SECTION IV

Design Calculations for a Septic Drip Disposal Systems

Size of the Absorption Area

The total amount of absorption area required generally depends on two factors:

1. the daily wastewater load of the facility or home being serviced by the system.
2. the absorption capacity and treatment ability of the soil.

STEP 1:                     Calculate the design flow. Example:

                    Flow = 150 GPD PER BED x 3 Bedrooms = 450 Gallons Design Flow

STEP 2:                     Determine the soil loading rate.

A field evaluation of the soils at the site must be completed by a qualified person, i.e. soil scientist as per section II. See Table 1 for loading rates which are stated in gallons per day per square feet and in gallons per day per linear foot.

STEP 3:                     Compute total tubing necessary for the absorption field.

                    From Area chart:

                    Tubing necessary = Area / 2 = (Daily Flow / AREA Loading Rate) / 2 = Linear Feet

                    Tubing necessary = (450 GPD / 0.1 Gal/Day/Sq.Ft.) / 2 = 4,500 Square Feet / 2 = 2250 LF

                    From Tubing chart:

                    Tubing necessary = Flow / Linear loading rate = Linear Feet

                    Tubing necessary = 450 GPD / 0.2 Gal/Day/LF = 2250 Linear Feet

STEP 4:                     Determine layout and shape of the dripper line absorption field.

Keep each individual dripper line lateral length no greater than 300 feet from its connection to the supply manifold to its connection to the return flush manifold because of excessive friction loss. Always configure the system supply line to feed laterals at the lower end of the dripper field lines and return from the highest elevations. When running a continuous dripper line, it may turn and make a loop or series of loops back to the return flush line before making a connection. Keep each lateral in a zone close to the same length.

STEP 5:                     Calculate pump head loss to prove standard pump is adequate.

General Layout Requirements Septic Drip System

The septic tank, added treatment units, dosing tank and distribution field are subject to set back regulations to keep required distances from wells, property lines building foundations and bodies of water according to local regulations.

Septic effluent Drip disposal absorption systems are typically shallow placed installed from 6-24" below the ground surface and in suitable soils as defined by code or a soils scientist. At grade or surface drip is suitable with treated effluent.

Topography

The dripper tubing should be placed on contour. Dripper tubing along any given lateral should not be off grade by more than 6" in 100 feet. Tubing installed on sloping sites (less than 5%) should have small diameter manifolds for supply and return and with sub zones not to exceed 1200 LF to minimize draindown which may overload the down slope laterals. Since pressure compensating dripper emission rates are consistent at varying pressures, no special design requirements to ensure proper soil loading rates are needed.

The central unit must be positioned to allow the backflush and field flush water to be discharged back to the septic tank with minimum back pressure. The waste line from the central unit is recommended to be gravity. If this line needs to go up hill or is over 30' long a special design may be necessary. Call American for special design considerations.

Each zone should be fed from the lowest elevation of the zone and the air release should be placed at the highest elevation. Accumulate the return return manifold piping to a common return line to save pipe costs. The sizing of the pipe will be determined later in this chapter.

General Information Requirements Summary

The site layout will determine those items of information needed in the detailed design including the following:

1. NUMBER OF ZONES
2. LONGEST LATERAL IN EACH ZONE
3. HIGHEST GPM ZONE
4. LENGTH OF RUN FROM THE PUMP CHAMBER TO THE CENTRAL UNIT
5. LENGTH OF RUN FROM THE CENTRAL UNIT TO THE FIELDS
6. LENGTH OF RUN FROM THE FIELDS TO THE CENTRAL UNIT
7. LENGTH OF RUN FROM THE CENTRAL UNIT TO THE SEPTIC TANK
8. ELEVATION DIFFERENCES OF ALL COMPONENTS

The above information is needed to perform the simple pump head loss calculation to determine if the standard pump which is necessary to backflush the disc filters is also adequate for dosing and field flushing. Standard head loss tables are used to compare to the pump curve.

Size of Central Hydraulic Unit

The cost of any system is impacted by the size of the hydraulic unit. The self contained units have a 15 gpm capacity or a 25 gpm capacity. The capacity includes dosing and field flush flow rates. Skid mounted units need to be enclosed in a heated and floor drained building. They start at 40 gpm increase in increments of twenty gpm.

