Common Issues with AOSS Design Kemper Loyd, P.E., Technical Services Engineer Div. of Onsite Sewage, Water Supplies, Environmental Engineering and Marina.

Common IssuesCommon Issueswith with

AOSS DesignAOSS Design

Kemper Loyd, P.E., Technical Services EngineerKemper Loyd, P.E., Technical Services EngineerDiv. of Onsite Sewage, Water Supplies, Environmental Engineering and Marina ProgramsDiv. of Onsite Sewage, Water Supplies, Environmental Engineering and Marina Programs

Virginia Department of HealthVirginia Department of Health

““Interesting” Soil Hydraulic Interesting” Soil Hydraulic Loading RatesLoading Rates

Regulations for Alternative Onsite Sewage Systems Regulations for Alternative Onsite Sewage Systems (AOSS Regulations)(AOSS Regulations)

Lets take a look at a single Table 1 Requirement:Lets take a look at a single Table 1 Requirement:

The way we sometimes see that requirement The way we sometimes see that requirement translated istranslated is

(Anybody see a pattern here?)(Anybody see a pattern here?)

Note the descriptive words in the table heading:Note the descriptive words in the table heading:

MaximumMaximum (not universal for the category)(not universal for the category)

Pressure-DosedPressure-Dosed (not gravity-dosed)(not gravity-dosed)

Trench BottomTrench Bottom (not drip area or pad area)(not drip area or pad area)

12VAC5-613-80 provides 12VAC5-613-80 provides some caveats for some caveats for designers:designers:

10.a: The designer is responsible for reducing loading rates according to the features and properties of the soils in the soil treatment area as well as for reducing loading rates for other types of dispersal.

10.e: Trench bottom hydraulic loading rates for gravity dosed systems shall be reduced from the values in Table 1.

10.f: Area hydraulic loading rates for systems such as drip dispersal, pads, and mounds shall be reduced from the values in Table 1 and shall reflect standard engineering practice.

Within a Category – Interpolate RatesWithin a Category – Interpolate Rates

Pressure-Dosed Trench Distribution:Pressure-Dosed Trench Distribution:

Note: This is just one method; there are other, equally valid Note: This is just one method; there are other, equally valid methods to interpolate other rates within a category.methods to interpolate other rates within a category.

Between Categories – Extrapolate RatesBetween Categories – Extrapolate Rates

Gravity-Dosed Trench Distribution:Gravity-Dosed Trench Distribution:

Between Categories – Extrapolate RatesBetween Categories – Extrapolate Rates

Drip Distribution:Drip Distribution:

* From 12VAC5-610-955.C.1 of the * From 12VAC5-610-955.C.1 of the Sewage Handling and Disposal Sewage Handling and Disposal RegulationsRegulations (as amended by emergency regulation). (as amended by emergency regulation).

Between Categories – Extrapolate RatesBetween Categories – Extrapolate Rates

Pad Distribution:Pad Distribution:

Equalization PlacementEqualization Placementand Volume Issuesand Volume Issues

Equalization (EQ) is commonly proposed for Equalization (EQ) is commonly proposed for facilities where you routinely have a day with a facilities where you routinely have a day with a large wastewater flow followed by several days large wastewater flow followed by several days with significantly reduced (or no) flows.with significantly reduced (or no) flows.

It allows a reduction in the size of the dispersal It allows a reduction in the size of the dispersal area and may also allow a reduction in the size area and may also allow a reduction in the size of the treatment unit.of the treatment unit.

A good example of a facility that often benefits A good example of a facility that often benefits from EQ is a church.from EQ is a church.

The placement of the EQ tank and its required The placement of the EQ tank and its required volume sometimes pose problems.volume sometimes pose problems.

ExampleExample: : A church seats 500 for Sunday morning services, A church seats 500 for Sunday morning services,

which run from 9:00 a.m. to 12:30 p.m.which run from 9:00 a.m. to 12:30 p.m.

No activities occur at the church Monday-Saturday No activities occur at the church Monday-Saturday (this is not typical).(this is not typical).

The designer proposes a wastewater generation of The designer proposes a wastewater generation of 5 gallons per church attendee, for a total Sunday 5 gallons per church attendee, for a total Sunday flow of 2,500 gallons.flow of 2,500 gallons.

