The Client Assumed the Tanks Had to Go in the Ground. We Drew It Both Ways.

The client had already decided. The tanks were going in the ground, behind the generator room, and the question he wanted answered was how deep. Nobody in the room had asked why they were going in the ground at all. It was just what a sewage treatment plant looked like to everyone who had ever seen one.

We drew it both ways. Sunk, the way he expected, and standing on a slab at grade. Then we put the two sheets side by side and went through what each one actually costs and what each one takes away from you later. He still chose the sunk option in the end. But he chose it, which is a different thing from inheriting it, and he knew before signing what he had given up.

We now show the above-ground, no-excavation option on every job, even the ones where we are fairly sure it will lose. A client who has never been shown it does not know it exists.

What the ground charges you, on top of the concrete

Our concrete rate sits at around ₱14,000 per cubic metre of tank capacity at a 3,000 psi mix, which is more than enough for a water-retaining structure. On one job we charged about ₱1.2 million for a 70,000-litre three-chamber tank with a heavier top slab. A client's own contractor, without our margin on the civil package, tends to land nearer ₱10,500 per cubic metre. Those are our numbers on our jobs. Your site will not be our site, and the level of filtration, the volume and the ground you are standing on will all move them.

The point of quoting them here is that the concrete rate is the part everybody compares, and it is not where the two options separate. The drawing is where the money on an STP is decided, not the build — that is the whole of why we spend longer on the sheet than on site.

Excavation is the obvious one, and the volume is always larger than the tank. You are digging the working space too. On one layout we redrew last year, blocking the tanks into one monolith instead of six free-standing boxes took roughly 45 cubic metres out of the dig and turned two separate pits into one — that is the arithmetic in our notes on how the tank layout decides the civil bill. Then there is the spoil, which has to go somewhere, and in a built-up Cebu site "somewhere" means trucks and a tipping fee.

Then the water table. On a coastal or reclaimed site you are pumping the hole dry for as long as the walls are going up, and a tank that is empty during a wet season can try to float. That is a design case, not a surprise, but it is a design case that only exists below grade.

Then the neighbours. If you are digging within a few metres of an existing structure, the cost of not undermining it is real and it belongs in the quote rather than in a surprise later.

And then the slab on top. A sunk tank's cover slab is not just a lid; if it sits where people or vehicles go, it is a floor, and it gets designed and priced as one. That is also its best feature, which we will come to.

What the ground gives you back

Two things, and they are both worth having.

The first is gravity. Sewage leaves a building at an invert level that was fixed when the building was built. If the tanks are in the ground, the wastewater arrives by falling, and the only pumping in the plant is the pumping you chose for process reasons. Put the same tanks on a slab at grade and you have to lift every drop that enters the plant, which means a sump, a duty pump, a standby pump, a level control and a small permanent power bill that lands in the running cost per cubic metre for the life of the plant. On an existing building that inlet lift is often the whole argument, and it decides the case before anyone gets to the concrete rate.

The second is the cover slab. Sink two rows of tanks and the slab at grade is your walkway — you do not build a service platform, because you are standing on one. That is also why we never draw a walkway between two rows of sunk tanks: the two rows should share one spine wall and the slab above does the walking. It was a client who put that to us as a question, and he was right.

What you can never do again once the slab is on

This is the part that gets decided in the drawing and cannot be revisited.

Underground, we do not fit a weir plate and we do not fit a scum board. Not because they are bad — above ground they are perfectly sensible — but because neither of them can be reached once the cover slab is poured. A weir plate has to be levelled, and levelling it means a person inside the tank. A scum board has to be skimmed, and skimming it means a person inside the tank. Anything you cannot service through a hatch is something that will quietly stop working in year two and nobody will know. Confined-space entry is not a maintenance procedure; it is an event, with gas testing, ventilation and a standby man.

So the underground clarifier we carry forward on every job is built to need nothing done inside it. Rectangular, poured together with the other chambers rather than dropped in as a separate vessel. A mass-concrete hopper with all four faces falling inwards to a central sump, and that sump's floor is the structural slab — nothing is cast below the foundation line. An inlet stilling baffle to kill the velocity, a submerged outlet draw-off cast through the cross wall about 200 mm below top water level so floating material can never leave with the treated water, and a submersible sludge pump sitting in the sump. The internals on that job priced out at about ₱60,000, against a circular clarifier that would have wasted roughly a fifth of its bounding box in the corners it does not use.

Above ground, the same list reopens. You can level a weir plate because you can see it. You can fit a scum board because you can reach it and, more to the point, because you can decide where the skimmings go — the honest reason we leave scum boards out is not only the hatch, it is that on most small plants nobody has answered that question. A tank you can walk around is a tank whose clarifier you can actually watch, and watching the clarifier is how most solids problems are caught early — along with a settled-volume reading, which is why we read a cone rather than chase MLSS.

What above ground costs you that nobody mentions

Sun and rain. Every piece of equipment on an above-ground plant is outdoors unless you build it a roof, and building it a roof is a structure you did not price. We have a live example in our own kit: our dosing pumps last about a year, and we think part of the reason is that they get left out in the open, which is wrong to begin with. We have written about treating that as a specification problem rather than a supplier problem. An above-ground plant makes it your problem on day one.

