Common Issues
AOSS Design
Kemper Loyd, P.E., Technical Services Engineer
Div. of Onsite Sewage, Water Supplies, Environmental Engineering and Marina Programs
Virginia Department of Health
“Interesting” Soil Hydraulic
Loading Rates
Regulations for Alternative Onsite Sewage Systems
(AOSS Regulations)
Lets take a look at a single Table 1 Requirement:
The way we sometimes see that requirement
translated is
(Anybody see a pattern here?)
Note the descriptive words in the table heading:
Maximum (not universal for the category)
Pressure-Dosed (not gravity-dosed)
Trench Bottom (not drip area or pad area)
12VAC5-613-80 provides some caveats for
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 Rates
Pressure-Dosed Trench Distribution:
Note: This is just one method; there are other, equally valid
methods to interpolate other rates within a category.
Between Categories – Extrapolate Rates
Gravity-Dosed Trench Distribution:
Between Categories – Extrapolate Rates
Drip Distribution:
* From 12VAC5-610-955.C.1 of the Sewage Handling and Disposal
Regulations (as amended by emergency regulation).
Between Categories – Extrapolate Rates
Pad Distribution:
Equalization Placement
and Volume Issues
Equalization (EQ) is commonly proposed for
facilities where you routinely have a day with a
large wastewater flow followed by several days
with significantly reduced (or no) flows.
It allows a reduction in the size of the dispersal
area and may also allow a reduction in the size
of the treatment unit.
A good example of a facility that often benefits
from EQ is a church.
The placement of the EQ tank and its required
volume sometimes pose problems.
A church seats 500 for Sunday morning services,
which run from 9:00 a.m. to 12:30 p.m.
No activities occur at the church Monday-Saturday
(this is not typical).
The designer proposes a wastewater generation of
5 gallons per church attendee, for a total Sunday
flow of 2,500 gallons.
Equalization will be provided to disperse the
treated Sunday flow over a 5 day period at a rate
of 500 gpd, and the dispersal field will be sized
for that daily flow.
Church → Treatment → EQ → Dispersal
If the EQ tank is placed after Treatment, the
Treatment unit must be able to treat a
wastewater flow of 2,500 gallons generated over
a 4-hour period. This may require a unit sized
for >2,500 gpd.
Church → EQ → Treatment → Dispersal
If the EQ tank is placed before Treatment, the
Treatment unit need only be sized to treat the
equalized flow of 500 gpd, and it can be fed to
that unit over 24 hours.
Church → Treatment → EQ → Dispersal
Church → EQ → Treatment → Dispersal
In either of the above configurations, what must be
the minimum working EQ volume?
a. 500 gallons
b. 2,000 gallons (2,500in - 500out = 2,000net)
c. 2,500 gallons
Church → Treatment → EQ → Dispersal
Church → EQ → Treatment → Dispersal
In either of the above configurations, what must be
the minimum working EQ volume?
c. 2,500 gallons is most nearly correct.
Remember, the 2,500 gallon influent flow is
generated over 4 hours, while the 500 gpd
forward flow occurs over 24 hours (averaging
20.8 gph). Therefore, only a small portion of
that 500 gpd is likely to be sent forward
during the 4 hours that influent is generated.
Generally, the wastewater generation schedule will be much
more complicated than this example, and tables such as the
following should be developed (and submitted) to clearly
identify the wastewater sources, flow timing, etc.
“New” Requirements
Drip Dispersal Systems
Often Overlooked
The Sewage Handling and Disposal Regulations were
amended by emergency regulation on March 14, 2014 to
include several “good engineering” practices for drip
dispersal systems.
Those practices are listed in 12VAC5-610-955 (with the
changes discussed in GMP 135.A).
Most of the following requirements are also included in
GMP 156 and must be followed for a shallow-placed drip
dispersal system to be credited with 50% TN reduction.
Since 50% TN reduction is now required over much of
Virginia, the use of shallow-placed drip dispersal is
Many designers have not yet become familiar with these
“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
points of the supply and return lines, but not specifically
at the high points of the supply and return manifolds.
Additionally, vacuum release has been found to be an
important design element. Without vacuum release,
drainage through lower-elevation emitters following a
dose will pull a vacuum on higher emitters. That vacuum
can and will suck fine soil particles (which were
suspended during dosing) into the emitters and drip
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
must be configured so that they do not drain to the
dripfield at the conclusion of each dose.
On sloping sites, the supply and return manifolds must not
allow upper driplines to drain to lower driplines after each
Pump Tank Located Up-slope from Dripfield
At the conclusion of each dose, the volume of effluent
within the supply and return pipes drains to the dripfield.
Also, the upper drip laterals can drain to the lower laterals.
The drain-down issue might be addressed by
placing the pump tank below the dripfield.
One manufacturer recommends separate supply
and return lines for each drip lateral on sites with
a “discernible slope”.
Properly-located check valves may also help
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
thinking that dripfields work best when dosed frequently
with small doses.
