How many tanks, what size? Aerobic or not? We have an aerobic system and routed our kitchen drains to a 500 gallon grease trap in front of the other tanks to increase the efficiency of the system and my installer raved about how wonderfully the system will function this way. The engineer had to take into consideration the number of bedrooms in order to design the system. Did he ask you questions about your entertaining habits? When I talked to our installer about adding a guest house, he said it shouldn't make a difference because the system is designed for a certain capacity on premises, whether they use the facilities indoors or out, there is still enough capacity.

dannie_gal, great questions--the paperwork shows we will have:
1. 1250 gallon two-compartment septic tank
2. 1000 gallon pump tank
3. non-aerobic
4. low pressure dose system.

What capacity is yours designed for?

Thank you!

Trudymom, I am much more familiar with the aerobic systems, but yours looks to be a little more than double the capacity of mine which would make sense because our can get rid of the extra water much quicker than the traditional septic system. We have a 4 bed/3.5 bath house and I told the designer we could entertain up to 15 people once per month and sometimes more a couple of times a year. As long as you let the engineer know about how many people you could anticipate, I'm sure his design will be fine. You could always get another opinion if it would make you feel better. I really like the idea of our kitchen waste being routed to a different chamber because it basically doubled our capacity and we won't have to worry about ever filling it up. We are still very careful about using the disposal, but it was relatively inexpensive for that added reassurance.

The capacity of your tank is determined by a code table based on the number of bedrooms in the structure. You could call your local code enforcement office and they can tell you what the local code requires.

What lazypup said. Your system is based on the number of bedrooms. The engineering assumes usage as if each bedroom were occupied. I have no idea how your guest house will impact this system. Would it have its own? Do the plans you submitted to the engineer include the guest house?

The biggest detail that hasn't been discussed is the type of drainage field you'll have. The tanks you have are probably the same that would be spec'd for a 3 bedroom home with a pump-up type system. The drainage field is what would be twice as big for a 6 bedroom home.

As has been noted by lazypup and bungeeii, the "engineering" will be based on the number of bedrooms in the house, whether occupied or not...that's how the plumbing inspectors usually treat things (I assume the septic designer may have added another bedroom or two for your future guesthouse). If your system is anaerobic, then you might be using something like the Infiltrator leach chambers. There are many other options that will be determined by your site conditions (sand filter, mound, at-grade is even possible....etc.) Depending on your soil type that is probably going to translate to 5 -600 feet of trenching. It will vary depending on how your soil specs out. As far as the impact of a large number of guests goes, it will be unnoticeable unless your guests are there for many days and/or your leachfield is really slow draining.

trudymom

the size of the septic tanks are designed for the number of bedrooms in your house which translates into a total number of people who could be living there (way more than you have). I think in our area they assume 2 people for the master bedroom and 1.5 for the other bedrooms: so that would be roughly 10 people for your house and the system is sized assuming those 10 people would be living there full time.

An occasional party with 50 people isn't going to bust the system - you do have a 1250 gallon tank. At 4 gallons per flush (which is high for a modern toilet) - the septic tank can handle over 300 flushes. With 50 people - do you think they will be flushing more than 6 times each! That's a lot of beer!

Another way to look at it - assume those 10 people that the system is designed for flushed a toilet 3 times per day - that's 30 flushes per day, or 900 flushes per month. If only 2 are living in the house full time, you have 720 flushes to use for parties.

Your septic system is big enough to handle an occasional party of 50. If you wind up having a giganitc party (like hundreds) - then rent some porta potty's for the event.

as far as the guest house goes, around here you would be REQUIRED to install a separate septic for it. each plumbed structure must have it's own system! my neighbor behind me foudn this out the hard way. he spent 12k on dirt work to build a "guest" house next to his house. his grandson was going to live in it full time, though the county did not know this. anyway, the county ended up nixing the guest house because they only have 1.75 acres and each system required a minimum 1 acre at teh time. they would not allow it to be tied in to the old system either, even if they doubled the tank size. so for the last 4 years there has been a 50'x50'x4' dirt pile in the side yard that was smoothed and compacted for building on.

