USCG STA Tybee_Sewage Mgmt Evaluation_FINAL_modified for solicitation.pdf

Prepared for:

United States Coast Guard Facilities Design and Construction Center 5505 Robin Hood Rd., Suite K, Norfolk, VA 23513‐2431

Contract No. 70Z05019DPHEBKE06 Task Order No. 70Z04719FATYBEE00, Conduct A/E Services for Topographic and Hydrographic Surveys, Geotechnical Investigation, and Site Soil Evaluation for On‐site Sewer Management at USCG Station Tybee, GA

ON‐SITE

SEWAGE MANAGEMENT SYSTEM EVALUATION

U.S. COAST GUARD STATION TYBEE ISLAND

FINAL

JUNE 2020

Prepared by:

PHE‐BAKER JV LLC

77 Upper Rock, Suite 302 Rockville, MD 20850

USCG STATION TYBEE ON-SITE SEWAGE MANAGEMENT SYSTEM EVALUATION

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USCG STATION TYBEE ON-SITE SEWAGE MANAGEMENT SYSTEM EVALUATION

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ON-SITE SEWAGE MANAGEMENT SYSTEM EVALUATION

U.S. COAST GUARD STATION TYBEE

SIGNATURE PAGE

Approved By:

Angela Nocera, PE Project Manager PHE-Baker JV, LLC

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TABLE OF CONTENTS

SIGNATURE PAGE .................................................................................................................................. i

Acronyms……….. ................................................................................................................................... ii

1 Background

2 Site Soil Evaluation

3 Existing Site Description

4 System Assessment

4.1 Current On-Site Sewage Management System Capacity

4.2 Current On-Site Sewage Management System Demand

4.3 Adequacy of Current On-Site Sewage Management System

4.4 Area A

5 Proposed Alternatives

5.1 Option 1 – Modify the Existing System

5.2 Option 2 – Modification to a Mound System

5.3 Option 3 – Modification to a Community System

5.4 Option 4 – Alternative Treatment Systems

6 Recommendations

7 References…. ........................................................................................................................... …….23

LIST OF TABLES

TABLE 4.1: CURRENT ON-SITE SEWAGE SYSTEM CAPACITY

TABLE 4.2: CURRENT STATION TYBEE SEWAGE DEMAND

Table 4.3: NPS Buildings and Trailer Pads Sewage Demand

Table 4.4: Total Estimated Max Day Sewage Demand

Table 6.1: Ranking of Alternatives and Current System

LIST OF FIGURES

Figure 1.1: USCG Station Tybee and National Park Service Areas

Figure 1.2: Area B

Figure 2.1: October 2019 Geotech Survey Boring Locations

Figure 2.2: April 2020 Geotech Survey Boring Locations

Figure 3.1: Existing West End Infrastructure

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Figure 3.2: Existing East End Infrastructure

Figure 3.3: Station Tybee Vicinity Map

Figure 3.4 FEMA Flood Zone Map

Figure 3.5: USCG 5 Feet Sea Level Rise Map

Figure 4.1: Absorption Fields (Numbered)

Figure 4.2: Trailer Pads and NPS Lots

Figure 4.3: Area A

Figure 5.1: Wisconsin Mound Size Comparison

Figure 5.2: Wisconsin Mound Alternate Configuration

Figure 5.4: Proposed Community System Modification………………………………………………………………………….19

APPENDICES

Appendix A: Geotechnical Exploration Letters

Appendix B: Sanitary System Site Plan

ACRONYMS

USCG United States Coast Guard Cal-Tech

DPH

DPH Manual

FDCC

Cal-Tech Testing, Inc.

Georgia Department of Public Health DPH Environmental Health Section Manual for On-Site Sewage Management Systems Facilities Design and Construction Center

JV PHE-Baker JV, LLC FDPO Chatham County Flood Damage Prevention Ordinance NPDES National Pollutant Discharge Elimination System

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1 BACKGROUND

The PHE-Baker JV, LLC (JV) performed a geotechnical site characterization in support of proposed facility improvements at the United States Coast Guard (USCG) Station Tybee at Cockspur Island, Georgia. This work was completed for the USCG Facilities Design and Construction Center (FDCC), under Contract Number 70Z05019DPHEBKE06, Order Number 70Z04719FATYBEE00.

A new station building and parking lot are proposed at the project site. The PHE-Baker JV understands that the station building will be an elevated three-story building consisting of a high bay ground-supported boat bay at the ground level with a mix of administrative and dormitory living spaces on the second and third floors. The parking lot is expected to be subject to HS-20 loading due to semi-trailer traffic. The camping trailer site is proposed to have a new sanitary absorption field and repair/replacement area installed. The existing boat ramp is proposed to be demolished and replaced with a longer and wider ramp with pavement added for maneuvering, access and parking. At the picnic shelter, a new, elevated pump house and emergency generator are proposed.

