Filtered-to-Drip Irrigation
Graywater Systems
The holy grail
of greywater systems accomplishing maximum coverage and uniformity with automated self-cleaning filters and greywater compatible drip irrigation.
Filtered-to-Drip Irrigation
In order to pass through tiny drip emitters graywater needs to be filtered and pressurized. Filters need to be automatically self-cleaning to prevent rapid clogging. Through using drip irrigation and automatic controls, Filtered-to-Drip Graywater Systems have the potential to offset significant quantities of potable water use. Turfgrass lawns and other small plants need a very broad and even distribution that can only be accomplished with graywater using drip irrigation. Many high tech designs utilize “Make-up water” (usually freshwater or rainwater) to supplement the Graywater supply or flush the filter, and are required to have a Backflow Prevention Device to protect the source supply from contamination that could occur from graywater back flowing into the potable water system.
Common features of filtered-to-drip systems
Pumping Basin
(aka surge tank or surge basin)
Graywater is collected into a pumping basin but the system needs to be designed to empty at least every day because its prohibited to store greywater for more than 24 hours ( high amounts of nutrients and suspended solids and bacteria in graywater rapidly multiply under storage to turn into noxious septic blackwater). Other Code Requirements for pumping basins:
Watertight
Made of solid, durable material
Vented
Sealed against vermin
Access opening for cleaning
Labeled with capacity, and "GRAY WATER SYSTEM, CAUTION- UNSAFE WATER"
Passive overflow drain to the sewer (same size as inlet pipe) with a backwater valve (sewer check valve)
Unions on piping to tank
If underground, lids must withstand 300 pounds per square foot
Install on 3” thick concrete slab or compacted baserock foundation.
1/16” screens on all vents, inlets, and overflow pipes
Designed to minimize time graywater is held in tank
Designed to distribute estimated amount of graywater on a daily basis
see Code 1602.8.3 for additional details
Effluent Pump
Rated to pass 3/4” solids
Pumps are different sizes (horsepower) depending on the distance and height you have to reach
Inline float switch attached to the pump activates pump as soon as basin fills with Graywater
submersible
Vortex impeller or similar technology approved for sewage effluent
Dirty water submersible pumps are designed to pump soft solids up to 1 1/2" or more in size with little to no filtration required before the pump
Include a check valve (alllows one-way flow only) to prevent Backflow into the basin.
A course mesh hair catch filter bag recommended be installed at the inlet into the basin, designed such that even after clogging it does not impede the passage of graywater into the basin.
Include Unions ( a plumbing coupling designed to be easily removable)to allow for easy removal and maintenance of the pump
Power for pump typically supplied by an existing plug-in 120 V GFCI outlet (cutting the plug off the end of the pump cord (for hard wiring) will void the pump warranty)
Electricity use of a 660 Watt pump (running for 1/2 hour per day in a typical 4-person household), would be an actual use of 450 Watts during operation, for a total daily power usage of 0.25 kilowatt hours which represents 0.6% of the average home power consumption.
Self-Cleaning Filters
(Vary widely according to design)
Graywater is pumped through an automated self cleaning “Disc filter” containing a stack of discs each with a series of microscopic grooves whose dimension determines the effective mesh size of the filter, ranging from 40 to 600 mesh. Graywater is filtered as it flows through the disc grooves, which contain up 100x the surface area of a screen filter, but nonetheless clog readily and thus require a self-cleaning mechanism that shifts from normal filtration flow to backwash by reversing flow direction. Triggering the backwash cycle can be done through the use of pressure sensors upstream and downstream of the filter that measure the pressure differential (pressure loss) across the filter (Which increases as the filter becomes clogged), or through the use of timers that flush the filter on a regular interval. Filter backwash water is usually provided by a pressurized feed from municipal water, with cross connection prevention provided by a code-compliant, Reduced Pressure Principal-type double-check valve Backflow prevention device.
Sub-Surface Drip
After filtration graywater is distributed to the landscape via a 1” supply line(PVC or PE) connected to 1/2” dripplerlines with built-in “in-line” drip emitters. High amounts of suspended solids (i.e.dissolved detergents, dirt) and bacteria in unfiltered graywater quickly clog traditional drip emitters so use emitters specially designed to resist root intrusion and irrigate with waste water. Dripperlines are laid on or under the soil surface, covered by 2” of mulch (aka sub-mulch) or soil (aka sub-soil), with the spacing between lines determined by the soil type and plant material.
