Preprint, Nutrient Recovery and Management 2011, January 9-12, 2011, Miami, FL Page 1 of 19

Analysis of Organic Nitrogen Removal in Municipal Wastewater by 

Reverse Osmosis 

Rion Merlo1*, Joe Wong1, Victor Occiano2, Kyle Sandera2, Anil Pai2, Seval Sen2, Jose Jimenez3, 

Denny Parker1 and John Burcham4

1 Brown and Caldwell, 201 N. Civic Drive, Suite 115, Walnut Creek, CA 94596

2 Brown and Caldwell, 9665 Chesapeake Drive, Suite 201, San Diego, CA 92123

3 Brown and Caldwell, 850 Trafalgar Court, Suite 300, Maitland, FL 32751

4 City of Escondido, 1521 S. Hale Avenue, Escondido, CA 92029

* To whom correspondence should be addressed. Email: rmerlo@brwncald.com

ABSTRACT

Organic nitrogen (Norg) removal by the reverse osmosis (RO) process is not well documented in 

the literature. Unlike inorganic nitrogen (i.e. ammonia, nitrate, nitrite), which is consistently 

removed across a RO membrane as a function of process operation (i.e. flux and recovery), pH 

and membrane type; the Norg removal will vary depending on its characteristics. Characteristics 

of Norg that could affect its removal include size, charge and hydrophobicity. The results of a 

pilot study that was conducted to determine the total nitrogen (TN) removal possible with the RO 

process are presented. The Norg removal rates are compared with removals observed from three 

full-scale RO facilities and four pilot studies. The results of this analysis suggest that RO may 

not consistently produced TN levels less than 1.0 mg/L without additional treatment. Biological 

nutrient removal (BNR) followed by RO treatment would be necessary to meet stringent TN 

limits. Additional processes such as coagulation or activated carbon may be necessary as well.

INTRODUCTION

The TN content of secondary effluent will consist of ammonia, nitrate, nitrite, and Norg. The Norg

fraction contains particulate, colloidal and soluble species. Dissolved organic nitrogen (DON) is 

defined as the Norg that passes through a 0.45-micron membrane filter. This definition means that 

both soluble and colloidal species will be in the sample filtrate and the sum will be reported as 

DON. If the RO process is used, a microfilter (MF) or ultrafilter (UF) is typically employed 

upstream of the RO as a pretreatment step. This means that the majority, if not all, of the Norg in 

the RO feed will be DON.

For the most part, the secondary effluent DON will consist of organic material present in 

drinking water, material excreted from the activated sludge biomass (termed soluble microbial 

products, SMP), and trace materials introduced in the collection system. Pehlivanoglu-Mantas 

and Sedlak (2008) characterized the effluent of three municipal wastewater treatment plants and 

concluded that less than 30 percent of the DON was amino acids and ethylenediaminetetraacetic 

acid (EDTA) while 70 percent was unidentified. Their results suggest that most of the DON in 

secondary effluent is low molecular weight and hydrophilic.
Preprint, Nutrient Recovery and Management 2011, January 9-12, 2011, Miami, FL Page 2 of 19

Pagilla et al. (2006) reported the results of a survey of United States plants that showed that 

DON accounted for greater than 20 percent of secondary effluent TN. Their survey showed DON 

could account for 2 to 85 percent of effluent TN in the plants surveyed. Bratby et al. (2008) 

presented the results of a survey of secondary effluent DON that showed values ranging from 0.4 

mg/L to 2.2 mg/L and an average value of 1.18 mg/L. Ikenberry et al. (2002) reported secondary 

effluent DON values ranging from 1.0 mg/L to 1.5 mg/L at the Truckee Meadows Water 

Reclamation Facility in Reno, Nevada. Brown et al. (2007) reported an average Norg

concentration of 2.9 mg/L in the secondary effluent of a biological nutrient removal (BNR) plant 

in Stamford, Connecticut.

DON content has been shown to increase across the activated sludge process by several authors

and is attributed to SMP production. Bratby et al. (2008) reported an increase in DON at 

different points in an oxidation ditch designed for nitrogen removal. The soluble organic 

nitrogen, where the colloidal fraction is removed by enhanced coagulation prior to filtration, 

increased from approximately 0.3 mg/L to approximately 0.9 mg/L. Sattayatewa et al. (2009) 

measured an increase in DON in the primary anoxic zone of a 4-stage Bardenpho process and 

concluded this was a result of either anoxic biological activity or heterotrophic denitrification. 

Parkin and McCarty (1981) reported that secondary effluent DON from laboratory-scale 

activated sludge reactors fed with synthetic wastewater was higher during non-steady-state 

conditions than during steady-state conditions. 

Norg removal was of particular importance at the Hale Avenue Resource Recovery Facility 

(HARRF) in the City of Escondido (City), California. The HARRF is an activated sludge plant 

that produces 5 million gallons per day (mgd) of recycled water and discharges approximately 12 

mgd of treated effluent to the ocean via an outfall pipeline. Discharge of treated effluent during 

wet weather is an important factor in the operation of the HARRF. While the influent wastewater 

flow is the greatest during wet weather, the demand for recycled water is lowest during the same 

time period. One infrequently used option is disposal of tertiary-treated effluent to the 

Escondido Creek (live stream discharge). 

