Effects of Aeration on Water Quality from Septic System Leachfields

Effects of Aeration on Water Quality from Septic System Leachfields
Effects of Aeration on Water Quality from Septic System Leachfields
David A. Potts, Josef H. Görres, Erika L. Nicosia, and José A. Amador*
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
ABSTRACT
tems, new technologies that lower nutrient and pathogen emissions have been developed.
The capacity of leachfield soils to enhance water quality can vary substantially with environmental conditions.
For instance, fluctuations in depth to the water table
and in soil temperature can affect leachfield functioning
(Bouma et al., 1975; Cogger and Carlile, 1984; Viraraghavan and Dickenson, 1991). One approach to improving the quality of water coming out of septic systems is to
promote conditions that enhance contaminant removal
and/or retention in the leachfield. The biogeochemical
transformations in leachfield soil are controlled by the
type and availability of electron acceptors and donors.
For example, sufficiently high levels of oxygen are necessary for microbial oxidation of ammonium to nitrate.
Nitrate may then be removed by denitrification, but only
if the amount of organic carbon is sufficient to support
the activities of denitrifying bacteria. In addition, most
of the soil meso- and microfauna thought to be involved
in pathogen removal use O2 as an electron acceptor. Levels of O2 may be suboptimal below the biomat due to
the combined effects of high rates of microbial activity
and low gas diffusion rates through leachfield soil. Enhanced aeration of leachfield soil may thus improve its
ability to remove nutrients and inactivate pathogens via
abiotic and biological processes that require oxygen. It
may also promote anaerobic processes in microsites,
such as denitrification, that rely on the products of aerobic processes. Finally, aeration may be expected to enhance wastewater infiltration by reducing or eliminating
the biomat (e.g., Erickson and Tyler, 2001).
A patented process for the rejuvenation of leachfields
using aeration (Potts, 2000) has been successful in restoring hydraulic function in more than 60 failed onsite
wastewater treatment systems in the eastern United States.
The effects of this process on water quality leaving the
leachfield, however, are not known. We conducted a
pilot-scale study to evaluate the effects of aeration levels
on the quality of water coming out of leachfields. The
effluent from a household septic tank was passed through
lysimeters filled with sand to a depth of 30 cm. Silica
sand with a high uniformity coefficient was used because
it is chemically inert and it represents the shortest retention time and thus the case with the least effluent treatment. Experimental treatments consisted of lysimeters
vented to the leachfield, representing the conditions in
a conventional system, and lysimeters aerated to maintain an oxygen concentration of 0.20 to 0.21 mol mol⫺1.
We measured the concentration of total N and P, pH,
ammonium, nitrate, sulfate, phosphate, reduced iron,
BOD5, fecal coliforms, and Escherichia coli in the septic
tank effluent coming into the lysimeters and in the drainage water from aerated and leachfield lysimeters. We
We conducted a pilot-scale study at a research facility in southeastern Connecticut to assess the effects of leachfield aeration on removal
of nutrients and pathogens from septic system effluent. Treatments
consisted of lysimeters periodically aerated to maintain a headspace
O2 concentration of 0.209 mol mol⫺1 (AIR) or vented to an adjacent
leachfield trench (LEACH) and were replicated three times. All lysimeters were dosed with effluent from a septic tank for 24 mo at a rate
of 12 cm d⫺1 and subsequently for 2 mo at 4 cm d⫺1. LEACH lysimeters
had developed a clogging mat, or biomat, 20 mo before the beginning
of our study. The level of aeration in the AIR treatment was held
constant regardless of loading rate. No conventional biomat developed
in the AIR treatment, whereas a biomat was present in the LEACH
lysimeters. The headspace of LEACH lysimeters was considerably
depleted in O2 and enriched in CH4, CO2, and H2S relative to AIR
lysimeters. Drainage water from AIR lysimeters was saturated with
O2 and had significantly lower pH, five-day biological oxygen demand
(BOD5), and ammonium, and higher levels of nitrate and sulfate than
LEACH lysimeters regardless of dosing rate. By contrast, significantly
lower levels of total N and fecal coliform bacteria were observed in
AIR than in LEACH lysimeters only at the higher dosing rate. No
significant differences in total P removal were observed. Our results
suggest that aeration may improve the removal of nitrogen, BOD5,
and fecal coliforms in leachfield soil, even in the absence of a biomat.