The total sewage demand for this report is based on the combined wastewater production of Station Tybee, two buildings belonging to the National Park Service (NPS), a crewed patrol boat, and a set of five trailer pads. The USCG also specified three areas to PHE-Baker JV, LLC (JV) for facility improvements, shown in Figure 1.1 as Areas A, B and C.

Figure 1.1: USCG Station Tybee and National Park Service Areas

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Area A is an NPS camping trailer site with trailer pads and a single absorption field. Area B is the proposed site for a new boat ramp with additional improvements, shown in Figure 1.2. Area C is an NPS picnic shelter with underground storage tanks, and the proposed site of a pump shelter and water treatment equipment.

Figure 1.2: Area B

2 SITE SOIL EVALUATION

Two separate geotechnical explorations of the site were conducted by Cal-Tech Testing, Inc. (Cal-Tech) in accordance with standards set out by the Georgia Department of Public Health Environmental Health Section (DPH) Manual for On-Site Sewage Management Systems (DPH Manual), revised June 2019. The first exploration was carried out in the immediate vicinity of Station Tybee during September of 2019 with a corresponding exploration letter presented in October 2019. The other exploration was carried out at

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the picnic shelter in Area C during April of 2020. Both geotechnical exploration reports are included in Appendix A.

For the October 2019 exploration, two Standard Penetration Test (SPT) borings, one Double-Ring Infiltration (DRI) Test, and one California Bearing Ratio (CBR1) Test were conducted as part of the exploration. The locations of the borings are shown in Figure 2.1.

Figure 2.1: October 2019 Geotech Survey Boring Locations

The results of the SPT show that the composition of the subsurface is mostly sandy and that the water table is approximately 6 feet to 8 feet below ground surface. The results of the DRI Test show the infiltration rate of the soil is 49.3 ft/day or 24.7 in/hr. Soils draining this quickly are considered very pervious. The required infiltration rate set by the DPH Manual for conventional septic tank systems on

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parcels of land three acres or more must not exceed 120 min/in, or 0.5 in/hr. For parcels of less than three acres, the maximum rate is 90 min/in. It should be noted that, for very pervious soils like those present in the borings, ASTM D3385 Paragraph 1.6 states DRI Test results may be unreliable.

The April 2020 exploration consisted of an SPT boring and a CBR1 test and showed a mostly sandy subsurface, with a water table only 2 feet below ground. The locations of the borings are shown in

Figure 2.2.

Figure 2.2: April 2020 Geotech Survey Boring Locations

3 EXISTING SITE DESCRIPTION

USCG Station Tybee is located on Chatham County Parcel ID 10022 01001 encompassing 571.98 acres of man-made land and salty, tidal marsh on Cockspur Island northwest of Tybee Island, GA. The civil site plan drawing (Replace Septic Tank System, Station Tybee, Cockspur Island, Drawing Number 0253-S) is included in Appendix B. According to the plan, the extent of USCG Station Tybee is approximately 2.68 acres. The southeast portion of the facility houses the existing septic tank system. The system is approximately 120 feet away from an on-site capped well and approximately 13 feet away from an on-site pressurized water

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line located directly south of the engineering building and mechanic building. A public sewer system is not located within 200 feet of the site. According to the site plan, the existing conventional septic system consists of a 5,000-gallon septic tank connected using a simple gate valve to a series of ten individual 4-inch perforated PVC pipes evenly spaced at 10 feet and each confined in a 16-inch deep by 36-inch wide gravel trench bottom with 7 feet of undisturbed ground between each trench. The dimensions of the absorption field are approximately 45 feet long by 100 feet wide.

Southeast of Station Tybee are two buildings owned by the NPS: a headquarters building (HQ) and a maintenance building. To the south and the southeast of Station Tybee are five trailer pads presumably connected to two absorption fields with unknown specifications. There are water lines connected to all five trailer pads and a total of two metering boxes between all of them.

Besides the field at Station Tybee, there are three additional absorption fields in the relevant areas for this study: one is within Area A, another is adjacent to the easternmost trailer pad, and the third is directly west of the HQ. No data has been made available on any of the three absorption fields.

Figure 3.1 and Figure 3.2 were provided by the USCG and show the locations of existing infrastructure within the general area of this study. In Figure 3.2, the HQ is marked as “FOPU HQ,” while the maintenance building is located at the southeast corner of the map.

