Image Credit: All images: Matt Bath
Image Credit: All images: Matt Bath
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Editor’s note: This is one in a series of blogs detailing the construction of a net-zero energy house in Point Roberts, Washington, by an owner/builder with relatively little building experience. You’ll find Matt Bath’s full blog, Saving Sustainably, here. If you want to follow project costs, you can keep an eye on a budget worksheet here.
Standard treatment of sewage hasn’t changed much over the years. Nature actually had it figured out pretty well before humans even attempted to manage it. Given enough time, soil and the organisms that inhabit it are extremely adept at breaking down harmful toxins and dispersing the safer compounds into underground waterways. The only thing a septic system does is harness this awesome power.
Designing a proper system starts with a soils test and/or “perc” test. A soils test involves removing a deep core of soil and analyzing what appears in the different layers. Soil is then classified into sand, gravel, loam, clay, and all sorts of combinations of those types. A perc test involves filling a deep hole with water and timing how long it takes water to percolate through the soil at the bottom of the hole. (The soil must be prepped by soaking it thoroughly first and most counties require you to have a license to complete the test.) Both tests can give a pretty good indication of how well a particular patch of soil will perform at breaking down the effluent — what sewage becomes after sitting for a period of time.
Soils and perc tests
In my state, the soils test or perc test must be completed by a licensed septic designer. The designer I hired charged $150 and found my soil to be “sandy loam” for the top inch and “medium sand” for the next 2 feet until reaching the water table at 28 inches. This is the depth at which dry soil becomes saturated with water due to an underground spring.
The county and state health codes dictate what kind of dispersal system can be used for a given type of soil. Conditions on my site allowed me to use a gravity distributed system, which is the simplest type.
The next step in the design called for locating the area of the lot where the drain field would be located. Health codes dictate setbacks for the field of 5 feet from property lines, 10 feet from water lines, and 100 feet from natural water supplies. I have a natural canal on one end of my property, and the 100-foot setback took up a substantial amount of the lot.
The drain field needed to be 400 square feet, and there also needed to be a reserve field of the same area set at least 6 feet away from the main field. I had a problem here because it was impossible to fit both fields into the setbacks. Fortunately, by using the next system up from gravity I was able to use smaller fields that fit within the setbacks.
A pressure distributed system
I will be installing what is called a pressure distributed system. It’s basically the same as a gravity type system but with the addition of a pump. This balances the distribution of effluent more evenly across the field, thus allowing a smaller field area.
The sewage from the house exits the main drain pipe and enters a three-compartment concrete septic tank (see Image #2 below).
The first compartment is aptly named the trash chamber, and allows the sewage to separate into solids on the bottom, a layer of sludge on the top, and a cleaner liquid in the middle. This liquid is allowed to enter the second compartment, called the digestion chamber. In order to exit this chamber, the sewage must decompose into small enough particles to pass through a filter. A pump sits at the bottom of the last compartment, the clarifier chamber. When the level of liquid in the chamber causes a float to reach a certain height, some of the liquid is pumped out of of the tank and through a pipe to the drain field.
There are two more floats: the first to ensure the pump doesn’t run too often and the second to sound an alarm if the pump isn’t working and the tank is getting full.
Excavation
The experienced backhoe operator I hired was able to dig the 9 foot by 15 foot hole to a depth of 77 inches in less than an hour (see Image #3 below). I would have liked to do it myself, and I realize now, after having seen it done, that with the right person checking the depth for me I could have done it. But it would have taken me at least five times as long. The hardest part about digging a hole that deep is that you aren’t able to see the bottom of the hole from inside the cockpit of the excavator, so you’re basically digging blind.
Once the hole was dug, I climbed down on a ladder and made sure the bottom of the hole was nice and flat. We hit the water table around 6 feet down, so I was working in about 6 inches or so of water. Once I was sure I had made a nice bed for the tank I climbed back out and we got started on digging the field. This part could have very easily been done by hand since the depth of the drain field is only 7 inches. But since the excavator was already there, it made quick work of the job.
