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Published online 20 April 2005
Published in J Environ Qual 34:936-942 (2005)
DOI: 10.2134/jeq2004.0024
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Waste Management

Nitrogen Removal in Laboratory Model Leachfields with Organic-Rich Layers

Marjorie E. Bedessem*, Thomas V. Edgar and Robert Roll

Department of Civil and Architectural Engineering, 1000 East University Avenue, Department 3295, University of Wyoming, Laramie, WY 82071

* Corresponding author (bedessem{at}uwyo.edu)

Received for publication January 20, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Septic system leachfields can release dissolved nitrogen in the form of nitrate into ground water, presenting a significant source of pollution. Low cost, passive modifications, which increase N removal in traditional leachfields, could substantially reduce the overall impact on ground water resources. Bench-scale laboratory models were constructed to evaluate the effect of placing an organic layer below the leachfield on total N removal. The organic layer provides a carbon source for denitrification. Column units representing septic leachfields were constructed with sawdust–native soil organic layers placed 0.45 m below the influent line and with thicknesses of 0.0, 0.3, 0.6, and 0.9 m. Using a synthetic septic tank effluent, NO3–N concentrations at 3.8 m below the influent line were consistently below 1 mg L–1 during 10 months of operation compared with a NO3–N concentration of nearly 12 mg L–1 in the control column. The average total N removal increased from 31% without the organic layer to 67% with the organic layer. Total N removal appeared limited by the extent of organic N oxidation and nitrification in the 0.45-m aerobic zone. Design modifications targeted at improving nitrification above the organic layer may further increase total N removal. Increased organic layer thicknesses from 0.3 m to 0.9 m did not significantly improve average total N removal, but caused a shift in residual nitrogen from organic N to ammonia N. Results indicate that addition of a layer of carbon source material at least 0.3 m thick below a standard leachfield substantially improves total N removal.

Abbreviations: BOD5, five-day biochemical oxygen demand • MCL, maximum contaminant level • TKN, total Kjeldahl nitrogen


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE MOST COMMON contaminant identified in ground water is dissolved N in the form of nitrate (Freeze and Cherry, 1979; Canter et al., 1987). This contaminant is becoming increasingly widespread because of agricultural activities and disposal of domestic wastewater on or beneath the land surface (Blevins et al., 1996; Liang and MacKenzie, 1994; Owens et al., 1992; Wilhelm et al., 1994). The largest discharge, by volume, of domestic wastewater into the ground water is from septic tanks (USEPA, 1977; Perkins, 1984). Nitrate N released from septic system drainfields often exceeds the drinking water maximum contaminant level (MCL) of 10 mg L–1 NO3–N and may threaten ground water drinking water sources (Robertson et al., 1991; USEPA, 1999; Walker et al., 1973). As a result, septic systems have been listed as one of the top 10 major sources of ground water pollution by 31 out of the 52 reporting states, tribes, and territories in the USEPA's 2000 national water quality inventory (USEPA, 2001). In Wyoming, a trend toward subdividing large ranch acreages has spawned the development of rural ranchettes, creating heightened concern about the significant numbers of wells and septic systems in previously unimpacted areas.

Previous research on N movement has been conducted at sites with septic tank systems placed within sandy soils (Walker et al., 1973; Childs et al., 1974; Robertson et al., 1991). Nitrate N concentrations were found to exceed the MCL in ground water up to 170 m from the drainfield. Because of the discovery that typical septic systems may not provide enough reduction of nitrate levels before the effluent wastewater reaches ground water supplies, research has been conducted to find reasonable alternatives that will have the appropriate effect (USEPA, 2002). Research in this area has proceeded in many different directions, from active systems that change or add processes to the septic system (Bunnell et al., 1999; Cogger and Carlile, 1984; Piluk and Hao, 1989) to passive systems that change the subsurface environment or add reactive zones around or within the septic system drainfield (Brooks et al., 1984; Gillette, 1993; Rock et al., 1984; Robertson and Cherry, 1995). Alternative technologies are often designed to provide environments that increase biological treatment and remove contaminants through filtration, adsorption, and absorption (USEPA, 2002).

Research by Robertson and Cherry (1995) used reactive porous media layers for passive in situ attenuation of nitrate. The system involved the placement of a layer of reactive material consisting of solid organic carbon below the drainfield that would promote denitrification. Robertson and Cherry (1995) tested sawdust, compost, and rye-seed as their organic sources, and modeled their system as both a reactive organic wall down gradient from the drainfield as well as a reactive layer below.

