Making a constructed wetland to process greywater in the farm
Lately I’ve seen a lot of news saying that this year might be the hottest ever recorded. That affects many aspects of life, but in my opinion, the most important issue is clean water. It’s only April now, and I can already tell the soil in our garden is drier than last year, even though I always mulch and try to increase soil carbon to improve water retention.
That’s why I feel deep gratitude every time I use clean water—for daily needs or for watering the garden. But it also makes me feel a bit guilty every time I take a shower, wash dishes, do laundry, or flush the toilet, knowing that this precious water is just being sent down the drain as “wastewater.”
In Permaculture, two core principles are “Catch and Store Energy” and “Produce No Waste.” From that perspective, household wastewater is a type of energy worth capturing and storing. And the system I’m about to share is how we do that here at home.
How We Built Our Household Wastewater Treatment System
1. Avoid Hard-to-Treat Chemical Products
Wastewater is really just clean water that’s picked up a few “extras” beyond H₂O and natural minerals. Some of those extras—like synthetic chemicals—are hard to break down. Others, though, are easy to deal with naturally.
Because our household wastewater is treated with natural systems and eventually flows into our food forest, we avoid any products that contain persistent chemicals.
For example, the soap we use is homemade with just three ingredients: potassium hydroxide (KOH), cooking oil, and water. While KOH is technically a chemical, it’s the same compound found in wood ash and can be broken down and absorbed by soil microbes and plant roots. So, no, it’s not “chemical-free”—but it’s easily processed by nature.
2. Separate Greywater and Blackwater
To make the system efficient, it's important to separate greywater (from sinks, showers, etc.) from blackwater (from toilets).Greywater is generally cleaner, but kitchen sink water can contain lots of grease—so some people consider that “blackwater” too. Either way, the key is to treat them differently because they require different treatment times and methods.
Here’s how we manage the two before combining them later:
Blackwater (Toilet Wastewater)
We use a septic tank, just like most rural homes. This is actually a natural treatment system—waste breaks down in water over 1–3 days thanks to microbial activity. The process releases a bit of ammonia and methane but leaves nutrient-rich water behind.
However, blackwater is too concentrated (and smelly) to go straight to plants. So after the septic tank, we pipe the water into a 150L drum filled with activated charcoal to remove odors. The charcoal needs replacing every few years.
Greywater (From Shower, Kitchen, etc.)
This is easier to treat. To remove grease from kitchen water, we built a small grease trap from an 18L paint bucket using a T-joint, a few plastic pipes, two unions, and two threaded connectors.
In this trap, grease floats to the top, while the cleaner water flows on. Every 1–2 months we scoop the grease out and compost it.
3. Use Plant Roots to Filter Wastewater Before Irrigation
Once blackwater is deodorized and greywater is degreased, we combine them and send them into a natural filtration system. This system uses aquatic plant roots and soil microbes to absorb and break down nutrients in the water.
To do this, we created a series of filter tanks with aquatic plants. Each tank can hold about one day’s worth of water for our two-person household (about 100L). We installed four 200L HDPE barrels to allow for longer retention—at least 4 days in the system before reaching the garden.
Pipes are arranged to slow the flow: water enters from the bottom corner and exits from the top on the opposite side to maximize contact with plant roots.
Some great plants for water filtration in Vietnam include:
Water hyacinth (bèo tây)
Umbrella palm (thủy trúc)
Canna lilies (bách thủy tiên)
We planted the first two barrels with canna lilies and umbrella palms growing in gravel, and the next two with water hyacinths floating freely. These plants grow vigorously thanks to the nutrient-rich water, and—if you’re not using harsh chemicals—you can compost them later as green manure.
After passing through the four tanks, the water goes into a small pond (1.2m deep) with shallow edges for more planting. The water here is odor-free but still contains nitrogen (as shown by algae growth), making it perfect for feeding trees—especially hardy perennials.
4. Irrigating the Garden
There are many ways to use this water—hoses, pumps, watering cans—but I chose a passive trench system to keep it low-maintenance. We dug shallow ditches with a gentle slope (about 2%) to let water flow slowly without eroding soil. The trenches follow keyline design principles, directing water from the pond to the driest parts of the orchard. The trenches run between fruit trees like oranges and macadamias, supplying both water and nutrients. Every month or two, we release pond water into the garden.
References:
Ability of umbrella palm to filter water: https://www.sciencedirect.com/.../abs/pii/S0925857413003601
Phytoremediation with water hyancinth: https://link.springer.com/article/10.1007/s10669-007-9047-2
Video from Andrew Millison: https://www.youtube.com/watch?v=f-sRcVkZ9yg
Keyline design: https://www.keyline.com.au/