A
pproximately 23% of households in the United
States rely on onsite wastewater treatment systems
for disposal of domestic sewage (United States Census
Bureau, 2003). Conventional septic systems are designed
for removal of solids in the septic tank, and dispersal of
wastewater in the associated leachfield. The passage of
effluent through leachfield soil results in the removal
of pathogens and biodegradable organic carbon at rates
generally exceeding 90% (USEPA, 2002). Removal rates
for N and P in the leachfield of conventional septic systems are more modest, ranging from 50 to 85% for P
and 0 to 40% for N (Kaplan, 1987; USEPA, 2002). The
water quality enhancement functions of leachfields are
thought to be associated with the biological and hydraulic processes that take place in the clogging mat, or
biomat, at the infiltration surface of the leachfield trench
(USEPA, 2002). There is growing concern among water
management and regulatory agencies that failing or improperly installed septic systems cause contamination
of ground and surface waters with pathogens, nutrients,
and biologically active compounds (Canter and Knox,
1985; Yates, 1985). In response to more stringent regulation of the quality of effluent delivered by septic sysD.A. Potts, Geomatrix, LLC, Killingworth, CT 06419. J.H. Görres,
E.L. Nicosia, and J.A. Amador, Laboratory of Soil Ecology and Microbiology, University of Rhode Island, Kingston, RI 02881. Received
8 Dec. 2003. *Corresponding author ([email protected]).
Published in J. Environ. Qual. 33:1828–1838 (2004).
 ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA
Abbreviations: AIR, aerated lysimeters; BOD5, five-day biological
oxygen demand; LEACH, lysimeters vented to leachfield.
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POTTS ET AL.: WATER QUALITY FROM SEPTIC SYSTEM LEACHFIELDS
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
also determined the composition of the atmosphere in
the headspace in both treatments. The effects of aeration were determined at loading rates of 12 cm d⫺1
(approximately 3 gallons ft⫺2 d⫺1) and 4 cm d⫺1 (approximately 1 gallon ft⫺2 d⫺1).
MATERIALS AND METHODS
Experimental Facility
The study was conducted in a laboratory facility built adjacent to a two-story, two-family home in southeastern Connecticut, USA. The home was built in 1983 and was fitted with a
new conventional septic system in 1996. The septic tank had
a maximum capacity of 4733 L (1250 gallons) and was not
pumped during the course of the study. The home was inhabited continuously by three to six people.
A schematic diagram of the experimental setup is shown
in Fig. 1. All of the effluent from the septic tank was diverted
to a pump station and stored in a high-density polyethylene
(HDPE) tank (1325-L [350-gallon] maximum capacity) housed
in a climate-controlled (17–19⬚C) room above the laboratory.
The contents of the tank were mixed at regular intervals using
a pump. Wastewater from the tank was pumped through a
3.75-cm-diameter (1.5-in) PVC manifold to a series of dosing
tanks in the laboratory. Cylindrical, HDPE dosing tanks
(30.5-cm [12-in] i.d., 45.7-cm [18-in] height) had a maximum
capacity of 38 L (10 gallons) and were dosed every 6 h. Dosing
was regulated using electronically actuated valves. Dosing
tank overflow was allowed to drain completely until only the
desired dose volume was retained.
Wastewater from the dosing tank flowed by gravity into a
lysimeter that consisted of a HDPE cylinder (43.2-cm [17-in]
i.d., 45.7-cm [19-in] height) fitted with a drainage fitting 1.91
cm (0.75 in) from the bottom, and water and gas input fittings
and inspection port on top. Wastewater was delivered to the
surface of the soil through a horizontal, 1.91-cm-diameter
(0.75-in) PVC pipe in which 0.64-cm-diameter (0.25-in) holes
were drilled into the top of 0.1-cm (0.04-in) slotted well screen
mesh. The bottom of the lysimeter was filled with 7.5 cm of
no. 4 silica sand (diameter ⫽ 4.75 to 1.40 mm; uniformity
coefficient ⬍1.8), on top of which was placed 30 cm (12 in)
of no. 00 silica sand (diameter ⫽ 0.71 to 0.21 mm; uniformity
coefficient ⬍1.6) (U.S. Silica Co., Berkeley Springs, WV), with
headspace constituting the volume above the sand.
Lysimeters were dosed with wastewater at a rate of 12 cm
d⫺1 (approximately 3.0 gallons ft⫺2 d⫺1) for the first 24 mo of
the experiment. LEACH lysimeters had developed a clogging
mat, or biomat, approximately 20 mo before the beginning of
our study, as indicated by periodic visual inspection. On 21
Apr. 2003 the dosing rate was changed to 4 cm d⫺1 (approximately 1.0 gallon ft⫺2 d⫺1) to determine the extent to which
the effects of aeration on leachfield water quality were affected
by dosing rate. The aeration level in the AIR treatment was
kept constant regardless of dosing rate.
Treatments
Treatments consisted of lysimeters with aerated (AIR) or
unaerated (LEACH) headspace, with each treatment replicated three times. Aeration was accomplished using a patented
process developed by Geomatrix, LLC (Potts, 2000). This
process has been used successfully in hydraulic rejuvenation
of failed septic system leachfields, but its effects on water
quality are not known. Ambient air was pumped at regular
intervals into the headspace of AIR lysimeters to maintain an
1829
O2 level of 0.20 to 0.21 mol mol⫺1 using a piston pump. This
resulted in a slight (approximately 2.5–6.7 kPa) positive pressure within AIR lysimeters. To mimic the in situ composition
of the atmosphere found in the leachfield, the headspace and
the gravel bed below the soil of LEACH lysimeters were
vented to the septic system leachfield.
