Installing Soil Treatment Systems - NEIWPCC
Transcript of Installing Soil Treatment Systems - NEIWPCC
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A to Z Soil Treatment Systems
Randy Miles
2019 Northeast Onsite Wastewater Short Course
April 3, 2019
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Soil Treatment & Dispersal Options
• Purpose
– Provide further treatment
– Assimilate treated effluent into the receiving environment so neither public health nor environmental quality are adversely impacted.
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Soil Treatment & Dispersal Options
• Subsurface dispersal
– Discharge effluent into soil
– Most cases – original, undisturbed, unsaturated soil
– As effluent quality increases, focus is more on dispersal
– Point of application – top 2-3 feet of soil
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Subsurface Dispersal
• Treatment provided by soil
– Certain depth of unsaturated soil needed
• Measured from point of application into soil
• Measured to water table, excessively coarse or impermeable soils
– Typical minimum vertical distance is 12-36″
• Function of soil and site conditions
• Function of degree of effluent treatment
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To Use Soil for Treatment
• You need
Unsaturated flow
Aerobic environment
Aerobic bacteria
Soil particles
Air spaces
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Onsite Wastewater System Design Options
• Major design options include:
–Depth of trench bottom
– Loading rates
• At infiltrative surface
• For most restrictive horizon
• Account for horizontal flow
–Distribution system
–Pretreatment options6
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Principles for Applying Effluent to Soil
• To achieve best treatment
– Distribute over large enough area to ensure unsaturated flow
– Small doses help maintain aerated conditions
– Apply at shallow depths
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System definitions
• Terminology of soil treatment areas is very regional and regulatory driven
• Our terminology is based on national glossary
– Online: www.onsiteconsortium.org/
• Long term goal - use the same language
• Don’t get caught up in terminology & definitions
• STA = Soil treatment area
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Conventional
ShallowUltra-shallow
At-grade
Mound
areal fill
Below-gradeAbove-grade
ORIGINAL
GRADE
Aerobic
soil
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Benefits of Shallow Depth• Easier to maintain vertical separation from
limiting condition
– Seasonal high water table
– Claypan
– Bedrock, etc.
• Proper separation distance allows for interactions with particles and bacteria
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Benefits of Shallow Depth
• Keeps effluent in biologically active portion of soil profile
– More oxygen present
– More soil microbes present
• Evapotranspiration
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SOIL BIOTA POPULATIONVS
SOIL DEPTH
98.7%
0.9%
0.4%
16 inches
30 inches
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WATER USE(WHERE ROOTS DRINK AND EAT)
41%
25%12 inches
24 inches
16% 36 inches
18%
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System Selection - Consider
• Effluent quality
– May need treatment beyond septic tank if
• High strength waste or
• Using smaller diameter perforations in distribution
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System Selection - Consider
Slope
• Setbacks
• Other
– Aesthetics
– Maintenance
– Cost
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Types of Soil Treatment Units
• Trench
• Bed
• Low pressure
• At-grade
• Drip distribution
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Trench System
• Definition - A trench containing media through which septic tank effluent is treated
• Many types of media
• Distribution– Gravity
– Pump to gravity
– Pressurized
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Well
Aerobic soil
Groundwater
Septic System Design Options
Must allow for aerobic zone beneath field
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Subsurface Dispersal• In ground: gravity drainfield
– Why / Where used?
