high gas yield with low residuals but avoid wastes with sulfides and sanitizers because they can cause process upsets, such as digester foaming, he said.

VillageCreek_Photo1
Photo 1. The co-digestion building is where the plant receives industrial wastes. Operators ensure that the wastes do not contain chemicals that would upset the anaerobic digestion process. (Credit: Kristina Twigg)

For 10 minutes every hour, the high-strength wastes are injected into six of the plant’s 14 anaerobic digesters. The plant has been capturing digester biogas for decades and uses it to power one of two 5.2-MW turbines. These turbines generate about half of the plant’s energy, most of which is used for the plant’s aeration system.

VillageCreek_Photo2
Photo 2. Biogas, used to generate energy via the plant’s turbines, is created in these anaerobic digesters fitted with linear motion mixers. (Credit: Kristina Twigg)
Steam heat provides return on investment
However, the Village Creak Water Reclamation Facility has also found a way to reduce the energy needed for its aeration basins.

In the process of using the turbines to generate electricity, heat is also created. The plant has harnessed this heat to make steam, which powers two of the plant’s blowers. The heat is also used to warm buildings and anaerobic digesters during winter. Even the steam itself is not wasted — it is condensed and reused.

VillageCreek_Photo3
Photo 3. The Village Creek Water Reclamation Facility generates both energy and steam. The steam is used to power two of the plant’s aeration basin blowers. (Credit: Kristina Twigg)
“The cost savings from the steam process has paid for everything else,” Rafalko said. The project, started in 2007, has saved $3 million so far, he said.

Improvements lead to other efficiencies
While the steam process is the largest part of the plant’s energy-efficiency program, staff have also taken advantage of low-hanging fruit, such as optimizing process controls, upgrading pumps and motors, replacing its SCADA system, and installing a web-controlled lighting system. “Going through and taking measures helped us to identify maintenance needs and further energy improvements,” Pressley said.

The plant also created anoxic zones in six of its 13 aeration basins. In the presence of oxygen, bacteria convert ammonia to nitrate (NO3). Then in the anoxic zones, the bacteria can utilize the oxygen present in the NO3. This eliminates mechanical aeration in these sections of the basins, further reducing the plant’s energy needs. These improvements bring the facility one step closer its goal of net-zero energy.

VillageCreek_Photo4

Photo 4. Using anoxic zones in the aeration basin improves energy efficiency at the Village Creek Water Reclamation Facility. (Credit: Kristina Twigg)

KONICA MINOLTA DIGITAL CAMERAKristina Twigg photo

 

 

 

 

 

 

 

Note: The information provided in this article is designed to be educational. It is not intended to provide any type of professional advice including without limitation legal, accounting, or engineering. Your use of the information provided here is voluntary and should be based on your own evaluation and analysis of its accuracy, appropriateness for your use, and any potential risks of using the information. The Water Environment Federation (WEF), author and the publisher of this article assume no liability of any kind with respect to the accuracy or completeness of the contents and specifically disclaim any implied warranties of merchantability or fitness of use for a particular purpose. Any references included are provided for informational purposes only and do not constitute endorsement of any sources.