The first step is to find out your county's requirements. I purchased land that already had an approved perc test. I want to build a small cabin of 800 square feet with one bedroom. The county will tell you how big a tank you need and how many feet of leach lines are required for a building of a given size. In my county it's completely dependent on the number of bedrooms, rather than square footage or number of bathrooms. If you want a garbage disposal, that counts as another bedroom, in terms of the required capacity.
All this can get much more complicated if you have trouble meeting your county's requirements for setbacks, or, the distance from various parts of your system to your house, your property line, streams and wells. I have plenty of space so this wasn't a problem. Another issue is if your ground is either too well-drained or too poorly drained. I don't have this problem either. If you do, you'll need a professional to design and install your system, and probably this will include hiring a septic engineer.
So, I needed an 1100 gallon tank and 250 feet of leach lines. I figured at first to do 300 feet in three lines, just to be safe. All lines had to be less than 100 linear feet and all lines had to be the same length. Lines had to be spaced at least 10 feet apart on center. The tank had to be at least 10 feet from the house. The leach lines have a "distribution box" to control the outflow to the different lines. I'm not sure why different counties apparently have opposite strategies on this topic. My county wants equal distribution to all lines. Other counties want sequential distribution, meaning that one line has to fill up completely before waste flows to the next line. We had to put the leach lines about 80 feet from the tank, in order to have a spot where we could run three hundred foot lines. Once we started, we changed this to 4 lines of 75 feet (well, actually 72 feet, more on that later) to minimize the number of trees we had to remove. You can get fiberglass septic tanks, but as long as you are not in a very remote area, there should be a septic tank fabricator that will deliver a concrete tank for a more reasonable price. Our local company didn't have an 1100 gallon tank, so we used their smallest, which is 1250 gallons.
In my county the line from house to tank has to slope betwen 1/8" and 1/4" per foot. From the tank to the leach lines it has to have some slope. The leach lines can be between level and 1/4" per foot. We made the leach lines level to keep things easy.
The first thing to do was rent a 5 ton mini-excavator. I rented mine from United Rentals which is probably the largest outfit of its sort. There may be other options in your area and it always makes sense to get multiple bids. Amazing they let anyone rent one of these! If you don't have any, or much experience with big equipment, be careful. You can literally kill someone very easily, or cause a lot of expensive damage. Take it slow. Make sure you know your surroundings at all times. Keep pets and children far away from the job. Make sure you know where anyone working with you is at all times. Use the seat belt, and probably avoid driving on significant slopes completely. If you feel uncomfortable with the slope, stop. These things tip over more easily than a car and if it does, you're in big trouble.
I had run a small backhoe for about 15 hours and learning basic operation with the excavator was fairly quick. Most ecavators and large backhoes have two different styles of controls. Modern big ones have a switch that lets you change from one style to the other. The little Kubota I was used to had one style with no switch. It was also different from either option on the excavator. The two styles are "Cat" and "John Deere". I set it to Cat since that's what my friend is most used to, so I figured that's what I should learn. I felt that in about 1/2 hour I had the hang of it. However, there's a big difference between being able to dig a basic trench or hole and being able to dig a level trench or hole without getting off the machine, or doing any of these things efficiently. I rented the excavator for a week. The cost per day goes down significantly when you rent for a longer period. It makes sense to plan out several tasks and rent the machine for more time, as opposed to renting it for a couple days at a time. They allowed 40 hours of run time and I got my money's worth by digging some trenches for water and power lines that I need to install later. Even so, it's hard to get hours on the equipment unless you have a lot of people working. I put 33 hours on it in 7 days, with two of us working on the septic. I had a few days alone digging trenches and we lost one day due to heavy rain. Depending on your terrain, wet ground may make it impossible to work. I'm lucky that I have a lot of rocks and well-drained soil, so heavy rain didn't stop the work, other than it was too cold and wet that one day to sit on the excavator, or have one guy down in the trench.
You'll need to lay out your tank, distribution box and leach lines. I used some 18" redwood garden stakes as markers. While this can be done with string and string levels, or a transit level, it makes more sense to buy or rent a laser level on a tripod. I used an inexpensive one (under $400) to lay out the rough system. My friend had a fancy self-leveling one that costs ten times that which we used once we actually started digging. I used "marking paint" to mark the location for the lines, which is spray paint that can spray upside down.
Once I had figured out the county's requirements and laid out the system I went with the perc test and a lot plan that I drew showing the layout to the county and got my permit. The permit fee was substantial, about $550.
The next thing was to dig the trench. You can dig an approximately level trench and then fill it a few inches deep with drain rock and level that with a rake. We bought 3/4" drain rock, but my friend did such a good job of digging a level pit, we wound up not needing it. You'll also need a level spot for the crane truck to park next to the pit.