Sampling and Analyses
Sampling and Processing
Raw wastewater samples were collected from a valve in
the input stream (Fig. 1) and placed in autoclaved, 125-mL
polypropylene screw-cap bottles. One to two liters of wastewater were allowed to flow through the valve before sample
collection. Water samples from the lysimeters were collected
in 3-L Tedlar bags (2-mil thick; SKC, Eighty Four, PA) at
ambient temperature (17–19⬚C). The bag was connected to
the lysimeter outlet using Tygon tubing. To ensure that water
samples were exposed to an atmosphere with the same composition as that found in the lysimeters, a connection was made
from the headspace of each lysimeter to the drainline connected to the sampling bag. Sampling of water from the lysimeters started coincident with a dosing event, and continued for
60 to 90 min. Between 700 and 900 mL of water were collected
from each lysimeter on each sampling date.
A portion of the raw wastewater and lysimeter water samples was analyzed immediately for dissolved oxygen and the
presence of Fe2⫹. The remaining sample was kept on ice during
transport to the laboratory in Kingston, RI (approximately
1 h). Unfiltered samples were assayed immediately on arrival
to the laboratory for BOD5, pH, and total fecal coliforms and
E. coli. A portion of the unfiltered sample was frozen for
subsequent determination of total N and total P content. The
remaining sample was filtered by passing through a glass microfiber filter (GF/F, 25-mm diameter; Whatman, Maidstone,
UK). The filtered samples were stored in plastic, screw-cap
scintillation vials at 4⬚C and analyzed for NH4⫹ and NO3⫺ within
24 h, for PO43⫺ within 48 h, and for SO4⫺2 within 72 h of collection.
Leachfield gases venting into LEACH lysimeters and headspace gases from the AIR and LEACH lysimeters were sampled using a portable soil gas monitor (SoilAir Technology,
East Longmeadow, MA). Carbon dioxide, CH4, O2, and H2S
were determined using infrared, catalytic bead, galvanic, and
electrochemical sensors, respectively. Measurements were
made approximately 2 h after dosing of lysimeters with septic
tank effluent. Gas samples were drawn at a rate of approximately 0.05663 m3 h⫺1 (2.0 standard ft3 h⫺1) for 30 to 60 s and
the maximum values detected during that sampling period are
reported for all gases except O2, for which the minimum value
is reported.
Analyses
Constituent analyses were performed according to methods
of the American Public Health Association (1998). Dissolved
oxygen was measured using the azide modification of the
Winkler titration method. The concentration of Fe2⫹ in water
was determined using EM Quant iron (Fe2⫹) test strips (EM
Industries, Gibbstown, NJ). The pH of water samples was
determined using a combination pH electrode and a Model
UB-10 pH meter (Denver Instruments, Denver, CO). The concentration of sulfate was measured using the barium chloride
turbidimetric method. Nitrate, ammonium, and phosphate concentrations of water samples were determined colorimetrically
using an automated nutrient analyzer (Flow Solution IV; Alp-
J. ENVIRON. QUAL., VOL. 33, SEPTEMBER–OCTOBER 2004
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
1830
Fig. 1. Schematic diagram of experimental facility (top), lysimeters (middle), and detail of leachfield gas intake (bottom). Drawings are not to scale.
kem, College Station, TX). The total N and total P content of
water samples was determined using the persulfate digestion
method. Samples were digested by autoclaving at 121⬚C and
analyzed colorimetrically for NO3⫺ and PO43⫺ as described
above. Fecal coliforms and E. coli were assayed using the
standard fecal coliform membrane filtration procedure. The
BOD5 was measured on undiluted, unamended samples by
manometric respirometry using an OxiTop BOD system
POTTS ET AL.: WATER QUALITY FROM SEPTIC SYSTEM LEACHFIELDS
(WTW, Fort Myers, FL) at 21 ⫾ 1⬚C. Volumes were 250 mL
for raw and LEACH samples, and 432 mL for AIR samples.
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
Statistical Analyses
Differences between AIR and LEACH treatments were
evaluated using Student’s t test at the 95% confidence level
(SigmaStat for Windows, Version 2.03; SPSS, 1995). Statistical
analyses were performed on untransformed data except for
total coliform bacteria, where data were log-transformed.
RESULTS
Operating Conditions
LEACH lysimeters had developed a clogging mat, or
biomat, approximately 20 mo before the beginning of
our study, as indicated by periodic visual inspection.
