Sand Island Wastewater Treatment Plant Sludge Characterization Study

Roger Babcock Jr., Ph. D., U.H. Civil Engineering, Water Resources Research Center


U. S. Army Garrison-Hawaii


09/01/00 - 03/31/01


The Schofield Army Barracks Wastewater Treatment Plant (SBWWTP) treats wastewater from the following facilities in central Oahu, Hawaii: U.S. Army's Schofield Barracks Military Reservation, Wheeler Army Air Field, Camp Stover Housing Area, Helemano Military Reservation, U.S. Navy's Field Station Kunia, and the private Helemano Plantation Development. The plant provides secondary treatment using the conventional activated sludge process and is designed to treat an average flow of 4.2 million gallon per day (MGD). Current average daily discharge is approximately 2.2 MGD. This effluent does not consistently meet the requirements of HAR 11-62 for total coliform bacteria, due to spikes that occur during some periods. While the requirements for HAR 11-62 specify total coliform, the requirements for effluent reuse discharge specify fecal coliform. The Army is attempting to meet the requirements of their National Pollution Discharge Elimination System (NPDES) permit by upgrading the plant to consistently meet the requirements of HAR 11-62. At present, the treated effluent from the plant is discharged to an irrigation ditch operated by the Dole Foods Company (DFC). The effluent is mixed with water drawn from the Wahiawa Reservoir and used for irrigation on DFC lands. An existing easement allowing discharge into the DFC irrigation system expires in December 2001. The water is used to irrigate diversified agricultural crops such as coffee, papaya, corn, and macadamia nuts. Prior to this the water was used to irrigate sugar cane on the same lands.

The U.S. Army Garrison-Hawaii asked the U.S. Army Engineer Research and Development Center at Waterways Experiment Station to assist with evaluating the potential impacts of using treated effluent on the groundwater. This scope of work and is designed to meet a requirement made by the State of Hawaii Department of Health, Wastewater Branch (DOH). This requirement is outlined in the attached December 22, 2000 letter from DOH to the U.S. Army Garrison-Hawaii. The work will be performed by the University of Hawaii and the U.S. Army Engineer Research and Development Center (ERDC) Geotechnical Laboratory.


The purpose and objectives of this study are to evaluate the following:

1) Assess the potential impact, if any, of using irrigation water containing effluent from the Schofield Barracks Wastewater Treatment Plant (SBWWTP), with an emphasis on evaluating the movement of fecal coliform bacteria and coliphage virus through the shallow soils.

2) Estimate the ultraviolet radiation (UV) and chlorine dosages necessary to reduce the fecal coliform bacteria and coliphage virus counts in the shallow soils from irrigating with water containing effluent from the SBWWTP.

Scope of Work

To meet the objectives listed above it is proposed that primarily field and laboratory studies, with some computer modeling be performed. A schedule for the proposed scope of work is also included.

Laboratory analyses to evaluate the wastewater conditions will measure fecal coliform bacteria and the coliphage virus as indicators. Fecal coliform bacteria are chosen as they are a more representative indicator of wastewater treatment plant performance than total coliform. The coliphage virus is chosen because it is a commonly occurring virus in wastewater, and is not commonly found as background in the environment.

Test Plots

The field portion of the study involves the installation of test plots either on the Dole Foods Company Lands (DFC), or in an area on Wheeler Army Airfield adjacent to the SBWWTP. The test plots will be built and managed with the assistance of the Water Resource Research Center (WRRC) at the University of Hawaii (UH). The location of the test plots would be chosen, if possible, in an area previously irrigated for either sugar cane or diversified agriculture on the Dole Food Company (DFC) Lands. This would help to evaluate actual conditions that might occur in the DFC lands. Representatives from the U.S. Army Garrison-Hawaii will assist with negotiating site access with DFC. Figure 1 shows a diagram of the test plots. Each test plot will be approximately 5 x 5 feet. Plot 1 would receive unaltered DFC ditch water, and Plot 2 would receive DFC ditch water supplemented with UV radiation or chlorine.

