Septic Systems
Mon, Jan 15, 2018If you’re looking to build a cabin, chances are you’ll have to research a septic system. Waste has got to go somewhere and sewer is almost certainly not an option. It isn’t for us. In fact, most of the area is on septic; not just the remote outskirts.
A common misnomer is to call a septic system a septic tank. A septic system will have a tank as part of its components, but it’s just one part of the overall system. The tank will hold waste, typically in one of two chambers. All waste is deposited into the first chamber, and the job of the first chamber is to hold onto that waste until it becomes a liquid. The first chamber is connected to the second chamber half way up, so that liquid can freely flow between the two, but solids will drop to the bottom (and not be passable), and oils/fats will float to the top (and will likewise not be passable).
A well-designed and healthy septic tank will successfully convert nearly all the waste to a liquid within some reasonable amount of time. Your tank gets sized appropriately to handle the amount of waste that may need to be queued up while its processed.
In the second chamber, the liquid is continued to be processed by microbes.
This tank (specifically the second chamber) will have an outgoing pipe that carries the effluent (liquified waste) to a distribution box, and then a drain field. The drain field and the septic tank are the two primary components of the system.
The drain field is responsible for reintroducing the effluent back to mother nature. The drain field is typically one or more long channels that are buried rather shallowly below ground. The effluent leaks out of the channels and seeps into the soil. The microbes living in the soil process the effluent further.
The distribution box helps ensure the effluent is being equally distributed to each of the channels in the drain field.
In a healthy and well-designed septic system, the waste is fully processed on site, and you don’t have to have a pump truck come and clean out your tank.
The soil that you bury your drain field in is a particularly important aspect of this overall system. In order to receive a septic permit, you have to have something called a Percolation Test, or “Perc Test” for short. This is where someone comes out, digs 6’ hole and studies how well liquid percolates (moves) through the soil. You want it to percolate well so that you don’t have to apply artificial pressure to the drain field in order force the effluent into the soil.
There are two different types of septic systems:
- gravity based
- pressurized
There is a third called ‘alternative’, which contains newer technology-based solutions for particular difficult installations. Gravity is the easiest to design, maintain, and construct. It’s also the cheapest. You want a gravity-based system, if possible.
Our system was designed as a gravity system. I also learned that our percolation test results were very good, which hopefully meant the system would operate well once in use.
Even though the septic permit was being renewed, we were considering redesigning the system to accommodate five bedrooms. As part of our wishlist, we wanted a fifth bedroom to be a kids bunk room. There was space for it even in the original floor plans, but it was marked as an ‘unfinished rec room’. We could leave it marked as such, but if we ever wanted to sell the place, we wanted to be able to say it was five bedrooms. The current permit only allowed for up to four bedrooms.
Kittitas County allows homeowners to design their own gravity system, assuming they take a short class and pay an additional fee. As I was finding, designing these systems is not too difficult. The amount of gallons the system must support per day is based on the number of bedrooms in the home. This also dictates the size of tank your system will require.
Once you know how many gallons per day your system will need, you must design a drain field large enough to accommodate that. The math gets a little trickier here, but its still pretty simple. Depending on what materials you use to construct the drain field, you can significantly reduce the size of your drain field.
The county prescribes a daily water flow rate of 120 gallons/day. This means that our drain field must accommodate 480 gallons of effluent per day (4 bedrooms * 120 gallons/bedroom).
Our soil was determined to have a loading rate of 0.6. We plug this into an equation of daily water flow / loading rate: 480 gallons/day / 0.6 = 800 sq ft of infiltration area. This is the amount of surface area our drain field must provide.
Here is where it gets interesting. By using Infiltrator Chambers for our drain field, we can discount this number by as much as 40%! Applying our discount, the amount of drain field we need comes to 480 sq ft.
Infiltrator Chambers
Washington State, and many others, allow for some pretty good discounts when you use what is called a gravel-less system. These gravel-less systems are ‘modern’, but have been around for years. They’re also easier to install, and for us, was quite a bit cheaper as getting gravel to our site is expensive. The reason you can discount your drain field when using these is because the gravel-less system creates larger air pockets in the drain field, allowing for more microbe growth and distribution of your effluent. The gravel-less drain field is like an unobstructed underground tunnel. Whereas with the gravel systems, there isn’t as much air movement and this inhibits the microbe activity.
Air movement is really important, and is why you don’t want to bury your drain field too deep. If its too deep, those microbes will suffocate. The point of the drain field is to give the microbes one more opportunity to process the effluent before returning it to mother nature. Without this process, you may be contaminating your ground.
You won’t get a discount for it in your permit, but you can improve the longevity of your system by adding drain rock to the bottom of these chambers. Just a thin 1-3” layer of rock. This layer creates more surface area for the liquid to interact with, and thus more microbe activity.
After our discounts, our drain field size results in two 83’ x 3’ long channels. These need to be at least 9’ apart, and we have to leave enough space around the drain field to accommodate two more channels, if we ever need to. I believe this is a state requirement. The idea is you have to ‘reserve’ an area for future drain field channels in case the first set ever fail. As I understand it, you are not allowed to excavate a drain field (or septic tank). The course of action is to leave it and place a new system adjacent to it. Makes sense.
In case it’s not obvious by now, we’re planning to install the septic system ourselves. Infiltrator manufacturers both the drain field chambers, and the tank. We buy our own PVC pipe, and the system basically connects together like a kit. The install seems easy, and budgeting it all out, if we do it ourselves, I think we can build this system for less than $5k. A professional installation may cost two or three times that or more.
Later, when we purchase the materials, I will learn all about different PVC ratings. For a septic system, you want to buy PVC 3034 (aka SDR 35). This flavor of PVC pipe has enough flex in it that as the ground shifts, it shouldn’t crack. Keep everything at 4” too; you don’t want some kind of clog halfway to your drain field, or not have enough capacity that things back up into your house.
Ultimately we decided not to expand the capacity of the system. This is going to be a weekend place for us, and it’s never going to be used to its maximum capacity anyway.
We’re going to work on the septic system while we have downtime in laying the foundation. The foundation work will require a number of inspections, and coordinating deliveries of materials, etc. When we have downtime from those, we will chip away at the septic system. We just need to have it installed and approved by next snowfall, since the renewed permit will expire Jan 2019.
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