Improved Sizing Methodology for Subsurface

Proceedings of the Eleventh Individual and Small Community Sewage Systems Conference, ASAE, October 20-24, 2007, Warwick, Rhode, Island.

Improved Sizing Methodology for Subsurface Wastewater Infiltration Systems Kevin D. White, Ph.D., P.E. University of South Alabama Department of Civil Engineering Mobile, AL 36609 [email protected] Abstract. A new framework for sizing subsurface wastewater infiltration systems, based on scientific principles that govern soil hydraulics, biomat development (including oxygen transport), and peak flow storage. Current design methods are typically based only on soil properties and hydraulic loadings. The proposed method considers several factors that influence wastewater infiltration: 1. native soil hydraulic conductivity, 2. organic loading and its effect on biological growth (and thus hydraulic conductivity), and 3. peak flow storage. The use of scientific principles to optimize infiltration trench design indicate that trench systems with large interface areas for infiltration and gas exchange; and minimal reductions in soil hydraulic conductivity due to biomat development are most effective. A long and narrow trench design is the most efficient geometry for maximizing interface area, for minimizing biomat development, and for maximizing gas transport (oxygen exchange) into and out of the trench. The proposed method is presented generically and can be used for sizing both conventional and alternative systems. A further advantage of the methodology is that it can be used to determine the effect of full or partial treatment of wastewater on drain field design and size. Keywords: trench sizing, infiltration, organic loading, biomat, and oxygen transport.

Introduction

Septic tank drain fields have traditionally been sized using a simple method of hydraulic loading, based on soil types and gravel-filled infiltration trenches. Native soil percolation rates ('perc' tests), or more recently soil texture analyses, are in some way correlated to a hydraulic loading rate (which generally reflects a soils ability to accept effluent--the long term acceptance rate). The use of alternative drain field products (non gravel) over the last 15 years has prompted regulatory agencies to question and evaluate existing sizing criteria and in some cases to formulate new sizing criteria or provide exceptions to existing criteria. Unfortunately, this has led to a wide variety of drain field sizing criteria being used across the United States, each using its own unique combination (and values) of design variables (daily flow estimates, soil type, percolation rate, bottom and/or sidewall infiltration area, storage volume, and long-term-acceptance-rates or LTARs). Application of the numerous design variables differs significantly from state to state. In many cases, the sizing methods currently used (and codified) have been in place for decades, are based on gravel-filled trenches, and are difficult to apply when considering new drain field technologies. A more consistent drain field sizing method is needed, based on known scientific principles and facts, to more appropriately address the sizing of not only gravel-filled trench systems, but also the various engineered drain field products now available and those yet to be developed. A great deal of scientific information is available to describe much of the physical science related to the infiltration of effluents into soils; however, the understanding of trench biomat growth and decay is still immature, particularly as it relates to organic loading, oxygen availability, and the associated changes in soil hydraulic conductivity. There are quite a few variables that in some way impact infiltration sizing: soil physical characteristics (texture, structure, pore size, moisture content, hydraulic conductivity, etc.), effluent hydraulic loading, effluent organic loading, trench architecture (infiltration area), oxygen availability (and transport into the soil), biomat development, and peak storage needs. To simplify, a sizing method based on hydraulic loading and organic loading is proposed, with consideration for peak flow storage.

Organic Loading, Oxygen Availability, and Biomat Development

One of the complicating factors that often limit the infiltration of effluent is the formation of a biomat on the soil-trench interface. It is this interface where liquid effluent first touches the soil into which it

