Page 1 of 17 

Performance of Passive Wastewater and Groundwater Nitrex TM Nitrogen Removal System 

Pio Lombardo 

ABSTRACT

Excessive nitrogen levels caused by inadequate wastewater treatment pose a threat to the 

ecological health of water resources in the United States. Very few appropriate options currently 

exist for nutrient removal from decentralized wastewater flows. Conventional nutrient removal 

technologies, including carbon feed systems for nitrogen removal are generally operator intensive 

and complex. Decentralized wastewater management systems utilizing nutrient removal 

technologies provide a wastewater management option that is cost effective and protective of the 

environment, and avoids the need for centralized sewering. Based upon research at the 

internationally acclaimed University of Waterloo, the new, patented, innovative NITREXTM

technology fulfills this need. 

Also the passive NITREXTM technology, which is a wood based reactive media, has been 

demonstrated to be capable of reducing wastewater total nitrogen levels to less than 3 mg/l and 

nitrate levels below 0.10 mg/l N. The NITREXTM technology can be incorporated into permeable 

reactive barriers (PRB) to remove nitrate-nitrogen from groundwater. The longevity of the 

NITREXTM filter is estimated to be 20+ years with little operation and maintenance requirements. 

The NITREXTM Filter is the only known passive treatment technology to provide virtually 

complete single-pass nitrate removal to produce an effluent quality that meets the new stringent 

surface water discharge criteria for nitrogen. 

The paper will present the results of independent EPA and State sponsored performance 

evaluations of NitrexTM wastewater nitrogen removal systems in Oregon, Montana, and Cape 

Cod, MA and field monitoring data from residential and commercial installations. Moreover the 

results from a pilot project installed in summer 2005 will be presented demonstrating the 

effectiveness of the NITREXTM permeable reactive barrier on Cape Cod for groundwater 

treatment. Testing and field applications show that the NITREXTM technology can achieve high 

process reliability and strong performance in both wastewater and groundwater applications. 

INTRODUCTION 

This paper presents the results of a number of independent testing programs of nitrogen removal 

systems. In these testing programs, the passive NITREXTM Filter system has demonstrated the 

ability to produce a wastewater effluent with a TN less than 3 – 4 ppm. The NITREXTM Filter 

has been evaluated in the following testing programs: 

__________ 

* Pio Lombardo, PE, President of Lombardo Associates, Inc. of Newton, MA, Phone Number: 

(617) 964-2924, Email: pio@lombardoassociates.com
 Page 2 of 17 

 LaPine, Oregon National Decentralized Wastewater Treatment and Disposal 

Demonstration Project funded by EPA evaluating 15 alternative, innovative denitrification 

systems. The Project is being performed by the Oregon Department of Environmental 

Quality, U.S.G.S. and the Deschutes County Environmental Heath Division. Project is 

ongoing. See http://www.deq.state.or.us/wq/lapinedata/SiteRptCriteria.asp The LaPine 

Oregon Project evaluated two NITREXTM Filters at the residences of Flemming and 

Stone. 

 Montana Department of Natural Resources evaluation of three on-site denitrification 

systems in Polson, MT. Project performed from Nov. 1999 to Aug. 2002. (Dupuis, MT 

DNR, 2001) 

 Massachusetts Septic System Test Center, Otis Air Force Base Cape Cod, MA (2001 - 

2005) 

 University of Rhode Island 

 6,000 gpd facility in Mashpee, MA 

 Groundwater nitrate removal systems in Falmouth, MA 

 Seven additional installations in Canada for individual and cluster wastewater systems, 

along with golf course and farmland runoff treatment systems (since 1997) 

A schematic of the NITREXTM Filter system is presented on Figure 1. 