Figure 3.1: Existing West End Infrastructure

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Figure 3.2: Existing East End Infrastructure

The JV combined features from Figure 3.2 and Figure 3.1 manually to create a vicinity map, shown in Figure 3.3. The water line, Fort Pulaski fiber line, and well locations on Figure 3.3 were obtained from the topographic survey completed by Hyatt Surveyors in April 2020. Locations of other features are approximate. A 50-foot buffer zone was drawn around the water lines and wells on Figure 3.3. This zone represents known areas where the construction of a septic tank or absorption field would not be legally permissible per DPH Manual Sections 511-3-1-.05 (2) and 511-3-1-.07 (3). This zone also represents where construction of Wisconsin Mounds would not be permissible per Sections F(6)(A)(4)(a) and F(5)(A)(8) of the DPH Manual.

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Figure 3.3: Station Tybee Vicinity Map

The USCG facility is bisected by two FEMA flood zones. The western and northern portion of the facility are designated as FEMA flood zone VE described as: Coastal areas with a 1% or greater chance of flooding and an additional hazard associated with storm waves. These areas have a 26% chance of flooding over

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the life of a 30-year mortgage. Base flood elevations derived from detailed analyses are shown at selected intervals within these zones.

The southeastern portion of the facility containing the septic system is designated as FEMA flood zone X described as: Area of moderate flood hazard, usually the area between the limits of the 100- year and 500-year floods.

A map of the effective FEMA flood zones at USCG Station Tybee is shown as Figure 3.4. According to the map, the absorption field is outside of the 100-year flood zone, but parts of Area A and its respective absorption field are within the flood zone.

Figure 3.4 FEMA Flood Zone Map

Data from the “Sea Level Rise Viewer” application on the USCG’s official website provided more detailed information by showing that Area A is among the last areas on Cockspur Island that would be covered by water during flooding, and would be safe up to an additional 5-6 ft of water. A map generated using the application with 5 ft of sea level rise is presented in Figure 3.5.

Zone X

Zone VE

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Figure 3.5: USCG 5 Feet Sea Level Rise Map

4 SYSTEM ASSESSMENT

To determine if the existing system is adequately sized, the capacity of the absorption field was compared to the current max day demand at USCG Station Tybee.

4.1 Current On-Site Sewage Management System Capacity

The system capacity is based on the infiltration capability of the trenches. There are 10 trenches measuring 16 inches deep, 36 inches wide, and 45 feet long each. The capacity of the system was calculated using the DPH Manual Section 511-3-1-.18 (1)(b) for Non-Conventional On-Site Sewage Management Systems. Per the DPH Manual, a 36-in wide gravel trench absorption shall be considered to have a storage volume of 7.85 gallons per linear foot. This factors in a reduction of infiltrative surface area of each trench by approximately 50% due to the combined effects of compaction, contact area, and fines associated with gravel aggregate.

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The total storage of the absorption field was calculated as:

Total Storage Capacity = 7.85gal/LF x 45 LF/trench x 10 trenches = 3,532.5 gallons

The capacity of the current treatment system is summarized in Table 4.1.

TABLE 4.1: CURRENT ON-SITE SEWAGE SYSTEM CAPACITY

Storage Volume1 (gal/LF)

Length per Trench (LF)

Number of Trenches

Total Storage Volume (gal)

Infiltration Rate (in/hr)

Existing 2 Required3

7.85 45 10 3,532.5 24.7 0.5 max 1 Per DPH Manual for On-Site Sewage Management Systems, Section 511-3-1-.18, January 2016 2 Per Cal-Tech Double Ring Infiltrometer Test, October 2019. See Appendix A.

3 Per DPH Manual for On-Site Sewage Management Systems, Table FT-7, January 2016

There are three additional absorption fields within the relevant area of this report, which do not currently service Station Tybee but are assumed to service the NPS buildings and trailer pads. No data on their actual capabilities, sizes or other specifications, were available. It is reasonably assumed that the layout of the pipes and trenches within the absorption fields is consistent with the general format of the field at Station Tybee in that it also follows the specifications and instructions of the DPH Manual. These fields are shown in the next section, in Figure 4.1.

4.2 Current On-Site Sewage Management System Demand

There are currently 69 personnel stationed at USCG Station Tybee, 11 of which stay on base and 58 of which regularly patrol on the coastal patrol boat. Additionally, the scope of work for this task order states the coastal patrol boat discharges wastewater from a 400-gallon tank every three days. There are also five trailer pads and two buildings belonging to the NPS. One building is the HQ building, and the other is the maintenance building. The maintenance building regularly has 30 occupants, but the HQ building’s regular occupancy is unknown.

Table JT-1 of the DPH Manual lists sewage flow schedules for a variety of facility types and recommends 75gpd for residents of Boarding Houses, 100gpd per room for hotels, and 75gpd per person for resorts.