Proceedings of the Eleventh Individual and Small Community Sewage Systems Conference, ASAE, October 20-24, 2007, Warwick, Rhode, Island. must infiltrate. The presence of natural microorganisms in the soil and microbial foodstuff (carbonaceous materials) in wastewater effluents provide the necessary ingredients for microbial growth. In particular, bacterial growth is accompanied by the formation of extracellular polysaccharides which contribute to the soil clogging characteristics of the biomat (Ronner and Wong, 1998). Microorganisms feeding on the materials in wastewater effluents grow and die-off based on some equilibrium between food, oxygen availability, and the microbial population present. Little biomat (and hydraulic clogging) is produced, for example, when treated effluent is applied to conventionally-sized trenches (Salthouse and Loudon, 1997). This indicates that when organic loading (mass of carbonaceous material applied per day to a unit infiltration area) is low, microbial growth is also low and microbial biomass does not sufficiently accumulate within soil pores to clog them completely. Our conventional trench sizing methods are codified based on hydraulic loading. However, the hydraulic loading rates are a function of LTARs resulting from specific organic loading and biomat development (Bouma, 1975). Empirically, we know that "resting" a trench for some period of time will increase (or restore) its hydraulic conductivity. This is because no organic load is "feeding" soil microorganism growth and the oxygen normally being consumed in degrading organic material in effluents can now be used to oxidize dead or dying microorganisms and growth byproducts. Increased amounts of oxygen in the trench not only help break down organic materials (including dead microorganisms and accumulated byproducts), but also keep the system from becoming totally anaerobic, resulting in a reduction of the bacterial metabolism. Erickson and Tyler (2000) recognized the relationship between organic loading rate, oxygen availability, and biomat development; and suggested that drain field design should incorporate knowledge of wastewater BOD and soil oxygen availability.

Design Methodology

An appropriate sizing method should apply known principles to affect drain field system design. Both hydraulic loading rate and organic loading rate should be incorporated into the design, as well as some recognition of total infiltration area available. Oxygen availability is key to organic material breakdown and biomat management, and should be addressed in the design, however, more data on oxygen availability and design relationships is needed before appropriate design criteria can be developed. Peak flow storage and some factor of safety should also be design consideration. The design method proposed employs a 5 step process; 1. 2. 3.

4. 5.

Determine the actual effluent flow rate. This is typically referred to as the "design daily flow". Utilize existing Hydraulic Loading and Organic Loading design parameters to size a system. Evaluate the resulting system sizes determined and select the sizing method which yields the largest design; i.e., the one with the most infiltration surface area. For example, in clay soils, hydraulic loading rate may be the most limiting feature while in sandy soils organic loading rate may be the most appropriate sizing criteria. Determine the system storage volume, to insure that peak flow volumes can be adequately stored. Apply a Safety Factor. In order to calculate a safety factor, a trench-type system will be sized using the proposed, science-based criteria (LTAR hydraulic loading, LTAR organic loading, and storage volume) and then compared to a standard gravel-trench sized using existing regulatory codes. The ratio of the existing code gravel trench size to the proposed, science-based sizing method is defined as the implied safety factor. Some minimum safety factor should be applied irregardless of calculation (1.2 to 1.5).

Design Daily Flows

Most onsite systems serve residential properties; therefore, onsite wastewater regulations often express wastewater flow requirements as a pre-determined number of gallons per day per bedroom in the residence to be served. The number selected varies widely from state to state. For example, Florida regulations require a system to be sized for 120 gallons per bedroom per day while Illinois requires 200 gallons per bedroom per day. Thus, in Florida a four bedroom house is expected to generate 480 gallons of wastewater while in Illinois, the same house would generate 800 gallons. The differences here do not reflect any difference in water use between residents of the two states; rather it reflects where the state regulatory system chooses to incorporate its “safety factor.” Increasing the number of gallons per day

Proceedings of the Eleventh Individual and Small Community Sewage Systems Conference, ASAE, October 20-24, 2007, Warwick, Rhode, Island. from a household provides one of the “conservative assumptions” that are often used to mask a lack of a scientific basis for sizing criteria in statewide onsite wastewater regulations. The size of a subsurface wastewater system for single family homes is typically based on peak daily flow and long term hydraulic acceptance rates of the soils. In most states the design flow is based on the number of bedrooms in the house. For example, a daily flow (Qd) of 150 gallons per day is commonly assumed for each bedroom (N). Using this procedure then a typical three bedroom house would have a design daily flow of 450 gallons per day, where Qd = Qb·N

