Presentation on theme: "Common Issues with AOSS Design Kemper Loyd, P.E., Technical Services Engineer Div. of Onsite Sewage, Water Supplies, Environmental Engineering and Marina."— Presentation transcript:
Common Issues with 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)
12VAC provides some caveats for 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 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 12VAC 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.
Example: 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 (2,500 in out = 2,000 net ) 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 for 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 12VAC (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 increasing. Many designers have not yet become familiar with these “new” drip requirements.
12VAC 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.
12VAC 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 dose.
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.
12VAC 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.
Example: This drip system contains: 800’ of drip tubing 350’ of 1½” Supply Pipe 350’ of 1¼” Return Pipe All piping drains between doses. 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 = 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? = 15 gallons Supply pipe will fill = 5 gallons Drip tubing will fill = (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.
12VAC 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. Example: 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… Etc…(Sigh…)
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.
Unfortunately: 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 functions. 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 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 branch. When the valve is closed, all flow is diverted to discharge through the branch to the right.
Example: 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 Tank.
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. (Q i = Q e = Q design ) The flow sent to the Media Filter is controlled by the recirculation pump and can be changed by changing the pump cycle timing Example: Pumping 5Q design to the Media Filter will provide a recirculation ratio of 4:1, since 4Q design will return to the Recirculation Tank and 1Q design will exit via the discharge side of the RV. 6Q design to the Media Filter will provide a ratio of 5:1.