Ponding in LEACH lysimeters was observed throughout the sampling period, with the volume of ponded
effluent increasing when the loading rate decreased
from 12 to 4 cm d⫺1, probably as a result of the reduced
hydraulic head at the lower loading rate. By contrast,
visual inspection of AIR lysimeters indicated that a biomat had not formed at the soil surface before the beginning of the study, and none formed by the end of the
sampling period. Ponding was not observed in AIR lysimeters regardless of dose. The temperature of septic
system effluent coming into the lysimeters was generally
2 to 3⬚C lower than that of drainage water from the
lysimeters (Fig. 2). Both values increased with time, and
were close to ambient temperature (17–19⬚C) by the
final sampling date. We did not observe treatment differences in the temperature of lysimeter drainage water.
Headspace Gases
Levels of O2 in the gases coming into the LEACH
lysimeters ranged from 0.06 to 0.160 mol mol⫺1 during
the period examined. Oxygen in the headspace of the
LEACH treatment ranged from 0.078 to 0.197 mol mol⫺1,
whereas in the AIR treatment the concentration of oxygen was 0.209 mol mol⫺1 on all sampling dates (Table 1).
The concentration of CO2 in leachfield gases ranged
from 0.013 to greater than 0.05 mol mol⫺1. Carbon dioxide levels were one to two orders of magnitude higher
in LEACH than AIR treatments throughout the measurement period. Methane levels in leachfield gases
ranged from 2 to ⬎50 000 ⫻ 10⫺6 mol mol⫺1. Levels of
CH4 in the headspace of LEACH lysimeters varied from
750 to ⬎50 000 ⫻ 10⫺6 mol mol⫺1. By contrast, methane
concentrations in the AIR treatment ranged from 0 to
65 ⫻ 10⫺6 mol mol⫺1. No hydrogen sulfide was detected
in the AIR lysimeters on any sampling date, whereas
the concentration of H2S in LEACH lysimeters ranged
from 0 to ⬎100 ⫻ 10⫺6 mol mol⫺1.
Water Quality Parameters
The pH of water from LEACH lysimeters was indistinguishable from that of the raw wastewater, regardless
of sampling date, ranging between 6.2 and 7.0 (Fig. 2).
By contrast, water from AIR lysimeters had pH values
1831
that were consistently and significantly lower than in
the LEACH treatment, with values ranging from 3.9
to 4.4.
Dissolved oxygen levels in incoming wastewater ranged
from 0 to 0.9 mg O2 L⫺1 (Fig. 2). Aeration enhanced
dissolved oxygen levels, with values ranging from 8.1 to
11.9 mg O2 L⫺1 in water from AIR treatments. Dissolved
oxygen in the LEACH treatment was significantly lower
than in the AIR treatment on all sampling dates, with
values ranging from 0 to 3.3 mg O2 L⫺1.
The BOD5 of incoming wastewater ranged from 73
to 198 mg L⫺1 (Fig. 2). Values of BOD5 in LEACH
lysimeters ranged from 38 to 168 mg L⫺1, with average
removal rates of 29.6 and 60.9% at loading rates of 12
and 4 cm d⫺1, respectively (Table 2). Values of BOD5
in AIR lysimeters were significantly lower than in the
LEACH treatment, and were below the detection limit
of 1 mg L⫺1, on all sampling dates, with removal rates
higher than 99% (Table 2).
Values for fecal coliforms and E. coli were identical
in all instances, and only levels of fecal coliforms are
reported (Fig. 3). Levels of fecal coliforms and E. coli
in wastewater ranged from 103 to 106 colony-forming
units (CFU) 100 mL⫺1 (Fig. 3). Numbers of fecal coliforms and E. coli were significantly lower in AIR than
in LEACH treatment on all sampling dates. Reduction
in fecal coliforms and E. coli ranged from 98.0 to 98.6%
(1 to 3 log units) in the LEACH treatment and 99.2 to
99.9% (2 to 6 log units) in the AIR treatment (Table 2).
The total N concentration in incoming wastewater
ranged from 22 to 48 mg N L⫺1 with inorganic N making
up 60 to 80% of the total N pool (Fig. 4). The concentration of total N was significantly lower in AIR than in
LEACH lysimeters only on the first three sampling dates,
when the nominal loading rate was 12 cm d⫺1. After the
wastewater loading rate was reduced to 4 cm d⫺1 (while
holding the aeration level constant) there were no statistically significant differences in total N between treatments. Removal of nitrogen in LEACH lysimeters at the
high loading rate was 1.3%, whereas in AIR lysimeters it
was 23.6% (Table 2). Little to no net removal of total
N was observed in either treatment at the low dosing
rate (Table 2). Inorganic nitrogen constituted between
60 and 90% of total N in water from AIR lysimeters,
and 90 to 100% of total N in LEACH lysimeters. Differences in NO3⫺ and NH4⫹ concentrations between treatments were statistically significant on all sampling dates,
with NO3⫺ dominating the inorganic N pool in the AIR
lysimeters and NH4⫹ constituting the bulk of the inorganic N pool in LEACH lysimeters (Fig. 4).