Equalization will be provided to disperse the Equalization will be provided to disperse the treated Sunday flow over a 5 day period at a treated Sunday flow over a 5 day period at a rate of 500 gpd, and the dispersal field will be rate of 500 gpd, and the dispersal field will be sized for that daily flow.sized for that daily flow.

Church → Treatment → EQ → DispersalChurch → Treatment → EQ → DispersalIf the EQ tank is placed after Treatment, the If the EQ tank is placed after Treatment, the

Treatment unit must be able to treat a Treatment unit must be able to treat a wastewater flow of 2,500 gallons generated over wastewater flow of 2,500 gallons generated over a 4-hour period. This may require a unit sized a 4-hour period. This may require a unit sized for >2,500 gpd.for >2,500 gpd.

Church → EQ → Treatment → DispersalChurch → EQ → Treatment → DispersalIf the EQ tank is placed before Treatment, the If the EQ tank is placed before Treatment, the

Treatment unit need only be sized to treat the Treatment unit need only be sized to treat the equalized flow of 500 gpd, and it can be fed to equalized flow of 500 gpd, and it can be fed to that unit over 24 hours.that unit over 24 hours.

Church → Treatment → EQ → DispersalChurch → Treatment → EQ → Dispersal

Church → EQ → Treatment → DispersalChurch → EQ → Treatment → Dispersal

In either of the above configurations, what must be In either of the above configurations, what must be the minimum working EQ volume?the minimum working EQ volume?

a. 500 gallonsa. 500 gallons

b.b. 2,000 gallons (2,5002,000 gallons (2,500inin - 500 - 500outout = 2,000 = 2,000netnet))

c.c. 2,500 gallons2,500 gallons

Church → Treatment → EQ → DispersalChurch → Treatment → EQ → Dispersal

Church → EQ → Treatment → DispersalChurch → EQ → Treatment → Dispersal

In either of the above configurations, what must be In either of the above configurations, what must be the minimum working EQ volume?the minimum working EQ volume?

c.c. 2,500 gallons is most nearly correct. 2,500 gallons is most nearly correct. Remember, the 2,500 gallon influent flow is Remember, the 2,500 gallon influent flow is generated over 4 hours, while the 500 gpd generated over 4 hours, while the 500 gpd forward flow occurs over 24 hours (averaging forward flow occurs over 24 hours (averaging 20.8 gph). Therefore, only a small portion of 20.8 gph). Therefore, only a small portion of that 500 gpd is likely to be sent forward during that 500 gpd is likely to be sent forward during the 4 hours that influent is generated.the 4 hours that influent is generated.

Generally, the wastewater generation schedule will be much Generally, the wastewater generation schedule will be much more complicated than this example, and tables such as the more complicated than this example, and tables such as the following should be developed (and submitted) to clearly following should be developed (and submitted) to clearly identify the wastewater sources, flow timing, etc.identify the wastewater sources, flow timing, etc.

““New” RequirementsNew” Requirementsforfor

Drip Dispersal SystemsDrip Dispersal SystemsOften OverlookedOften Overlooked

The The Sewage Handling and Disposal RegulationsSewage Handling and Disposal Regulations were were amended by emergency regulation on March 14, 2014 to amended by emergency regulation on March 14, 2014 to include several “good engineering” practices for drip include several “good engineering” practices for drip dispersal systems.dispersal systems.

Those practices are listed in 12VAC5-610-955 (with the Those practices are listed in 12VAC5-610-955 (with the changes discussed in GMP 135.A).changes discussed in GMP 135.A).

Most of the following requirements are also included in Most of the following requirements are also included in GMP 156 and must be followed for a shallow-placed drip GMP 156 and must be followed for a shallow-placed drip dispersal system to be credited with 50% TN reduction.dispersal system to be credited with 50% TN reduction.

Since 50% TN reduction is now required over much of Since 50% TN reduction is now required over much of Virginia, the use of shallow-placed drip dispersal is Virginia, the use of shallow-placed drip dispersal is increasing.increasing.

Many designers have not yet become familiar with these Many designers have not yet become familiar with these “new” drip requirements.“new” drip requirements.