The Intermittent Wet Weather Discharge permit issued by the San Diego Regional Water Quality 

Control Board (RWQCB) places stringent requirements on such discharges. The permit limits 

are aimed at protecting water quality in the creek by preventing discharge of excessive amounts 

of nutrients. Although the permit stipulates many conditions and discharge limits on several 

analytical constituents typically found in treated effluent, TN and total phosphorus (TP) are of 

key importance. The permit currently in effect stipulates the following limitations for the key 

analytes listed below:

• TN not to exceed 35 mg/L

• TP not to exceed 3 mg/L

• 5-day carbonaceous biochemical oxygen demand (CBOD5) not to exceed 25 mg/L; and

• Total suspended solids (TSS) not to exceed 30 mg/L

Limitations on TN and TP may be subjected to more stringent criteria in the future; the discharge 

limits could be reduced to 1.0 mg/L TN and 0.1 mg/L TP. The RO process was identified as a 

potential means to reduce effluent TN and TP to meet these levels. A reduction in these 
Preprint, Nutrient Recovery and Management 2011, January 9-12, 2011, Miami, FL Page 3 of 19

constituents would provide the City more flexibility for increasing reuse options. This paper will 

report the results of the HARRF pilot study and compare the Norg removal to other RO pilot 

projects and full-scale RO facilities. 

MATERIALS AND METHODS

Pilot Site

Pilot testing of UF and RO was performed at the HARRF using three different feed waters: 

secondary effluent, pilot-scale cloth media filter (CMF) effluent and full-scale tertiary effluent 

(from existing continuous backwash filters). The HARRF secondary system is a conventional 

activated sludge system with secondary clarification. The system is operated at a solids retention 

time (SRT) between 3 and 5 days and partially nitrifies. 

Ultrafilter Pilot Unit

The UF was a pilot-scale GE-Zenon system. The UF was a submerged membrane system that 

utilizes a vacuum pump for filtration. The UF system performed a daily tank drain after every 

other backwash cycle (approximately every 90 minutes) to remove material that accumulated in 

the process tank. The system was operated at a flux rate ranging from 20 to 25 gallons per square 

foot per day (gfd) during the study. 

Reverse Osmosis Pilot Unit

Prior to the pilot study, a review of RO membrane technologies was performed to identify the 

membrane type that would provide the highest nitrogen removal. In general, data specific to Norg

removal on real world applications are limited. The TML-10 membrane manufactured by Toray

was selected for the pilot study based on available data. Details of the TML-10 membrane are 

provided in Table 1. The RO skid was operated in a 3-stage configuration. During the pilot 

study, the RO was operated at an average flux ranging from 10 gfd to 12 gfd and 80 percent 

recovery. Sulfuric acid was used for pH adjustment of the RO feed to optimize ammonia 

removal. A proprietary anti-scalant chemical was added to mitigate inorganic fouling.

Table 1. Summary of TML-10 Membrane Manufactured by Toray Used 

for the HARRF Pilot Study

Parameter Value

Material Cross-Linked Fully Aromatic 

Polyamide Composite

Diameter 4 inch

Surface Area 73 square feet

Salt Rejection 99.7 percent

Production Flow Rate 1,850 gallons per day

Maximum Operating Pressure 600 pounds per square inch (psi)

Maximum Feed Water Temperature 45 degrees C

Acceptable Feed Water pH Range 2-11
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Analytical Methods

Samples were collected from the UF influent and RO effluent throughout the test period. 

Refrigerated composite samplers were used for sample collection. The composite samplers were 

time based; since the pilot systems were operated at a constant flow there was no need for flowpaced sampling. Table 2 summarizes the methods and detection limits that were performed 

during the pilot study. For purposes of data analysis of pilot data and survey results, data points 

that were reported as below the detection limit were assumed to be half the detection limit value 

for calculation and reporting purposes. Norg is determined by subtracting the measured ammonia 

concentration from the measured total Kjeldahl nitrogen (TKN) content.

Table 2. Summary of Analytical Methods to Determine Norg Removal Used for the 

HARRF Pilot Study

Parameter Sample Location Method Detection Limit

TKN UF Influent/RO Effluent SM4500-N(org) B 0.13 mg/L

Ammonia UF Influent/RO Effluent SM4500-NH3 B, C 0.13 mg-N/L

Nitrate UF Influent/RO Effluent EPA 300 0.015 mg-N/L

Nitrite UF Influent/RO Effluent EPA 300 0.02 mg-N/L

Pilot Schedule

The pilot study consisted of three operating periods. During Period 1, the UF/RO system was fed 

with water from a pilot-scale CMF (Aqua Aerobic-Systems, Inc., Rockford, IL). The CMF 

treated secondary effluent. During Period 2, the UF/RO system was fed with secondary effluent. 

During Period 3, the UF/RO system was fed with tertiary effluent from the full-scale continuous 

backwash filters. Operating the UF/RO system on the continuous backwash filter effluent was 

performed last because there was concern with potential irreversible fouling of the UF and RO 

membranes that may be caused by the coagulant and polymer that is used prior to filtration.

Figure 1 presents a process flow diagram of the different operating periods.

Figure 1. Process flow diagram of illustrating each operating period during pilot study.