Levels of total P in incoming wastewater ranged from
2.8 to 11.8 mg L⫺1, with phosphate constituting 30 to
50% of total P (Fig. 5). No significant differences between treatments were observed in the concentration
of total P or phosphate coming out of lysimeters on any
sampling date (Fig. 4). Furthermore, there was no net
removal of total P in either treatment (Table 2); rather,
there was a net increase of 1 to 13.4% in total P lysimeter
drainage water relative to effluent inputs.
The concentration of sulfate in incoming wastewater
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
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J. ENVIRON. QUAL., VOL. 33, SEPTEMBER–OCTOBER 2004
Fig. 2. Time course of pH, dissolved oxygen, five-day biological oxygen demand (BOD5), and temperature of raw wastewater input and of
drainage water from aerated lysimeters (AIR) and lysimeters vented to the leachfield (LEACH). The wastewater loading rate was changed
from 12 to 4 cm d⫺1 on 21 Apr. 2003. Temperature was not measured on 12 Feb. 2003. Values are means (n ⫽ 3). Bars represent one standard
deviation. Significant differences between AIR and LEACH treatments (P ⬍ 0.05) for a particular sampling date are indicated with an
asterisk (*).
ranged from 2.9 to 7.4 mg L⫺1 (Fig. 6). Water from
LEACH lysimeters had significantly lower concentrations of SO24⫺ than AIR lysimeters. Sulfate levels were
reduced 40 to 50% in LEACH lysimeter water relative
to incoming values, whereas the concentration of sulfate
increased by a factor of 2 to 3 after passage through
AIR lysimeters. No Fe2⫹ was detected in raw wastewater
or in water from AIR lysimeters, whereas low levels of
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POTTS ET AL.: WATER QUALITY FROM SEPTIC SYSTEM LEACHFIELDS
Table 1. Concentration of O2, CO2, CH4, and H2S in the headspace of aerated lysimeters (AIR) and lysimeters vented to the leachfield
(LEACH) and in the gas inputs to LEACH lysimeters on different sampling dates.†
Sampling date
Treatment
O2
CO2
mol
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
12 Mar. 2003
7 Apr. 2003
14 May 2003
4 June 2003‡
25 June 2003
LEACH, input gases
LEACH, headspace
AIR, headspace
LEACH, input gases
LEACH, headspace
AIR, headspace
LEACH, input gases
LEACH, headspace
AIR, headspace
LEACH, input
AIR, headspace
LEACH, input
AIR, headspace
CH4
mol⫺1
0.160
0.140–0.197
0.209
⬍0.021
0.085–0.135
0.209
0.040
0.078
0.209
0.006
0.209
0.093
0.209
H2S
10⫺6
0.013
0.007–0.024
0–0.0003
⬎0.05
⬎0.05
0.0004–0.0008
⬎0.05
⬎0.05
0–0.0004
⬎0.050
0–0.0012
⬎0.050
0–0.0002
mol
30 000
750 to ⬎50 000
15–35
⬎50 000
⬎50 000
15–65
31 250
⬎50 000
0
20 000
0–2
3
0–5
mol⫺1
⬎100
0–65
0
⬎100
⬎100
0
⬎100
⬎100
0
12
0
0
0
† Headspace values represent range for three replicate lysimeters. Values for LEACH input gases represent a single measurement. No measurements
were made on 12 Feb. 2003.
‡ The headspace in LEACH lysimeters was flooded on 4 and 25 June 2003, preventing sampling of headspace gases.
Fe2⫹ (0–3 mg L⫺1) were observed in LEACH lysimeters
on the first four sampling dates (data not shown).
DISCUSSION
Headspace Gases and Dissolved Oxygen
Aeration of soil in AIR lysimeters resulted in levels
of O2 identical to those found in ambient air, and resulted in drainage water saturated with oxygen. By contrast, LEACH lysimeters, which were vented to the
leachfield trench, had relatively low headspace O2 levels
and associated low concentrations of O2 in drainage
water. There was little or no measurable O2 in incoming
wastewater, so O2 dissolved in the drainage water of
LEACH lysimeter must have come from gases in the
headspace and the gravel below the sand bed. Both of
these were vented to the septic system leachfield, and
O2 was present in varying concentrations in the incoming
vent gases (Table 1). Walker et al. (1973) found levels of
O2 and CO2 of 0.196 and 0.007 mol mol⫺1, respectively,
in soil 5 to 10 cm below the biomat of a leachfield from
a conventional septic system, suggesting that the soil
was fairly well aerated. However, our own field measurements, using the same equipment and methodology
as in the present study, suggest that O2 levels in the
atmosphere of the soil below a leachfield can vary widely
(0–0.09 mol mol⫺1), even at depths much greater than
the 30 cm used in our study. The presence of high levels
of H2S and CH4 in the gases venting from the leachfield
supports the contention that anaerobic conditions prevailed in the leachfield trench above the biomat on most
sampling dates. Methane was also found in the headspace of AIR lysimeters, albeit at concentrations approximately 1000 times lower than in the LEACH treatment
(Table 1). The presence of CH4 in the AIR treatment
may be the result of out-gassing of methane dissolved
in incoming wastewater and/or the establishment of anaerobic conditions in microsites within the lysimeters,
perhaps shortly after dosing with wastewater.