12VAC5-610-955.C.4: Air/vacuum release valves shall be located at the high points of the supply and return manifolds to each zone.

Air release valves have always been necessary at the high Air release valves have always been necessary at the high points of the supply and return lines, but not specifically points of the supply and return lines, but not specifically at the high points of the supply and return manifolds.at the high points of the supply and return manifolds.

Additionally, Additionally, vacuum releasevacuum release has been found to be an has been found to be an important design element. Without vacuum release, important design element. Without vacuum release, drainage through lower-elevation emitters following a drainage through lower-elevation emitters following a dose will pull a vacuum on higher emitters. That vacuum dose will pull a vacuum on higher emitters. That vacuum can and will suck fine soil particles (which were can and will suck fine soil particles (which were suspended during dosing) into the emitters and drip suspended during dosing) into the emitters and drip tubing, causing them to eventually clog and fail.tubing, causing them to eventually clog and fail.

12VAC5-610-955.D: All drip dispersal systems shall be equipped with devices or methods to restrict effluent from draining by gravity to portions of a zone or laterals lower in elevation. Variable distribution due to gravity drainage shall be 10% or less within a zone.

Lengthy dripfield supply and return pipes and manifolds Lengthy dripfield supply and return pipes and manifolds must be configured so that they do not drain to the must be configured so that they do not drain to the dripfield at the conclusion of each dose.dripfield at the conclusion of each dose.

On sloping sites, the supply and return manifolds must not On sloping sites, the supply and return manifolds must not allow upper driplines to drain to lower driplines after each allow upper driplines to drain to lower driplines after each dose.dose.

Pump Tank Located Up-slope from DripfieldPump Tank Located Up-slope from Dripfield

At the conclusion of each dose, the volume of effluent At the conclusion of each dose, the volume of effluent within the supply and return pipes drains to the dripfield.within the supply and return pipes drains to the dripfield.

Also, the upper drip laterals can drain to the lower laterals.Also, the upper drip laterals can drain to the lower laterals.

The drain-down issue might be addressed by The drain-down issue might be addressed by placing the pump tank below the dripfield.placing the pump tank below the dripfield.

One manufacturer recommends separate supply One manufacturer recommends separate supply and return lines for each drip lateral on sites with and return lines for each drip lateral on sites with a “discernible slope”.a “discernible slope”.

Properly-located check valves may also help Properly-located check valves may also help resolve the drain-down issue.resolve the drain-down issue.

12VAC5-610-955.F: Each drip dispersal zone shall be time-dosed over a 24 hour period. The dose volume and interval shall be set to provide unsaturated flow conditions. Demand dosing is prohibited. Minimum dose volume per zone shall be 3.5 times the liquid capacity of the drip laterals in the zone plus the liquid capacity of the supply and return manifold lines (which drain between doses) accounting for instantaneous loading and drain back.

The minimum dose requirement runs contrary to typical The minimum dose requirement runs contrary to typical thinking that dripfields work best when dosed frequently thinking that dripfields work best when dosed frequently with small doses.with small doses.

ExampleExample::

This drip system contains:This drip system contains:

800’ of drip tubing800’ of drip tubing

350’ of 1½” Supply Pipe350’ of 1½” Supply Pipe

350’ of 1¼” Return Pipe350’ of 1¼” Return Pipe

All piping drains betweenAll piping drains between

doses.doses.

It serves a 3-bedroom It serves a 3-bedroom

residence (450 gpd).residence (450 gpd).

800’ of drip tubing800’ of drip tubing

800 / 81 = 9.9 gallons800 / 81 = 9.9 gallons

9.9 x 3.5 = 34.7 gallons9.9 x 3.5 = 34.7 gallons

350’ of 1½” Supply Pipe350’ of 1½” Supply Pipe

350 / 10 = 35.0 gallons350 / 10 = 35.0 gallons

350’ of 1¼” Return Pipe350’ of 1¼” Return Pipe

350 / 13 = 26.9 gallons350 / 13 = 26.9 gallons

34.7 + 35.0 + 26.9 = 34.7 + 35.0 + 26.9 = 96.6 gallon minimum dose96.6 gallon minimum dose

450 gpd / 96.6 = 4.7 doses/day450 gpd / 96.6 = 4.7 doses/day

Round down to 4 doses/day at 113 gal/doseRound down to 4 doses/day at 113 gal/dose

What happens if you set for 9 doses/day at 50 gal/dose?What happens if you set for 9 doses/day at 50 gal/dose?