Noise. On a sunk plant the slab absorbs a good deal of it. On a slab at grade next to occupied rooms you hear everything, which is one reason we stay with air pumps rather than air blowers on small plants unless the strength of the wastewater really calls for a blower. A loud machine next to a function room gets switched off by somebody, and a plant that gets switched off at night is a plant that fails on ammonia. Small plants are the unforgiving ones here — a 5 m³/day plant is harder to run than a 500, and it has less water to hide a mistake in. The recovery from a stopped blower is slow and it comes back in two pieces, so this is not a small thing.

Footprint at grade. Underground, your tanks cost you nothing above the slab. Above ground they occupy parking, or a setback, or the space the building wanted for something else. On a tight commercial lot that alone ends the discussion.

And the inlet lift, again, which is the first thing to check and the one people check last.

Where above ground wins outright

Leased premises. A sunk tank is a permanent alteration to somebody else's land. A plant on a slab can be extended, relocated or removed at the end of the term, and that optionality is worth real money to a tenant who cannot say what happens in year six.

Rock, or a high water table, or an existing structure too close to dig beside. If the ground is fighting you, stop fighting it.

Sites where the sewage already has to be pumped. If there is already a lift station because the building's drainage sits below the street, the inlet-lift argument disappears, and with it most of the case for going underground.

Speed, sometimes. Civil works, not equipment, is the long pole on almost every job we have built — that is the shape of the programme on a typical STP. Excavation, dewatering and the sequence of pours are the part that slips when it rains. A plant that needs only a plain slab starts treating sooner.

And retrofits into a running site, where the plant has to go in while the building keeps operating. A retrofit and a new build are two different jobs, and an excavation next to a working kitchen is a different animal from a slab poured in a corner of the car park.

The joints, not the cubic metres

If you do go underground, one thing is worth saying plainly, because it is where water-retaining tanks are won and lost. The tank is not decided by how much concrete is in it. It is decided by the joints: the waterstop in the base, the kicker on every wall line, the waterstop at each lift joint. Then a fill-and-mark test on every chamber before anything goes on top — fill it, mark the level, and the drop after 24 hours should be no more than 1/500 of the depth, witnessed, as a hold point that somebody signs.

We ask to see the build method on every civil quote now, ours or anybody else's. Not the contractor's trade secrets — the dimensions, the mix, the waterstop and kicker detail, and that test. We ask the same of ourselves, and we say so on our contracting page. If a quote for a sunk tank does not describe how the tank will be proven watertight, the cheapest line on the page is the one you should be least comfortable with. That is the same instinct behind the questions we would ask before signing.

What we actually put in front of a client

Both drawings, with the differences named rather than implied. Same process train in both — the treatment does not change because the tanks moved; the technology choice is a separate decision and it is rarely the one that decides the money. Same capacity, sized to the same influent band and specified the same way — the sizing method does not change with the tank elevation — because the influent is the number most clients do not have and an assumption made bigger is an assumption the client pays for in concrete either way.

What we show next to each: the excavation and dewatering exposure, the inlet lift and what it costs to run, the equipment enclosure, the noise position relative to occupied space, and the maintenance items that are or are not reachable. Then the risks and assumptions in writing, including the unflattering ones. A client has the right to see all of that before signing, not after.

One thing we hold constant across both: we keep the discharge line and design to the discharge standard the receiving water class sets, whatever the client's reuse plans are. And if reuse is on the table, the treated-water storage gets sized from the use pattern, not the daily flow — which at ₱14,000 per cubic metre of capacity is a decision you want made before the slab, not after. Breaking into a finished tank to add one is the expensive way to learn that.

If you want to see roughly where your own numbers land before you talk to anybody, our STP price estimator and the design generator will both take a flow and a building type and give you something to argue with. They are rough by design. They are still better than a figure somebody remembered from another project.

Where this stops being transferable

Every one of the arguments above flips on a site we have not seen. The inlet lift that kills the above-ground option on a finished building is irrelevant on a site still at foundation stage, where the drainage has not been set yet. The water table that rules out digging on a reclaimed coastal lot does not exist 3 kilometres inland. The ₱14,000 per cubic metre we quote in Cebu, where we fabricate locally and do not pay freight in, is not the number in a province where everything arrives by truck.

And the maintenance argument cuts differently depending on who will be standing in front of the plant. A plant with a dedicated in-house maintenance team can carry equipment that a plant run by whoever started last month cannot — and turnover is a design input, not an operations problem. A good operator can carry a plant, but he cannot carry a bad design, and he certainly cannot carry a weir plate he is not allowed to reach.

What holds, as far as we can tell, is the process rather than the answer: draw it both ways, price the ground rather than only the concrete, and never let the client find out in year three that the option existed. It has worked for us over and over. We can only hope it works on your site, because every site is different.

If you are at the stage of deciding where the tanks go, book a free 15-minute consultation and we will go through both layouts for your site. If you already know your scope, tell us what you need and we will come back with both drawings priced. Our completed work is on the projects page if you would rather look at what we have built first.