The field is 9 feet by 35 feet, and it must be flat and level. The first step is finding the lowest spot, then and digging down 7 inches from there. Once that is done it’s simply a matter of matching that depth to the remainder of the field. I used a 6-foot level and also a rotary laser level to check the depth, and a rake and shovel to fine-tune things.
Adding the piping
The pipes that disperse the effluent to the drain field are made from 2-inch PVC reduced to 1 1/4-inch PVC using two cross fittings and a tee fitting. In image #4 below, you can see how the 2-inch pipe enters the field and is attached to cross fittings underneath the black tunnels. The design allows the 2-inch pipe to branch into six separate 1 1/4-inch pipes and disperse the effluent into the the field evenly.
The PVC is very simple to glue together with primer and cement. Once the pieces are glued, a 1/8-inch hole must be drilled at the top of each 1 1/4-inch line every 2 feet, starting 1 foot from the place it tees off from the main 2-inch pipe. Those pipes are then covered with the large, black gravelless chambers. The holes allow the effluent to spray out and cover as much of the area of the entire field as possible.
The gravelless chambers are exactly what they sound like. Instead of covering the PVC lines with gravel, you use the plastic chambers. They are cheaper and much easier to install than shoveling gravel. The chambers help direct the effluent sprayed from the pipes into the soil and also ensure that the pipes will be protected from human activity above.
Installing the tank
Around this time the septic tank manufacturer arrived with the tank on the back of a flatbed. The driver used a small crane on the back of the truck to hoist the massive tank into the air and lower it into the hole we had prepared.
I had a hose all ready to start filling the tank with some water as soon as it was dropped. This helps to give it some added weight and ensure it settles into the soil. The backhoe operator also added soil around the sides but left the top and the inlet and outlet ports on the sides exposed.
The electrical cable was too short
The tank installation went smoothly, but for the second time in my build I underestimated the amount of wire I needed and now I’m stuck with a couple pieces that are too short. I’ll have to return to the electrical supply store to get longer pieces. An important part of working solo is knowing where your deficiencies are and I tend to cut things just a little too close so I don’t waste a thing. With wire, hopefully this will be the last time I underestimate the length I need!
I began the day by running 1 1/2-inch PVC from the tank to the drain field. I learned a lot from the mistakes I made running electrical conduit, and the end result was a well planned and perfectly aligned run. The next step was attaching the float switches inside the pump chamber of the septic tank. There are three floats that will work together to operate the pump.
The float switches are bell-shaped plastic parts attached to low-voltage cords. They are very simple but ingeniously designed. When the water is low, gravity pulls the float down so the heavier bell end is down. When the water level raises, the bell end floats upward, causing a sliding metal part inside the float to meet another metal part in the middle and complete the circuit.
Placing floats correctly
The floats must be placed on the PVC pole precisely (see Image #5 below). The first float was to be placed 13 inches from the bottom of the tank. This switch controls the “redundant off” function in the control panel. Basically, it ensures that the pump continues to run until the effluent level is low enough to lower the float. Without it, the pump would wear out a lot faster because it would run so often. Imagine if you took out the garbage to the street every time you had a piece of trash! It’s much more efficient to have a small container and only empty it when the container is full.
The second float is placed 20 inches from the bottom of the tank. Once the effluent level is high enough to raise this float, the pump activates and continues to run until the “redundant off” float is lowered.
The third float is the high-level switch, and is placed 35 inches from the bottom of the tank. This activates an alarm and a siren if the float is raised, and will allow me sufficient time to figure out what is wrong and fix it before the effluent level gets so high that the tank is full (see Image #6 below).
The control panel is mounted on a post
You can see in Image #6 how I’ve mounted the control panel on a 2×6 pressure-treated post and run wires through 3/4-inch conduit from the panel down towards the pump chamber riser. Tomorrow I will buy a rubber grommet to make a seal as I drill a hole through the side of the riser and run the conduit into the junction box. The last step will be running some 12/2 UF cable from my temporary power pole underground to the control panel.
The other part I still need is the flexible hose assembly that will connect the pump to the 1 1/2-inch PVC I talked about in the beginning of this post. Once that is hooked up, it’s just a matter of filling the tank with water and checking to ensure everything is working properly.