50 - 35 = 15 gallons50 - 35 = 15 gallonsSupply pipe will fillSupply pipe will fill

15 - 10 = 5 gallons15 - 10 = 5 gallonsDrip tubing will fillDrip tubing will fill

5 - 27 = 5 - 27 = (22 gallons)(22 gallons)Return pipe will not fillReturn pipe will not fill..

Since the pressurizationSince the pressurizationvalve is typically on the endvalve is typically on the endof the return line (in the of the return line (in the pump tank), the system will not achieve proper operating pump tank), the system will not achieve proper operating

pressure, and the rate of flow from the emitters will not pressure, and the rate of flow from the emitters will not be uniform as desired. Emitters on the lower laterals will be uniform as desired. Emitters on the lower laterals will discharge more than those on the upper laterals.discharge more than those on the upper laterals.

12VAC5-610-955.H: A means for measuring or estimating total flow dispersed to the soil absorption area and to verify field dosing and field flushing rates shall be provided. (Note: This requirement is not included in GMP 156.)

Although a “new” requirement, providing some means to Although a “new” requirement, providing some means to measure/estimate both dosing and flushing flows should measure/estimate both dosing and flushing flows should be something that has always been provided in a design.be something that has always been provided in a design.

The “new” aspect of the requirement is that the means to The “new” aspect of the requirement is that the means to measure/estimate flows needs to be made clear in the measure/estimate flows needs to be made clear in the design submittal.design submittal.

The “Cadillac” way to comply with this requirement is to The “Cadillac” way to comply with this requirement is to provide two flow meters in the design – one on the provide two flow meters in the design – one on the supply line and one on the return.supply line and one on the return.

Subtracting the return meter reading from the supply meter Subtracting the return meter reading from the supply meter reading will give the dosing flow.reading will give the dosing flow.

Compliance can also be accomplished via typical pump Compliance can also be accomplished via typical pump drawdown testing, flow measurement, etc. drawdown testing, flow measurement, etc.

ExampleExample::

Dosing Flow: After system pressurizes, close the return Dosing Flow: After system pressurizes, close the return valve and measure the rate of flow via a pump valve and measure the rate of flow via a pump drawdown test. (Note that pressure-compensating drawdown test. (Note that pressure-compensating emitters will provide a pretty uniform flow rate over a emitters will provide a pretty uniform flow rate over a wide range of operating pressures.)wide range of operating pressures.)

Flushing Flow: Set the return valve to the operating Flushing Flow: Set the return valve to the operating position and either directly measure the return flow position and either directly measure the return flow (bucket) or measure it indirectly by diverting it to a (bucket) or measure it indirectly by diverting it to a drum/tank while performing another pump drawdown test drum/tank while performing another pump drawdown test (measuring the sum of the dosing and flushing flows).(measuring the sum of the dosing and flushing flows).

The procedure should be stated in the submittal and/or the The procedure should be stated in the submittal and/or the Operation and Maintenance Manual.Operation and Maintenance Manual.

Design Calculations OmittedDesign Calculations Omitted(i.e. Show Your Work!)(i.e. Show Your Work!)

How can we review this?

No elevations…No elevations…No pipe sizesNo pipe sizesNo pipe lengths…No pipe lengths…No pump curve…No pump curve…Etc… Etc… (Sigh…)(Sigh…)

Hopefully, in the course of preparing a project Hopefully, in the course of preparing a project submittal, the designer is justifying design submittal, the designer is justifying design assumptions, performing design calculations, assumptions, performing design calculations, consulting pump curves, etc.consulting pump curves, etc.

Help us help you (the designer) by providing that Help us help you (the designer) by providing that information and work as part of your submittal.information and work as part of your submittal.

If VDH reviewers do not have the information If VDH reviewers do not have the information necessary to review critical system components necessary to review critical system components and cannot readily figure out what you did, then and cannot readily figure out what you did, then we have to ask, and that we have to ask, and that willwill slow things down. slow things down.

