Process for Designing Onsite Systems

prepared by
Dr. Paul Trotta, Northern Arizona University

Objectives

Major Steps of Onsite System Design

Drainfield Design Example

Solution to Drainfield Design Example

1      Objectives:

1.1     To become aware of the issues related to the design and review of a plan for an onsite wastewater system.
1.2     To structure and program the steps needed to develop or review a plan for an onsite wastewater system.
1.3     To become familiar with the design philosophy of the proposed Arizona rules.

2      The Major Steps of an Onsite System Design or Design Review:

Step #1 Collect and consolidate all the available site data.
Step #2 Evaluate the available site data.
Step #3 Evaluate the site's subsurface condition.
Step #4 Determine the appropriate Soil Absorption Rate (SAR) and Minimum Vertical Separation (MVS).
Step #5 Compute the wastewater burden.
Step #6 Compute the required total absorption area.
Step #7 Specify the hydraulic requirements.
Step #8 Develop hydraulic system features. 

2.1     Step #1 Collect and consolidate all the available site data.

Including:

• County, NRCS, & USDA soil maps
• Onsite inspection report from county environmental health department
• Engineer’s soil report (qualitative soil analysis and/or percolation test report)
• Subdivision maps and Assessor’s maps
• Property legal descriptions

2.2     Step #2 Evaluate the available site data for its geographic constraints to ascertain available sites for effluent treatment and dispersal.

Consider:

2.2.1     Regulatory setbacks

Determine the setbacks to property lines, wells, drainage ways, driveways, existing or proposed structures, cut banks (steep excavated slopes).

2.2.2     Obvious additional constraints

Examples: rock outcroppings in the middle of a proposed soil treatment area, clusters of trees which will likely be considered important by the property owner, archeological sites, and/or protected species habitat.

2.3     Step #3 Evaluate the site's subsurface condition to determine the suitability of the soil profile for treating and dissipating the treated effluent.

2.3.1      Depth to limiting layer(s)
The existence of a regulatory limiting layer of impervious rock, fractured rock, semi-impervious clays, sands, gravels, cinders, rock dominated soil layer, and/or evidence of high groundwater. The depth to the limiting layer will influence the choice of treatment technology chosen. Higher treatment levels which result in lower Biochemical Oxygen Demand (BOD) and Suspended Solids (SS) may result in higher allowable Soil Absorption Rates (SARs). This results in less total absorption area for a given soil type and less total trench length and/or shallower trenches for an overall smaller soil treatment area footprint. Higher treatment levels that result in lower Fecal Coliform (FC) levels may allow for deeper trenches that also can decrease the total trench length and therefore the total footprint of the system. The designer should consider the various combinations of treatment and dispersal technologies and choose (with the advise and consent of the client) the most appropriate system for the conditions.

2.3.2      Depths and types of major soil bands found in the soil profiles of the site
These first three steps will be discussed in greater detail in the module "Site Evaluation &Characterization" and in separate soils evaluation classes. We have presented a brief overview of this information as a part of the larger process. The following steps in the design process relate to the application of the site evaluation and characterization data.

2.4     Step #4 Determine the appropriate Soil Absorption Rate (SAR) and Minimum Vertical Separation (MVS).

The nominal SAR is based upon qualitative soil analysis and/or percolation tests. Each major soil band can have a different SAR. The appropriate overall SAR may be a weighted average of the individual SARs for the major soil bands if horizontal dispersal is assumed or the most limiting SAR if downward dispersal is assumed. Historically, the SAR was developed entirely from percolation test data. Recently, the use of qualitative soil analysis has gained significant credibility in Arizona as an alternative approach. Both percolation, "perc," tests and qualitative soil analysis are subject to errors. The perc test can have significantly different results from location to location in soils that seem to be identical because of minor fluctuations in such things as root density (especially dead roots), rocks, and fractured soils. Differences in test procedures and conditions not explicitly standardized in the test protocol can also affect the test results (i.e., temperature of the soils and test water or use of gravel packs and forms to support soils with low wet strength). Qualitative soil analysis can also have small-scale variability and has the added problem of more evaluator subjectivity. Both tests can easily be used with the most conservative results in forming the design.

2.4.1      SAR and Adjusted SAR (SARa)
Arizona has moved toward an approach which will allow an increase of the application rate over the nominal SAR developed from percolation tests or qualitative soil analysis if higher treatment levels are attained prior to dispersal into the native soils. Higher treatment levels which result in lower Biochemical Oxygen Demand (BOD) and Suspended Solids (SS) result in higher allowable SAR values. The rationale for this approach is found in the assumption that the clogging mat, which forms at the soil interface, will remain (in the aggregate) more permeable because of the lower availability of nutrients for the bacteria which form the clogging mat. Higher SAR values result in less total absorption area for a given soil type. This can result in less total trench length and/or shallower trenches or smaller footprints for horizontal systems. The following chart (full-page picture) illustrates the adjustments proposed for the SAR for various technologies based upon their nominal final total concentration of BOD and SS added together. It should be noted that the adjustments are largest for soils with higher initial absorption rates and are dramatically reduced for dense clay soils. This reflects the hypothesis that for dense clay soils the clogging mat is less likely to be the hydraulically limiting layer. The soils themselves have a low absorption rate which may be more restrictive than the bacterial layer.