Ready for another inspection
You can see my $7,000, high-powered sprinkler system at work in Image #7 below. Hard to believe that underneath the plastic “infiltrators” you see in the background and 12 inches of soil, that spray is going to be the last step in safely disposing of my sewage.
Today, the septic designer brought over a hose assembly and we hooked it up. The hose assembly attaches to the pump at the bottom of the tank and makes three 90-degree turns before exiting the tank and going through the PVC pipes I assembled a few days ago.
Now it was time to fill up the pump chamber with water and test the system. It was necessary to fill up the chamber with water for two reasons. First, you never want to run any kind of water pump without water in it because air has much lower resistance than water and the motor will burn out without that resistance (so make sure you aren’t out of windshield wiper fluid!). Second, the panel won’t operate the pump unless both the redundant off and pump on floats have been activated, and I’m not too keen on climbing down into the tank and flipping them upside down by hand.
After 10 minutes or so I had enough water in the tank to activate both floats and I turned on the power to the control panel. The pump activated and we got the beautiful water show you see in the photo. The septic designer called the inspector for an appointment so I can replicate the display for him and he will sign off on it.
Finishing up
The final steps involved making inspection ports from 6-inch PVC and placing them over each of the ends of the drain lines. Each port has a plastic cap that can be lifted off to expose the 90-degree long sweep at the end of the line. If for any reason the drain lines get clogged, you can take off the cap, reach down inside the PVC and unscrew the plug for the line in order to flush it out.
Toward the end of the day, the backhoe operator came back and expertly returned the soil over everything, driving back and forth to compact the soil (see Image #8 below).
Totally out of sight now, you can hardly tell that anything was done! What better way to “save sustainably” than to build your own septic system on your lot rather than take up a ton of land with a giant sewage treatment plant? Obviously, those are necessary with high-density residential areas, but in neighborhoods like mine, it really is too bad that more homes don’t have their own septic systems.
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30 Comments
I've often wondered about
I've often wondered about well and septic systems. Are they considered net zero water? All of the water being used (well, ok most of it) is being returned back to the source which is the definition of net zero water.
Response to Calum Wilde
Calum,
What program are you thinking of? This is the requirement of the Living Building Challenge: "One hundred percent of the project’s water needs must be supplied by captured precipitation or other natural closed-loop water systems, and/or by recycling used project water, and must be purified as needed without the use of chemicals. All stormwater and water discharge, including grey and black water, must be treated onsite and managed either through reuse, a closed loop system, or infiltration."
In a typical suburban house, the amount of water that percolates into the soil through the septic pipes is much lower by volume that the amount of clean water (from a well or municipal system) used by the occupants. Much of the irrigation water is lost to evaporation; cooking water is consumed, water used to mop floors evaporates; and so on.
Moreover, the water that emerges from the holes in the septic pipe isn't as clean as the water that enters the house from the well or municipal water source.
But I understand your instinctive feel for the issue -- water comes into the house, and it returns to Nature without being destroyed. If we all lived like the Native American communities did on this continent before the European invasion, there wouldn't be a water issue. It flows. Who can measure it? It's a closed system.
Now, unfortunately, we manage to pollute our water sources, and the Colorado River never makes it to the sea. It's all siphoned up and consumed. So we need to develop new ways of thinking about water.
I live in the woods at 1800 feet of elevation, with no one living at a higher elevation, and clean water flows abundantly on our land. We can't really waste water as long as we have a well-functioning septic system and we don't pour the wrong chemicals down the drain. Nature doesn't care if I use 5 gallons a day or 50 gallons a day; the water keeps flowing downhill whether I use it or not.
Martin,Thanks for the
Martin,
Thanks for the detailed reply.
I didn't have a specific program in mind. (that's another debate but I'm not a fan of teaching to the test so to speak.) I tend to ignore the specifics of each program and look for commonalities between them. My thought is that in those communities will be the true heart of the issue. Passive House (Germany) vs Passive House (US) vs LEED, vs.... In the end the commalities are better air sealing, better insulation, etc. Shooting for 10W/m^2 heating load is just an arbitrary number.