The JV discussed capacity with the City of Savannah’s Sewer Engineers on 10 October 2019, and they recommended 100 gallons per day (gpd) per person for sewage. This is a conservative estimate compared to the DPH recommendations and was therefore used to calculate demand.

The average daily demand on the septic system was calculated as follows:

Average Daily Demand = 69 personnel x 100gpd = 6,900gpd

The total max day demand on the septic system was calculated as follows:

Max Day Demand = (69 personnel x 100gpd) + 400gpd for Patrol Boat = 7,300gpd

The current sewage demand is summarized in Table 4.2.

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TABLE 4.2: CURRENT STATION TYBEE SEWAGE DEMAND

Coast Guard

Patrolmen

Coast Guard Watch-

Standers

Average Per Capita Daily

Demand (gppd)

Average Daily Demand

(gpd)

Coastal Patrol Boat Discharge

(gal)

Max Day Demand (gpd)

58 11 100 6,900 400 7,300

There are several absorption fields presumably servicing the NPS buildings and trailer pads. The absorption fields in question are shown numbered in Figure 4.1. The unmarked absorption field at Station Tybee in the northwestern corner is a generalized representation, and its dimensions do not match the site plans provided by the USCG. Absorption fields 1 and 2 correspond to the trailer pads, while absorption field 3 is assumed to only include the HQ building. The NPS maintenance building is assumed to have its own absorption field, the location of which was not provided.

Figure 4.1: Absorption Fields (Numbered)

The JV made multiple attempts to contact the NPS to inquire about the nature of the HQ and maintenance buildings in order to ascertain sewage flow; no correspondence was ever received. Estimation was therefore used to determine the sewage flows of both buildings. The maintenance building is assumed to be equivalent to an office building with a kitchen when calculating flow according to the DPH Manual, which is 25 gpd for the office and an additional 5 gpd for the kitchen. At 30 people, this comes to 900 gpd

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total for the maintenance building. No information was available on the HQ, nor on the frequency of use for the trailer pads. For reference, a trailer pad is considered to be equivalent to 50 gpd per capita if the number of users is known.

Table 17.M of the DPH Manual states that for the purposes of lot sizing requirements, a lot should have a maximum sewage flow of 1200 gallons/acre-day when dealing with public non-single family dwellings. For Station Tybee Island calculations, this value will be used to determine not just the HQ building, but the trailer pads as well because it represents a reasonable worst-case scenario for sewage demand compared to other available methods of estimation.

Actual lot data were not available. Assumed lot sizes for purposes of estimating the sewage demand of the trailer pads and the HQ building are shown in Figure 4.2. Trailer pads are numbered for reference.

Calculations for the sewage demand of the three lots shown as well as the NPS maintenance building are given in Table 4.3.

Table 4.3: NPS Buildings and Trailer Pads Sewage Demand

Lot # of People gpd per Capita

Max Flow (gpad) Acreage Sewage Demand

(gpd)

Trailer Pads 1-4 N/A 50 1200 0.879 1054.8

Trailer Pad 5 N/A 50 1200 0.152 182.4

NPS HQ N/A N/A 1200 0.895 1074

NPS Maintenance 30 30 - - 900

Total - - - - 3211.2

Figure 4.2: Trailer Pads and NPS Lots

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Sewage flow schedule would obtain the Average Day Demand via the following equation:

Average Day Demand = Demand of (Station Tybee On-Site Personnel + NPS Buildings + Trailer Pads)

Max day demand was calculated as follows:

Max Day Demand = Average Day Demand + Patrol Boat Demand

The current max day sewage demand is summarized in Table 4.4.

Table 4.4: Total Estimated Max Day Sewage Demand

Sewage Demand

Item

Station Tybee

Personnel Patrol Boat Trailer Pads NPS HQ NPS

Maintenance Total

Sewage Demand

(gpd) 6900 400 1237.2 1074 900 10511.2

4.3 Adequacy of Current On-Site Sewage Management System

According to the information provided, the existing septic tank capacity for the sewage system is 5,000 gallons. According to the calculations, the existing absorption field capacity is 3,532.5 gallons while max day demand exceeds both the septic tank and field capacity at 10,511.2 gallons per day. The current system is overtaxed by demand and would require roughly two times more septic system capacity to meet demand.