(1)

Qd = design daily flow Qb = daily flow per bedroom N = number of bedrooms The value of 150 gallons per day per bedroom is a very conservative number and includes an implicit safety factor ranging from 2.3 to 3.6. The USEPA , in its onsite wastewater manual, estimates significantly less wastewater generation. Three independent studies show that household wastewater generation averages about 70 gallons per day per person (USEPA, 2002). Thus, it seems more reasonable that a wastewater generation estimate would be on the order of 75 gallons per day per person. For a 3-bedroom residential home, the first bedroom would typically house 2 persons, and each additional bedroom would typically house one additional person. Thus, actual wastewater generation from a 3-bedroom home would be about 300 gallons per day.

Hydraulic Loading Rates

Soil acceptance of wastewater effluent is the design goal. Soil permeability has been studied more than perhaps any other aspect of onsite wastewater systems and there is a scientific basis for determining the effluent acceptance rates (or loading rates) of soils based on soil characteristics. One of the most elegant expressions of a soil classification system for determining loading rates can be found in the NOWRA Model Code (2005). This system reports a range of soil acceptance roughly between 0.1 and 1.0 gallons per day per square foot of absorption area. In practice, Hydraulic Loading Rates (H) are derived for most soil types and are listed typically by state code. For example, a typical hydraulic loading rate (or hydraulic acceptance rate) specified for sandy soil is about 1.0 gpd per square foot of trench infiltration area. Note that many regulatory codes utilize only trench bottom area for sizing. A clay or clay loam soil might have an hydraulic loading rate of 0.25 gpd per square foot. However, it is recognized that hydraulic acceptance rates may vary based on the organic content of the wastewater. Hydraulic Loading Rate Sizing. The following step-by-step method is applied for establishing trench length based on Hydraulic Loading rate. Step 1. Determine the Design Daily Flow (Qd). Step 2. Select the allowable hydraulic loading rate (H) for the specifically characterized soil immediately below the trench, as dictated by applicable regulations. Step 3. Determine the required trench infiltration area using the following equation; Qd At = H Where At is defined as trench infiltration area in total square feet. Step 4. Determine trench length by dividing the required trench area A by the soil interface area (As) per trench foot of the system. This soil interface area per trench foot is variable and dependent on the particular trench type and geometry.

Proceedings of the Eleventh Individual and Small Community Sewage Systems Conference, ASAE, October 20-24, 2007, Warwick, Rhode, Island. A L= A s

Organic Loading Rates (OLR)