Anyway, back on track, when I looked a various definitions of net zero water nothing mentioned well and septic systems, but they all had a variation of sending the water back where it came from. Now this could easily be taken way too loosely to mean the water cycle in general, but that's clearly not in the spirit of the definition. What I wasn't sure about was the idea that if you consider the amount of juice and milk that flows through my walking filters (ahem children) and back into the local ground, then it's likely a wash between how much we pull out of the ground and how much we put back in. It's still a loose interpretation, but that's why I was asking.
Nevertheless, I think you've answered the question sufficiently. It may not be net zero water usage, but as long as I'm not contaminating the ground water with harsh chemicals then it should be sustainable. And that's my goal, sustainability. It doesn't have to conform to any single standard or program, I just want it to be sustainable.
Water conservation efforts such as low flow end use devices should also be a part of this. I don't have any direct measurements of my water usage, but making some educated guesses based on my water heaters power consumption, our ground water temp, device flow rates, and our typical habits, the last time I calculated it we were well below California's maximum daily per person usage, though slightly higher than Cape Town's. I would like to say, even if my measurements are off by a factor of 2, that we're still doing ok.
I think that's enough tangents for one comment. Thanks again.
Gravity Fed
My system is gravity fed, in that it doesn't need pumps to move the effluent, it relies on gravity to move the effluent. I have an aerobic system which means oxygen is pumped into the 1st chamber of the 2 chamber tank. This creates an effluent that is 20x cleaner than an anaerobic (no oxygen) septic system. In addition, the aerobic system has no smell, it smells earthy like (soil). The anaerobic systems produce toxic gases like methane, sulfur, etc.
It costs me maybe $4.00 per month to run the air pump. The effluent is clear and requires less ground filtration to become "safe" as it returns to the aquifer. One can say I pretty much treat my own waste and return a "Net Zero" water. Of course like Martin mentioned, there's evaporation & such but I can confidently state that about 90% of the water I pump out of my well goes back into the ground as clean effluent as it makes its way into the water table.
I also DO NOT use harsh chemicals. Only baking soda to clean toilets and earth friendly laundry soap for my washing machine. No bleach or bad chemicals go into my septic system.
Response to Calum Wilde (Comment #3)
Calum,
You wrote, "As long as I'm not contaminating the ground water with harsh chemicals then it should be sustainable."
What's true for a small house in Vermont isn't necessarily true for a large homestead in Nebraska. Everywhere is different.
For many households, most residential water use goes to irrigation -- and most of that water evaporates. If you live in a dry area, and if you and your neighbors are pumping enough irrigation water out of the ground to lower the water table, your water use rate is unsustainable -- even if you have a septic system.
There are many locations in the U.S. where the situation I described -- with people pumping water out of the ground at a faster rate than it can be replenished -- is already happening.
Response to Peter L
Peter,
You wrote, "In addition, the aerobic system has no smell, it smells earthy like (soil). The anaerobic systems produce toxic gases like methane, sulfur, etc."
In this context, smell is irrelevant. The only person who ever smells my septic tank is the guy who comes to pump it out every 20 years. (As it turns out, when I paid to have my septic tank pumped, inspection proved that the pumping was unnecessary. I could have gone 40 years without pumping.)
A septic system that smells is seriously defective. All properly designed septic systems, where aerobic or anaerobic, are odor-free to people standing nearby. Just don't get out a shovel, and don't open the tank lid. (That would be dangerous to your kids if you tried it, for a variety of reasons.)
Weird question
What flood zone are you in?
Calum
Septic fields are designed close to the ground surface so that the majority of the effluent can evaporate and not percolate down into the water table. One way conventional septic fields fail is by being buried too deep. They are closed systems in that the water is part of a cycle, but the cycle isn't limited to one property.
PeterL
I had to put in an oxygenator too because of the limited area I had for a drain field. It's the only septic system I've ever installed that does smell. The air pumped into the tank has to go somewhere and where it vents there is an odor.