If the infiltration rate results are a true indication of the speed at which the effluent is infiltrating through the absorption field, the existing system cannot provide 24-hr retention for the max day demand per DPH Manual Section 511-3-1-.05 (1). Section A(3)(c) of the DPH Manual states that, on any parcel of land greater than three acres, a conventional leach field and septic tank system can only be utilized if percolation rates do not exceed 0.5 in/hr. If the measured 24.7 in/hr infiltration rates are correct, the existing system does not thoroughly filter pollutants and would instead allow quick leaching into the shallow water table which was measured at 6-8 ft below ground surface. For sites with excessive infiltration rates, DPH requires a Level IV Soil Study be conducted and a site plan be developed by a licensed engineer to demonstrate the adsorption limitations can be overcome by design. Design elements could include replacing existing soil with acceptable media and installing underdrains to lower the water table. Data from a Level IV Soil Study is required to determine if these measures would be effective for the existing system.

4.4 Area A

Area A, shown in

Figure 4.3, was examined to determine if it would be suitable for another sanitary absorption field and repair/replacement area. It is about 2.61 acres and partially lies within a FEMA-defined VE flood zone.

There is also an absorption field already beneath Area A, as well as several water lines that intersect with each other across the area and connect to the trailer pads. Additionally, based on percolation rate testing at nearby areas, the percolation rate at Area A is most likely well above the legally permissible maximum rate of 90 minutes/inch. An exact zone showing available space within Area A for an absorption field could

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not be identified; however, much of Area A overlaps with the 50-foot buffer zone presented on Figure 3.3, making it unsuitable for on-site sewage management.

Figure 4.3: Area A

Section B(4)(D) of the DPH Manual states that sites subject to frequent flooding should not be considered for on-site sewage management. Some parts of Area A could potentially still be used for above-ground treatment solutions if given proper flood protection. Legal requirements for flood protection are contained in the Chatham County Flood Damage Prevention Ordinance (FDPO) and include a determination of the “Regulatory Flood Protection Elevation” value, which is a function of the area’s base flood elevation. This sets a minimum height of elevation requirement for the site of the treatment solution. It should be noted that the Chatham County FDPO states "There shall be no fill used as structural support or to elevate areas used for septic tank drain fields." However, the FDPO only applies to unincorporated parts of Chatham County. §24-122 sets ordinances for V-zone flood zones, but part of Area A is in an X flood zone. §24-124 allows variances to the ordinances under extenuating circumstances.

If flood protection is chosen, the method of protection may not present adverse effects to neighboring areas, including the unnatural diverting of floodwaters or increasing flood hazards to nearby areas. First Flush requirements must also be met. A drainage report will identify the appropriate measures required by permitting authorities.

5 PROPOSED ALTERNATIVES

Four alternatives to the current system have been selected for consideration based on feasibility. The advantages and disadvantages of each of the options are explored in the following sections. A Level 4 Soil Survey would be required to confirm whether there is sufficient land available for each proposed

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treatment alternative. If insufficient land is available, alternative treatment systems like membrane package plant systems could be considered. Fluence’s membrane aerated biofilm reactor and Koch Membrane bioreactor package plants are considered to be leaders in this industry.

5.1 Option 1 – Modify the Existing System

Option 1 includes the following: Excavate and line the trench bottoms of the existing absorption fields at Station Tybee and Area A with new material to bring the infiltration rates down to legally required levels.

The absorption field at Area A cannot be expanded due to Sections 511-3-1-.05 (2) and 511-3-1-.07 (3) of the DPH On-Site Sewage Management Systems Manual, which require water lines be kept at least 50 ft from septic tanks and absorption fields, respectively. Replacing the field at Area A could mean losing legal access to much of the already used space. The absorption field at Area A appears to be roughly the same size as the one at Station Tybee and would be insufficient on its own, even with improvements.

This option would also require reconfiguration of the existing septic system with an aerobic treatment unit (ATU), dosing station, and the conventional absorption fields with a pressure distribution system. A 10,000-gallon septic tank to precede the current 5,000-gallon tank in series would be added. Section 511- 3-1-.05 (5) of the DPH Manual permits the use of septic tanks in series if they are single-compartment and the first tank has a volume of at least 2/3 of the required liquid capacity. A reconfigured distribution device, the additional septic tank, and the altered absorption fields will allow the maximum surface area available to be utilized in leaching. Sewage from the NPS buildings and the easternmost trailer pad would lead into a single pipe and be sent to a central treatment point, where the remaining sewage from the four western trailer pads, the patrol boat, and Station Tybee would also be piped for treatment.

Additional measures, such as fencing, may be required to prevent vehicles from parking on top of the Area A absorption field once it is completed. Given the legally required infiltration rates and the sizes of the absorption fields, this system will not be able to handle the full treatment requirements and would need to be supplemented by an additional treatment solution.

Advantages:

• The system is low-cost and simple to install.

• It reduces carbon footprint and maximizes infiltration.

• It has fewer operation and maintenance requirements compared to other methods.

• It meets the 24-hr requirement for septic tank retention time.

• It requires no additional space and occupies the existing absorption field location.