In one of Robertson and Cherry's field studies using an organic layer placed below the drainfield, NO3–N concentrations were attenuated by an average of 80% through the organic layer; however, ammonia N levels increased by about 12% (Robertson et al., 2000). While the NO3–N concentrations were shown to substantially decrease, the impact on total N, including potential continuing sources of NO3–N such as organic N, remained unclear. In addition, the organic layer field trials were conducted in Ontario, Canada, in a significantly wetter environment than in Wyoming's high altitude semiarid climate. A laboratory column study was thus developed to evaluate potential system performance in sandy Wyoming soils with very low organic content and negligible incident precipitation. Other column studies have been conducted using sawdust or woodchips for nitrate removal (Vogan, 1993; Carmichael, 1994); however, these thesis studies again focused primarily on NO3–N concentrations as they were designed to treat synthetic and natural nitrate-contaminated ground water, respectively. The laboratory simulation described here focuses on an assessment of total N removal from synthetic septic tank effluent, using a sawdust organic layer system constructed of selected Wyoming soils and without precipitation input.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A laboratory model was constructed to simulate conditions in which an organic layer was placed below the drainfield at the time of a septic system installation. Four column units were constructed, one without the organic layer and three with varying organic layer thicknesses.

Laboratory Models
Each column consisted of a 4.6-m-long, 7.6-cm-diameter PVC pipe mounted vertically on a wall, as shown schematically in Fig. 1 . A synthetic wastewater was introduced into each pipe 0.8 m below the top of the pipe to simulate Wyoming design criteria for frost depth. Synthetic wastewater was introduced into a gravel layer, with a native topsoil sample above it. One column contained only the native soil and was the control for the experiments. Columns were filled vertically in small lifts and were carefully and repeatedly agitated to promote settlement of the media to the appropriate depths. Reactive organic layers were placed 0.45 m below the gravel layer in the three test columns. For each column model configuration, the organic layer had a different thickness, either 0.3, 0.6, or 0.9 m. The rest of the PVC pipe, above and below the organic layer, was filled with native soil sample.



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  		Fig. 1. Schematic of deep ground water model, consisting of control and organic layer columns.

 
Sample ports were placed at 0.9-m intervals along the pipe, with other ports placed at important locations, such as just above and below the organic layer. Sample ports were constructed of aquarium filter porous stones with tapered ends inserted into the column to assure good soil contact for sample collection under potentially unsaturated conditions. The porous stones were attached to a brass fitting and collection tubing. A small reservoir was placed to maintain a constant head 0.6 m from the bottom of the column. Oxygen could enter passively from the top of the open column surface and through the ports along the length of the column.

Soils
The native soil used for the laboratory models was a fine sand with a 2% clay content and 4% silt content from Laramie, WY. A number 30 sieve was used to remove grass and other large objects from the native soil. This soil had a 0.4% organic matter content on a dry weight basis after sieving, a soil paste pH of 8.3, and a NO3–N content of 2 mg kg–1. The organic layer was comprised of 30% pine sawdust (v/v) combined with the native sand. Although we use the term "organic layer" to describe this soil mixture, the mixture is not classified as an organic soil horizon using USDA terminology. The sawdust–sand mix had an organic matter content of 5.5% on a dry weight basis, a soil paste pH of 7.7, and a NO3–N content of 13.0 mg kg–1. A 13-mm-diameter gravel was used around the influent piping.

Sample Collection and Chemical Analysis
Samples were collected from the columns biweekly for nitrate and nitrite for approximately ten months, following a two-month acclimation period. Samples were collected monthly for ammonia, total Kjeldahl nitrogen (TKN), and five-day biochemical oxygen demand (BOD5). Ammonia analysis was conducted by the phenate method (4500-NH3 F) using a 10-mL sample volume (American Public Health Association, 1998). Total Kjeldahl N was determined using the Macro-Kjeldahl method (4500-NORGC) and a digestion–distillation unit (Labconco, Kansas City, MO). The phenate method, as mentioned above, was used to determine the ammonia concentration after the distillation procedure. The ammonia nitrogen was then subtracted from the TKN values to determine the organic nitrogen. Five-day BOD analyses were conducted using Method 5210-B along with the azide modification Method 4500-O (American Public Health Association, 1998). Samples were also collected weekly for a four-month period for fecal coliform testing. The sample port porous stone material was tested to confirm that the sampling process would not artificially reduce fecal coliform numbers. Fecal coliform enumeration was determined using the Fecal Coliform Membrane Filter Procedure (9222 D) (American Public Health Association, 1998) using M-FC media with a sample size of either 1 or 0.1 mL.

