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Comparing Sewage Systems: Study determines best domestic system for reducing nitrogen.

Linda Atkins and David Christensen are Environmental Health Specialists with the Jefferson County Environmental Health Division.


Nitrate nitrogen contamination in drinking water is a public health concern today throughout the United States.

The U.S. Environmental Protection Agency has set a maximum (10 milligrams per liter) limit for nitrate nitrogen in drinking water. Many state and local governments, including the Washington State Department of Ecology (WDOE), have developed regulations to protect ground water from nitrate contamination.

Ground and surface water nitrate contamination has been associated with on-site sewage systems. This contamination occurs when the systems are used at high densities or when ground water flow patterns prevent disbursement and dilution of sewage effluent into an aquifer.

In Jefferson County, Wash., public water supply wells in areas with excessively coarse soils are protected from excessive nitrogen loading through wellhead protection regulations. These rules require reducing nitrogen from waste streams on individual septic systems. If it can be determined that a shallow trench or drip line can enhance plant growth and reduce nitrogen loading to the aquifer, the requirement for pretreatment may be unnecessary.

Testing available systems

A recent study evaluated technologies approved for nitrogen reduction based on literature and assessed performance under local soil, climate and rainfall conditions. To assess nitrogen reduction in on-site sewage systems, specialists at Jefferson County Health & Human Services in Port Townsend, Wash., evaluated four alternative sewage disposal systems. They are: a shallow pressurized trench system, an intermittent sand filter followed by drip irrigation, a pressurized sand-lined bed and a proprietary aerobic treatment unit followed by drip irrigation.

The researchers measured the decrease in total nitrogen (TN) concentration in the waste stream provided by the treatment unit and in the soils up and downgradient of the drain field. They assessed plant uptake in the drain field and dilution by ground water.

None of the treatment units reached the goal of a 50 percent reduction in TN. Downgradient soil water samples had TN concentration 50 percent less than in the dosing chamber. Overall, the intermittent sand filter system operated the most consistently and reduced TN concentration the most.

Parameters and procedures

Precipitation in the test area averages between 16 and 35 inches (40 and 88 centimeters) annually. This range occurs because of a "rain shadow" effect from the Olympic Mountains that minimizes precipitation over the northeastern part of the county. Elevation ranges from sea level to 495 feet (150 meters) in the study area, which is served by public water and private wells. Selected study site systems:

* met current sizing and treatment standards,

* were occupied full time,

* were in use at least one year,

* had waste strength within residential parameters,

* had waste flows measured by at least one reliable method,

* were not subject to catastrophic events such as flooding and

* had occupants who could be interviewed about household and landscaping practices.

The study used daily or weekly precipitation and temperature minimum and maximum records. Above normal rainfall occurred during the sampling period for the areas studied. Normal temperatures ranged from 32[degrees]F to 77[degrees]F (0[degrees]C to 25[degrees]C). Each wastewater treatment system was tested before treatment at the dosing chamber and after treatment before discharge to a disposal field. The aerobic treatment unit had no trash trap or other sampling port for an influent sample to be taken, so only post treatment samples came from this system.

Suction lysimeters and piezometers were installed in pairs 5 to 8 feet (150 to 240 centimeters) upgradient and downgradient of the disposal system. One pair was upgradient and two pairs down gradient to a depth 12 to 18 inches (30 to 45 centimeters) below the disposal system. On the site using shallow pressurized trenches, only suction lysimeters were used.

No upgradient soil moisture monitoring was attempted at the site with the unlined pressurized sand filter beds, due to coarse soil conditions and no documented shallow water table. Ports were installed in the drain field beds during construction to allow installation of suction lysimeters 12 inches (30 centimeters) below the sand lining. Three samples were collected within 7 to 10 days to characterize wastewater quality during weekly cycles. A Washington certified laboratory used standard methods for sample analysis.

Data analysis

Overall, no treatment system tested met the goal to reduce TN 50 percent before discharge into soil. The intermittent sand filter system reduced the most nitrogen. The average reduction was 34 percent within this treatment unit, with additional nitrogen reduction measured in the drain field. The unlined sand filter beds reduced TN an average 26 percent.

Determining the effect of dilution versus plant uptake will require more study. Nitrogen loading to the aquifer is reduced by plant uptake -- not dilution. Attempts to measure downgradient soil moisture offered mixed results. Researchers determined that a suction lysimeter could be used to draw samples from unsaturated soils.

The piezometers had trouble obtaining samples in dryer soils. No difference was found in TN concentration between side-by-side piezometers and lysimeters -- with one exception. Differences between the upgradient lysimeter and piezometer at the aerobic treatment unit site were attributed to hydrological and construction differences.

Data collected from the aerobic treatment unit site indicate that the system may provide nitrogen treatment. However, the installation did not allow for testing raw waste water before treatment. To assess system performance via ongoing operation and maintenance, wastewater must be sampled at multiple points in on-site sewage systems.

At one site, no differences were found between using suction lysimeters and piezometers for sampling. At another site, differences did occur. Because ground water flow patterns are complex and temporal variability is high, future study requires more test replicates to assess the differences.

Test data also indicate that nitrogen concentrations were reduced in the drain field component of each system. The reason for the reduction, whether due to dilution, treatment or a combination of these two mechanisms, remains undetermined. A multi-season study could help determine each mechanism's contribution.
Four Treatment Systems Compared

Total nitrogen summary statistics include mean, standard deviation
(std), coefficient of variation (CV) and number of samples collected

Sand Filter

 Pump Treatment Upgradient Pooled
 chamber unit lysimeter downgradient

Mean 4.96 92.64 61.73 12.75
std 4.49 5.51 7.18 5.42
CV 0.91 0.06 0.12 0.42
n 9 10 10 28
Aerobic Treatment Unit

 Upgradient Upgradient Treatment Downgradient Downgradient
 lysimeter piezometer unit lysimiter piezometer

Mean 22.52 6.31 24.30 2.98 7.57
std 5.31 4.21 7.49 1.82 1.70
CV 0.24 0.67 0.31 0.61 0.22
n 10 10 10 10 10
Unlined Pressurized Sand Filter Beds

 Siphon Pooled
 chamber downgradient

Mean 63.88 45.4
std 18.24 18.7
CV 0.28 0.41
n 9 18
Shallow Pressure Trenches

 Upgradient Pump Downgradient Downgradient
 lysimeter chamber lysimeter 2 lysimeter 3

Mean 4.96 92.64 61.73 12.75
std 4.49 5.51 7.18 5.42
CV 0.91 0.06 0.12 0.42
n 9 10 10 28
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Article Details
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Author:Atkins, Linda; Christiansen, David
Publication:Resource: Engineering & Technology for a Sustainable World
Geographic Code:1USA
Date:Nov 1, 2001
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