Figure 1. SAR Adjusted values for various percolation tests and treatment systems.

2.4.2      Minimum Vertical Separation (MVS)
Similarly, higher treatment levels that result in lower Fecal Coliform levels may allow for deeper trenches that also can decrease the total trench length and therefore, the total footprint of the system. The designer should consider the various combinations. The rationale for this approach is found in the assumption that lower (log order) concentrations of bacteria in the effluent have a decreased likelihood of migrating or being carried down to the limiting layer (groundwater, fractured rock, etc.). The accompanying chart, developed from the proposed new Arizona rule, illustrates these approaches.

The coliform value presented is the log value of the bacteria concentration per mL. Each change of value of 1 unit represents a 10 fold change. Septic tank effluent (Log-10; 8) has 100 times as many Fecal Coliform as does the effluent from pad systems (Log-10; 6). With 100 times less bacteria, the assumption is that there is a reduced public health threat, and therefore the effluent can be released within 3.5 feet of the limiting layer rather than the required 5 feet for septic tank effluent. It should be noted that for all treatment technologies the minimum vertical separation (MVS) is doubled for soils that show a high soil absorption capability. This reflects the possibility of unimpeded downward movement of bacteria in such soils when subjected to bacteria-laden effluent discharges.

Table #1 Nominal Performance Characteristics and Minimum Vertical Separations (MVS)

NOMINAL PERFORMANCE
	TSS	BOD5	TSS+BOD	N(total)	Coliform	MVS (ft)	MVS (ft)
System Type	mg/l	mg/l	mg/l	mg/l	L0G-10	(SAR 0.2-0.6)	(SAR 0.63 -1.1)
Septic Tank	75	150	225	53	8.0	5.00	10.00
Graveless Trench	75	150	225	53	8.0	5.00	10.00
Wisconsin Mound	30	30	60	53	5.5	3.25	6.50
Pad System	50	50	100	53	6.0	3.50	7.00
Intermittent Sand Filter w/ Underdrain	10	10	20	40	3.0	2.00	4.00
Intermittent Sand Filter w/Bottomless	20	20	40	53	5.0	3.00	6.00
Peat System	15	15	30	53	5.0	3.00	6.00
Textile Filter	15	15	30	30	5.0	3.00	6.00
Ruck	30	30	60	15	6.0	3.50	7.00
Aerobic- Subsurface Dispersal	30	30	60	53	5.5	3.25	6.50
Aerobic- Surface Dispersal	30	30	60	53	0.0	0.00	0.00
Cap System	75	150	225	53	8.0	5.00	10.00
Constructed Wetlands	20	20	40	45	5.0	3.00	6.00
Sand Lined Trench	20	20	40	53	5.0	3.00	6.00
Sequencing Batch Reactor	30	30	60	53	5.5	3.25	6.50

2.5     Step #5 Compute the wastewater burden from the proposed home.

The wastewater burden from a house is a combination of many indicators including income level, number of family members, age of family members, house size (sq. ft.), number of bedrooms, cultural background, education, number of bathrooms, number and kind of plumbing features, and likely others. A rational design would address any other obvious considerations that could increase or decrease the estimated wastewater burden (e.g., dehumidifiers connected to drains, indoor pools or steam rooms, water using hobbies like wine or beer brewing). Currently, Arizona requires the analysis of the wastewater burden two ways: the number of bedrooms and the number of plumbing fixtures. Essentially, the bedroom count is simply multiplied by 150 to achieve the daily design flow. Determine the fixture count using the table in Arizona code R18-9-A314(4)(a)(ii), the use the table in Arizona code R18-9-A314(4)(a)(i) to determine the design flow. Choose the design flow method that gives you the highest design flow.

2.6     Step #6 Compute the required total absorption area required for the proposed house.

Consider the tradeoffs that may influence the final system selection if more than one alternative can be identified. The total footprint of the dispersal system may be decreased by increasing the level of treatment. The engineer should discuss with the property owner the tradeoff between using more of the available site and possibly increasing costs for additional treatment.

2.7     Step #7 Specify the hydraulic requirements of the proposed system.

Once the type, size, and location of the treatment area is determined and the type size and location of the treatment facilities are determined, a design of the hydraulic features can proceed. Such a hydraulic design will begin with the definition of the hydraulic requirements of the proposed system.

Some of these requirements are derived from the site plan:

1. change in elevation (positive or negative) between the pump tank and the drainfield
2. effluent transmission line length
3. elevation changes within the drainfield

1. daily dose allowed per square foot of drainfield area
2. number of doses recommended per day
3. dose quantity per dose
4. time intervals between doses

1. effluent delivery configuration within the drainfield
2. the number of discharge points in each drainfield zone
3. the type of discharge points in each drainfield zone
4. the pressure required at each discharge point

2.8     Step #8 Develop derived hydraulic system features. Including:

1. the total pressure needed from the pump
2. the number of dispersal zones
3. total flow rate needed from the pump
4. working storage within the pump tank
5. emergency storage within the pump tank

3      Drainfield Design Example:

Determine the smallest footprint for a proposed home’s drainfield.