Nitrate and nitrite were analyzed using a DX-120 ion chromatograph system (Dionex, Sunnyvale, CA) equipped with an AS-40 automatic sampler and Peaknet chromatography software (Dionex). The DX-120 utilized an IonPac AS11-HC analytical anion exchange column (4 x 250 mm) (Dionex). The eluent used with this column was a 30 mmol L–1 solution of 50% NaOH. The flow rate was 1.0 mL min–1.

Synthetic Wastewater
To minimize introduction of pathogens into the laboratory environment, researchers often use synthetic wastewaters in laboratory treatment studies (Cao et al., 1992; Chen et al., 1995; Kargi and Karapinar, 1995; Kuai and Verstraete, 1998; Noto et al., 1998; Raj and Murphy, 1998; Rodgers, 1999; Romanski et al., 1997; Yoo et al., 1999). Laboratory chemicals were combined to develop a synthetic wastewater with characteristics similar to typical septic tank effluent (Brooks et al., 1984; Crites and Tchobanoglous, 1998; Kristiansen, 1981; Piluk and Hao, 1989; Rock et al., 1984; USEPA, 1977, 2002). The synthetic wastewater used in this study contained the following ingredients per liter: 0.5 mg MnCl2·4H2O, 83.6 mg MgCl2·6H2O, 20.6 mg (NH4)2SO4, 83 mg NH4Cl, 66.6 mg (NH4)2CO3, 133.7 mg Na2CO3, 49.9 mg CaCO3, 3.1 mg FeCl3·6H2O, 74.7 mg KH2PO4, 21.4 mg urea, and 277 mg glucose. Seed from the effluent from the Laramie wastewater plant was added at a ratio of 0.3% (v/v). In addition, four species of fecal coliform bacteria (Escherichia coli, Klebsiella pneumonia, Enterobacter aerogenes, Enterococcus fecalis) were cultured on plates of tryptic soy agar (TSA). Colonies were transferred from the plates into the tryptic soy broth and incubated for at least 24 h at 35°C. After incubation, the broth containing the colonies was added to the synthetic wastewater at a ratio of 0.5% (v/v).

The flow rate of the synthetic wastewater was approximately 200 mL d–1 or 4 cm d–1 through each column. This flow rate was chosen based on the expected wastewater loading from a family of four, assuming a wastewater generation rate of about 200 L cap–1 d–1 and a leachfield area of 1 by 25 m, given the selected soils.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Nitrogen Movement in the Control Column
Results from the control column reflect the use of a native Wyoming soil in the drainfield environment of a septic system. The control column provides the conventional comparison to the organic layer columns. The concentration of NO3–N and NO2–N (Table 1) is very low to undetectable in the influent, typical of septic tank effluent, as illustrated by data from the sample port located at 0.8 m below the surface. Within the first 0.15 to 0.6 m of the column below the influent line, NO3–N levels increased up to an average of 15 mg L–1 at the 1.5-m sample port. The dramatic increase in NO3–N concentrations can be attributable to the rapid decrease in ammonia N within this same area of the column. Depletion of organic N did not appear to take place until far below the drainfield. For the first 1.9 m below the drainfield, organic N remained relatively unchanged. The change in organic N levels coincided with an increase in ammonia near and within the ground water level of the column.


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  		Table 1. Statistics for the conventional leachfield model, control column. Influent data is at the 0.8-m depth.

 
Figure 2 illustrates the average concentrations of the various nitrogen components throughout the depth of the control column. Average NO3–N concentrations were over the MCL of 10 mg L–1 NO3–N at all sample ports deeper than 1.4 m. The control column can be said to illustrate the nitrate contamination problem that prompted this study.



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  		Fig. 2. Average N concentration profiles. Comparison between the conventional leachfield model (native soil, control column) and the modified leachfield models (0.3- and 0.9-m organic layer columns). Note "sawdust" refers to a sawdust–sand mix as the organic layer. Hatch marks illustrate the depths of the organic layers in the respective columns. Nitrate and nitrite N values are averaged over the sampling period of October 2001–July 2002 (15–23 samples). Ammonia N values are averaged from March–July 2002 (6 samples) and organic N values were averaged from April–July 2002 (4 samples). Synthetic wastewater was applied from August 2001–August 2002.

