Volume 14, Number 3, Autumn 2005

The push is on at the USEPA toreduce eutrophication in our nation’s

receiving waters through increasedremoval of nitrogen and phosphorusfrom our wastewater treatment plant

discharges. Requirements for en-hanced nutrient removal are here formany wastewater treatment facilities

and are coming for others. As aresult, prudent stewards of wastewa-ter plants in all areas of our country

are seeking greater awareness andunderstanding of nutrient removalissues to guide them in their facility

and financial planning.

Nutrient removal is not a new issue,

but it can be confusing to implement,particularly with the progressionfrom BNR to EBNR in recent years.

When required for your wastewaterfacility, you will need to understandhow regulations will apply now and

in the future, and then determine theappropriate means of removal basedon the specifics of your existing

Nutrient Removal – An Evolving Challenge

CSO Research Effort ExploresLower Cost Approach

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Moline, Illinois Showcases Water Treatment Plant Improvements ...................................................... 5Upcoming Presentations at WEFTEC 05 ............................................................................................. 6

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A research effort to develop and test a

lower-cost approach for combinedsewer overflows (CSOs) is currentlyunderway in Indiana. Results from

this effort could aid the more than700 U.S. municipalities with com-bined sewer systems. When waste-

water and stormwater flows exceedsystem capacity, sewage can be

discharged into local waterways or

backup in residential areas.

University of Notre Dame assistant

professor of civil engineering andgeological sciences, Dr. Jeffrey Talley,has assembled a research team that

brings together a diverse group ofpartners. These include the Univer-

facilities. Nutrient removal require-ments differ depending on the impactof nutrients on the receiving water

and the way regulations are struc-

tured in your area to attain selectedlevels of removal. Some facilities will

At the H.L. Mooney Water Reclamation Facility, EBNR is accomplished in the aeration basins and finalfilter facility, which includes 10 deep-bed filters.

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NUTRIENT REMOVAL, continued from page 1—

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face greater requirements for removalthat increase over time, while

requirements for other facilities willbe less stringent. In some cases,facilities may receive “seasonal”

nutrient removal requirements thatwill necessitate different levels ofremoval at different times of the year.

Chemical or Biological

There are many choices of treatment

protocols capable of removing nitro-gen and phosphorus from wastewater.These protocols fall into two main

categories – chemical and biological.Chemical protocols include ammoniastripping and ion exchange for

nitrogen removal, and the addition ofa metal salt or lime for phosphorusremoval. Biological nutrient removal

(BNR), though, is often the process ofchoice due to the familiarity of plantoperators with biological treatment,

and the capital and operating coststhat can be achieved compared tocosts for alternative chemical and

physical treatment processes. Incases where BNR alone cannot meetnutrient removal requirements,

chemical or physical treatment can beused as a supplement.

Biological nutrient removal iscommonly the more economicalmeans of removing nitrogen and

phosphorus from wastewater. Biologi-cal nutrient removal uses naturallyoccurring microorganisms with

oxygen. Facilities for BNR are similarto those for a normal activated sludgeprocess, except that anaerobic and

anoxic zones are added. Generally, anexisting activated sludge plant can bemodified to include these additional

zones quite easily. This results insignificant capital cost savingscompared to the implementation of

chemical or physical nutrient re-moval. Additionally, operational costs

are lower due to reductions inchemical consumption, waste sludgeproduction, and energy consumption.

From BNR to EBNR

Enhanced biological nutrient removal

(EBNR) protocols are needed wheneffluent nutrient removal require-ments exceed those that can be

accomplished by BNR alone. Biologi-cal nutrient removal typically reducesnutrient concentrations to 5-8 mg/l of

total nitrogen, although some plantshave achieved lower concentrations,and 1-3 mg/l total phosphorus (see

table below).

Unlike nitrogen, phosphorus has nogaseous form and must be concen-

trated into a biomass and removed asa particulate with the waste sludge.Biological removal of phosphorus

requires an anaerobic zone with theabsence of dissolved oxygen andnitrate in the process. This allows

phosphorus removal bacteria tothrive and accumulate phosphorus intheir cells in excess of nutritional

requirements. These bacteria areremoved along with the waste sludge.

Biological Choices

Choices of facilities for BNR andEBNR can include just about any of

the commonly used biological treat-ment processes, including activatedsludge, rotating contactors, and

filters. The key to economical imple-mentation of BNR is to select appro-priate removal protocols that can be

advantageously adapted to existingfacilities. For instance, in activatedsludge plants, existing reactors can

be modified to include both nitrifica-tion and denitrification zones inaddition to retaining their capability

to remove BOD. Depending onremoval protocol selection, additionalreactor volume may be required. If

this were the case, the economics ofperforming denitrification in reactorswould need to be compared with that

of performing denitrification usingcontactors or filters to determine themore cost-effective alternative. There

are many BNR protocols, each ofwhich could be appropriate depend-ing on site-specific conditions. The

nitrogen protocols vary in the numberand arrangement of treatment steps,but all employ one or more of the

following:

■ Sequential nitrification followed

by denitrification■ Denitrification using influent

organics to achieve substrate

level denitrification■ Mixed systems where nitrifica-

tion and denitrification occur in

the same reactor

Concentrations in mg/l

Nitrogen Phosphorus

Influent Raw Sewage 30-40 6-10

Secondary Treatment 15-30 4-8

Biological Nutrient Removal 5-8 1-3

Enhanced Nutrient Removal 3 or less 0.5 or less

Facilities for BNR aresimilar to those for a

normal activated sludgeprocess, except thatanaerobic and anoxic

zones are added.