The test plots will be operated for 1 year, with UV disinfection for 6 months and chlorine disinfection for 6 months. The test plots will be outfitted with drip irrigation lines so that water from the DFC ditch can be applied. A valve operating on a timer system will be programmed to provide the correct amount of irrigation. The amount of irrigation applied will be approximately 72 inches per year, which is 50% higher than average irrigation application to the DFC lands. Experience from previous studies has shown that increased application rates are necessary to obtain samples in the lysimeters. The exact amount applied may change based on discussions with DFC representatives. This irrigation water contains about 5% effluent from the SBWWTP, with the remaining 95% coming from Wahiawa Reservoir. If the test plots are built adjacent to the SBWWTP then effluent will be taken directly from the plant.

The test plots will be planted with Bermuda grass and cut with a hand trimmer. The cuttings will be removed with a hand rake. Three pan lysimeters will be installed in each test plot. Pan lysimeters will be used to allow movement of coliform bacteria; ceramic cup lysimeters do not allow coliform bacteria to pass through the cups. The lysimeters will be installed at depths of 12 inches, 18 inches, and 24 inches. All lysimeters will be installed laterally from a central access ditch to avoid disturbing the in-situ soil profile and field compaction conditions. Plot 2 will be irrigated with DFC ditch water augmented with either UV or chlorine. The UV disinfection dose will 140 mW-s/cm2 with 3 banks in series. This UV doseage is representative of dosages in sewage treatment plants for R-1 reclaimed water production.

The concentration of added chlorine will be 5 mg/L which is about the maximum amount of chlorine added to effluent discharging from a treatment plant. The pan lysimeters should provide sufficient sample volume for fecal coliform and virus analyses by traditional agar plate methods. At least half of the samples will also be analyzed using the latest molecular biological techniques including polymerase chain reaction (PCR), reverse transcription PCR (RT-PCR), restriction fragment length polymorphism (RFLP), and 16s rRNA genetic fingerprinting. These techniques do not require large sample volumes and are capable of determining microbial community structure and the origin of the coliforms (soil, animal, human) and other bacteria. This will assist in differentiating between coliforms in the background soil and colifroms from the sewage effluent. These techniques do not apply to viruses which can only be quantified by plating techniques.

Lysimeter samples will be collected approximately twice monthly, for a total of 24 sampling rounds. All 24 sample rounds will be analyzed for fecal coliform and coliphage virus counts (pending sample availability). Samples will also be analyzed for nitrates, phosphorus, and total dissolved solids. Irrigation rates may have to be increased to provide sufficient sample volume for plate counting methods.

This study will compare the fecal coliform and coliphage virus counts of the applied irrigation water with the coliform counts after the water has percolated through the grass and soil to be captured by the lysimeters. The results from Plot 1 will be used primarily to evaluate the potential for fecal coliform bacteria or coliphage virus to be transported through the soil due to the application of effluent water on irrigated lands. The results of the plot 2 will be compared to plot 1 to evaluate what impact UV or chlorine disinfection might have on the fecal coliform bacteria counts in the effluent stream and soil.

Laboratory Analysis

The laboratory portion of this study will be used to more accurately evaluate the UV and chlorine disinfection dosage necessary to impact the fecal coliform bacteria and coliphage virus counts in Oahu soils. The effect of UV and chlorine disinfection on the entire bacterial community structure will be evaluated from the 16s rRNA genetic library data. This will be performed in a laboratory under controlled conditions with the soil samples placed in laboratory columns. Approximately 20 soil samples will be collected with a 2-1/2" inside diameter split-spoon sampler from the DFC lands to a depth of approximately 48 inches.

These samples will be returned to a laboratory and placed in soil columns. A schematic diagram of the column set-up is shown in Figures 2 and 3. Waters with different concentrations of chlorine will be added to the top of the soil samples. Samples will be collected at the bottom of the soil columns after the water has percolated through the columns. A vacuum will be applied to the bottom (or pressure to the top) of the soil samples to induce percolation. The water samples will then be analyzed for fecal coliform count, virus count, and other parameters including 16s rRNA libraries.

The effluent water will be collected at the SBWWTP from the clarifier prior to the effluent entering the chlorine tank. This water, that approximately meets R-3 standards (as defined in the 1993 Effluent Reuse Guidelines), will be transported to the laboratory and stored in tanks for percolation through the soil columns. The amount added to the columns will be approximately the maximum amount that can be moved through the column under a vacuum and will proceed until at least 100 pore volumes have passed.