HARRF 

Secondary 

Effluent

UF/RO Pilot 

System

CMF Pilot 

System

Tertiary Filter

Period 1

Period 3

Period 2
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RESULTS OF PILOT STUDY

Table 3 summarizes the UF influent nitrogen species for each of the periods and the total of all 

periods. The TN content of the UF influent did not vary significantly throughout the testing 

period; the average for all periods was 19.4 mg/L. For all periods, the secondary system partially 

nitrified, which is evidenced by the presence of ammonia, nitrate and nitrite in the UF influent. 

The high amounts of nitrite (2.12 mg-N/L, average) indicates the nitrification process is 

incomplete in the secondary system. This is because the secondary process is operated at a 

relatively low SRT (approximately 3 to 5 days) and the dissolved oxygen (DO) concentration in 

the aeration basins is approximately 1.0 mg/L. The Norg content of the UF influent was 1.97 

mg/L when averaged for all periods. The highest Norg value measured was 2.57 mg/L. The Norg 

content was similar for all periods, whether or not filtration was performed upstream of the UF. 

This is attributed to the low TSS content of the secondary effluent during the testing period (<10 

mg/L); most of the Norg was assumed to be soluble.

Table 4 summarizes the RO effluent data collected during the pilot study. The average RO 

effluent TN concentration was 1.88 mg/L. A significant fraction of the RO effluent TN consisted 

of Norg (0.61 mg/L, average). Table 5 summarizes the rejection of each nitrogen species by the 

UF/RO system. The average TN removal was 91 percent and the inorganic nitrogen average 

rejection rates ranged from 89 to 95 percent. The Norg removal was the lowest of all nitrogen 

species with an average rejection of 68 percent. There was not a significant difference in UF/RO 

performance during each operation period; effluent nitrogen concentrations and rejection rates 

were relatively consistent.
Preprint, Nutrient Recovery and Management 2011, January 9-12, 2011, Miami, FL Page 6 of 19

Table 3. Summary of UF/RO Influent Data from HARRF Pilot Study

Parameter Unit Count Average Min Max

Period 1

TKN mg/L 5 6.82 5.43 7.50

Ammonia mg-N/L 5 4.95 3.43 5.44

Nitrate mg-N/L 6 8.63 7.99 9.29

Nitrite mg-N/L 6 2.81 1.99 4.11

Norg mg/L 5 1.86 1.57 2.06

TN mg/L 5 18.3 16.2 19.4

Period 2

TKN mg/L 5 13.6 9.48 15.4

Ammonia mg-N/L 5 11.7 7.29 14.3

Nitrate mg-N/L 9 6.39 3.44 8.77

Nitrite mg-N/L 9 1.73 0.81 2.50

Norg mg/L 5 1.93 0.81 2.54

TN mg/L 5 20.9 19.4 23.6

Period 3

TKN mg/L 5 11.9 8.11 18.4

Ammonia mg-N/L 5 9.73 6.38 16.0

Nitrate mg-N/L 6 4.79 3.93 6.04

Nitrite mg-N/L 6 2.17 0.29 3.17

Norg mg/L 5 2.12 1.73 2.57

TN mg/L 5 19.1 17.3 22.6

All Periods

TKN mg/L 15 10.8 5.43 18.4

Ammonia mg-N/L 15 8.78 3.43 16.0

Nitrate mg-N/L 21 6.57 3.44 9.29

Nitrite mg-N/L 21 2.17 0.29 4.11

Norg mg/L 15 1.97 0.81 2.57

TN mg/L 15 19.4 16.2 23.6
Preprint, Nutrient Recovery and Management 2011, January 9-12, 2011, Miami, FL Page 7 of 19

Table 4. Summary of UF/RO Effluent Data from the HARRF Pilot Study

Parameter Unit Count Average Min Max

Period 1

TKN mg/L 7 0.99 0.83 1.15

Ammonia mg-N/L 7 0.47 0.35 0.54

Nitrate mg-N/L 7 0.53 0.35 0.75

Nitrite mg-N/L 7 0.20 0.10 0.55

Norg mg/L 7 0.52 0.41 0.66

TN mg/L 7 1.72 1.40 2.04

Period 2

TKN mg/L 10 1.27 0.82 2.11

Ammonia mg-N/L 10 0.67 0.37 1.56

Nitrate mg-N/L 11 0.48 0.18 2.06

Nitrite mg-N/L 11 0.19 0.01 0.89

Norg mg/L 10 0.60 0.32 1.35

TN mg/L 10 1.96 1.34 5.06

Period 3

TKN mg/L 5 1.46 1.36 1.64

Ammonia mg-N/L 5 0.71 0.61 0.75

Nitrate mg-N/L 5 0.29 0.14 0.49

Nitrite mg-N/L 5 0.19 0.14 0.28

Norg mg/L 5 0.75 0.61 1.03

TN mg/L 5 1.94 1.66 2.16

All Periods

TKN mg/L 22 1.22 0.82 2.11

Ammonia mg-N/L 22 0.61 0.35 1.56

Nitrate mg-N/L 23 0.45 0.14 2.06

Nitrite mg-N/L 23 0.19 <0.02 0.89

Norg mg/L 22 0.61 0.32 1.35

TN mg/L 22 1.88 1.34 5.06
Preprint, Nutrient Recovery and Management 2011, January 9-12, 2011, Miami, FL Page 8 of 19