• Where soil and site meet code requirements
• Simplest and least expensive option
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Subsurface Dispersal
• In ground: Drainfield
– Design considerations
• Sized by soil and design flow (gallons/ft2/day)– Bottom: Is the design criteria
– Sidewall: Is extra if it occurs
• Need aerated soil below
• Many configurations
• Maximum slopes typically of 20% - 45%
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Purpose of Media
• Provide void space
– For effluent to enter
– For effluent to contact soil infiltrative surface
• All have advantages and disadvantages
• Soil loading rates must be established for each media material
– Hydraulic loading rates
– Organic loading rates
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Ideal Trench System
• Narrow trenches• Shallow placement• Oriented on contour• Time dosed
– Surge capacity (septic tank/pump chamber)
• Multi cells – for resting• Monitor flow and
ponding
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In-ground
trench
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Soil Absorption System
– Rock size
• Too small, more likely to plug, pore space too small;
• Too large, hard to work with and level
– Rock must be clean and free of fines
• Largest reason for trench plugging
– Purpose of rock is to hold the trench open and allow access by the waste to the soil base
• There have been other materials developed to do that
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Options for Distribution Media• Gravel/Rock
– Functions:
• Supports distribution pipe
• Transfers effluent from pipe to infiltrative surface
• Provides storage of peak flows
• Dissipates energy of surges
• Supports sidewall and overlying soil
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Options for Distribution Media
• Gravel
– Typically ¾ to 2 ½ inches in diameter
– Usually 6 inches below the pipe and 2 above
– Concerns:
• Amount of “fines” in gravel
• Dropping it on infiltrative surface may compact soil
• Machinery to place gravel may compact soil
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Placing Tire Chips in Conventional Trench
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Gravity Distribution on the Contour
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Distribution Box
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Distribution Devices
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Serial Distribution (Step-down)
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Alternative Systems
• Gravelless drainfield systems– Component: part of a disposal component
– Function: to take the place of gravel in drainfields, hopefully overcoming some of the limitations of gravel
– Considerations:• Different types
• Each type has different sizing criteria
• Each type can be used with gravity or pressure distribution.
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Options for Distribution Media
• Gravelless technologies
– Can be used for gravity & dosed flow networks
– Can be used where gravel is used
– Avoids many of the installation problems of aggregate
– Have unique sizing and installation requirements
– Have their own unique precautions
– Do NOT make the soil “work” better
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Distribution Media
• What’s the difference?
– Choices
– No ‘Value Judgment’
• Rock
• Chamber
• Gravelless pipe
• Synthetic media
• Other
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Gravelless Technologies• Aggregate-free technologies - chambers
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Chamber Technology
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Polystyrene Wrapped Pipe
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Gravelless Technologies
• Aggregate-free technologies – gravelless pipe
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Geotextile Wrapped Pipe
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Other Examples
Tire Chips Eljin In-Drain
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Trench cross-section example: Crowned backfill; shedding water
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Subsurface Dispersal
• In ground: Drainfield
– Design considerations – parallel vs. serial
Minnesota Coop. Ext.
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Type (cont.)
a. Method for dosing to field:
– Pump-to-gravity
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Subsurface Dispersal
• In ground: Drainfield
– Installation considerations
• Bottoms of each bed/trench must be level
• Soils must be dry to install
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What is a bed?
• Excavation greater than 3 feet in width
• Filled with distribution media
• Often with multiple pipes
• Gravity or pressure distribution
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Distribution Device
Distribution Box
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Bed System with Chambers
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Low O2 flow to
infiltrative surface –
poor treatment
High O2 flow to
infiltrative
surface – good
treatment
Trench versus Bed
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Application/Distribution
• Two general options available:
– Gravity-flow
– Dosed-flow
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Application/Distribution Options
• Dosed-flow distribution
– Predetermined volume of effluent is held in chamber and dosed to next component.
– Provides:
• More uniform loading to next component
• Resting times between doses
To next component
(usually soil)
Pretreatment
unit
Holding chamber & dosing
device
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Dosed-flow Distribution
• Dosing methods - Demand
–Dose occurs when sufficient, pre-determined volume of effluent has been collected.
–Dosing frequency depends on how much wastewater generated.