nitrate. Nitrification results in proton release that, in
the poorly buffered quartz sand used in this experiment,
results in acidic conditions. The low pH values observed
in the AIR lysimeters support this interpretation. The
net loss of total N in AIR lysimeters on the first three
sampling dates is probably due to denitrification, which
is promoted by the presence of high levels of nitrate in
the effluent. Conversely, passage of wastewater through
LEACH lysimeters only resulted in ammonification of
wastewater N, with little or no NO3⫺ production or net
removal of N. The absence of net N losses in LEACH
lysimeters suggests that denitrification did not take
place in this treatment, probably limited by very low
nitrate levels.
Denitrification may occur in anaerobic microsites
even when oxygen concentrations in the bulk medium
are high (Conrad, 1996; Sexstone et al., 1985). In addition, denitrification can take place under aerobic conditions. For example, Robertson and Kuenen (1991) have
shown that in batch culture experiments, Thiosphaera
pantotropha, a known denitrifier, can remove O2 and
nitrate simultaneously, and the presence of both electron acceptors resulted in more rapid growth on acetate
than when either O2 or nitrate was present. Studies of
nitrogen removal in buried sand filters (of similar age
and under similar loading and temperature conditions
to ours) have also suggested that denitrification, rather
than microbial immobilization, is responsible for total
Table 2. Average extent of removal for total N, total P, fiveday biological oxygen demand (BOD5), and fecal coliforms in
aerated lysimeters (AIR) and lysimeters vented to the leachfield (LEACH) at different loading rates.
Removal rate at a loading rate of
Parameter
Treatment
Total N
LEACH
AIR
LEACH
AIR
LEACH
AIR
LEACH
AIR
Aeration had a strong effect on N removal and speciation and it clearly promotes oxidation of ammonium to
4 cm d⫺1
%
Total P
BOD5
Transformation and Removal of Nitrogen
12 cm d⫺1
Fecal coliforms
1.3
23.6
(8.2)†
(13.4)
29.6
99.5
98.0
99.1
† Values in parentheses indicate average increases.
0.8
2.9
(1.0)
(10.6)
60.9
99.3
98.9
99.9
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
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J. ENVIRON. QUAL., VOL. 33, SEPTEMBER–OCTOBER 2004
Fig. 3. Time course of fecal coliform bacteria in raw wastewater input and in drainage water from aerated lysimeters (AIR) and lysimeters
vented to the leachfield (LEACH). The wastewater loading rate was changed from 12 to 4 cm d⫺1 on 21 Apr. 2003. Values are means (n ⫽ 3).
Bars represent one standard deviation. Significant differences between AIR and LEACH treatments (P ⬍ 0.05) for a particular sampling
date are indicated with an asterisk (*).
N losses (e.g., Gold et al., 1992). Measurement of the
gaseous products of denitrification (e.g., NO, N2O)
should help in elucidating the role this process has in
N removal in leachfield soils.
The fact that net N removal in AIR lysimeters was
limited to those sampling dates when wastewater was
applied at the higher rate (12 cm d⫺1) suggests that the
length of time the soil remains saturated with wastewater has a strong influence on whether N is removed
by denitrification in the leachfield. The additional organic
C associated with higher wastewater loading may also
have enhanced denitrification by providing additional
electron donors and/or promoted localized anaerobic
conditions. In addition, transient increases in soil water
content of a greater magnitude at the higher dosing rate
may also have interfered with soil aeration. Denitrification in septic system leachfields is thought to be limited
by the availability of organic C in wastewater (Sikora
et al., 1976). Plósz et al. (2003) observed a similar effect
of organic substrate input on denitrification rates in an
anoxic reactor exposed to oxygen, which they attributed
to high dissolved oxygen consumption, which in turn
supported denitrifying conditions. Gaseous losses of N
may also be due to the activities of nitrifying bacteria,
which produce NO and N2O under anaerobic or microaerophilic conditions (Groffman, 1991). Such conditions
may be established temporally in areas of the lysimeter
after dosing with wastewater.
The presence of nitrate in drainage water from
LEACH lysimeters, albeit at low levels, indicates that
nitrification is taking place in the soil, since there is no
detectable nitrate in incoming wastewater. However,
the low concentration of nitrate suggests that nitrification is inhibited to some extent under these conditions.