now in my area you have to have a minimum of 2 acres per septic system for new build. older systems are grandfathered in, but if you decide to add on to your home you cannot if you have to increase the size of the septic and don't have teh 2 acres to do it.

and as stated above, the health dept here sets the size and type system you get. you give the house plans to them, they come do a perc test if they don't already know that area, and then they tell you what type system you require. sometimes they will give you no choice at all, but usually they give you the option of a minimum and a suggested system size/type.

my house has 2 septics, one for the laundry and kitchen and one for the bath rooms. both are 1000 gallon with at least 350 ft of leach field line each.

OT but had to share:

We have friends that built a guest house and didn't want to pay the $12k to have a septic tank installed for the house. Instead they opted for a waterless compost toilet. So while they get to poop in nice toilets their guests have to poop in compost? I think not!

"An occasional party with 50 people isn't going to bust the system - you do have a 1250 gallon tank. At 4 gallons per flush (which is high for a modern toilet) - the septic tank can handle over 300 flushes. With 50 people - do you think they will be flushing more than 6 times each! That's a lot of beer!"

The tank is already full.
A large volume WILL push the extra effluent into the leach field.
Most often some solids get pushed out when the tank is overloaded.

Once in a while is not harmful, but repeatedly saturating a leach field WILL shorten its useful life.

"The tank is already full.
A large volume WILL push the extra effluent into the leach field.
Most often some solids get pushed out when the tank is overloaded"....

Yes, that is the main problem, the actual septic tank size/capacity has little influence (as brickeye notes, it is always pretty much full until the pump sends the next batch of effluent to the field); it is the ability of the leachfield to absorb the extra effluent that will ultimately determine "capacity". If it is clogged or slow draining, the pump is trying to push the effluent towards the field but if the field is saturated the effluent is being backed up into the tank which starts to overflow above the outlet pipe and can cause the settled solids to migrate past the tank baffle towards the pump compartment.

Of course solids sent to the leachfield will negatively impact its ability to absorb the primarily liquid effluent and will lead to future problems.

"...the pump sends the next batch of effluent to the field)..."

While some septic systems have pumps, the vast majority work on plain old gravity.
The tank is lower then the house plumbing, the leach field is lower than the top of the tank.
Wast in from the house displaces water that flows out on the other end.
A baffle from the top of the tank down towards the bottom prevent direct flow from in to out.

Aerobic systems pump air through the waste, then transfer it to s second tank for chlorination, then a third tank for de-chlorination before discharge.
These systems are not allowed in all locations, but are often the only systems that can be used when the ground will not pass perc for the leach field.
The discharge water can go on the surface.
They require power and maintenance and an annual inspection in some places.
It is essentially a reduced size sewage treatment plant for a single house.

"While some septic systems have pumps, the vast majority work on plain old gravity. The tank is lower then (sic) the house plumbing, the leach field is lower than the top of the tank. "

Thanks brickeyee.....Didn't know most were gravity (obviously it depends on your site and climate); nonetheless, the same idea applies concerning the saturation of the field, gravity or pumped, and potential mixing of solids within the tank due to back up.

I live in Canada where we have to bury the tank compartment quite deep to go below the frost line thus the discharge outlet from the tank is usually at least 6 feet deep. If you have a realtively level lot and you want your field some distance from your house it is not practical to bury your laterals for the field too deep if you are trying for the gravity method (I don't think it is recommended to go much past 4 or 5 feet on the laterals either...you don't want to start contaminating groundwater before the soil microbes have a chance to treat the leachate for one). If you have sloped land then it might be fine. All the systems I have seen in my neck of the woods use a pump.....

the OP has two tanks - the 1250 tank for solid waste the second 1000 gallon tank for fluid. Tank 1 drains into tank 2 - fluid from tank 2 is either pumped or gravity fed the leach field.

the level at which tank 1 drains is not at the full line - it is below the full line but above where the solid waste line is. as long as you have processed beer flowing - it won't be an issue for an occasional party with 50 people given the size of the tanks.