–No control on how much effluent is dosed daily (Socially controlled)
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Dosed-flow Distribution
• Dosing methods - Timed
– Timer controls number of doses per day & dose volume
– Allows only a certain amount of effluent to be dosed daily (24-hour period)
– Protects downstream components from being overloaded
– Useful for controlling surges or big-flow days
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Pump Systems
• Pump to gravity
• Low pressure pipe (LPP) system
• At-grade or mound system
• Drip irrigation
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Dosed-flow Distribution
• Dosing devices - Pump
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Type (cont.)
a. Method for dosing to field:
– Siphon-to-gravity
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What is a low pressure distribution (LPD) system?
AKA Low Pressure Pipe (LPP)
• Small diameter laterals ~ 1 ¼ inch
• Small orifices ~ 1/8 to 3/16 inch
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Pressure Distribution
• Contents
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LPP: Spraying Upward and Distal Turn-up
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Lateral turn up showing EZ FLOW option – sleeved line shown at rear
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Low Pressure Pipe
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What is a shallow-narrow-pressurized trench?
• Type of LPD
• Dosed via 1/8″ orifices in low-pressure 1″ distribution laterals covered with 12″ diameter half pipe
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What is a shallow-narrow-pressurized trench?
• Installed in upper portion (8 to 12″) of soil
• Highly-treated effluent applied
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Above-grade system terms & options
• Terms referred to in above-grade systems
– Absorption and basal area
• At-grade
• Mound
– Modified mound
• Areal fill
• Bottomless media filter
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Absorption versus basal area
• Absorption area is critical for acceptance
• Basal area is largely a function of landscaping
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Distribution
Absorption
Basal
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What is an at-grade?
• Above-grade STA installed with some part of the infiltrative surface located at the original ground elevation
– Excavation 0 to 6 inches into native soil
– Distribution media in contact with native soil
– Pressure or gravity distribution
– Orifices above the original ground elevation
– Suitable cover
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Subsurface Dispersal
• At-grade system
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Subsurface Dispersal
• At-grade system
– Uses native, original soils
– Insufficient soil depth for typical subsurface system but enough so mound isn’t required
– Similar horizontal and vertical separations
– Used with gravity or pressure distribution
• Pressure distribution or drip recommended
– Low linear loading rate – long & narrow
– Usually max slope of 25%
– Proper preparation of surface is critical
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At-grade
• Similar installation techniques as mound and areal fill except distribution media is in direct contact with native soil
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Initial Site Preparation for At-Grade System
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Coarse Aggregate and Pipe for At-Grade System
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Soil and Geotextile for At-Grade System
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What is a mound and a modified mound?
• Mound: above-grade STA with
– At least 12 inches of clean sand treatment media between bottom of infiltrative surface and original ground elevation
– Pressure distribution
• Modified mound: has < 12 inches of sand, but a minimum of 6 inches
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Mound system
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Mound system
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Mound components
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1.Top soil
2. Sand
3. Distribution
media
4. Piping
6. Cover5. Geotextile
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What is an areal fill system?
• Above-grade STA with entire infiltrative surface located above original ground elevation
– Uses imported soil fill treatment material
• Typically Group I or II (sandy or loamy) soils
– Pressure or gravity distribution
– Difference from a mound
• Uses media other than clean sand
• May use gravity distribution
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Areal fill
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Limiting layer
Imported
suitable fillDistribution system
Aerobic soil
•Similar construction techniques
•Varying media specifications
•Gravity distribution is sometimes used
Scarified surface
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What is a bottomless media filter?• Bed of specified media, typically with pressurized distribution
• Wastewater trickles down through media & disperses into soil directly under filter
• Constructed sand and modular peat are most common, but others exist
• Typically aggregate or soil cover or modular lid/container
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Bottomless media filter (BMF)
• Can be above or below original grade
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Liner or
Prefabricated Box
Backfill
material or
container
ORIGINAL
GRADE
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Pressure Distribution
• Objectives:
–Quickly pressurize network
–Be fully pressurized for most of dose
–Minimize draining into lower laterals
–Have about the same amount of effluent reach each square foot of infiltrative surface
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Pressure Distribution
• How it works
– Predetermined quantity of effluent collected
– At predetermined volume or time, dosing device discharges effluent to distribution network
– Because of small pipe and orifices, distribution network pressurizes quickly
– Current design objectives want equal amounts of effluent from each orifice
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Pressure Distribution
• Why/Where used?