Levels of dissolved oxygen in LEACH water were between 0 and 4 mg L⫺1 (Fig. 2) and O2 was present in
the leachfield gases (Table 1), although generally at low
concentrations. The high concentration of methane may
have inhibited ammonia oxidation. Methane is a competitive inhibitor of ammonia oxidation (Bedard and
Knowles, 1989) and was present at concentrations exceeding 20 000 ⫻ 10⫺6 mol mol⫺1 on most sampling dates
(Table 1). By contrast, extensive nitrification appeared
to take place in AIR lysimeters, as indicated by levels
of NO3⫺ in drainage water that accounted for up to twothirds of the total N inputs from septic tank effluent
(Fig. 4). High levels of dissolved O2 and NH4⫹ and methane levels that were three orders of magnitude lower
than in LEACH lysimeters probably created conditions
conducive for nitrification in AIR lysimeters. Our results suggest that the role of methane as an inhibitor of
ammonia oxidation in leachfield soils warrants further
study.
Removal of Biological Oxygen Demand
Greater BOD5 removal in AIR than in LEACH lysimeters indicates that microbial decomposition of organic carbon is restricted under LEACH conditions.
The enhanced availability of O2 as a terminal electron
acceptor in AIR lysimeters probably results in a shift
toward aerobic respiration, which is more energetically
efficient than fermentative and anaerobic respiration
pathways (Fuhrmann, 1998). This may explain the absence of biomat development in AIR lysimeters, since
a greater proportion of organic C inputs would be oxidized to CO2. Removal of BOD5 in AIR lysimeters was
not affected by loading rate, whereas removal in LEACH
lysimeters loaded at 4 cm d⫺1 was double that at 12 cm
d⫺1. The effect of loading rate on BOD5 removal in the
LEACH treatment may reflect the limited capacity of
the LEACH soil to provide the microbial community
with the types and levels of electron acceptors necessary
to metabolize organic compounds efficiently.
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
POTTS ET AL.: WATER QUALITY FROM SEPTIC SYSTEM LEACHFIELDS
1835
Fig. 4. Time course of total N, ammonium N, and nitrate N in raw wastewater input and in drainage water from aerated lysimeters (AIR) and
lysimeters vented to the leachfield (LEACH). The wastewater loading rate was changed from 12 to 4 cm d⫺1 on 21 Apr. 2003. Values are
means (n ⫽ 3). Bars represent one standard deviation. Significant differences between AIR and LEACH treatments (P ⬍ 0.05) for a particular
sampling date are indicated with an asterisk (*).
Sulfate Production
Higher levels of sulfate in AIR than in LEACH lysimeters are indicative of enhanced microbial oxidation of
reduced S compounds found in septic tank effluent. For
example, a number of species within the genus Thiobacillus derive energy from the oxidation of H2S and
S2O23⫺ (formed from bacterial reduction of organic sulfur
compounds) within the pH values observed in our ex-
periments (Germida et al., 1992). Furthermore, some
of these bacteria are also capable of denitrification
(Kanter et al., 1998; Robertson and Kuenen, 1991), possibly contributing to the loss of N in AIR lysimeters.
Total Phosphorus and Phosphate
The lack of PO34⫺ or total P removal in AIR or
LEACH lysimeters is not surprising. The main mecha-
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
1836
J. ENVIRON. QUAL., VOL. 33, SEPTEMBER–OCTOBER 2004
Fig. 5. Time course of total P and phosphate P in raw wastewater input and in drainage water from aerated lysimeters (AIR) and lysimeters
vented to the leachfield (LEACH). The wastewater loading rate was changed from 12 to 4 cm d⫺1 on 21 Apr. 2003. Values are means (n ⫽ 3).
Bars represent one standard deviation. Significant differences between AIR and LEACH treatments (P ⬍ 0.05) for a particular sampling
date are indicated with an asterisk (*).
nisms for removal of organic and inorganic phosphate
in leachfield soil involve sorption and binding of the
PO34⫺ to iron and aluminum oxides and oxyhydroxides
(Robertson, 2003; Zanini et al., 1998). The sand used
in the present study was manufactured from monocrystalline industrial quartz and did not contain appreciable
amounts of these oxides. Oxides could have formed on
the surface of sand particles originating from reduced
forms of iron in incoming water. However, we did not
detect Fe2⫹ in the wastewater input stream. Even if such
mineral coatings did form, they did not appear to have
an effect on P removal. The increase in total P levels in
drainage water from both LEACH and AIR lysimeters
relative to raw wastewater was unexpected (Table 2),
although it has been reported by others (e.g., Gold et
al., 1992). A large number of microorganisms has been
shown to accumulate excess phosphate in the form of
polyphosphate (Kulaev and Vagabov, 1983). Shifts in
redox status and availability of carbon substrates can induce release of phosphate from polyphosphates (Kornberg, 1995). This process has been implicated in the
release of phosphate from enhanced biological phosphorus removal processes in wastewater treatment
plants (Carucci et al., 1999) and may account for our
results.