Table 5. Summary of UF/RO Rejection Data from the HARRF 

Pilot Study

Parameter Count Average Min Max

Period 1

Ammonia 5 89.3% 85.7% 91.2%

Nitrate 6 93.8% 91.3% 95.6%

Nitrite 6 92.7% 84.3% 95.6%

Norg 5 71.4% 67.0% 80.1%

TN 5 90.4% 88.7% 91.7%

Period 2

Ammonia 5 94.5% 93.3% 95.8%

Nitrate 7 95.3% 94.4% 96.3%

Nitrite 7 91.5% 77.1% 99.4%

Norg 5 66.1% 32.1% 78.0%

TN 5 92.4% 91.7% 94.0%

Period 3

Ammonia 4 91.7% 89.0% 95.3%

Nitrate 5 94.2% 91.8% 96.3%

Nitrite 5 82.6% 47.6% 95.4%

Norg 4 66.9% 59.2% 75.2%

TN 4 89.9% 87.5% 92.7%

All Periods

Ammonia 14 91.8% 85.7% 95.8%

Nitrate 18 94.5% 91.3% 96.3%

Nitrite 18 89.4% 47.6% 99.4%

Norg 14 68.2% 32.1% 80.1%

TN 14 91.0% 87.5% 94.0%

RESULTS OF SURVEY OF FULL-SCALE FACILITIES

Data from three full-scale facilities using the RO process to treat municipal wastewater were 

analyzed and compared with the HARRF pilot study results.

Groundwater Replenishment System 

The Groundwater Replenishment System (GWRS), located in Fountain Valley, California 

purifies secondary effluent wastewater for groundwater injection and for spreading basins. The 

GWRS system treats secondary effluent wastewater from the Orange County Sanitation District 

(OCSD) Plant Number 1. Plant Number 1 effluent consists of a combination of effluent from a 

non-nitrifying activated sludge process and a trickling filter process.

The treatment process at GWRS consists of MF (Siemens-Memcor), RO (Hydranautics ESPA2 

membranes), advanced oxidation, decarbonation and lime addition. The RO membranes are 
Preprint, Nutrient Recovery and Management 2011, January 9-12, 2011, Miami, FL Page 9 of 19

operated at a 12-gfd average flux and an 85-percent recovery. Sodium hypochlorite is added 

upstream of the RO to form chloramines from the ammonia present in the wastewater at a target 

of 3 mg/L total chlorine. The RO influent pH is adjusted to 6.8 using sulfuric acid. The MF 

backwash is discharged back to Plant Number 1 upstream of the primary clarifiers.

Table 6 summarizes the TKN, ammonia and Norg values for MF/RO influent and effluent for the 

GWRS from February 2008 through February 2010. For Norg samples where the difference 

between TKN and ammonia is zero, a value of 0.05 mg/L is assumed for reporting and 

calculations. The MF influent average TKN was 23.0 mg/L and the average ammonia 

concentration was 21.0 mg/L. These values are higher than the values measured during the 

HARRF pilot test because Plant Number 1 does not produce a nitrified effluent, whereas the 

HARRF produces partially nitrified secondary effluent. The average Norg concentration for the

MF influent is 2.03 mg/L, which is similar to the average value measured during the HARRF 

pilot test (1.97 mg/L). It is interesting to note that the GWRS shows a much lower Norg value in 

the RO effluent (0.10 mg/L) compared to the value determined for the HARRF pilot study (0.61 

mg/L).

Table 7 summarizes the ammonia and Norg removal through the MF/RO system. The average 

ammonia removal for GWRS (93%) is similar to the value measured for the HARRF data (92%). 

There is a large difference in Norg removal between the two data sets where GWRS produces 

94% average Norg removal and the HARRF pilot study only had 68%.

Table 6. Summary of MF/RO Influent and Effluent Data for the GWRS 

(February 2008 through February 2010)

Parameter Unit Count Average Min Max

MF Influent

TKN mg/L 115 23.0 2.50 33.0

Ammonia mg-N/L 115 21.0 1.20 30.8

Norg mg/L 115 2.03 0.30 4.20

RO Effluent

TKN mg/L 116 1.32 0.20 2.10

Ammonia mg-N/L 116 1.24 0.20 1.90

Norg mg/L 116 0.10 <0.05 0.80

Table 7. Summary of MF/RO Rejection Data for GWRS 

(February 2008 through February 2010)

Parameter Count Average Min Max

Ammonia 115 93.3% 72.7% 96.0%

Norg 115 94.4% 73.3% 98.4%

Water Replenishment District of Southern California

The Water Replenishment District (WRD) of Southern California owns and operates the Vander 

Lans Facility that treats tertiary effluent from the Long Beach Water Reclamation Plant 
Preprint, Nutrient Recovery and Management 2011, January 9-12, 2011, Miami, FL Page 10 of 19

(LBWRP). The LBWRP is a BNR plant and has tertiary filtration. The Leo J. Vander Lans 

Advanced Water Treatment Facility (AWTF) consists of MF, RO, advance oxidation, 

decarbonatoin, and sodium hypochlorite addition. The MF is a Pall Microza system and the RO 

membrane is Hydranautic ESPA2. The RO process is operated at an 85-percent recovery and an 

average flux of 9.9 gfd. Sulfuric acid and threshold inhibitor are added upstream of the RO unit. 