Figure 1: NITREXTM Filter system 

Septic 

Tank 

Nitrifying 

System 

NITREXTM

Filter 

Drain field 
 Page 3 of 17 

The NitrexTM nitrogen removal mechanisms consist of ammonification and nitrification in a sand 

filter or other aerobic treatment system that provides complete nitrification as follows: 

 

Organic Nitrogen → NH4 + NH4+ + 1.5 O2 → 2 H+ + H2O + NO3- 

Denitrification in the NitrexTM filter occurs by heterotrophic denitrification using the NITREXTM

media as a slow release carbon source by the following reaction: 

4NO3 - + 5CH2O → 2N2 + 5CO2 + 3H2O + 4OH - 

As stated in the EPA Nitrogen Control Manual (1993), in nitrification: 

 4.6 mg oxygen required/mg nitrogen oxidized 

 7.1 mg CaCO3 alkalinity depleted/mg nitrogen oxidized 

 0.10 – 0.15 mg net volatile solids/mg nitrogen oxidized 

in denitrification (anaerobic conditions required) 

 2.9 mg oxygen liberated/mg nitrogen reduced 

 3.6 mg CaCO3 alkalinity recovered/mg nitrogen reduced 

 net volatile solids removed/mg nitrogen reduced is dependent upon carbon source and can 

be similar to other biological systems 

For nitrification, alkalinity can become the limiting factor and is often added in high rate systems 

and in low alkalinity waters. With higher nitrogen strength wastewater and in low alkalinity 

waters, alkalinity may limit complete nitrification and thereby restrict nitrogen removal in 

heterotrophic denitrification systems. For wastewaters with 40 – 60 TN, alkalinity levels need to 

be 284 – 426 mg/l for single pass filters and some other treatment processes. With recirculating 

media filters, the alkalinity requirements are reduced as the recirculating media systems 

regenerate some alkalinity and thereby net alkalinity requirements for complete nitrification are 

reduced. Typical alkalinity sources are (EPA, 1993): 

 1.8 mg CaCO3 alkalinity added/mg CaO (quicklime) added 

 1.2 mg CaCO3 alkalinity added/mg NaOH (caustic) added 

In biological denitrification, carbon feed/availability is usually the limiting factor and TN levels 

cannot generally be < 10 mg/l without a carbon feed system. 

As shown on Table 1, the passive NITREXTM Filter system was determined to be the best 

performing nitrogen removal system based upon testing 15 technologies at the LaPine, Oregon 

project (http://www.deq.state.or.us/wq/onsite/LaPine.htm ), as it achieved an effluent TN 

quality of 3 ppm. All data for all the tested systems is publicly available at the above web site and 

was subjected to extensive QA/QC procedures performed by the Oregon Department of 

Environmental Quality. 
 Page 4 of 17 

Technology

Mean 

Effluent TN

Standard 

Deviation

1 NITREX 2.4 0.3

2 Biokreisel 14.0 0.9

3 AX-20 17.0 1.1

4 RX-30 18.8 1.1

5 Amphidrome 26.3 2.0

6 EnviroServer 32.3 1.5

7 FAST, w/o RV 36.4 1.6

8 Nayadic 37.2 1.9

9 Dyno2 50.2 2.5

10 Puraflo 51.4 2.0

11 Lined Sand Filter 51.4 2.6

12 Bottomless Sand Filter 56.5 2.5

13 NiteLess 61.0 2.3

14 Septic Tank 66.1 1.1

15 IDEA 96.8 30.5

Technology Total Nitrogen Effluent Quality (mg/L)

Figures 2 through 7 present the LaPine Total Nitrogen Test Results for septic tank effluent, sand 

filter effluent and NITREXTM Filter system effluent and NITREXTM Filter effluent temperature 

for the LaPine Flemming residence, Stone Residence and Polson, MT site, respectively. 