Removal of Fecal Coliform Bacteria and E. coli
Fecal coliform bacteria removal was similar in both
treatments, with a removal rate of 98 to nearly 100%,
depending on loading rate. Drainage water from AIR
lysimeters had levels of fecal coliform bacteria that were
one to four orders of magnitude lower than LEACH
lysimeters. Removal of fecal indicator bacteria in soil
absorption fields is attributed to a number of factors,
including straining, temperature, soil moisture, pH, organic matter, type of bacteria, and antagonistic microflora (Bitton and Harvey, 1992; Hagedorn et al., 1981).
The more acidic conditions of AIR lysimeters may have
contributed to the greater extent of total coliform removal, as suggested by others (Gold et al., 1992; Reddy
et al., 1981; Reneau et al., 1989). We did not observe
formation of a biomat in the AIR lysimeters, suggesting
it may not be necessary for effective removal of pathogens in well-oxygenated soil absorption fields. Concerns
with reduced treatment efficiency in systems that do
Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved.
POTTS ET AL.: WATER QUALITY FROM SEPTIC SYSTEM LEACHFIELDS
1837
Fig. 6. Time course of sulfate in raw wastewater input and in drainage water from aerated lysimeters (AIR) and lysimeters vented to the leachfield
(LEACH). The wastewater loading rate was changed from 12 to 4 cm d⫺1 on 21 Apr. 2003. Values are means (n ⫽ 3). Bars represent one
standard deviation. Significant differences between AIR and LEACH treatments (P ⬍ 0.05) for a particular sampling date are indicated with
an asterisk (*).
not develop a biomat (e.g., Postma et al., 1992; Tyler
and Converse, 1994) may not be warranted in the present case, since aeration appears to enhance removal of
fecal coliform bacteria.
Comparison with Previous Studies
Water quality parameters in LEACH lysimeters appear to differ from previous studies. For example, we
observed consistently low dissolved O2 levels in drainage
water from LEACH lysimeters (Fig. 2), whereas it is
generally thought that the soil beneath leachfields is
relatively well oxygenated (USEPA, 2002) as a result
of unsaturated flow below the biomat. We also observed
that ammonium dominated the inorganic N pool (Fig. 3),
whereas a number of studies have found that nitrate is
the main form of inorganic N in the soil beneath a
leachfield (e.g., Anderson et al., 1994; Bunnell et al.,
1999), suggesting that nitrification is an active process
in this zone. In addition, significant (30–40%) net loss
of total N has been reported in leachfield soils (USEPA,
2002), whereas we observed no net N loss in LEACH
lysimeters. There are a number of possible explanations
for these discrepancies. The unstructured nature of the
sand used in the present study could account for some
of the differences. Soils generally exhibit some level of
aggregation, not present in the sand used in our lysimeters, that affects the spatial distribution of pore sizes,
water, and air, and leads to the formation of anaerobic
microsites where denitrification may take place (Sexstone et al., 1985). In addition, determinations of nitrate
concentration are often made at depths considerably
greater than the 30 cm in this study (e.g., Stewart and
Reneau, 1988). In other instances nitrate was measured
in ground water receiving the leachfield water (e.g.,
Arravena et al., 1993). These situations are not comparable with our experimental setup, since they allow for
the possibility of contact with oxic soil or water outside
the area immediately below the leachfield. Nitrification
could be taking place in oxygen-rich areas of the soil
profile or in ground water. Nitrate could also have other
sources, such as fertilizers or decomposing vegetation.
Finally, laboratory experiments simulating leachfield
conditions may not take into account the composition
of the atmosphere above the leachfield (e.g., Van Cuyk
et al., 2001), which our data indicate differs significantly
from ambient air.
CONCLUSIONS
We found that aeration has a strong effect on the
speciation of nitrogen, and enhances significantly the
removal of nitrogen, BOD5, and fecal coliforms and E.
coli in leachfield lysimeters. Furthermore, this enhancement took place in the absence of a conventional biomat.
These effects have implications for the functioning of
conventional septic systems. Water managers and regulatory agencies are increasingly concerned with the effects of effluent from septic system leachfields on
ground and surface water, especially in the case of failed
or improperly constructed fields (USEPA, 2002). The
high costs and unpredictable outcome of leachfield replacement makes this an economically unattractive alternative for most homeowners. Aeration may be an
effective alternative to both prevent failure and rejuvenate septic system leachfields.
ACKNOWLEDGMENTS
We thank Tracey Daly and Kevin Johns for technical assistance and George W. Loomis and Arthur J. Gold for helpful
comments on this manuscript.
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