Graywater systems designed for homes with conventional plumbing are well-covered in the references noted at the bottom of this page, so here I'll just describe a couple systems I've designed that aren't featured there.
Casa Juliana Kitchen Graywater System
Wastewater from the kitchen sink is often problematic to use because it is relatively high in organic solids, oils, grease and soap which over time tend to seal up soil pores, preventing water from infiltrating the soil. Eventually the graywater becomes black, anaerobic and smelly. For these reasons, most U.S. jurisdictions classify kitchen wastewater as blackwater. These problems can be mitigated by installing an interceptor (grease trap) that traps both grease and solids.
The system described here was designed to evenly distribute the kitchen graywater exiting the interceptor. A distribution hub made from a 5 gallon bucket, cement and 1/2" PVC pipe distributed graywater to 6 small infiltration chambers, irrigating papayas and bananas. The plants produced prolifically during our years at Casa Juliana (in south Texas). Note that the small 1/2" distribution pipes occasionally clogged up. 1" or larger pipes would have performed better.
Kitchen sink graywater enters interceptor through pipe on left, then exits through pipe on right, leaving grease and solids behind.
Graywater then flows into distribution hub and is evenly distributed through 6 pipes.
Close-up of distribution hub.
Detail of infiltration chambers at the end of each distribution pipe. Note mulch around chambers.
The papayas and bananas irrigated from this kitchen sink graywater system.
This is a low-maintenance graywater system I designed for a family of four in Oregon's Willamette Valley. It accepts all the graywater from the lavatory sink, bathtub/shower and washing machine. Since it does not receive the high solids/fats of the kitchen sink graywater, we decided to design it without an interceptor. A basin was excavated in native soil which had relatively low permeability, containing some clay and a high silt content. The excavated soil was used to construct a berm around the basin to prevent rainwater runoff from entering the basin. The berm also insured that graywater is contained within the basin even during times of heavy rainfall. A line of connected infiltration chambers was placed in the center of the basin which was then backfilled with mulch, in this case wood chips, to a level several inches above the top of the chambers.
(Above) A graywater system of infiltration chambers in a basin that was subsequently backfilled with mulch.
(Above) After 4 years of full-time use, the system continues to work well. The mulch basin is just behind the lawn and irrigates the surrounding copse which includes a large willow tree, aspens, blackberry bushes and plum trees. The only ongoing maintenance has been adding several inches of wood chip mulch per year to replenish the older chips that have composted.
(Above) The composting mulch surrounding the infiltration chambers teems with worms, arthropods, fungi, molds, roots, etc. As compared with a conventional septic system and leach field, the level of water treatment in this system is almost certainly higher due to greater oxygenation through the wood chip mulch and to high levels of biological activity within the mulch. Also, scientific research has shown that tree roots are effective in absorbing significant quantities of nitrogen, one of the two pollutants health authorities are most concerned with in residential wastewater systems.
(Above) Note the composting wood chips, interlaced with a mat of roots uptaking water and nutrients.
| Update, summer 2008: After 6 years of full time use, this system temporarily failed due to roots in the graywater pipe where it entered the chambers. While cleaning out the roots, we also discovered that worm castings had built up in the bottom of the chambers. As the castings were deposited, a mass of small tree roots grew up into them. The solution was to excavate the chambers with a shovel, remove the castings and roots and replace the chambers. The nutrient rich castings were used to fertilize flower beds. The side louvers of the chambers appeared to be a primary entry point for worms and roots. To help prevent this in the future, the chambers were covered with 2 layers of 6 mil black plastic sheeting prior to backfilling. The chambers were then backfilled with sawdust to facilitate future excavations. The system has been back in full time use for 6 months and will continue to be monitored and reported on here.
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| Partially excavated chambers showing worm castings and roots
| Close up view of worm castings
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| Excavated and inverted infiltration chamber, showing worm castings and roots entering through side louvers | Infiltration chambers cleaned, replaced and covered with 2 layers of 6 mil plastic sheeting, prior to backfilling with sawdust |
2013 Update:
The Oregon graywater system described above has continued to be used full-time since the 2008 modifications (also described above). Compost from around the chambers and worm castings from inside them will be excavated and the system inspected in the summer of 2014. The results will be posted here at that time.
2014 Update:
The Oregon graywater system is now in its 12th year of full-time use and continues to work well. It was inspected in the summer of 2014 by looking through the two inspection ports. The ports can be seen protruding through the plastic sheeting in the last photograph above. Some small roots were visible, but appeared minimal and were not interfering with the functioning of the system. The owner preferred to wait until next summer to excavate the compost from around the infiltration chambers and worm castings from inside them. The results will be posted here at that time.
Recommended reading:
Rainwater Harvesting for Drylands by Brad Lancaster
Graywater Guides by Art Ludwig:
--Create an Oasis with Greywater
--Branched Drain Greywater Systems
--Building Professional's Greywater Guide