By using multiple zones, this problem is easily managed. You can design a system with up to two (2), four (4), eight (8), or sixteen (16) zones for self contained central units, or multiples of eight (8) zones for central units which need heated housing.

REMEMBER: Always configure the layout to avoid draindown of the system. On sloped sites zones are feed from the top and return to the top using the special supply and return manifolds.

Flow Rate during Absorption Field Dosing

The flow rate during absorption field dosing depends on the amount of dripper line required for any particular installation. The flow rates are easily calculated as follows:

Note:

                    GPH = Gallons per Hour, GPM = Gallons per Minute

                    Dripper Line Length in Absorption Field / 2 Foot Emitter Spacing = Number of Emitters

                    Number of Emitters x .65 GPH = Absorption Field Dosing Rate in GPH

                    GPH / 60 minutes = Absorption Field Dosing Rate in GPM

Field Flushing Flow Rates

Since automatic flushing of the dripper lines in the absorption field is an integral function of the total system, it should be considered as part of the overall flow rate generated by the system. It has been established that proper scouring and flushing of any pipe system will require at least 1.6 gallons per minute flow at the outflow end (distal end) of any pipe. Therefore, we should design for a flow of at least 1.6 gallons per minute out of each dripper line connection that has been made to the return flush manifold pipe. Multiply each return manifold connection by 1.6 GPM to get the field flushing flow requirement.

Total Flow Requirement of System

The total flow used in calculating the operating flow requirement of the absorption field would be the combination of both the field dosing flow and the field flushing flow.
                    field dosing flow + field flushing flow = total hydraulic design flow

Size of Septic and Pump/Dosing Tanks

Septic tank volume is determined according to state and local regulations, and is the same as conventional systems. The pumping tank should provide flow equalization and one day for emergency storage; therefore, the same volume of the septic tank would be sufficient.

                    Example: For a 450 GPD waste flow.
                    Volume of Pumping Tank = 450 Gallons x 2 = 900 Gallons, use 1000 gallons

Calculating Head Losses

Friction and static losses to consider include, pump discharge line, static head, supply line to absorption field, return flush line, central unit loss, dripper line laterals and elevation.

You will use a standard head loss chart. The length of the supply line from the pump unit to the farthest point in the absorption field area should be used in calculating this dynamic head loss. You should use the total flow requirement (dosing & flushing) of the absorption field in GPM in the chart. Find the column which corresponds to the supply line size intended. Use the loss column for that size pipe at the absorption field total flow rate.

Note: A supply line and manifold size of 1" diameter is possible in most residential cases and will serve to reduce the amount of possible draindown into the drip field.

The return flush line loss is figured in the same manner as the absorption field supply line loss. Always attempt to use a return flush pipe size equal to the supply line size.

HYDRAULIC UNIT LOSSES: All the figures for pressure loss in the valves, flow meter, filters, fittings, etc. of the pre assembled pump/filter unit have been calculated and are found in Chart 2A (Filter Unit Losses) for 2 filter units (15 gpm max design). For larger flow rates up to 25 gpm use the three filter head loss curve. In order to find the flow loss through the hydraulic Unit, you will use the total flow rate calculated (dosing plus field flush) for the absorption field and find that flow rate in the first column of Chart 2A or the X-axis of the three filter chart.

PIPE FITTINGS: During all calculations in determining pressure/head losses and requirements, you can generally round up to the nearest whole number. The pressure loss through fittings in the residential system is insignificant and will not require any calculations and will not effect operation of the system. Use as few fittings as possible.

ELEVATION CHANGES: Changes in elevation will contribute to the total losses that need to be considered. The area of the system which must be considered for losses due to elevation changes is the elevation difference from the Pump to the highest point in the absorption field. The calculation sheets are divided into the elevation from the pump enable float to the hydraulic unit and separately from the hydraulic unit to the highest elevation in the field.

DRIPPER LINE LATERALS: Pressure/friction losses for the dripper line laterals and loops must be considered in the overall pressure loss calculation. This pressure loss is usually the largest single amount. The calculation is made simple by the use of Chart 3A (Drip Line Pressure Requirements). The pressure loss for maintaining the minimum required 1.6 gpm through the dripper line lateral during flushing has already been calculated and put on chart 3A for varied lengths of dripper line.