• It maximizes surface areal usage.

Disadvantages:

• The limiting horizon depth of the water table is relatively high, only about 6 ft – 8 ft below existing grade. The lack of water depth does not allow thorough filtration to occur before contact with the water table.

• Both absorption fields must be utilized to meet sewage demand requirements. Any changes to land use that reduces the size of an absorption field will require additional land and piping to replace the absorption field capacity.

• The rapid percolation rates of the soil limit the applicability for conventional treatment methods.

• It requires a large dosing tank and a timed dose delivery control unit to be maintained.

• Flood protection may be required for the Area A field.

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• There is no additional room for expansion beyond simply combining the existing absorption fields.

• Both fields would need to be excavated and re-lined.

• Barriers to prevent vehicle parking will need to be placed over the Area A field.

• It consumes additional power and incurs greater operational expenses and maintenance costs.

• An additional treatment solution will be necessary to meet the full treatment demands outlined in this report.

5.2 Option 2 – Modification to a Mound System

The current system may be supplemented or replaced entirely by a Wisconsin Mound soil absorption system, as described in the DPH On-Site Sewage Management Systems Manual, Section F (5).

Advantages:

• The mound can be placed over failing, in-ground absorption fields with no requirement to repair or replace the existing absorption fields.

• The required system pump provides the infrastructure necessary for additional treatment or other absorption field locations.

• This system is considered a conventional treatment system under DPH standards and operates using the same principles as a below-ground absorption field.

• The mound can be combined with other specific treatment systems to help reduce its size.

Disadvantages:

• It requires a Level IV Soil Study to determine adequate materials and sizing for the mound.

• It requires a pump, force main, and electrical components be installed, adding additional costs for operation and maintenance.

• It requires flood protection.

• It requires imported fill for the installation, further adding to construction costs.

• The area required is larger than just an absorption field on its own.

Figure 5.1 shows the approximate size of a proposed Wisconsin Mound southeast of Station Tybee around Area A.

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Figure 5.1: Wisconsin Mound Size Comparison

This shows what would be considered a worst-case scenario. It includes the area contributed by the septic tank, absorption trenches, and other distribution infrastructure. The limits of disturbance represent what would be the outermost reaches of the mound and extend 20 feet around the outermost parts of the system. Fiber lines are located within the limits of disturbance and may need to be relocated or protected.

The septic tank is large enough to accommodate the daily treatment demand, and the absorption trenches follow the same standards as those for regular in-ground absorption fields. All 12 trenches shown are 120 feet long, 3 feet wide, and have 4 feet of separation from each other. The 50-foot buffer zone may not be crossed by any of the treatment system’s infrastructure. The actual dimensions of a mound system would require a soil classification to identify key values necessary in determining the dimensions. Section F(5)(B) of the DPH Manual states that the mound’s size is ultimately reliant on three factors: sand fill loading rate, soil (basal) loading rate, and linear loading rate for the site. Some of the standards for determining these are linked to USDA standards.

It should be noted that at this time, the Georgia DPH has a limited number of approved septic tanks which can meet the requirements identified in the DPH Manual and the daily sewage treatment demand. The approval process for a new model will require filling out the Georgia DPH “Product Review Application” form found on the state’s website and submitting a $500 product review processing fee. If desired, multiple tanks can be used in series instead of a singular tank. However, the DPH Manual requires these tanks to be single-compartment only, and the first tank in the series must accommodate at least 2/3 of the required sewage demand.

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Figure 5.1 only considers the traditional setup of a Wisconsin Mound. It does not consider the substitution of alternative treatment systems for the absorption field that can help reduce the Wisconsin Mound’s size. It uses an alternating drainfield setup according to the DPH Manual. Figure 18.F of the Manual gives an example.

Section F (3)(D) of the DPH Manual states that if soil percolation rates are determined to be between 60 and 90 minutes per inch, you can choose to utilize two separate but equal absorption fields in an alternating fashion, switching between them in 1-year increments or as soon as one of the fields becomes saturated. This is a division of the necessary absorption field area into two parts, and does not require the creation of another full-size absorption field. This would allow the reorientation and redistribution of the absorption trenches into two separate groups in order to better maintain minimum required separation distances from water sources. The DPH Manual does not specify that the two fields be oriented in a specific manner relative to each other, it only states that they need be separate but equal. A proposed alternative orientation with a size reduction is presented at the end of this section in Figure 5.2. The size reduction is discussed further in this section.