 
Nitrogen Movement in the Organic Layer Columns
The three test columns containing organic layers proved to be more capable of reducing total N compared with the control column. The control column removed, on average, only 31% of total N, while the test columns removed up to 67% of the total N (Tables 2 and 3, Fig. 3) . Nitrogen concentration profiles for the 0.3- and 0.9-m layer columns are presented in Fig. 2 and Fig. 3 for visual comparison with the control column. Nitrogen concentration profiles for the 0.6-m organic layer column followed the same trends; however, development of preferential flowpaths caused greater variability in the data. Preferential flow appeared to be along the column walls of the 0.6-m organic layer column and was noted by an increased wastewater flow rate through the column and saturated conditions at selected sampling ports.


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  		Table 2. Statistics for the modified leachfield model, 0.3-m organic layer column. Influent data is at the 0.8-m depth.

 

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  		Table 3. Statistics for the modified leachfield model, 0.9-m organic layer column. Influent data is at the 0.8-m depth.

 


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  		Fig. 3. Average total N concentration profiles. Comparison between control and organic layer columns. Nitrate and nitrite N values were averaged over the sampling period of October 2001–July 2002 (11–21 samples). Ammonia N values were averaged from March–July 2002 (5 samples) and organic N values were averaged from April–July 2002 (4 samples). Synthetic wastewater was applied from August 2001–August 2002.

 
The average NO3–N concentrations were not above the MCL at any point below the organic layers and the average concentration of NO3–N leaving the columns was less than 1 mg L–1. The highest average NO3–N concentration below the organic layers was 5.1 mg L–1 in the 0.6-m layer column (data not shown). Above the layers, individual sampling events sporadically recorded very high NO3–N concentrations (205 mg L–1 NO3–N in the 0.9-m organic layer column), but the layers provided a sufficient denitrification environment to limit the movement of these elevated concentrations. Sporadic high nitrate values above the organic layers have also been noted in field studies (Robertson and Cherry, 1995). Indicator tests for the presence or absence of denitrifying bacterial activity were conducted on three occasions, all with positive results, using water samples from the organic layers and no additional growth media (Focht and Joseph, 1973; Vinther et al., 1999).

The average organic N concentration in the effluent from the organic layer columns was less than 1 mg L–1 compared with the average organic N concentration in the control column effluent of 10 mg L–1. The decrease in organic N concentration coincided with an increase in ammonia N concentrations. Effluent ammonia N in the organic layer columns was approximately 13 mg L–1 compared with approximately 6 mg L–1 in the control column.

An independent t test was conducted to assess whether the average concentrations in the control column and the organic layer columns were significantly different at the 95% confidence level. Statistically significant effluent values are denoted in the data tables. Significant differences between the control and organic layer columns for nitrate and ammonia were noted at all sample ports below 1.8 m from the surface for the 0.3-m layer, and 2.1 m from the surface for the 0.9-m layer.

Five-Day Biochemical Oxygen Demand and Fecal Coliform Removal
Five-day BOD samples were taken monthly at each sample port for four consecutive months. The average influent BOD5 concentration was 210 mg L–1 and removal averaged 93% in both the control and 0.3-m organic layer columns for an average effluent value of 15 mg L–1. Five-day BOD removal averaged 83% in the 0.9-m layer column for an average effluent BOD5 concentration of 36 mg L–1. Fecal coliform concentrations at the influent sample port averaged 7.0 x 104 colony forming units (CFU) per 100 mL. Fecal coliforms were reduced to less than one CFU 100 mL–1 in the majority of samples from the control column within the first 1.1 m below the influent piping. Similarly, fecal coliforms were reduced to less than one CFU 100 mL–1 in the majority of samples from the organic layer columns within the first 0.8 m below the influent piping. The columns can be said to be capable of at least 4 log removal of fecal coliform.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Typical soil infiltration systems remove an average of 10 to 40% total N (USEPA, 2002). The control column used in this study showed a 31% total N removal, consistent with our expectations of typical systems. Jenssen and Siegrist (1990) reviewed numerous laboratory and field studies and found that denitrification accounts for an average of 20% of the total N lost from wastewater infiltrating through the soil.

In ground water environments, the lack of usable organic carbon has been cited as the most common limitation to denitrification (Walker et al., 1973; Wilhelm et al., 1994). Nitrate is formed from the conversion of organic and ammonia N in the septic tank effluent after the majority of the carbonaceous biochemical oxygen demand is depleted. Organic carbon from the septic tank effluent itself is therefore unavailable and the subsurface sediments may provide only a very small amount of organic carbon that may or may not be biodegradable. For example, sand aquifers typically contain less than 0.5% (w/w) organic carbon, as was the case in this study. The addition of the organic layer appeared to provide the organic carbon source for denitrification and increased the total N removal in the layer columns to as much as 67%. Adsorption within the organic layer likely also contributed to the reduction in total N.