Treated effluent is pumped to the Alamitos Barrier Blend Station.

Table 8 summarizes final effluent data from the LBWRP effluent and AWTF effluent for 2009

and Table 9 summarizes the rejection rates. The average influent Norg was 1.91 mg/L, which is 

similar to the values reported at GWRS and the measured values for the HARRF pilot study. 

Similar to GWRS, the Norg in the AWTF effluent is low (<0.2 mg/L). However, the average TN 

is 1.47 mg/L. 

Table 8. Summary of WRD Data

(January 2009 through December 2009)

Parameter Unit Count Average Min Max

LBWRP Final Effluent (January 2008 through December 2009)

Ammonia mg-N/L 12 1.07 0.58 1.72

Nitrate mg-N/L 12 5.85 3.92 7.71

Nitrite mg-N/L 12 0.08 0.03 0.18

Norg mg/L 12 1.91 0.90 2.79

TN mg/L 12 8.91 6.82 11.51

AWTF Effluent (January 2009 through December 2009)

Ammonia mg-N/L 74 0.17 <0.025 0.52

Nitrate mg-N/L 74 1.22 0.43 3.00

Nitrite mg-N/L 73 0.06 <0.005 0.18

Norg mg/L 72 <0.2 <0.2 0.26

TN mg/L 72 1.47 0.61 3.40

Table 9. Summary of MF/RO Rejection Data for WRD

(January 2009 through December 2009)

Parameter Count Average Min Max

Ammonia 6 81.2% 69.7% 88.7%

Nitrate 6 89.8% 87.4% 94.3%

Nitrite 6 26.3% 0.00% 91.7%

Norg* 6 93.5% 86.9% 96.4%

TN 6 89.5% 82.6% 94.7%

*Calculated using half the detection limit to represent RO effluent (half of 0.2 mg/L 

or 0.1 mg/L)

Scottsdale Water Campus

The Scottsdale Water Campus located in Scottsdale, Arizona is an advanced treatment facility 

that produces high-quality effluent using MF/RO treatment. The secondary process consists of an 
Preprint, Nutrient Recovery and Management 2011, January 9-12, 2011, Miami, FL Page 11 of 19

activated sludge process operated in a Modified Ludzack-Ettinger (MLE) configuration for 

nitrogen removal. Table 10 summarizes the nitrogen species measured in the MF feed and RO 

effluent for January 2009 through June 2010. The average Norg concentration for the MF feed 

was 1.05 mg/L, which is the lowest value measured compared to the HARRF pilot results, 

GWRS and WRD. The average RO effluent Norg concentration was 0.14 mg/L, which is similar 

to the results obtained from the other full-scale facilities. Table 11 summarizes the ammonia and 

Norg removal through the MF/RO process. The average Norg removal was 81 percent, which is 

lower than the other full-scale facilities, but higher than the HARRF pilot results. However, the 

lower Norg removal measured at the Scottsdale Water Campus is attributed to the low MF feed 

Norg concentration (1.05 mg/L, average) and the RO effluent Norg detection limit. In reality, the 

Norg removal may be similar to the other full-scale facilities if lower detection limits were 

employed.

Table 10. Summary of Scottsdale Water Campus MF Feed and RO 

Effluent (January 2009 through June 2010)

Parameter Unit Count Average Min Max

MF Feed

Ammonia mg-N/L 12 0.88 0.49 1.99

Norg mg/L 12 1.05 0.14 1.93

RO Effluent

Ammonia mg-N/L 13 0.16 <0.1 0.51

Norg mg/L 13 0.14 <0.1 0.39

Table 11. Summary of MF/RO Rejection Data for Scottsdale 

Water Campus (January 2009 through June 2010)

Parameter Count Average Min Max

Ammonia* 12 78.6% 49.9% 95.7%

Norg* 12 80.7% 17.1% 98.6%

* Calculated using half the detection limit to represent RO effluent 

(half of 0.1 mg-N/L or 0.05 mg/L for ammonia; half of 0.5 mg/L or 

0.25 mg/L for TKN)

RESULTS FROM OTHER PILOT STUDIES

Data from four pilot studies where the RO process was used to treat municipal wastewater were 

analyzed and compared with the HARRF pilot study results and full-scale facilities.

Miami-Dade Pilot Study

Miami-Dade County, Florida conducted a pilot study evaluating several RO pilot processes as a 

part of a nutrient removal study (Botero et al., 2010). The results of the pilot study for each of the 

systems tested are presented in Table 12. With the exception of the Hydranautics RO, all RO 

systems were operated in a single-stage configuration; the Hydranautics RO was operated as 

both single-stage and 3-stage configurations. For the single-stage systems, which were conducted 

mainly for performance comparison of RO membranes from various vendors, the average RO 
Preprint, Nutrient Recovery and Management 2011, January 9-12, 2011, Miami, FL Page 12 of 19

effluent Norg concentrations ranged from 0.32 mg/L to 0.54 mg/L. It is interesting to note that the 

3-stage system had a higher Norg concentration (0.66 mg/L) than when it was operated in a 

single-stage configuration (0.39 mg/L). This is expected since the higher recovery will result in 

higher Norg concentration at the membrane surface. The lowest average TN concentration was 

1.49 mg/L for all systems. These results are similar to the results obtained from the HARRF pilot 

study.