As can be seen from Figures 2 – 7, the NITREXTM Filter system was able to consistently produce 

an effluent with a total nitrogen of less than 3 – 4 ppm. As it is known that denitrification slows 

with lower temperatures (EPA, 1993), the NITREXTM Filter systems did exhibit less than 

complete denitrification during cold wastewater periods. The data shows that denitrification in 

the NITREXTM Filter system occurs at reduced rates when wastewater temperatures are below 

48°F. The lower reaction rate at lower temperatures can be addressed with longer detention times 

in the Nitrex filter. 
 Page 5 of 17 

Figure 2 - La Pine, Oregon – Fleming Residence – Total Nitrogen Influent & Nitrex Effluent Quality 

Figure 3 - La Pine, Oregon – Fleming Residence, Wastewater Effluent Temperature

Total NItrogen

0

10

20

30

40

50

60

70

80

90

10/1/2000

1/9/2001

4/19/2001

7/28/2001

11/5/2001

2/13/2002

5/24/2002

9/1/2002

12/10/2002

3/20/2003

6/28/2003

Date

TN (mg/L)

Septic Tank

Effluent

Sand Filter

Effluent

Nitrex Effluent

Temperature (C)

0

2

4

6

8

10

12

14

16

18

20

10/1/2000

1/9/2001

4/19/2001

7/28/2001

11/5/2001

2/13/2002

5/24/2002

9/1/2002

12/10/2002

3/20/2003

6/28/2003

Date

Temp (C)

Nitrex Effluent
 Page 6 of 17 

Total NItrogen

0

10

20

30

40

50

60

70

80

10/1/2000

1/9/2001

4/19/2001

7/28/2001

11/5/2001

2/13/2002

5/24/2002

9/1/2002

12/10/2002

3/20/2003

6/28/2003

Date

TN (mg/L)

Septic Tank

Effluent

Sand Filter

Effluent

Nitrex Effluent

Figure 4: La Pine, Oregon – Stone Residence – Wastewater Effluent Temperature

Figure 5: La Pine, Oregon – Stone Residence – Total Nitrogen Influent & Nitrex Effluent Quality

Temperature (C)

0

5

10

15

20

25

10/1/2000

1/9/2001

4/19/2001

7/28/200111/5/20012/13/2002

5/24/2002

9/1/2002

12/10/2002

3/20/20036/28/2003

Date

Temp (C)

Nitrex Effluent
 Page 7 of 17 

Figure 6: Polson, MT – Total Nitrogen Influent & Nitrex Effluent Quality 

Figure 7: Polson, MT – Wastewater Effluent Temperature

Total NItrogen

0

10

20

30

40

50

60

70

Jan-00

Jan-00

Jan-00

Jan-00

Feb-00

Feb-00

Feb-00

Mar-00

Mar-00

Date

TN (mg/L)

Septic Tank

Effluent

Sand Filter

Effluent

Nitrex Effluent

Effluent Temperature (C)

0

5

10

15

20

25

30

Jan-00

Jan-00

Jan-00

Jan-00

Feb-00

Feb-00

Feb-00

Mar-00

Date

Temp (C)

Nitrex Effluent
 Page 8 of 17 

Figure 8: Mashpee, MA 5,750 gpd NITREXTM Wastewater Systems

 
 Page 9 of 17 

Recent data for the 5,750 gpd design flow Nitrex system in Mashpee, MA is presented below: 

Date Total Nitrogen 

May 10, 2006 < 5.0 mg/l 

June 14, 2006 < 7.0 mg/l 

July 18, 2006 < 2.6 mg/l 

MDL

Septic Tank 

Effluent

Waterloo 

Effluent

Nitrex 

Effluent

BOD 2 270 9 130 1. NitrexTM Filter removed all nitrate

TSS 1 174 6 16 2. Waterloo Biofilter did not completely nitrify - 1.3 mg/l NH3 -N 

TKN 0.5 57 4.5 4.4

Ammonia as N 0.02 47 1.3 1.3

Nitrite as N 0.01 < 0.2 0.68 < 0.2

Nitrate as N 0.02 < 0.4 15.9 < 0.4

Org-N Calculated 10 3.2 3.1

TN - Calcualted 57.6 21.08 5.0

Alkalinity 1 252 62 137

pH 0.01 7.2 6.6 6.33

MDL

Septic Tank 

Effluent

Waterloo 

Effluent

Nitrex 

Effluent

BOD 2 320 < 7 160 1. NitrexTM Filter removed all nitrate

TSS 1 218 2 14 2. Waterloo Biofilter did not completely nitrify - 3.2 mg/l NH3 -N 