This drip system contains:
800’ of drip tubing
350’ of 1½” Supply Pipe
350’ of 1¼” Return Pipe
All piping drains between
It serves a 3-bedroom
residence (450 gpd).
800’ of drip tubing
800 / 81 = 9.9 gallons
9.9 x 3.5 = 34.7 gallons
350’ of 1½” Supply Pipe
350 / 10 = 35.0 gallons
350’ of 1¼” Return Pipe
350 / 13 = 26.9 gallons
34.7 + 35.0 + 26.9 = 96.6 gallon minimum dose
450 gpd / 96.6 = 4.7 doses/day
Round down to 4 doses/day at 113 gal/dose
What happens if you set for 9 doses/day at 50 gal/dose?
50 - 35 = 15 gallons
Supply pipe will fill
15 - 10 = 5 gallons
Drip tubing will fill
5 - 27 = (22 gallons)
Return pipe will not fill.
Since the pressurization
valve is typically on the end
of the return line (in the
pump tank), the system will not achieve proper operating
pressure, and the rate of flow from the emitters will not
be uniform as desired. Emitters on the lower laterals will
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
measure/estimate both dosing and flushing flows should
be something that has always been provided in a design.
The “new” aspect of the requirement is that the means to
measure/estimate flows needs to be made clear in the
design submittal.
The “Cadillac” way to comply with this requirement is to
provide two flow meters in the design – one on the
supply line and one on the return.
Subtracting the return meter reading from the supply meter
reading will give the dosing flow.
Compliance can also be accomplished via typical pump
drawdown testing, flow measurement, etc.
Dosing Flow: After system pressurizes, close the return
valve and measure the rate of flow via a pump
drawdown test. (Note that pressure-compensating
emitters will provide a pretty uniform flow rate over a
wide range of operating pressures.)
Flushing Flow: Set the return valve to the operating
position and either directly measure the return flow
(bucket) or measure it indirectly by diverting it to a
drum/tank while performing another pump drawdown test
(measuring the sum of the dosing and flushing flows).
The procedure should be stated in the submittal and/or the
Operation and Maintenance Manual.
Design Calculations Omitted
(i.e. Show Your Work!)
How can we review this?
No elevations…
No pipe sizes
No pipe lengths…
No pump curve…
Hopefully, in the course of preparing a project
submittal, the designer is justifying design
assumptions, performing design calculations,
consulting pump curves, etc.
Help us help you (the designer) by providing that
information and work as part of your submittal.
If VDH reviewers do not have the information
necessary to review critical system components
and cannot readily figure out what you did, then
we have to ask, and that will slow things down.
The pump calculations
are based on 250’ of
1½” force main.
The LPD calculations
are based on five 1½”
laterals with 3/8”
holes on 3’ centers.
What is the contractor
likely to construct?
Recirculation Valve
Recently, I have gotten several questions
regarding how the recirculation valve (“splitter”)
that is used in AdvanTex and EZ-Treat ATUs
Instead of a fixed flow division (like a D-box), this
recirculation valve allows the recirculation ratio
to be changed by merely changing the volume
that the pump delivers to the media filter.
Typical Flow Splitter
F lo w C o n tro l O rifice s In
R e m o va b le S ta n d p ip e
L .L .
T o p V ie w - F lo w S p litte r B a sin
2 " T yp .
W a te rtig h t G ro m m e t
S cre e n e d
In flu e n t
D isch a rg e
S id e V ie w - F lo w S p litte r B a sin
Adapted from Orenco Systems, Inc. drawing
Recirculation Valve (RV)
The valve is mounted in
the Recirculation Tank
on the Media Filter
return line.
When the valve is open,
all flow drops into the
Recirculation Tank
through the lower
When the valve is closed,
all flow is diverted to
discharge through the
branch to the right.
3-bedroom residence (450 gpd)
Recirculation pump timer controlled to send 50 gallons to
media filter every 32 minutes.
Assume no influent enters the Recirculation Tank
before/during pump operation.
Pump sends 50 gallons to the Media Filter.
All flow from the Media Filter returns to the Recirculation
Pump again sends 50 gallons to Media Filter.
Before flow returns from the Media Filter, 20 gallons of
influent enters the Recirculation Tank.
30 gallons of flow from the Media Filter returns to the
Recirculation Tank, closing the Recirculation Valve.
The closed Recirculation Valve forces the remaining flow
from the Media Filter to exit the Recirculation Tank via
the discharge pipe.
In summary:
Influent flow to the Recirculation Tank equals effluent flow
from that tank. (Qi = Qe = Qdesign)
The flow sent to the Media Filter is controlled by the
recirculation pump and can be changed by changing the
pump cycle timing
Pumping 5Qdesign to the Media Filter will provide a
recirculation ratio of 4:1, since 4Qdesign will return to
the Recirculation Tank and 1Qdesign will exit via the
discharge side of the RV.
6Qdesign to the Media Filter will provide a ratio of 5:1.

Common Issues with AOSS Design - Virginia Environmental Health