With the tank in place we were able to dig the trench from the tank to the distribution box location and leach lines. Their slope doesn't need to be exact, but does need definitely to be downhill! There was a little slope in the ground in the way, so the start of the trench was fairly deep. Because of the length of my line, having a "cleanout" was a good idea, so one could reach into the line with a snake without digging up the line if there is a blockage some day. There's a "T" junction called a "2-way cleanout" that has a gradual bend to both sides of the T. You can see the cleanout sticking up in the distance in the second picture below. The tank also needs a horizontal "T" inside the tank, that prevents liquids and solids from flowing too suddenly into or out of the tank. Your county will have a specification for the dimensions of those T's. Tanks and supply lines also have a minimum and maximum amount of dirt cover.
The most time consuming part of the job, and the most exacting, is digging the leach lines. I believe the standard in my county is that they can't vary more than 4 inches vertically over the course of the line. That's hard to do without a lot of raking. I was the guy raking and taking measurements with the laser level while my friend dug the trench. Fortunately, with someone who does this for a living, I had a mostly level trench to work with. It took two days to do the trenches. I wasn't used to that much raking and digging, so it was hard work even so. I would take a measurement every 5 feet or so, and my friend could then adjust how deep to dig. I'd then even up the trench with a rake.
Leech lines used to consist of a perforated pipe laid on a bed of gravel. Many are still done this way. Now however, many places allow the use of "infiltrators", which are plastic parts 4 feet long and various widths (we used 1.5 feet wide ones) that are perforated on top and open at the bottom, in an arc shape when viewed in cross-section. They can be placed directly on the bottom of the trench without gravel. There are no holes or gravel to get clogged with sewage and a much greater surface area for the liquid to drain into the soil. They just snap together and since there's no gravel, that saves a time consuming step. A system of this sort is somewhat more in materials, given the cost of the infiltrators, but it save a lot of time and whether you are doing this part yourself or paying for someone else to do the whole system, it appears to be worth it. Another nice feature is that the system can curve without having to glue in angle fittings as you would with pipe. Each line requires an "end cap" at both ends. You cut a hole with a utility knife in the plastic end cap at the start of each line to accept the inflow line. 1.5 foot infiltrators require a 2 ft wide trench. Excavators have different size buckets so we got one with a 1ft bucket for my water and power trenches and a 2ft bucket for the septic project. The trenches are supposed to be 2 feet deep. The infiltrators are 18" high and you need a minimum of 6" of cover. I guess they figure you have some margin for error by putting the fill displaced by the infiltrators on top of the lines when done.
After placing the infiltrators they get covered with filter fabric that allows rainwater to get in through the holes in the top of the infiltrators, but not dirt.
Next I set the distribution box and the lines. We used 4" ABS from the tank to the box and 4" SDR35 non-perforated sewer pipe. The SDR is weaker and cheaper than ABS and allowed from the distribution box. It can come in 20' lengths with "bell ends" so that you don't need couplers to connect them. If you have a truck with a rack, or can get it delivered, it saves time over having to use 10' lengths with couplers. SDR35 can be glued with PVC pipe glue, no primer is needed. ABS pipe has its own different glue, and no primer is used. The box has plastic gaskets embedded in the concrete. You cut the appropriate size hole in the plastic with a utility knife and then just slide the pipe into the hole. No glue is needed. At the tank, there is a "knockout" or a weak spot in the concrete that you break with a hammer. Then slide the ABS pipe into the knockout and "grout" around the pipe and hole with mortar to seal it. Some tanks will also have plastic fittings like the distribution box we used, but my tank didn't.
To complete the distribution box per the county's requirements, I got caps for each line, drilled a 1" hole in each and then placed them without glue on the end of each outflow line in the box. Then I poured water in the box and rotated the caps so that the water flowed evenly into each line. Apparently it's a little simpler if the inflow pipe comes from the side. In my case, since I put the inflow at the top I had to mortar in a single brick to the box to act as a "baffle" and prevent the force of the flow from the inflow pipe pushing an uneven flow into the outflow pipe directly opposite.
At this point you'd typically call the inspector. Since I wanted to have my friend present for the inspection. We were lucky that the inspector signed off before we had all the lines and distribution box in place. She signed off that the leach lines were properly placed and I just have to finish the system and go into the county office to sign that it is complete to get my final use permit. My friend backfilled all the leach line trenches and graded the area with the blade on the excavator. It's amazing how easy he makes that look and how hard grading is. I backfilled all the supply line trenches once I had the distribution box connected. I then went to the county and signed off the I had completed the system.
1250 gallon tank, delivered $1000 75 infiltrators, ends caps, D-box $1200 7 day excavator rental $1600 filter fabric $ 500 pipe, fittings $ 300 permit $ 550 expert excavator operation - priceless!
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