Table 12. Summary of Miami Dade Pilot RO Effluent

TN Norg

RO Pilot Process Average Minimum Maximum Average Minimum Maximum

CSM 1.49 0.20 2.10 0.34 0.10 0.60

Koch 1.51 0.90 2.10 0.32 0.10 0.80

Toray 2.01 1.20 2.80 0.38 0.10 0.90

Dow 2.30 0.12 3.60 0.54 0.10 1.50

Hydranautics 1.81 0.87 2.60 0.39 0.10 1.10

Hydranautics (Three Stage) 2.67 1.10 4.20 0.66 0.10 2.10

Luggage Point Advanced Water Treatment Plant

Schimmoller et al. (2008) presented results from a seven month pilot study. The pilot system was 

designed to represent the performance of the Luggage Point Advanced Water Treatment Plant in 

South East Queensland, Australia. The final effluent objectives for TN were less than 0.8 mg/L. 

The pilot system treated secondary effluent wastewater using flocculation and sedimentation, 

MF, RO and advanced oxidation. Ferric chloride was added to the flocculation and 

sedimentation at an average dose of approximately 100 mg/L. Ferric chloride was added to 

mitigate RO fouling by removing phosphorus (to reduce calcium phosphate scaling) and 

organics. An average total organic carbon (TOC) removal of 24 percent was reported during the 

study. The MF system was a Pall pilot system and the RO membranes were Toray TML-4040. 

Table 13 summarizes the nitrogen removal reported by Schimmoller et al. (2008). Using the 

values reported in Table 13 for TN removal, the MF/RO feed TN value must be less than 6.67 

mg/L in order to meet the treatment objective of less than 0.8 mg/L of TN. The Norg removal rate 

reported in Table 13 is similar to the removal determined from the full-scale facilities.

Table 13. Summary of Nitrogen Removal (25 

Samples) Reported by Schimmoller et al. (2008)

Parameter Value

Ammonia 83%

TKN 88%

Nitrate 87%

Nitrite 94%

Norg 92%

TN 88%
Preprint, Nutrient Recovery and Management 2011, January 9-12, 2011, Miami, FL Page 13 of 19

Bureau of Reclamation – Membrane Bioreactors for Water Reclamation

Adham et al. (2000) presented the results of a pilot study where MBR effluent was fed to a single 

pass RO system. Table 14 summarizes the nitrogen species measured in the MBR effluent and 

RO effluent. The results in Table 14 were collected from a GE-Zenon pilot MBR and a single 

stage Dow RO (BW30-4040) operated with a 50-percent recovery. It is important to note that 

chloramines were fed to the RO feed to mitigate biofouling on the RO membrane. The average 

Norg concentration in the MBR effluent was 0.76 mg/L. The average Norg concentration in the RO 

effluent was 0.10 mg/L; the average Norg removal across the RO was 86.6 percent. These values 

are similar to the full-scale facilities.

Table 14. Summary of Bureau of Reclamation Data (Adham et al., 2000)

Zenon MBR Effluent RO Effluent

Date

Ammonia, 

mg-N/L

TKN, 

mg/L

Nitrate, 

mg-N/L1

Norg, 

mg/L

Ammonia, 

mg-N/L2

TKN, 

mg/L

Nitrate, 

mg-N/L

Norg, 

mg/L

Norg

Rem.

08/09/99 2.08 3.04 0.96 0.96 0.54 0.65 0.50 0.10 89.2%

09/07/99 0.11 0.94 4.23 0.83 0.21 0.31 0.30 0.10 88.1%

09/21/99 0.31 1.18 2.67 0.87 0.13 0.27 0.12 0.13 84.6%

11/09/99 0.56 1.24 12.07 0.68 0.10 0.22 0.58 0.13 81.1%

01/04/00 1.28 1.91 10.85 0.63 0.07 0.11 0.63 0.04 93.2%

02/14/00 0.38 0.99 20.00 0.61 0.16 0.26 0.71 0.10 83.2%

Average 0.79 1.55 8.46 0.76 0.20 0.30 0.47 0.10 86.6%

1 MBR was operated with anoxic zone for nitrogen removal (8/9/99 through 9/21/99).

2 Chloramines were added downstream of the MBR permeate sample and upstream of the RO feed to control biological fouling 

during the study. 