Increasing the available organic carbon by increasing the layer thickness, however, did not appear to substantially increase total N removal. In the layer columns, then, one would conjecture that the availability of organic carbon was no longer the limiting factor for denitrification. Examination of the nitrogen speciation throughout the column profiles indicates that a considerable portion of the total N was still in the form of organic and/or ammonia nitrogen at the top of the organic layers. A lack of nitrification in the zone above the layers would tend to preclude subsequent denitrification.

In other leachfield systems, nitrification of wastewater has been shown to occur between 0.6 to 1.2 m of vertical travel (Andreoli et al., 1980). Placement of the layers at only 0.45 m below the influent line in this study may have been too shallow for sufficient nitrification of the ammonia present above the organic layer. A shallow installation depth is attractive for a new drainfield construction to minimize excavation costs. This depth had been selected because it had been proposed for an experimental installation in Nebraska, and the column design did yield a dramatic improvement over the conventional system. However, the control column results suggest that installing the layer at least an additional 0.5 m deeper would have increased the amount of NO3–N entering the organic layer by an average of 72%. This suggests that an improvement in total N removal beyond the 67% shown in this study is more likely to occur by utilizing design modifications targeted to improve nitrification above the installed organic layer, as opposed to increasing the thickness of the organic layer. Design modifications could include methods to increase aeration in the existing aerobic zone or provide an extension of the aerobic zone by deeper installation of the organic layer.

A review of the organic N results for the control column does not show reductions in concentration until very deep in the column, probably below a reasonable depth for placement of the organic layer. In the 0.9-m layer column, organic N concentrations decrease within the organic layer with a concomitant increase in ammonia N. This is consistent with increases in ammonia N noted toward the bottom of the organic layer in previous studies (Robertson and Cherry, 1995). However, organic N concentrations were not specifically evaluated in earlier work. A small amount of N, in the form of organic N (on the order of 2 mg L–1 for the 0.3-m layer column), appeared to be retained within the organic layer and could accumulate only to be flushed or mineralized in the future. However, this estimate is based on assumptions that ammonia increases are due to breakdown of organic N and nitrate and nitrite decreases are attributed to denitrification. The data is insufficient to clearly establish this retention estimate. Further data collection, perhaps at a field site, is warranted. In addition, the ammonia N at depth is a concern as it is also a potential source of nitrate in aerobic ground waters.

Five-day BOD removal in the organic layer columns was comparable with that of the conventional system except in the case of the 0.9-m column. Five-day BOD removal was approximately 10% less, and this may be related to some excess leaching of the solid carbon material. Again this is consistent with reported increases in dissolved organic carbon at the bottom of the organic layer in previous studies (Robertson and Cherry, 1995). Excellent fecal coliform removal was achieved in the uppermost part of the columns before the organic layer. As a result, specific impacts of the organic layers on fecal coliform removal could not be ascertained. Similar coliform removals to that shown in the columns have been documented within the first 0.6 m of subsurface disposal systems (Andreoli et al., 1980; Hagedorn, 1984; Reneau et al., 1989).

This laboratory study is only an estimate of what is likely to occur in the field as column studies are a more controlled environment, minimizing wastewater heterogeneities, temperature fluctuations, precipitation events, or other disturbances. Column studies may be subject to size and edge effects but can provide a picture of potential removal mechanisms. Additional data collection is recommended for field demonstration units taking into account the design recommendations included in this study.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Results from this laboratory column study showed that total nitrogen removal from septic tank effluent increased from 31% in the conventional system to as much as 67% in the organic layer amended systems. Addition of the sawdust–native soil layers effectively reduced the average NO3–N concentrations to less than 1 mg L–1, compared with an average of nearly 12 mg L–1 from the control column. Residual N below the layers was primarily in the form of ammonia N as opposed to organic N in the control column and may still represent a potential source of nitrate contamination for aerobic ground waters.

Total N removal did not substantially improve with an increase in thickness of the organic layer from 0.3 to 0.9 m. Nitrogen removal appeared to be limited by the extent of conversion to nitrate above the organic layers, rather than availability of organic carbon in the organic layer. These results suggest that an improvement in total N removal beyond the 67% shown in this study is more likely achieved by extending the aerobic zone or perhaps improving aeration in the existing zone thickness, rather than increasing the organic layer thickness.


   ACKNOWLEDGMENTS
 
This project was funded through the Wyoming Department of Environmental Quality, Water Quality Division, by a Section 319 Non-Point Source Grant from the United States Environmental Protection Agency.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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JEQ 2005 34: ix. [Full Text]  




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