Our health department discourages gravity-fed systems here. Their weakness is that the great majority of the effluent is dispersed into the runs in the first 20 feet or so, leaving the rest doing no work. A pressure field avoids that problem.
Malcolm
That depends on the system design. In my area the leach field is about 36" below grade and covered with plastic chambers and then buried with natural soil. Highly doubt much evaporation is happening 3 feet below grade with the effluent being covered with plastic chambers, geo-textile and that much soil.
The main reason septic systems "fail" is not due to the burial depth of the leach field but the black biomat sludge that forms on the soil with ANAEROBIC systems that prevents the effluent from draining into the earth. They have dug up 20 year old leach fields that were aerobic systems and found no biomass. Anaerobic systems will develop biomats as this is part of the process of lacking oxygen. That's why some systems will have "resting" chambers that you turn a bull valve and let a leach line "rest" for a year before putting it back into use. That give time for the biomat to dissolve and once again allow effluent to drain into the ground.
Chambers
Highly doubt that much evaporation is going on here in this type of system, being 36" below grade, covered with soil. As per the photo, the effluent is shown draining into the soil below.
PeterL
You may have noticed as they installed your plastic infiltrator tubes that they had perforations on all the ribs and that the geo-textile fabric was permeable. That's because septic fields disperse effluent through three mechanisms. Evaporation, transpiration and absorption. The radio with which each mechanism functions depends on climate and design, but for any of them to work they need to be close to the surface, and not covered by an impenetrable material. A drain field with 36" deep trenches has the tops of the infiltrator tubes around 12" from the surface. That's ideal as long as freezing isn't an issue, or a high water table.
Years ago I GCed a large open-marsh system for a townhouse complex. It was a series of shallow ponds filled with 12" of gravel and planted with native species. The level of the effluent was kept several inches below the surface so you could walk the ponds, and within a year of so it resembled a vibrant, natural wetland. It could only work in climates that sustain vegetation year round, but I'm surprised smaller versions haven't become more popular. It as simpler and much more interesting than the large sand filter system I put in around the same time for a similar project.
Great info as always Malcolm
I hadn't thought about the role evaporation played in the system. PeterL, I do like the advantage of having the pump because I check it every now and then and I can get a ton of information. I know how many times it has pumped since inception, how long until the next pumping, and have a basic idea of the effluent level because usually the pump off float has been activated (meaning the effluent level is substantially low). Then, of course, there is the greatest advantage of just having the alarm, giving you an extra week or so to fix the problem before system failure.
Matt
Hopefully you never need to change the pump, but if you do, the way you plumbed the flex-hose so the coupling is the highest thing in the tank will make any repairs a lot easier. I wish more were set up that way.
Funny story....
I actually already had to pull it up. About a month after full install the alarm went off. I did some troubleshooting and realized it had to be the pump, which was receiving power but not turning on. I pulled it up, unscrewed the bottom, and sure enough there was a loose wire. I reattached it and haven't had a problem since. If it wasn't on a flex hose it would have been a nightmare.
Gravity fed wastewater
PeterL
"....Our health department discourages gravity-fed systems here. Their weakness is that the great majority of the effluent is dispersed into the runs in the first 20 feet or so, leaving the rest doing no work. A pressure field avoids that problem..."
True. This is called 'creeping failure' and is the main fault with gravity fed systems.
Generally, pressure fed systems are labelled 'dosed pressurised effluent systems'. This means that pumps are run for a limited amount of time so as to not hydraulically overload the soakage field.
In my system, the pump runs for approximately 2 mins, pumps about 200 litres, shuts down, waits 2 hours, then if the level floats are OK, the cycle is repeated.
Evaporation or evapotranspiration?
Readers may find it easier to follow the controversy on this page concerning the issue of whether evaporation is significant above leach fields if we use a more accurate term -- evapotranspiration -- to describe what's going on.
Most leach fields have green plants above them during the growing season, and nearby trees often stretch their roots (with effects that are both benign and unfortunate) into a leach field. So much of the moisture that evaporates is really evaporating through the leaves.