Aqua 2000 Pilot Summary

The City of San Diego pilot tested the RO process at the San Pasqual Water Reclamation Plant

from 1996 to 1997 (Montgomery Watson, 1997). At this time, the City considered implementing 

a reuse program to supplement the drinking water supply. This initiative has not been 

implemented in San Diego to date. As a part of the pilot study, several RO processes were tested 

at pilot scale in conjunction with UF and MF. The UF and MF systems were fed effluent from 

water hyacinth ponds (fed with municipal primary effluent) followed by ferric chloride 

coagulation, sedimentation and granular filtration. The pilot units were operated in a two-stage 

configuration at an approximate 75-percent recovery. Table 15 summarizes the results of this 

study. The membrane types were kept anonymous for the testing results. Table 15 shows that the 

MF/UF influent Norg concentration was 0.67 mg/L and the average RO effluent ranged from 

<0.18 mg/L to 0.25 mg/L. These effluent values are similar to the values measured at the fullscale facilities.
Preprint, Nutrient Recovery and Management 2011, January 9-12, 2011, Miami, FL Page 14 of 19

Table 15. Summary of Aqua 2000 Pilot Results for Norg

RO Pilot Process

Average, 

mg/L

Minimum, 

mg/L

Maximum, 

mg/L

MF/UF Influent 0.67 0.18 2.80

MF-RO-A Effluent 0.25 0.18 0.77

MF-RO-B Effluent <0.18 --- ---

MF-RO-C Effluent <0.18 --- ---

MF-RO-D Effluent 0.20 <0.18 0.27

UF-RO-A Effluent <0.18 --- ---

UF-RO-B Effluent <0.18 --- ---

UF-RO-C Effluent <0.18 --- ---

DISCUSSION

Figure 2 presents a summary of the average Norg concentration measured in RO effluent from the 

HARRF pilot study results and the survey results. The Miami-Dade and HARRF pilot studies, 

which treated partially nitrified effluent, had the highest RO effluent Norg concentrations. In 

general, the full-scale plants had relatively low Norg content (<0.1 mg/L) in the RO effluent. It is 

not possible to draw any significant conclusions regarding the differences in RO performance 

between data sets since the nature of the Norg has not been characterized at each site. The Norg

concentration in the secondary effluent will consist of material present in the drinking water, 

SMP produced in the secondary system and trace materials introduced in the collection system. 

This means that not only the influent wastewater, but also the secondary process operation, will 

affect the Norg concentration and characteristics. Figure 3 summarizes the Norg removal measured 

by the MF/RO (or UF/RO, depending on the data set) process. The removals rates determined 

from these data sets vary from less than 70% to greater than 90%. 
Preprint, Nutrient Recovery and Management 2011, January 9-12, 2011, Miami, FL Page 15 of 19

Figure 2. Summary of average RO effluent Norg values (values that were less than the 

detection limit are expressed as half the detection limit value)

Figure 3. Summary of average Norg Removal through UF/RO or MF/RO.

0.01

0.10

1.00

Average RO Effluent Norg Concentration, mg/L

0

10

20

30

40

50

60

70

80

90

100

GWRS WRD Luggage Point Scottsdale Water 

Campus

HARRF Pilot Study

Norg Removal, percent
Preprint, Nutrient Recovery and Management 2011, January 9-12, 2011, Miami, FL Page 16 of 19

Table 16 presents the results of a sensitivity analysis designed to evaluate the impact of upstream 

nitrogen removal on the final TN concentration after RO. The average effluent TN

concentrations for the two nutrient removal processes that would feed the RO were assumed to 

be 10 mg/L and 4 mg/L. Representative technologies that could achieve a TN of 10 mg/L would 

be the MLE or step feed BNR processes. Representative technologies that could achieve a TN of 

4 mg/L would be the Bardenpho process, or the MLE (or step feed) process followed by 

denitrifying filters. A range of secondary effluent Norg (1.0 mg/L to 3.0 mg/L) and a range of Norg

removal by RO (50 percent to 90 percent) were used for the analysis. The average ammonia and 

nitrate removals by RO measured during the HARRF pilot study were assumed.

Table 16 shows that significant nitrogen removal upstream of the RO process would be 

necessary to achieve a TN concentration of less than 1.0 mg/L. As the level of nutrient removal 

increases, the RO process is more likely to achieve a TN concentration of less than 1.0 mg/L.

Less upstream nutrient removal could result in the necessity of additional treatment to remove 

nitrogen species in addition to RO. In addition, higher concentrations of Norg in the secondary 

effluent could result in the necessity for additional treatment.

In order to meet an annual average limit for TN of less than 1.0 mg/L, a safety factor would need 

to be assumed using the results in Table 16 to reduce the likelihood of a violation occurring in a 

permit cycle. It is important to note that the analysis presented in Table 16 only considers the 

average removal rates and assumed average nitrogen content in the secondary effluent. If permit 

requirements were written such that monthly or daily maximum TN value of less that 1.0 mg/L 

were necessary, additional nitrogen removal will be necessary.

The results presented in Table 16 agree with the data presented by Schimmoller et al. (2008) who 

showed that RO effluent could produce TN less than 1.0 mg/L. This required nutrient removal in 

the secondary system to remove the majority of the inorganic nitrogen. In addition, it is possible 

that the FeCl3 addition used to mitigate phosphorus reported in Schimmoller et al. (2008) could 

have contributed to a significant reduction in Norg.

There are other processes that could be used to remove Norg in addition to RO. Bratby et al. 

(2008) presented results of survey that showed that enhanced coagulation removed, on average, 

31 percent of DON in secondary effluent wastewater. Arnaldos and Pagilla (2010) performed 

batch coagulation and microfiltration experiments using secondary effluent from a full-scale 

wastewater plant. Their results showed that DON could be reduced from approximately 1.2 mg/L 

to approximately 0.3 mg/L using aluminum sulfate at a molar ratio dose of 1.4 Al(III)/initial 

DON. They attributed most of the removal to the coagulation, rather than the microfiltration. 