The amount of water that percolates through the columns will be measured.

A total of 10 columns will be constructed with water types added as listed below:

# Characteristics Purpose

    1. C1 Pure tap water (Board of Water Supply water at UH) Background
    2. C2 R3 water Worst case
    3. C3 R3 water dosed - chlorine concentration 0.5 mg/l Minimum chlorine
    4. C4 R3 water dosed - chlorine concentration 1.0 mg/l Moderate chlorine
    5. C5 R3 water sand filtered and dosed chlorine, 5 mg/l R-1 chlorine dose
    6. C6 Pure tap water dosed - chlorine concentration of 5 mg/l Background
    7. C7 R3 water sand filtered and dosed UV of 70 mW-s/cm2 Low UV dose
    8. C8 R3 water sand filtered and dosed UV of 140 mW-s/cm2 R-1 UV dose
    9. C9 R3 water sand filtered and dosed UV of 200 mW-s/cm2 High UV dose
    10. C10 Pure tap water dosed UV of 140 mW-s/cm2 Background

The minimum chlorine concentration of 0.5 mg/l is the amount of chlorine necessary to meet R-2 standards (with 15 minutes detention time). The moderate chlorine concentration of 1 mg/l is sometimes used to provide greater reduction of coliform in the effluent waste stream (30 minutes detention time will be used). The high dose of chlorine (5 mg/L for 120 minutes) following sand filtration is currently required for R-1 reclaimed water. The UV dose will be applied in three banks in series following sand filtration using a minimum dosage of 70 mW-s/cm2, a moderate dosage of 140 mW-s/cm2, and a maximum dosage of 200 mW-s/cm2. Most sewage treatment plants use a dose of approximately 140 mW-s/cm2. The columns containing tap water and R-3 water will be used for background and comparison with the columns having water with added disinfection. Each column run will also include a conservative tracer (chloride or bromide) to evaluate the column dispersion coefficient and soil porosity.

The object of the laboratory columns is to investigate what UV dosages and chlorine concentration would be necessary to impact the coliform bacteria and coliphage virus counts in water percolating through the soil sample. The effects on the entire bacterial community will also be determined using 16s rRNA fingerprinting. The columns using tap water and undisinfected R-3 water will be used for background information.

Approximately 6 additional soil samples will be collected and analyzed in the laboratory for agronomic trace elements (such as manganese, iron, aluminum), moisture content, and clay content. Information from these soil samples will be used to assist with evaluating fate and transport of the bacteria and viruses in the saturated/unsaturated zones.

Modeling and Numerical Analysis

The results of the field and laboratory portions will be used as input to an unsaturated zone/saturated zone model to predict the impact, if any, of coliforms and viruses on the groundwater below the DFC lands. Since depth to groundwater is 150 to 400 feet below the DFC lands, and it is not practical to complete field measurements to these depths, modeling will be used to estimate impacts to groundwater below the test plots. A groundwater model together with water balance calculations will be used to estimate the infiltration travel times. The modeling portion of the study allows the data collected in the laboratory and shallow field portions to be extrapolated to a greater depth.

A FEMWATER model will be built using the Department of Defense Groundwater Modeling System (GMS). The groundwater model will likely extend from the DFC lands (just north of Schofield Barracks) to the town of Waialua. The eastern boundary will be the extent of the Dole Food Lands, and the western boundary will be Kaukonahua Stream. It will incorporate portions of the following aquifers: The Schofield High-Level Water Body, Northern Groundwater Dam, Waialua Basal Water Body, and Waianae Dike Impounded Water Body.

It will consist of building surfaces called triangular irregular networks (TINs). These TINs will be used to build the FEMWATER groundwater model. The surfaces will likely include land surface, elevation of the bottom of the saprolite, and the freshwater/saltwater contact. The model will incorporate the unsaturated zone in areas irrigated with effluent from the SBWWTP. The coliform bacteria and coliphage virus will first be modeled using a conservative tracer approach assuming that they travel at the approximate rate of water infiltrating through the unsaturated zone. It is possible that an additional computer model may be used that includes the interaction of fecal coliform bacteria and coliphage viruses with the soil. Data collected from analysis of the soil samples will be incorporated into the groundwater model. The use of additional models will be investigated as part of this study, and reported to DOH in the progress review meetings.