InconsistenciesInconsistencies

Unfortunately:Unfortunately:

The pump calculations are The pump calculations are based on 250’ of 1½” based on 250’ of 1½” force main.force main.

The LPD calculations are The LPD calculations are based on five 1½” based on five 1½” laterals with 3/8” holes laterals with 3/8” holes on 3’ centers.on 3’ centers.

What is the contractor What is the contractor likely to construct?likely to construct?

Questions?Questions?

Recirculation ValveRecirculation Valve

Recently, I have gotten several questions Recently, I have gotten several questions regarding how the recirculation valve (“splitter”) regarding how the recirculation valve (“splitter”) that is used in AdvanTex and EZ-Treat ATUs that is used in AdvanTex and EZ-Treat ATUs functions.functions.

Instead of a fixed flow division (like a D-box), this Instead of a fixed flow division (like a D-box), this recirculation valve allows the recirculation ratio recirculation valve allows the recirculation ratio to be changed by merely changing the volume to be changed by merely changing the volume that the pump delivers to the media filter.that the pump delivers to the media filter.

H

Flow Control Orifices InRemovable Standpipe

Watertight Grommet

Discharge

Screened Influent

Side View - Flow Splitter Basin

Top View - Flow Splitter Basin

3"

3"

2" Typ.

L.L.

Typical Flow SplitterTypical Flow Splitter

Adapted from Orenco Systems, Inc. drawing

Recirculation Valve (RV)Recirculation Valve (RV)

The valve is mounted in The valve is mounted in the Recirculation Tank the Recirculation Tank on the Media Filter on the Media Filter return line.return line.

When the valve is open, When the valve is open, all flow drops into the all flow drops into the Recirculation Tank Recirculation Tank through the lower through the lower branch.branch.

When the valve is closed, When the valve is closed, all flow is diverted to all flow is diverted to discharge through the discharge through the branch to the right.branch to the right.

ExampleExample::

3-bedroom residence (450 gpd)3-bedroom residence (450 gpd)

Recirculation pump timer controlled to send 50 gallons to Recirculation pump timer controlled to send 50 gallons to media filter every 32 minutes.media filter every 32 minutes.

Assume no influent enters the Recirculation Tank Assume no influent enters the Recirculation Tank before/during pump operation.before/during pump operation.

Pump sends 50 gallons to the Media Filter.Pump sends 50 gallons to the Media Filter.

All flow from the Media Filter returns to the Recirculation All flow from the Media Filter returns to the Recirculation Tank.Tank.

Pump again sends 50 gallons to Media Filter.Pump again sends 50 gallons to Media Filter.

Before flow returns from the Media Filter, 20 gallons of Before flow returns from the Media Filter, 20 gallons of influent enters the Recirculation Tank.influent enters the Recirculation Tank.

30 gallons of flow from the Media Filter returns to the 30 gallons of flow from the Media Filter returns to the Recirculation Tank, closing the Recirculation Valve.Recirculation Tank, closing the Recirculation Valve.

The closed Recirculation Valve forces the remaining flow The closed Recirculation Valve forces the remaining flow from the Media Filter to exit the Recirculation Tank via from the Media Filter to exit the Recirculation Tank via the discharge pipe.the discharge pipe.

In summaryIn summary::

Influent flow to the Recirculation Tank equals effluent flow from that tank. (QInfluent flow to the Recirculation Tank equals effluent flow from that tank. (Qii = Q = Qee = Q = Qdesigndesign))

The flow sent to the Media Filter is controlled by the recirculation pump and can be changed by changing the pump cycle timingThe flow sent to the Media Filter is controlled by the recirculation pump and can be changed by changing the pump cycle timing

Example:Example:

Pumping 5QPumping 5Qdesigndesign to the Media Filter will provide a recirculation ratio of 4:1, since 4Q to the Media Filter will provide a recirculation ratio of 4:1, since 4Qdesigndesign will return to the Recirculation Tank and 1Q will return to the Recirculation Tank and 1Qdesigndesign will exit will exit via the discharge side of the RV.via the discharge side of the RV.

6Q6Qdesigndesign to the Media Filter will provide a ratio of 5:1. to the Media Filter will provide a ratio of 5:1.

Questions?Questions?