Very low flow dripper zones may release graywater slower than it is being generated, causing the water level in the pumping basin to build up and overflow to the sewer. While very high flow dripper zones may require more water to pressurize the dripperline than the volume of graywater normally generated in a typical house at any time.
Optimal zone size generally ranges from 500 to 1000 square feet and should be divided according to geography of the site and water requirements of plants.
Backflow Prevention Device
Backflow prevention devices ensure water quality by preventing contamination of the potable (drinking) water supply due to back-pressure or back-siphonage of graywater into the municipal system. Local regulations vary as to acceptable backflow standards but generally speaking the Reduced pressure (RP) backflow device ( RP valve) is the most reliable backflow prevention device and can protect against both back- pressure and back-siphonage. Above ground installation is required with concrete stabilization and a minimum 12-inch gap between the relief valve and finished grade to allow the RP to discharge water when a back-siphon situation occurs. Backflow preventer devices are legally required to be inspected annually by a certified backflow inspector.
Key points
High Tech Graywater Systems
Graywater is collected into a surge tank instead of being diverted directly to the landscape
Can pump water uphill
Can irrigate lawns and small plants
Can Use drip irrigation (with proper filtration)
High potential for irrigation-efficiency
Able to control other freshwater or rainwater irrigation systems
Can function as a complete irrigation system for a property
Permit relatively easy and straightforward Distribution exclusively via gravity
Smart Controllers
Typical functions for a controller include opening and closing solenoid valves, managing filter backwash cycles, water budgeting according to zone characteristics and changing weather conditions. The chief challenge with controlling greywater is that the supply is not always reliably there at a predictable time. The controller must be able to sense when the basin is full of water and then determine the correct zone to irrigate. Smart Controllers are able to self-adjust according to changing weather conditions, and seasonally changing evapotranspiration rates, usually based on realtime ET (evapotranspiration) values taken from active weatherstations, historical climate data for the appropriate CIMIS zone location, and/or soil moisture sensors. Some systems use a water meter that measures all graywater and alternate water that is distributed to each zone.
Make-up water
refers to the supplementary water (usually freshwater) that is distributed through the greywater system to cover any irrigation requirement not fulfilled by the amount of greywater produced. Make-up water can be added to the pumping basin through an “air gap” situation or can is allowed to be directly connected to the graywater system (allowed in some jurisdictions) if protected by an approved RPZ Backflow Prevention Device.
Designing and Calculating Drip Zone Flows
Greywater drip zones are usually designed to irrigate at approximately the maximum flow rate that graywater is being produced into basin, which avoids ever losing water through over flow. Theoretically if multiple fixtures in a whole house system were being used at the same time (i.e. 2 people showering @ 2.5 gpm + Laundry machine (2.5)and sink(2.5)) then producing 10 gpm is a good rough estimate of the maximum theoretical rate of production. To determine the flow rate of a drip zone, add up the total linear feet of dripper line and multiply by the flow rate of the drip emitter which is measured in gallons per hour, and divide by 60 (minutes in a hour) to get the flow rate in gallons per minute: (i.e. 300 ft x 2 gph emitters =600 gph/60 = 10 gpm.
Avoiding manually cleaned filters
High amounts of dissolved solids, nutrients and associated bacteria in graywater rapidly clog filters such that relying on manually cleaning a filter is NEVER recommended. Abandonment of greywater systems due to filter & maintenance failure is the inevitable situation occurring with manually cleaned filter systems. All successful filter systems are automatically self-cleaning and regardless tend to require the most troubleshooting of any part of the system.
Operation and Maintenance Manual
Required info:
Diagram(s) of the system and location of components
Instructions on operating the system
Details on start-up, shut-down, and deactivation
Applicable testing, inspection and maintenance
Contact info of installer/designer and component manufacturer
Directions that the O&M manual needs to remain with the building for the life of the structure
Other helpful info:
What kinds of soaps to use
A picture of the tubing before it is buried