– Where uniformity of distribution is vital to achieve unsaturated flow in soil or media
• Coarse or shallow soils
• Many pretreatment components
• Larger soil dispersal systems
– Helps provide predictable level of treatment in soil or media by minimizing hydraulic overloading
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Pressure Distribution
• Design considerations
– The smaller the orifice diameter: the greater the potential for plugging
– The smaller the orifices and the greater their spacing:
• The faster the network will pressurize
• The smaller the pump must be (gpm)
– Orifices at 12 o’clock will help network pressurize faster but are more subject to:
• Plugging
• Freezing in cold weather areas
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Supply Lines
• Free drainback to either the pump basin or drainfield, minimizes freezing in cold weather
• If drainback is directed towards the pump basin do not use a check valve
• Check valves must be used when using duplex pumps and a common discharge assembly
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Types of alternating or switching valves
• Bull run valves
• Gate valves
• Ball valves
• Sequencing valves (also called hydraulic switching valves)
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Dosed-flow Distribution
• Options – Drip distribution
– A small diameter pressurized distribution network that delivers small, precise volumes of pretreated effluent at slow controlled rates.
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Drip Distribution
• Why/where used?– Where most efficient uniform distribution is desired
or required
– May be used anywhere where gravity- or dosed-flow distribution is used
– Some locations allow use on soils shallower or finer-textured than allowed for other technologies
– Can be placed on more difficult sites – slopes, trees, etc.
– Causes less site disruption than other options
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The Benefits of Drip Irrigation
Controlled & Uniform Dosing– Even dosing over area
– Even dosing over time
– Minimizes soil saturation: encourages unsaturated flow
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Subsurface Drip Dispersal:Wastewater Treatment Approach
• Small doses of wastewater into the soil.
• Uniform distribution over the entire area.
• Wastewater moves through soil under unsaturated flow conditions, thus effectively treating the wastewater.
• Wastewater can be dosed into the active surface layer of the soil.
• Facilitates reuse of nutrients and water.
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Gravelless technologies
• Can be used for both gravity & pressure
• Can be used where washed rock is used
• Avoids the concerns of rock
• Have unique sizing & installation requirements
• Have their own unique precautions
• Check with manufacturer & local regulations
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Final cover
a. Depth - 6″ minimum recommended
b. Loamy topsoil material ideal, import if needed– Appropriate for vegetative
growth/landscaping
c. Crowned to allow for settling and shedding of surface water
d. Vegetation planted
e. Description of landscaping
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Backfilling
• Store topsoil separately so it can be used as final cover
• If site conditions (and inspector) allow, topsoil from one trench can be used to cover adjacent trench
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Landscaping importance
• Erosion protection
– Plants help hold topsoil in place
• Protect the system from freezing
– Plants trap snow
– Provide insulation
• Soften “look” of system so more aesthetically pleasing
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For Most Effective Wastewater Treatment in Soil
• Utilize aerobic soil conditions
• Encourage unsaturated flow
• Distribute evenly over soil infiltrative surface
• Place effluent shallow in soil profile to
– Increase evapotranspiration
– Use more biologically active portion of soil
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What controls LTAR?
• Infiltrative surface – biomat/soil interface
• Least permeable layer in profile
• Horizontal hydraulic conductivity above the least permeable layer
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Assessment of treatment potential
• Aerobic (unsaturated) soil conditions are required
• Aerobic (unsaturated) conditions results in greater contact time, thus greater treatment
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Conventional
ShallowUltra-shallow
At-grade
Mound
areal fill
Below-gradeAbove-grade
ORIGINAL
GRADE
Aerobic
soil
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Thank You!
Questions?
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