Determine the length of the longest dripper line lateral or loop in your system design and refer to Chart 3A . Find the corresponding pressure loss. For example, an absorption field requiring 1,200 feet of dripper line may have four lateral lines. The longest lateral is 300 feet. Chart 3A shows an inlet pressure requirement corresponding to a 300 foot lateral length is 15.2 psi or 35.1 feet.

Please Note: If your system design can not be modified to meet layout requirements discussed in this section to match the performance capabilities of the pump shown on the Chart, call American Manufacturing Company, Inc. at 1-800-345-3132 for pump specification and design.

Unequally Sized Absorption Areas Dosed as One Zone

When an absorption field has been designed in two or more unequally sized areas and these areas are dosed by a single zone (dosed at the same time) no special considerations will need to be addressed for this type of design. Since all the absorption field zones are being dosed at the same time, each zone will only accept the amount of waste water from that dosing cycle that it was designed to receive. This is because the flow rate for a smaller zone will be proportionally less than the larger zone due to the pressure compensating emitter. Also unequally sized zones should be designed to flush separately.

Unequally Sized Absorption Zones Dosed Separately

When it becomes necessary to design a system that doses two absorption fields of unequal size, no special considerations will need to be addressed for this type of design. Dosing unequal sized zones will provide equal distribution again due to the pressure compensating emitter. Each zone will have a different flow rate while dosing since they have different amounts of dripper line in them and different size absorption areas; therefore, they self adjust to the proper amounts of water.

Time Operated Dosing

Time dosing for waste water disposal is a major factor in contributing to keeping a soil absorption system functioning properly. Dosing the absorption field on a time delay basis instead of on demand, as waste water flows into the dosing tank, will avoid the over saturation of the soil.

How to Set Up Dosing of the American Septic Drip System

Each zone control doses 4 doses per day at standard. Look at the control panel and determine zone capacity. A two zone panel will dose eight (8) times per day or 1440/8=180 minute rest. It will rest 108 minutes at peak enable. A four zone panel (3 or 4 zone hydraulic unit) will dose 16 times per day or 1440 min per day / 16 doses =90 rest. It will rest 54 minutes at peak enable.

For example, in a three zone system with a 4 zone controller, each zone has 900 linear feet and the total gallons per day were designed for 600 gallon, the pump run time will be as follows;
          900 feet / 2 X .65 / 60           = 4.9 gpm,

The "pump run time dial" will adjust the pump run time from zero (0) minutes to 17.5 minutes. If the dial is set to run for 11.5 minutes, this run time will dispose of 450 gallons per day when two 900 linear feet zones are in use. Use a small phillips screw driver to adjust the "pump run time dial". After adjustment press and hold the reset button until a pump event is initiated (approx. 5 seconds ). Determine the total gallons pumped in each cycle by the flow meter and time the pump run time after backflush to determine if the setting is adjusted. Readjust if necessary. The gallons per day can be set by determining the gallons per dose in each zone (use the flow meter), find the doses per day from the rest time between doses, and adjust the pump run time accordingly.

To estimate the dosing time with a predetermined amount of waste water is a simple function of total daily flow divided by the number of doses per day and the flow rate of your absorption field. Use each absorption field dose at the same run time and frequency to insure equal loading, the formula to set up dosing is as follows:

                    Amount of daily waste water flow / Number of Doses per Day X # Zones = Amount of Each Dose

Assume a 450 gallons in 2 zones:           450 GPD (60%) / 4 Doses/Day X 2 = 33.75 Gal. per Dose

For example, a calculated dose of 33.75 gallons for a 2 zone controller will be programmed to dose every 3 hours (8 times per day). At 33.75 gallons per dose a dose time of 16.9 minutes is required for 900 linear feet of tubing. The only variable that will need adjustment in any system of this type will be the amount of time for each dose in minutes. The system designer will designate this dose amount.

Example of Dosing Separate Fields of Equal Size

Dosing separate fields of equal size is basically the same as in single dosed fields. Let us assume we have two equal fields with 1,100 linear feet of dripper line in each field. The flow rate for each field while dosing is 6.0 GPM or:

The average daily flow of this system is 270 gallons in 8 doses at 33.75 gallons each dose. Therefore, at 6 GPM per field, one field of 6 GPM will run approximately 5.6 minutes. Then in this example, the first of the equal absorption fields will dose 33.75 gallons in approximately 5.6 minutes and 3 hours later the next equal size field will dose for 5.6 minutes. Then another 3 hours later, the first field will dose again and so on. This scenario is favorable in that each field will be "at rest" for 6 hours between doses.