Mass loading refers to the strength of the wastewater being applied to onsite subsurface absorption systems. The importance of organic loading rates, sometimes called mass loading, should be recognized in the design of onsite wastewater systems. For example, the US Environmental Protection Agency (USEPA) Onsite Wastewater Treatment Systems Manual (2002) states: “Increasingly, organic loading is being used to size infiltration surfaces. Based on current understanding of the mechanisms of SWIS operation, organic loadings and the re-aeration potential of the subsoil to meet the applied oxygen demand are critical considerations in successful SWIS design.” Very few studies have been conducted on any aspect of long-term acceptance rate in seepage trenches. However, those that have suggest that mass loading is an important factor. Matejcek, Erlsten & Bloomquist (2000) conducted an experiment in which wastewater with known characteristics (CBOD5, TSS, and O&G) from a number of Florida restaurants was applied to lysimeters designed to simulate onsite wastewater seepage trenches. The experiment showed that columns in which mass loading exceeded 0.0015 lbs/ft2/day eventually clogged while those columns in which mass loading was 0.0015 lbs/ft2/day or less did not. While the authors made no specific claims regarding this result, the study suggests that if the mass loading is kept below the 0.0015 lbs/ft2/day threshold, the long-term acceptance rate of seepage trenches is greatly improved: “Failure occurred primarily in the lysimeters with two feet of unsaturated soil that were dosed with highand medium-strength wastewater. Twenty-four lysimeters failed during the 112- day study with 20 failures occurring between days 20 and 47. No failures were recorded in lysimeters with low strength wastewater, which received a daily mass loading of 0.0015 lb/ft2/day or less. In addition, total mass loaded on the low strength soil columns has exceeded the mass loading of the failed columns dosed with medium strength wastewater.” The proposed methodology sizes systems based on the understanding that soils have a limited ability to process or treat organic matter. A mass loading rate of 0.0015 lbs/ft2/day is used as a general guideline and for illustration. Others may choose to use more or less conservative numbers. Organic Loading Rate Sizing. The following step-by-step method is applied for establishing trench length based on Organic Loading Rate. Step 1. Determine the Design Daily Flow (Qd) based on the type of facility and number of rooms or occupants. Step 2. Establish an allowable organic loading rate (Ro) for the soil. A loading rate of 0.0015 pounds per square foot is supported by research data as a good number to use for design purposes. Step 3. Determine the required trench surface area using the following equation; Qd A=R o Where At is defined as trench area in total square feet. Step 4. Determine trench length by dividing the required trench area A by the soil interface area (As) per trench foot of the system. A L= A s

Surge Volume Requirements

Proceedings of the Eleventh Individual and Small Community Sewage Systems Conference, ASAE, October 20-24, 2007, Warwick, Rhode, Island. The determination of Surge Volume requirements in this method is based on the work of Rubin, Janna and Otis. The authors establish that system volume requirements are dependent on static volume and on dynamic volume. Subsurface soil based wastewater treatment systems should be designed to accommodate an inflow of wastewater until the outflow from the treatment unit has dispersed the liquid into the surrounding soil media. The estimated storage volume requirement is therefore dependent on a balance of the inflow rate or daily loading rate, and on the wastewater dispersal rate or discharge rate. The wastewater flow into a system depends on the fixtures in the residence or facility which discharge liquid into the wastewater system. Some fixtures discharge low volumes of liquid, while others such as washing machines or bathtubs discharge relatively high volumes of liquid. On occasion, the volume of liquid entering the system exceeds the average daily flow. This event results in a surge in wastewater flow. The surge volume capacity requirement can them be defined as the maximum discharge rate less the average daily flow. The method outlined below allows for finding the trench length required when the system is new and again after the system has matured. Surge Volume Sizing. A recommended sizing method for determining trench length is given below: Step 1. Determine the Design Daily Flow (Qd) based on the type of facility and number of rooms or occupants. Step 2. Determine the soil hydraulic loading rate as dictated by applicable regulations. Step 3. Determine the required trench surface area using the following equation; Qd At = H Where At is defined as trench area in total square feet. Step 4. Determine trench length by dividing the required trench area A by the soil interface area (As) per trench foot of the system. A L= A s 5. 6. 7.

Express the soil surface area in terms of the trench length L. Solve for trench length. This result is for a new trench. For a mature trench, divide the cross sectional area into two regions; one is for an area that has a fines/biomat layer and the other is for a soil surface. 8. Express the total flow rate as the sum of the flow passing through the fines/biomat surface and the flow that passes through the soil surface. 9. Express the area for both of these regions in terms of trench length. 10. Solve for the trench length required. This result is for a mature trench. This calculation procedure should provide a useable estimate of the volume and length required of a trench that will satisfy Surge Volume Capacity sizing for a given application.

Example Calculations

Examples are provided below which illustrate how the design parameters can be used to size a system. One example shows a system design limited by hydraulic capacity while the other is limited by organic loading. Consider a three bedroom home whose occupants wish to install a subsurface drainage system whose cross section is as shown in Figure 1. Determine the length of the trench required to accommodate this installation. Assume a conventional gravel system in a clay soil.