The Georgia DPH has lists of approved alternative treatment systems available, some of which reduce the legal requirements for the amount of land required for an absorption field. Some of these systems can be used in a Wisconsin Mound to reduce its overall size. The DPH Manual does not list any separation requirements between absorption fields or Wisconsin Mounds, which allows for multiple smaller systems to be constructed in proximity to one another of equal size. The only list published by the Georgia DPH which addresses reductions to the required area is the “Alternative and Chamber Absorption Field Product Approvals” list. The JV investigated the treatment options in the list and found two systems/configurations that would allow for a reduced land requirement: the EZFlow Drainage System (configuration EZ1303T), and the ICC Flowtech Drainage System (configurations FTS133T-1 and FTSG133T-1).

Both systems rely on similar treatment methods. Absorption field trenches are dug and a perforated pipe in placed in the center of the trench. The pipe is then surrounded in different configurations by bundles of EPS aggregate and a layer of filter paper. The specifics of the configurations for both the Flowtech and EZFlow systems are found in the same product list.

Instructions for calculating the required absorption field area of both these systems as published in the list by the Georgia DPH are identical. Calculation results are given in linear feet of absorption trench. The instructions cite Table DT-2 of the revised January 2016 version of the DPH Manual and rely on three things: an Equivalency Factor, the desired percolation rate, and the number of bedrooms present. Because both the 2016 and 2019 versions of the DPH Manual repeatedly list a bedroom as being equivalent to 150 gpd, we convert our sewage output of 10,500 gpd to an equivalent 70 bedrooms for calculation purposes.

The maximum percolation rate in Table DT-2 is 5 min/inch and is the optimal rate for reducing the size of the absorption field. The Equivalency Factor is a static value set by the DPH for different systems/configurations which changes the required absorption trench length. The Equivalency Factor for both systems and their different configurations is 0.65.

Doing the calculations according to the list’s instructions results in 1895.83 linear feet of absorption trench. This can be met using 16 trenches with a length of about 120 feet. Both the Flowtech and EZFlow systems use 2.5 foot-wide trenches instead of 3-feet wide like with conventional absorption fields. It is assumed by the JV that any conventional design standards which were not contradicted by the list’s system description/instructions are to be maintained for the alternate system. This includes the 4-foot separation distance between adjacent absorption trenches and the maximum permissible absorption trench length of 125 ft.

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By utilizing the Flowtech or EZFlow systems and their respective configurations, as well as by splitting the necessary absorption field’s area into two separate fields, we can alter the shape and size of the treatment area to better fit within the legal and physical constraints, whether by using a Wisconsin Mound system or a standalone absorption field. Figure 5.2 shows one possible configuration for a Wisconsin Mound system utilizing Flowtech or EZFlow systems within the absorption trenches.

Figure 5.2: Wisconsin Mound Alternate Configuration

Each trench is 120 ft long and 2.5 ft wide. The septic tank and distribution box have the same dimensions as the model in Figure 5.2. The limits of disturbance can be replaced by a solid barrier if necessary to prevent them from crossing into roads and driveways. The limits of disturbance remain at 20 ft from the outermost parts of the treatment infrastructure, but can be reduced around the septic tank if a smaller model of septic tank is approved.

5.3 Option 3 – Modification to a Community System

There are four septic systems currently in operation on the 571.98-acre parcel housing USCG Station Tybee. The absorption field of these four individual systems could be combined and centralized to form a single, larger absorption field capable of handling total tank demand across the entire parcel such as that shown on

Figure 5.3. The low-pressure collection system could connect nearby dwellings with the centralized septic tanks. The centralized system could then utilize either mounds, absorption fields, or a complete treatment plant. Existing absorption fields are outlined in purple and the proposed absorption field is outlined in white on

Figure 5.3. Field 1 is 4,500 square feet, Field 2 is 2,500 square feet, Field 3 is 3,200 square feet, and Field 4 is 2,100 square feet totaling 12,300 square feet of existing absorption field area. The area of the

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proposed absorption field exceeds the total at 15,800 square feet. All absorption field areas were estimated using Google EarthTM mapping service measurements. Because the infiltration rate throughout the project site is rapid, the centralized system should be constructed at another location on the island with acceptable percolation rates and a deeper water table. Alternatively, a mound system should be constructed on the project site with an acceptable media and soil to provide the required percolation rate.

Advantages:

• It centralizes the local absorption fields.

• It allows for shared operational and maintenance costs.

• It simplifies maintenance by allowing the servicing of the entire parcel at one location.

• It has potential to service additional facilities.

Disadvantages:

• It requires costly infrastructure improvements to transport and treat raw sewage.

• It mandates the removal of the existing system and the restoration of the ground to its undisturbed condition.

• It requires repair or replacement of existing drain fields with areas of the same size. Smaller or repair or replacement areas can be utilized in conjunction with alternate treatment products.

• It requires flood protection.

• It has the highest construction, operational, and maintenance costs of all three options.