TKN 0.5 58 5.6 6.7

Ammonia as N 0.02 44 3.2 2.9

Nitrite as N 0.01 0.29 <0.1

Nitrate as N 0.02 32.4 < 0.2

Org-N Calculated 14 2.4 3.8

TN - Calcualted 58 38.29 <7.0

Alkalinity 1 262 10.5 147

pH 0.01 6.82 6.06 6.32

MDL

Septic Tank 

Effluent

Waterloo 

Effluent

Nitrex 

Effluent

BOD 2 65

TSS 1

TKN 0.5

Ammonia as N 0.02 0.23

Nitrite as N 0.01

Nitrate as N 0.02 < 0.4

Org-N Calculated

TN - Calcualted

Alkalinity 1

pH 0.01

MDL

Septic Tank 

Effluent

Waterloo 

Effluent

Nitrex 

Effluent

BOD 2 290 7 49

TSS 1 84 1 2

TKN 0.5 48 1.2 2

Ammonia as N 0.02 35 0.29 0.5

Nitrite as N 0.01 <0.2 <0.2

Nitrate as N 0.02 16 <0.4

Org-N Calculated 13 0.91 1.5

TN - Calcualted 48 17.4 2.6

Alkalinity 1 184 72 155

pH 0.01 6.45 6.9

4. Valves need repaired and by taking off line not needed NitrexTM 

Filter modules, effluent BOD expected to be reduced to be < 30 

McShane Main Street Villages - Mashpee

Design Flow 5,750 gpd

COMMENTS

10-May-06

COMMENTS

26-Jul-06

1. System adjustments appear to be working to reduce ammonia 

from Biofilter and BOD from Nitrex

3. Valves need repaired and by taking off line not needed NitrexTM 

Filter modules, effluent BOD expected to be reduced to be < 30 

COMMENTS

14-Jun-06

3. Insufficient recirculation in the Waterloo appears to be the cause 

of the not complete nitrification as alkalinity was virtually completely 

consumed and pH reduced to 6.06. 

18-Jul-06

COMMENTS

1. System adjustments made on July 11 appear to be working to 

reduce ammonia from Biofilter and BOD from Nitrex
 Page 10 of 17 

Nitrex performance at the Massachusetts Alternative Septic System Test Center over five years 

has been a median of 5.1 m/l TN and a mean of 6.4 mg/l – of which 2.9 mg/l has been due to 

incomplete ammonia nitrification by the recirculating sand filter. 
 Page 11 of 17 
 Page 12 of 17 

Summary of NitrexTM Use Results 

 Numerous installations have shown NitrexTM filter systems to achieve <3 - 5 mg N/l 

 No maintenance required 

 Wastewater Temperatures above 40 to 48 °F optimal - lower temp requires larger systems. 

In colder climates, system insulation is important

The NitrexTM Permeable Reactive Barrier (PRB) Groundwater Treatment has been used for 

groundwater nitrate removal (Robertson, W.D. and J.A. Cherry. 1997) and was recently tested by 

the Woods Hole Marine Biological Laboratory at two coastal installations in Falmouth, MA 

(Cape Cod) under a project funded by the Cooperative Institute for Coastal and Estuarine 

Environmental Technology (CICEET) funded by NOAA. 

Figure 9 illustrates how a PRB operates in removing nitrogen by creation of a reactive zone in the 

groundwater in which heterotrophic denitrification occurs. 

Figure 9 Permeable Reactive Barrier for Groundwater Nitrate Removal 

 

Freshwater Aquifer PRB 

Coastal area PRB 
 Page 13 of 17 

 Figure 10 

The benefits of the PRB are: 

 Treats all sources of N 

 Immediate impact on water quality 

 Significant cost savings potential 

 Localized recharge to aquifers 

PRB Demonstration systems were installed in July 2005, Figure 10, adjacent to the Childs River 

and Waquoit Bay as illustrated in Figures 11 and 12. Groundwater nitrate levels from 1.5 – 3.0 

mg/l have been reduced to non-detect levels. 