Figure 1. Subsurface drainage system; Trench Type = gravel; Trench Size = 3 feet wide x 1 foot high; Soil interface area per trench foot; As = 5 ft2 per trench ft. Solution—Hydraulic Loading Rate Step 1. Determine the Design Daily Flow (Qd) based on the type of facility and number of rooms or occupants. Here we assume an estimated 70 gallons per day per bedroom, so that: Qd = 3(70) = 210 gallons per day Step 2. Determine the allowable sewage application rate (H) for the soil immediately below the trench as dictated by applicable regulations. Step 3. Determine the required trench surface area using the following equation; Qd A= H Where At is defined as trench area in total square feet. The soil characteristics are as follows: Soil Type = Clay Long Term Acceptance Rate H = 0.2 Waste water strength = 0.0013 lb/gal Design Organic Loading Rate Ro = 0.0015 lb/ft2/day The Soil Interface Area required is calculated as Qd 210 gal/day A = H = 0.2 gal/day/ft2 or A = 1050 ft2 of interface area Step 4. Determine trench length by dividing the required trench area At by the soil interface area (As) per trench foot (Lf) of the system. A 1050 ft2 L = A = 5 ft2/(trench ft) s L = 210 ft This is the trench length required without the inclusion of a safety factor. Solution—Organic Loading Rate Step 1. Determine the Design Daily Flow (Qd) based on the type of facility and number of rooms or occupants.


Qd = 3(70) = 210 gallons per day Step 2. Establish an allowable organic loading rate (Ro) for the soil. The mass of material that must be infiltrated is: m = (210 gallons/day)·(0.0013 lbm/gallons) or m = 0.27 lbm Step 3. Determine the required trench surface area using the following equation; m 0.27 A = 0.0005 lbm/ft2 = 0.0005 or A = 540 ft2 Step 4. Determine trench length by dividing the required trench area At by the soil interface area (As) per trench foot (Lf) of the system. A 540 ft2 L = A = 5 ft2/ft s or L = 108 ft Solution—Surge Volume Consider a three bedroom home whose occupants wish to install a subsurface drainage system as shown in Figure 1. Determine the length of the trench required to accommodate this installation. Assume 150 gallons/day/bedroom, and that the soil is sand. Solution Step 1. Determine the Design Daily Flow (Qd) based on the type of facility and number of rooms or occupants. Qd = Qb·N = 70(3) = 210 gallons/day Step 2. Determine the soil hydraulic loading rate as dictated by applicable regulations. For clay, the hydraulic loading rate is taken to be 0.2 gallons/day/(ft2 of soil interface area). At this point, we are not accounting for a surge component. Solving, Qd 210 A = H = 0.2 e Substituting values, we get 210 gallons/day A = 0.2 gallons/day/(ft2 of area) Solving, A = 1050 ft2 of soil interface area Suppose we planned to use a trench that is 1 ft deep and 3 ft wide. Having selected a cross section, we can now determine the length L of trench required. The surface area for both sides is 1·L + 1·L. For the bottom, the surface area is 3·L. The total surface area then is 5·L. The trench length required would then be

Proceedings of the Eleventh Individual and Small Community Sewage Systems Conference, ASAE, October 20-24, 2007, Warwick, Rhode, Island. L= or

1050 5

L = 210 ft

(1 ft deep x 3 ft wide new)

This result is what would be needed for a new trench, without fines/biomat formation and without meeting a surge requirement. Now suppose that the trench has matured and the hydraulic loading rate along the fines/biomat surface has become changed. In addition, suppose that a surge requirement is included, and that periodically, we anticipate an additional surge volume flow rate of 100 gallons/day. We identify two flows for a total flow rate of Qt = 210 + 100 = 310 gallons/day The hydraulic loading rates for the various regions will all be different. For the bottom region labeled c-d, the hydraulic loading rate is 0.05 gallons/day/(ft2 of surface area). For the lower sidewall regions b-c and d-e, the hydraulic loading rate is 0. gallons/day/(ft2 of surface area). For the upper sidewall regions a-b and f-e, the hydraulic loading rate is 1.8 gallons/day/(ft2 of surface area). These figures are summarized below:

Region a-b and e-f b-c and d-e c-d

Hydraulic loading rate 0.2 gallons/day/(ft2 of surface area) 0.1 gallons/day/(ft2 of surface area) 0.05 gallons/day/(ft2 of surface area)

The total flow that the trench must attenuate is the sum of these two flows; written in terms of surface area, we have: Qout = HdAd + HslAsl + HsuAsu

(2)

We assume that the fines/biomat layer is along the trench bottom, and that it has formed along the vertical sides to a distance of, say, 4 inches. If we plan to use a trench that is 1 ft deep and 3 ft wide, we calculate the following areas: Ad = bottom area = 3L Asl = lower side area (4 inches) = 2(4/12)L = 0.667L Asu = side area (8 inches) = 2(8/12)L = 1.333L Substituting numbers into Equation 2 above gives 550 gal/day = (0.05 gal/day/ft2)(3L)

Proceedings of the Eleventh Individual and Small Community Sewage Systems Conference, ASAE, October 20-24, 2007, Warwick, Rhode, Island. + (0.01 gal/day/ft2)(0.667L) + (0.02 gal/day/ft2)(1.333L) or 550 = 0.15L + 0.00667L + 0.0266L= 1.53L Solving for L, we get or

L = 358 ft

(1 ft deep x 3 ft wide mature

trench)

Volume Check (Check the enlarged figures in the paragraph that follows.)

The volume check is performed by applying the Rubin equation as follows. In this particular example a calculated surge volume capacity is 690 gallons while the available surge volume capacity is 5302 gallons. The system design based on HLR is determined to have sufficient volume.

Application of Safety Factor

The newly designed gravel system will then be compared to the size of the standard gravel system. The ratio of the current gravel size to the design size is defined as the implied safety factor (Fs). In this case the standard gravel length is 200 feet. The safety factor can then be determined as follows; 210 Fs = 108 or Fs = 1.94 The factor of 1.94 would than be applied to any new non gravel system that is developed.

Comparison Table

A table of calculations showing the application of the model to a gravel system in a range of soil types is given below.

Conclusions

This paper proposes a rational, scientifically-based method of determining the infiltration area required for onsite wastewater soil adsorption systems. The method developed for determination of trench sizing is intended to accommodate new technologies that have been developed in recent years and those that will be subsequently developed. Application of the proposed sizing method will allow for the appropriate sizing of infiltration systems that accept wastewaters of different waste strengths (i.e. systems with or without treatment), different trench geometries, and the use of pretreatment methods which fully or partially reduce the organic content of the wastewater. The methods and formulas described in this document are based on research conducted over the years by a number of experts in soil science, soil physics, hydraulics and wastewater engineering Glossary A Fs H L N Qb Qd

= = = = = = =

Soil interface area Safety Factor Hydraulic loading rate Trench length Number of bedrooms Daily flow per bedroom Design Daily Flow

ft2 gallons/day/(ft2 of soil interface area) ft gallons/bedroom gallons/day


References Ronner and Wong. 1998 Salthouse and Loudon, 1997

Bouma, 1975. Unsaturated Flow During Soil Treatment of Septic Tank Effluent. Journal of Environmental Engineering, American Society of Civil Engineers, 101 (EE6):967-983 Erickson, J. and Tyler, E.J., 2001. A Model for Soil Oxygen Delivery to Wastewater Infiltration Surfaces. In On-Site Wastewater Treatment: Proceedings of the Ninth National Symposium on Individual and Small Community Sewage Systems. American Society of Agricultural Engineers, St. Joseph, MI. USEPA, 2004. Onsite Wastewater Treatment System Manual. EPA/625/R-00/008.