Figure 5.3: Proposed Community System Modification

5.4 Option 4 – Alternative Treatment Systems

The JV investigated and identified numerous potentially suitable treatment systems available in the Georgia DPH approved treatment systems lists. According to Section A 511-3-1-.17 (1) of the DPH Manual, it is not permissible to discharge effluent from a treatment system to a ground surface or body of water, including drainage ditches and trenches. However, the National Pollutant Discharge Elimination System

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(NPDES) allows for discharge to a body of water if a NPDES permit is obtained. Applying for a NPDES permit requires the effluent to be treated to standards which are easily met by many of the approved packaged treatment systems. The JV also contacted several vendors not on the approved list who have confirmed that meeting the NPDES standards with their treatment systems would be achievable. Additionally, Section 7.2.13 of the NPDES Permit Writer’s Manual references Section 303(d)(4)(B) of the Clean Water Act, stating that effluent limitations may be relaxed in the event that the receiving water - the Savannah River in this case - is identified as not meeting applicable water quality standards.

Despite many of the approved packaged treatment systems meeting NPDES standards, none of them come in a size that can meet the daily treatment demand without utilizing multiples of the same system working in parallel. In addition, all of them are required to meet the effluent discharge standards set forth by the DPH Manual. If a selected product does not treat the wastewater to NPDES standards, a filtration system will still be necessary to meet the discharge requirements and would require additional design.

One possible alternative packaged treatment system the JV has examined is the Purifics Ceramic Ultra- Filtration System. Such a system could fit in an enclosure 8 feet wide and 20 feet in length, and would easily handle the daily treatment demands outlined in this report. Effluent quality is intended to reach less than 0.1 NTU turbidity, and less than 1 ppm of TSS. Many manufacturers provide wastewater treatment products, including Kubota Membrane USA Corporation and AUC Group.

The number of physical units in a packaged treatment system would be determined during a detailed analysis with the manufacturer; however, a minimum of two units would be recommended for redundancy. Cornerstone H2OTM is a manufacturer’s representative in Georgia which specializes in helping customers find wastewater treatment solutions from a variety of manufacturers with various treatment options.

Advantages:

• Highest quality of effluent available from treatment options

• Systems are typically serviced by vendor/manufacturer

• Fastest installation

• Reduced land requirements

Disadvantages:

• Typically the most expensive option to meet STA Tybee needs

• Requires either a NPDES permit or a replacement absorption field

• Most complex option available, requires an operator

• If using an approved system, will require multiples of the same system to meet daily demand

• Using a single packaged treatment system capable of meeting the daily demand will require

Georgia DPH approval

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6 RECOMMENDATIONS

Table 6.1 ranks the current system against the three options in terms of Capital cost, Operations & Maintenance (O&M) Cost, Ease of Construction, Environmental Impacts/Regulations, Land Required, and ability to Solve Existing System Problems. Each category is ranked 1 through 4 with 1 being best and 4 being least desirable.

Table 6.1: Ranking of Alternatives and Current System

Option Capital Cost

O&M Cost

Ease of Construction

Environmental Impact /

Regulations Land Required

Solves Existing System

Problems Current System (Do Nothing) 1 3 1 4 1 4

1. Modify the Existing System

3 2 2 3 3 3

2. Modification to a Mound System

2 1 3 2 2 1

3. Modification to a Community System

4 4 4 1 4 2

4. Alternative Treatment System

4 2 2 4

3 (without NPDES permit)

2 (with NPDES permit)

Systems were compared and ranked in each category on a scale of 1 to 4, with 1 being best, and 4 being least desirable.

Based on the overall rankings, the recommended alternative is Option 2 – Modify to a Mound System.

This method has been developed since the 1950s, has proven high performance, and has a success rate of over 95%. It will meet immediate sewage demand and operate on site with no more land footprint than the existing system uses. It will also protect the shallow water table from further contamination by pollutants. A future centralized, community system is also possible with the infrastructure required to build out this option. If land availability prohibits the feasibility of a mound or community system, the alternative treatment system would be recommended.

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7 REFERENCES

ASTM International. ASTM D3385, Standard Test method for Infiltration Rate of Soils in Field Using

Double-Ring Infiltrometer. 2018.

Georgia Department of Public Health (DPH). Manual for On-Site Sewage Management Systems. January 2016.

Georgia Department of Public Health (DPH). Manual for On-Site Sewage Management Systems. June 2019.

Georgia Department of Public Health (DPH). Product Approvals: On-Site Sewage Management Systems.

https://dph.georgia.gov/environmental-health/wastewater-management/product-approvals.

June 2019.

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APPENDIX A

Geotechnical Exploration Letters

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APPENDIX B

Sanitary System Site Plan

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