Figure 11 – Childs River 

The project interim results are presented at http://www.ciceet.unh.edu/ and indicate that 

groundwater nitrate has been eliminated by the PRB installations. 
 Page 14 of 17 

Flow

TN Effluent 

Requirement Total Cost/gpd

6,000 < 10 $167,000 $27.83

9,500 < 10 $267,000 $28.11

15,000 < 7 $450,000 $30.00

30,000 < 7 $850,000 $28.33

Turnkey Price Includes Engineering and Construction

NitrexTM Treatment Systems Capital Costs 

(Design, Install, & Start-Up) 

 Figure 12 Waquoit Bay 

NitrexTM Design – Build Delivery Mechanism 

 Footprint and not to exceed Budget provided at no cost to assess project feasibility 

 Phase I services – development of fixed cost, final design and permitting 

 Phase II – installation, start-up and performance certification 

Individual residences only available on volume basis

Costs 

•Nitrex Unit only - single family home $4,000 installed 

•Large systems sized based upon: 

•Minimum Temperature 

•Total N to be removed – i.e. from 20 to 5 ppm of N 

•Total System costs for residential applications: 

 New System $18,000 - $24,000 

•Comparison with Membrane Bioreactors 

•Generally 25+ % savings on capital costs 

•40+ % savings on annual O&M costs 
 Page 15 of 17 

OPERATION AND MAINTENANCE AND DISCUSSION

The NITREXTM Filter systems have not required any maintenance (Rich, 2003). These 

installations have demonstrated the ability of passive NITREXTM Filter system to consistently 

achieve a high degree of nitrogen removal during full-scale operation. During start-up elevated 

BOD occur as illustrated in the following Figures. However, initial elevated BOD values have 

not been a problem in drainfields. As can be seen in the Figures, the BOD is virtually all soluble 

as the TSS levels are extremely low, typically < 5 mg/l. In cases where low BOD concentrations 

are required, a post-polishing step is provided. During longer-term operation, BOD stabilizes to < 

20 mg/l depending on factors such as actual flow. Current filter designs incorporate hydraulic 

controls and recycle lines to reduce the start-up period. Monitoring evidence indicates complete 

nitrate removal can be achieved (<0.1 mg/l) while maintaining BOD <20 mg/l. 

NitrexTM Effluent BOD

0

20

40

60

80

100

120

140

160

180

200

220

240

260

0

100200300400500600700

800900

1000

Days After Start-Up

BOD (mg/L)

LaPine Stone Data

LaPine Fleming

MA Otis Data

Mashpee, MA

Start-up occurred in winter for LaPine sites, 2nd peak occurred during warmer 

weather.

Mashpee site is only one with hydraulic controls -adjusted at day 60.
 Page 16 of 17 

REFERENCES: 

Crites, R. and G. Tchobanoglous, 1998. Small and Decentralized Wastewater Management 

Systems, McGraw Hill, Boston, MA. 

Robertson, W.D. and J.A. Cherry. 1997 “Long Term Performance of Waterloo Denitrification 

Barrier”. Land Contamination & Reclamation, Vol. 5 No. 3, pp 183 – 188. 

Robertson, W.D., D.W. Blowes, C.J. Ptacek and J.A. Cherry. 2000. Long-term performance of 

in situ reactive barriers for nitrate remediation. Ground Water 38: 689-695. 

U.S. EPA (Environmental Protection Agency). 1993. Nitrogen control manual. Technical report 

EPA/625/R-93/010, Washington, DC. 

NitrexTM Effluent TSS

0

10

20

30

40

50

60

70

80

90

100

0

100

200

300

400

500

600

700

800

900

1000

Days After Start-Up

TSS (mg/L)

LaPine Stone Data

LaPine Fleming

MA Otis Data

Mashpee, MA

Start-up occurred in winter, 2nd peak occurred 

during warmer weather
 Page 17 of 17 

Valiela, I., M. Geist, J. McClelland and G. Tomasky. 2000. Nitrogen loading from watersheds to 

estuaries: Verification of the Waquoit Bay nitrogen loading model. Biogeochem. 49: 277-293.