@Malcolm
Funny you mentioned that wetland/leech field. That idea has always been one of my bucket list items when I finished my hydrology class. Live in a warm enough climate which would allow a septic system to be designed like a wetland.
Martin,
We don't water our
Martin,
We don't water our lawn, or use water for irrigation in any way. I live in Halifax, Nova Scotia, there's not much need for watering outside plants and I refuse to plant things that would need such wasteful water usage. I've tried to change my user name to include my location, but it didn't work. As I said, our water usage is pretty low, almost as low the Cape Town standards by my estimations.
With regard to my situation being sustainable or not, I'm not really concerned with the water needs of someone in Nebraska. I'm asking about my own situation. The water needs of people in other location is certainly important, it's just not what I was asking about.
Malcolm,
Thanks! I didn't know that. Now I'm really glad I haven't put top soil on top of my septic field to help the bare patch in the center to grow better.
Response to Calum Wilde
Calum,
Your family's water use sounds very responsible to me.
And you're right, of course, the water usage patterns in Nova Scotia don't help or hurt the residents of Nebraska in any way.
Funny Story
As part of the open-marsh installation, we had to plant small individual balls of plants in dirt specially shipped up from the States. We diligently placed hundreds of them on the gravel, but the next day found that crows picked up and moved every one. We repeated the exercise with the same results. Finally we got permission to use local bullrushes and other plants we could find in nearby old mine-tailing ponds. They have thrived.
John Clark
Encouraged by a guy I went to school with who designs aquatic systems, when I built my own house I explored incorporating a small greenhouse-based treatment. The engineering fees involved for approval were (20 years ago) over 10k. I didn't bite, but sometimes wish I had. Our local health authority said they discourage systems like that because they don't think they can rely on homeowners maintaining them.
My school friend Kim's website: http://www.ecotek.ca/ECO-TEK_Ecological_Technologies/Team.html
I wonder if the problem of creeping failure in gravity systems might be mitigated by using pipes with differing number of perforations depending on where they were in the field? Athough maybe that's just introducing unnecessary complexity to their design.
Cold Weather Septic Systems
How do you install this kind of septic system (i.e. only 1 foot below grade) in cold weather climates? And assume the owner is gone to Florida for several months during the winter, so no use of the septic system during that time.
Climate I'm referring to is winter 6 months of the year, average winter temps. 0F to -20F for several months. Frost level for building code is 4-5 feet below surface.
Response to T. Barker
T. Barker,
Most leach fields don't freeze to a depth of 4 feet, for several reasons:
1. Cold climates tend to be snowy climates, and snow acts as insulation.
2. Regular flows of warm water from the septic tank tend to raise the temperature of the soil. (In late April or early May, when our snow finally melts, the first place we see bare ground is often above the septic tank, because the warm water in the septic tank helps to melt the snow.)
I live in Vermont, and I've seen temperatures drop to -35 degrees F several times, and -38 degrees F once. My gravity-fed leach field has worked fine for 27 years, without any problems, even though the perforated pipes in the leach field are only about 12 inches below grade.
That said, if you live in Alaska, or anywhere with permafrost, you probably can't have a conventional septic system and leach field.
'Given enough time, soil and
'Given enough time, soil and the organisms that inhabit it are extremely adept at breaking down harmful toxins and dispersing the safer compounds into underground waterways'
Ok, this really isn't about creating an ecologically acceptable solution.Since when did nature run to water to dispose of waste the way humans do.
Brian
Doesn't all offal and excrement from every creature follow the same path?
I was thinking of heating water with a liquid to liquid heat pump which would suck heat from the septic tank, field and well water supply. Has anybody done this? It seems like it would be a low cost way of installing a geothermal heat source. I am a year or two away from installing the septic system so I have time to plan for it. Would the heat pump cool off the sewage too much for it to decompose?
By the way where are the pictures mentioned in the article?
Point Roberts, WA
Nils,
Matt gave you one answer. I'll give you another: The pictures are in the photo gallery. You can view them by clicking the green button at the top of the article -- the button labeled "View Gallery."
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