Six Tanks, Six Sets of Walls. We Redrew It and the Footprint Fell by a Third.

The drawing came back to me with six tanks on it, each one standing on its own, with a gap of about 150 millimetres between them. It looked tidy. It was also going to cost the client a small fortune in concrete he would never see again.

Nobody can form a 150 millimetre gap. The carpenter cannot get a form into it, and he certainly cannot strip one out of it. So what actually happens on site is that every tank gets priced with four walls of its own, and the gaps between them become a permanent trench full of nothing. The footprint on that first layout was about 65 square metres. By the time we had finished redrawing it, it was about 38. Roughly six cubic metres of concrete came out, and around 45 cubic metres of excavation went with it. Two separate pits became one.

We did not change the technology, the capacity, or a single process calculation to get there. We moved walls. This is the part of a sewage treatment plant that quietly decides what the client pays, and it is almost never what he is looking at when he compares two quotations.

The civil works is most of the money, and it is the part nobody argues about

Clients will spend an hour questioning the blower selection and thirty seconds on the tank. The tank is the bigger number. Our working benchmark in Cebu is around ₱14,000 per cubic metre of tank capacity at a 3,000 psi mix, which is more than enough for this duty. As a reference point, we charged about ₱1.2 million for a 70,000 litre three-chamber tank with a proper top slab. A client's own contractor, with no margin of ours in it, tends to land nearer ₱10,500 per cubic metre.

Those are our numbers, not a market rate. Ground conditions, access, water table and how far the mixer has to travel will move them on any given site. But the ratio holds: on most of the plants we build, the concrete outweighs the equipment. So when we are looking for money to give back to a client, we look at the geometry first, and at the equipment list second. I wrote about where the rest of that money gets decided in what we are actually deciding while the drawing takes shape.

Two rules that took a third off the footprint

The first one: never draw free-standing tanks with small gaps between them. Block them into one monolith with shared cross walls. One wall does the work of two, it gets poured once, and the chamber either side of it is the same volume it always was. The client is not buying a smaller plant. He is buying the same plant without the walls that existed only because of the way it was drawn.

The second one came from a question the client asked me, and it was a fair question. He looked at two rows of sunk tanks with a service walkway drawn down the middle and asked whether he did not save money by having one wall between the two rows instead. He was right. On sunk tanks the cover slab at grade is the walkway — you are standing on the plant. There is no reason to hold the two rows apart at the cost of a spine wall each. They share one.

Between those two changes the pits merged, the excavation dropped, and the plant got easier to build as well as cheaper. That last part matters more than it sounds. A layout that is awkward to form is a layout that gets built badly, and a badly built tank is a leak you find later. The equipment was never the long part of the programme — the civil works is.

Underground or above ground is a decision, not a default

Most of our plants go underground because the client wants his land back. That is a legitimate reason and usually the right call. But we show the above-ground, no-excavation option alongside the concrete one on every job now, because a client who has never been shown it assumes it does not exist. On a tight site with rock near the surface, or where the water table is going to fight us the whole way down, the above-ground route can be the cheaper plant even though the tank itself costs more.

Going underground also takes some things off the table permanently, and those have to be decided at the drawing stage, not discovered at commissioning. We do not put a weir plate or a scum board in an underground clarifier. Neither one can be levelled, adjusted or skimmed once the cover slab is on — reaching them means a confined space entry, and a confined space entry is not something I am going to design into somebody's weekly routine. I have never installed a weir plate. I do not install scum boards either, because in most of these plants there is nowhere sensible to send what you skim.

What goes in instead is an inlet stilling baffle, a submerged outlet draw-off cast through the cross wall a little below top water level, and a submersible sludge pump sitting in the sump. The clarifier itself is rectangular, poured with every other chamber, with a mass concrete hopper whose four faces all fall inwards to a central sump — and the floor of that sump is the structural slab. Nothing gets cast below the foundation line. No hopper dropping past it, no suction point lower than the slab.

People expect a circle. A circular clarifier wastes about a fifth of its bounding box in the corners you cannot use, and it has to be formed separately from everything around it. Even drinking water plants run rectangular clarifiers; I have seen plenty. If we can do it in concrete we do not need an insert, and concrete is still better here. What the clarifier can and cannot fix for you is a different conversation, which I have had at length in the clarifier is not the problem, it is where the problem shows up.

A water-retaining tank is won or lost on the joints

The cubic metres are the price. The joints are whether it works. Every wall line gets a kicker and a waterstop, the base slab goes in as one continuous pour, and every chamber gets filled and marked and left standing before anything else happens to it — a drop of more than about one five-hundredth of the depth in 24 hours and we are not moving on. That test is a hold point on our own programme, witnessed, and it is in the quotation as one.

Clients sometimes ask why we insist on showing how the tank will be built when they have their own contractor. It is because the failure mode we see most often is not a cracked wall. It is a construction joint that was never detailed, poured on a Friday, and now weeps into the ground three years later where nobody can get at it. If the client's contractor is building it, he gets the dimensions, the mix, the waterstop and kicker specification and the fill-and-mark test. Those are the things he has to hit. Our own build sequence stays on our sheet.

Where this shows up in the quotation

We always offer three routes on civil works: we build it, the client builds it, or the client's contractor builds it under our supervision at a stated fee. The third one is more popular than people expect, particularly where the client already has a contractor on site pouring something else. But the design has to be ours either way, because the layout decisions above are made on the drawing and cannot be recovered afterwards. A plant that was drawn as six free-standing tanks is six free-standing tanks forever.

If you want a rough sense of the numbers before speaking to anyone, the STP price estimator and the design generator will get you into the right order of magnitude, and the DENR discharge standards page sets out what the plant has to achieve in the first place. What the plant has to remove depends on your industry — the significant parameters by industry page is the shortest way to check. None of it replaces somebody standing on your site with a tape measure, but it stops the first conversation from starting at zero.

What does not transfer

The two wall rules travel well. Shared cross walls and a shared spine wall between sunk rows are close to free money on any layout that does not already have them, and I would apply them on any site in the country.

Almost everything else here is site-specific. The ₱14,000 per cubic metre is our rate in Cebu with our fabrication network, at today's material prices, on ground we can dig. Whether underground beats above ground on your plot depends on your water table, your rock, your access for a mixer and what you intend to do with the land on top. The no-weir-plate rule is right for an underground tank with a cover slab over it and wrong for an above-ground one where an operator can reach the launder from a walkway. The clarifier hopper shape assumes a sludge pump and an operator who will run it, which brings you back to what an operator can and cannot carry.

And none of the geometry saves you if the flow and strength on the drawing were wrong to begin with. That number is still the one most clients cannot give us — I have written about how rarely anyone has an influent result, and about why the design, not the build, decides the cost. A perfectly economical tank of the wrong size is still the wrong tank. If you are looking at an existing plant rather than a new one, a retrofit and a new build are two different jobs, and the civil constraints on a retrofit are usually what drive the design.

We do this work across Cebu and the rest of the country — you can see the plants we have completed, how we work as an STP contractor in the Philippines, what we cover on wastewater treatment in Cebu, and what our broader wastewater treatment services include. If you already have a plant running and it is the operation rather than the build that is giving you trouble, that sits with maintenance and operation and our O&M support.

If you have a layout in front of you and you are not sure whether it is drawn economically, send it over and book a free 15-minute consultation. Fifteen minutes on a layout is usually enough to tell whether there are walls in it that do not need to be there.