One lid or two? What a full septic pump-out empties

From the yard a septic tank looks like one thing with one lid. Underneath, plenty of them are a single concrete box with a wall across the inside, and the house’s water has to pass through both sides before it leaves. Which kind you have decides how many lids the crew opens.

Does septic tank pumping empty every compartment of the tank?

It should. On a two-compartment tank, a full pump-out goes in through the lid over each compartment and empties both, and on a system with a pump chamber behind the tank the crew opens that too and decides whether it needs the hose. When we quote septic tank pumping in Pocatello, that is the job being priced.

The reason it has to be done lid by lid is the wall. Wastewater lands in the first compartment, usually the larger one, where the heavy solids drop out and the grease floats. What is left passes through an opening partway up the dividing wall into the second compartment, and leaves the tank from there. Put the hose down the first lid only and the first side empties. The second side drains down to the height of that opening and stops. Its bottom layer and whatever floats on it are still there when the truck pulls out.

The lid that gets found is usually the one nearest the house. The one that gets missed tends to sit a few feet farther out along the same line, under sod the last owner rolled across the whole run.

How do you know how many compartments or lids your tank has?

Start with paper and the tank’s size. The permit file, the same records our guide on finding your septic tank starts with, can list the size and show a drawing of the system. In the yard, two lids in a line along the tank’s length point the same way. Then tell the pumper what you found before the visit.

The places to look, cheapest first:

1. Old pump-out invoices. A previous crew that pumped two compartments often wrote that down, or billed a gallon count that only makes sense for the whole tank.
2. The permit file for the system, for the tank size and whatever the drawing shows, if there is one.
3. The ground over the tank. Two lids or two risers a few feet apart, running away from the house in a straight line, usually mean two compartments.
4. An alarm panel on the house wall or a post near the tank. A panel means a pump, and a pump means a chamber behind the tank with its own lid.

Two lids do not always mean two compartments. Some single-compartment tanks have a lid over each end, one at the inlet and one at the outlet. The job is the same either way. Every lid gets opened.

On the phone, the useful things to say are how many lids you know of, whether any of them have risers, and whether there is an alarm panel anywhere on the property. If you know none of it, that is normal for an inherited tank, and the crew finds out at the lid.

Why does the second compartment matter if most solids settle in the first?

It is the last stop before the drain field. The first compartment catches most of the sludge and scum, and the second catches the finer solids that make it through the wall. Whatever builds up in the second compartment sits right at the outlet, so when the tank goes too long, that is what reaches the field first.

That makes the second side the one that matters most to the most expensive part of the system. A drain field on a rocky bench lot or a river-bottom lot near the Portneuf runs fine on clear water and clogs on solids, and the second compartment is the tank’s last chance to hold them back.

One skipped compartment does little harm on its own. A tank pumped through the front lid visit after visit, for years, has never had its outlet side emptied, and the layer on that floor has been growing the whole time. Many tanks also carry an effluent filter at that outlet end, and rinsing it is part of a full septic tank cleaning, which means the outlet lid has to come off for that too.

Is the pump chamber pumped on the same visit?

Often, but not by default. A system that pushes effluent uphill, or out to a pressurized drain field, has a separate chamber behind the tank that holds the pump. The crew opens it, looks at what has settled on its floor, and pumps it when solids have built up there. The crew makes that call at the lid.

Pump chambers turn up most where gravity cannot do the work. Bench properties where the best ground for a drain field sits above the house are the usual case, and so are lots where the field had to be pressurized to spread water evenly through the soil. If the chamber is there, it adds a lid to open and sometimes gallons to haul, so mentioning the alarm panel when you book keeps the quote honest.

What the float in that chamber is measuring, and what to do when its alarm sounds, is the subject of our septic alarm guide. Between visits, leave the chamber lid closed. A pump chamber is a confined space with sewer gas in it and a pump wired to live power, and nothing inside it is a homeowner job.

Can a tank be pumped through a small inspection port?

Not properly. The 4-inch capped pipe in the lawn is an inspection port, set over a baffle so someone can look down at it. The manhole lid over each compartment is the opening a tank is pumped through, wide enough for the hose to reach the sludge on the floor and for the crew to break up the scum mat on top.

A hose can be fed down a port, and some outfits do it when the real lids are buried. What comes out is mostly the liquid. The scum mat stays where it is, the sludge in the corners stays where it is, and the hose works straight down onto the baffle the port was put there to watch. Baffles in older concrete tanks here have been through decades of freeze-thaw, and a hose knocking against one can crack it or knock it loose.

If the only openings showing in your yard are ports, the manholes are still underground and every visit includes digging to them, until a riser goes over each lid.

How can you tell afterward that the whole tank was pumped?

Read the invoice. It should show the gallons hauled, and on a tank that was due, that number usually comes out somewhere in the neighborhood of the tank’s rated size, which counts both compartments together. A 1,000-gallon tank billed at a few hundred gallons is worth asking about.

The gallon count also sets the price. A routine pump-out on a 1,250 - 1,500 gal tank runs $325 - $475, and the cost page lays out what a larger-tank pump-out costs here next to the 1,000-gallon case.

When we do septic tank pumping, how full the tank was and what shape it is in go on the invoice in plain words. If the paperwork from your last pump-out shows a gallon count well under the tank’s size, the second compartment is the place the next crew should start.