Peak flows are accommodated by increasing the dose frequency to 13.3 doses per day or every 108 minutes. This is equal to 6.6 doses per day at peak enable for a 2 zone system.

Important: Always try to keep each dosing cycle a minimum of 6 minutes and a maximum of 17.5 minutes each.

Dosing Unequal Absorption Fields Separately

A little more calculation is required to prove the dosing on unequally sized absorption fields. Since each absorption field is unequal in size, each field will have a different flow rate while dosing and a different amount of the daily flow to be attributed to disposal in that particular field or zone. The daily flow has been calculated and established for that particular system, it is automatically divided proportionately to each field that doses separately by time due to the pressure compensating emitter.

Example: 450 Gallons per Day peak flow with two separately dosed fields of 1,100 linear feet and 700 linear feet. Divide the daily flow proportionately as follows:

To find what percentage of the daily flow will go to each zone you will need to know the ratio of difference between each field size. The formula to do so is as follows: Take the number in feet of the total in either field one (1,100 feet) or field two (700 feet) and divide that number by the total footage amount in both fields. That will tell you what percentage of the daily flow will need to be dosed to that particular field.

Field One:           1,100 Linear Feet / 1,800 L.F. (both fields) = .61 or 61%

61% of the total daily flow of 360 gallons will be dosed to Field One. 360 gallons x .61 = 219 gallons. The remaining amount of daily flow will go to Field Two. 360 Gallons per Day (100%). Field One Flow, 360 - 219 Gallons = 141 Gallons per Day (39%).

The next step will be to determine dose time for each field followed by the same 3 hour resting cycle (allows each field to rest 6 hours). To determine this you must calculate the total dosing flow rate for each field.

We have now established the total dosing time necessary per day for each field to dispose of the daily flow proportionally is equal. You could also design more zones for lower flow rates and shorter dosing cycles. (Time dosing if for units up to 8 zones). See previous discussion for peak flow considerations. It works the same.

YOU DO NOT NEED TO CONFIRM PROPORTIONAL FLOW TO UNEQUAL ZONES!
PRESSURE COMPENSATING EMITTERS AUTOMATICALLY PROPORTION FLOWS!

Dosing Volume and Pump Float Switch

The minimum depth of draw down in your dosing tank should be the maximum dosing volume for any zone. Therefore, you will need to know the volume of water per inch in depth of the dosing tank intended for use in your design.

For example, suppose your standard 1,000 gallon dosing tank has a volume of 20 gallons per inch of water depth. Assume you have two fields that dose separately and the larger field requires at least 60 gallons per dosing cycle. Therefore, your float switches must be set to allow a draw down of 3" of water.

REMEMBER EQUAL DISTRIBUTION WITH UNEQUAL ZONES !

One of the unique features of this system is that equal distribution is achieved with different size zones in the same system. The fact that the pressure compensating emitters discharge at the same rate from 7 to 70 psi means that if all zones have the same run time and the same number of doses each day, then they will receive the same amount of effluent per linear foot.

LARGE SYSTEM SIZING

Large systems are sized using the same criteria as small systems. The only difference is the number and size of each zone. The only difference in the receiving environment is the increased volume of water per unit of area may become more critical due to the large size of the area. Groundwater mounding and nutrient dilution may be effected.

To size a large system take the total flow rate of all tubing:

Large system filtration equipment's comes in multiples of 8 zones so estimate the number of zones to determine one zones dosing rate, example 1270/16 = 79 gpm.

The trick is if each lateral in all the zones is 300 feet long then the flushing flow is twice the dosing flow rate. therefore in the example the total filtration capacity will be two time the dosing flow or 2 X 79 = 158 gpm. Look in the list of large system filtration equipment to determine the model number needed. In this case an American Septic Drip system from 120 gpm to 200 gpm will be required. Model ASD200 with 16 zones. If you double the number of zones, one Model ASD 90 would be required. This may be cheaper. Call American for support for all large systems.

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