The proposed home has the following features:

1. 4 bedroom house
2. 4 full bathrooms
3. 1 kitchen
4. 1 laundry with a utility sink
5. 1 bar sink in the living room

1. The soils have been described as Silty Loam with moderate strength
2. A percolation test was also conducted which resulted in a “Perc” rate of 5 minutes per inch (mpi)
3. Seasonally saturated conditions were detected at a depth of 63 inches
4. The proposed soil treatment area is relatively flat and homogeneous

1. The trenches will be 1.5 feet wide.
2. Maximum trench length will be 100 ft.
3. A distance equivalent to twice their total depth will separate the trenches (min 5 feet).
4. A minimum of 9 inches of cover is required.
5. The sidewall area will be counted from the bottom of the 4-inch distribution pipe to the bottom of the trench and the distribution pipe will be covered by a 2 inches of distribution rock.

A textile filter treatment technology will be considered as pretreatment before dispersal into the trench system.

Use the Arizona rule as expressed in the data presented and determine the overall size of the drainfield. (Provide total length, total width, and total square feet).

4     Solution to Drainfield Design Example:

To solve this problem, you will be using information from the problem statement and steps 1-6 of the design process.

Step #1 Collect and consolidate all the available site data.

This step was completed in the problem description.

Step #2 Evaluate the available site data for its geographic constraints to ascertain available sites for effluent treatment and dispersal.

This step was completed in the problem description.

Step #3 Evaluate the site's subsurface condition to determine the suitability of the soil profile for treating and dispersing the treated effluent.

This step was completed in the problem description.

Step #4 Determine the appropriate Soil Absorption Rate (SAR) and Minimum Vertical Separation (MVS).

First, you must determine the appropriate SAR for the soil texture (Figure 1 of this module or table in Arizona rules [R18-9-A312(D)(2)(b)]), then compare it to the SAR estimated from perc test (also Figure 1 of this module or table in Arizona rules [R18-9-A312(D)(2)(a)]), and then choose the most conservative value for the SAR. Convert the SAR to an adjusted SAR (SARa) if applicable. Then, using the table in the Arizona rules [R18-9-A312(E)], you can determine the minimum vertical separation (MVS).

Determining the SAR.
From Figure 1, for a silt loam of moderate structure, the SAR is 0.6 g/day/ft². For a perc rate of 5 mpi, Figure 1 indicates that the SAR would be 0.9 g/day/ft². Choose the most conservative value, therefore the SAR = 0.6 g/day/ft².

Determining the SARa.
From Figure 1, using the SAR = 0.6 g/ft²/day and the Peat & Textile filter curve, the SARa = 0.9 g/ft²/day.

Determining the MVS.
It states in the Arizona rules [R18-9-A312(E)(2)], "[t]he allowable minimum vertical separation from the bottom of the constructed drainfield to the top of the nearest limiting subsurface condition is dependent on the ability of the facility to reduce the level of harmful microorganisms..." You will be using the table in section R18-9-A312(E)(2) and the nominal performance rating of the pretreatment from Table 1 of this module. Find the nominal performance of a textile filter. The MVS rules are only interested in the Coliform Log-10 level which is 5.0. Go to the table in the rules and pay attention to the first footnote in the table (i.e., "Soil absorption rate from percolation testing or soil characterization, in gallon per square foot per day." Therefore, using a Total Coliform Concentration of 5 cfu/100 mL and a SAR of 0.6 g/ft²/day, MVS = 3 ft.

Step #5 Compute the design flow from the proposed home.

Determine the number of bedrooms, multiply the number of bedrooms by 150 gpd/bedroom to obtain the bedroom-method design flow of the home. Then compute the number of fixture units and use the table in R81-9-A314(4)(a)(i) to obtain the fixture-count design flow of the home. Choose the number that is the largest for the design flow.

Design Flow (bedroom method) = 4 bedrooms x 150 gpd/bedroom = 600 gpd
Design Flow (fixture unit method): 37 fixture units, table in R18-9-A314(4)(i), design flow = 900 gpd

Choose the most conservative value, therefore, the design flow = 900 gpd

Step #6 Compute the required total absorption area required for the proposed house.

In this step, you will be calculating the required absorption area, trench length, overall size of the drainfield.

Required absorption area = wastewater burden (gpd) ÷ SARa (g/ft²/day) = 900gpd ÷ 0.9 g/ft²/day
= 1000 ft²

Available sidewall depth is determined by subtracting the cover soil and cover over pipe depth and MVS from the soil depth.

Available absorption area.
Remember the problem statement indicated that for trenches less than 3 ft deep, the bottom area may be counted as absorption area in addition to the sidewall area. Therefore, available sidewall area = 1 ft side + 1 ft side + 1.5 ft bottom = 3.50 ft.

Required trench length = required absorption area ÷ available absorption area = 1000 ft² ÷ 3.5 ft = 286 ft

Summary

Material last reviewed: April 3, 2006

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