Parkin and McCarty (1981) showed that activated carbon could be effective for DON removal 

with an average measured value of 72 percent. 
Preprint, Nutrient Recovery and Management 2011, January 9-12, 2011, Miami, FL Page 17 of 19

Table 16. TN Removal Sensitivity Analysis Using Different Nutrient Removal Processes and a Range of Secondary Effluent Norg

Nutrient 

Removal 

Process

Sec. Eff. 

Norg

Conc., 

mg/L

Sec. Eff. 

Ammonia 

Conc., 

mg-N/L

Sec. Eff. 

Nitrate 

Conc., 

mg/L

Norg 

Removal, 

percent

Ammonia 

Removal, 

percent

Nitrate 

Removal, 

percent

RO Eff. 

Norg

Conc., 

mg/L

RO Eff. 

Ammonia 

Conc., 

mg-N/L

RO Eff. 

Nitrate 

Conc., 

mg-N/L

RO Eff. 

TN Conc., 

mg/L

Add. Norg

Removal, 

mg/L

TN = 10 mg/L

Representative Technologies: MLE, Step Feed 

BNR

1.0 0.2 8.8 65 91.8 94.5 0.35 0.02 0.48 0.85 NA

1.0 0.2 8.8 75 91.8 94.5 0.25 0.02 0.48 0.75 NA

1.0 0.2 8.8 85 91.8 94.5 0.15 0.02 0.48 0.65 NA

1.0 0.2 8.8 95 91.8 94.5 0.05 0.02 0.48 0.55 NA

2.0 0.2 7.8 65 91.8 94.5 0.70 0.02 0.43 1.15 0.15

2.0 0.2 7.8 75 91.8 94.5 0.50 0.02 0.43 0.95 NA

2.0 0.2 7.8 85 91.8 94.5 0.30 0.02 0.43 0.75 NA

2.0 0.2 7.8 95 91.8 94.5 0.10 0.02 0.43 0.55 NA

3.0 0.2 6.8 65 91.8 94.5 1.05 0.02 0.37 1.44 0.44

3.0 0.2 6.8 75 91.8 94.5 0.75 0.02 0.37 1.14 0.14

3.0 0.2 6.8 85 91.8 94.5 0.45 0.02 0.37 0.84 NA

3.0 0.2 6.8 95 91.8 94.5 0.15 0.02 0.37 0.54 NA

TN = 4 mg/L

Representative Technologies: Bardenpho, MLE

(or step feed) with dentrifying filters

1.0 0.2 2.8 65 91.8 94.5 0.35 0.02 0.15 0.52 NA

1.0 0.2 2.8 75 91.8 94.5 0.25 0.02 0.15 0.42 NA

1.0 0.2 2.8 85 91.8 94.5 0.15 0.02 0.15 0.32 NA

1.0 0.2 2.8 95 91.8 94.5 0.05 0.02 0.15 0.22 NA

2.0 0.2 1.8 65 91.8 94.5 0.70 0.02 0.10 0.82 NA

2.0 0.2 1.8 75 91.8 94.5 0.50 0.02 0.10 0.62 NA

2.0 0.2 1.8 85 91.8 94.5 0.30 0.02 0.10 0.42 NA

2.0 0.2 1.8 95 91.8 94.5 0.10 0.02 0.10 0.22 NA

3.0 0.2 0.8 65 91.8 94.5 1.05 0.02 0.04 1.11 0.11

3.0 0.2 0.8 75 91.8 94.5 0.75 0.02 0.04 0.81 NA

3.0 0.2 0.8 85 91.8 94.5 0.45 0.02 0.04 0.51 NA

3.0 0.2 0.8 95 91.8 94.5 0.15 0.02 0.04 0.21 NA
Preprint, Nutrient Recovery and Management 2011, January 9-12, 2011, Miami, FL Page 18 of 19

CONCLUSIONS

The RO process is designed to remove charged species. Manufacturer reported rejection rates for 

ammonia, nitrate and nitrite can be used to estimate the removal of these parameters at full-scale. 

However, the nature and concentration of Norg in secondary effluent will vary depending on the 

wastewater. Therefore, it is not possible to know the actual Norg concentration in RO effluent 

without testing. The Norg in secondary effluent can consist of low molecular weight and 

uncharged species that may not be removed through the RO process. In short, the RO process is 

not designed to effectively remove Norg, it is designed to removed charged ionic species.

For municipalities that are evaluating processes to achieve low TN levels, the concentration and 

characteristics of Norg is crucial and may require additional treatment. Upstream BNR to target 

inorganic nitrogen coupled with a physical/chemical process (i.e. coagulation or activated

carbon) to target Norg could be potential solutions.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the HARRF operations, maintenance and laboratory staff who 

contributed to the pilot study. Specifically, we would like to acknowledge Dennis Sperino, James

Larzalere, Pete Klein, and Vasana Vipatapat. The authors would like to acknowledge all of the 

people who contributed data to this effort. These include: Mehul Patel (GWRS), Paul Fu (WRD), 

Binga Talabi (Scottsdale Water Campus) and David Faber (Woongjin CSM).

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