To achieve the lower effluent concen-trations required, EBNR uses a

supplemental carbon source, such asmethanol.

Although nitrogen and phosphorusconcentrations can both be reducedusing biological processes, the

mechanisms for their removal aredifferent. Nitrogen removal isaccomplished in two steps. In the

nitrification step (which is commonlypracticed in conventional wastewatertreatment facilities), nitrifying

bacteria use oxygen to convertammonia nitrogen to nitrate. Theactual removal of nitrogen from the

wastewater requires a further stepcalled denitrifica-tion. In denitrifica-

tion, nitrates areconverted tonitrogen gas under

anoxic conditions.Nitrogen gas isthen discharged

harmlessly into theatmosphere.

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NUTRIENT REMOVAL, continued —

■ Manipulation of the environmentand feed patterns to achieve

nitrification and denitrificationat different times

■ Simultaneous nitrification and

denitrification at differentlocations in the same reactor

Biological phosphorus removalrequires an anaerobic step ahead ofBOD and nitrogen removal steps as

described previously.

Evolving the Process

The City of Tampa, Florida pioneerednitrogen removal from wastewater inthe early 1970’s in an effort to save

environmentally impacted TampaBay from further degradation.Greeley and Hansen designed

improvements to the city’s Howard F.Curren Advanced WastewaterTreatment Plant that featured a

60-mgd two-stage activated sludgesystem using pure oxygen and deep-bed denitrification filters. Subse-

quently, Greeley and Hansen de-signed an expansion of the plant to96 mgd.

As a result of the expansion, existingpure-oxygen reactors are now usedonly for single-stage carbonaceous

treatment, and existing aerobicdigestion tanks have been modified toserve as the second nitrification stage

in lieu of constructing additional

tankage. This saved the city anestimated $20 million.

Flexibility is the main characteristicthat can be credited for the success of

the Howard F. Curren Plant. Thisflexibility provides the capability tochange from a two-stage system to a

single-stage system and the ability todenitrify in the nitrification tanks,when needed, to reduce the nitrate

load on the filters. As a result of theEBNR improvements at the HowardF. Curren AWTP, Tampa Bay is in the

best condition it has been in decades.

Pilot Studies UncoverSpecific Needs

The Prince William County ServiceAuthority’s 18-mgd H.L. MooneyWater Reclamation Facility in

Woodbridge, Virginia has agreed to atotal nitrogen discharge limit of8 mg/l into Neabsco Creek, which is a

tributary to the Potomac River. Theplant also removes phosphorus downto 0.18 mg/l based on a monthly

average. Pilot studies conductedahead of the design of the EBNR

efficiencies and determined that theplant could not provide reliable

nitrogen removal with the existingtankage alone. Due to insufficientcarbon in its primary effluent,

modifications for nutrient removalrequired an EBNR protocol includingmethanol addition to achieve efficient

denitrification.

In addition to the use of methanol,

this Greeley and Hansen designedupgrade includes aeration basinsmodified to allow operation for step

feeding primary effluent or as aModified Lutzack Ettinger Process,as well as a new final filter facility

consisting of 10 deep-bed filters.Denitrification is accomplished in theaeration basins and/or the final

filters. Total phosphorus concentra-tions are reduced from an average of6 mg/l down to 0.1 mg/l with chemical

treatment using ferric chloride andpolymer. Since start-up of the BNRfacilities in 2001, the plant has

consistently met or exceeded effluentnutrient requirements. An addedbenefit of the new EBNR facilities

has been the reduction in the quan-tity of ferric chloride and polymer fedfor the reduction of phosphorus.

Greeley and Hansen engineers canassist with developing innovative

nutrient removal strategies forwastewater utilities. Our extensiveexpertise in the theory, design, and

operation of nutrient removal facili-ties results in user-friendly solutionsthat are practical, flexible, reliable,

and easy to operate and maintain. �

For more information regarding

biological nutrient removal and other

nutrient removal technologies,

contact Carl Koch, Ph.D. at 302-428-

9530 or Jong Lee, Ph.D. at 312-578-

2314. Koch is a co-chair for develop-

ment of an upcoming Water Environ-

ment Federation Manual of Practice

on the operation and maintenance of

biological nutrient removal facilities.

The Howard F. Curren AWTP features one of the world’s largest deep-bed denitrification filter facilities.

facilities at the Mooney WRF evalu-

ated alternative BNR operatingprotocols at a variety of tempera-tures, anoxic volumes, recycle rates,

methanol dosage rates, and solidsretention times. These studies,conducted over an 18-month period,

identified the benefits of methanoladdition to improve nitrogen removal