They Sent Us a Finished Drawing and Asked for a Price. We Read It First.

A drawing reached us with a one-line message: please quote this. Six chambers, a technology named in the title block, dimensions on every tank, and a bill of quantities already typed out. The sender was not asking us to design anything. He was asking us to be the cheapest supplier of a design somebody else had already finished.

We quoted it. But we read it first, and what went back had six sections instead of one price. That structure is now how we answer any wastewater drawing that arrives already drawn, and it is deliberately different from how we answer a blank site. A new plant from scratch follows our own sequence — flow, strength, end goal, then chambers. Somebody else's drawing has to be answered in the order a reader can check, which is not the same order at all.

First we draw their design back to them, exactly as issued

Section one of the document is their plant, with nothing corrected. Chamber volumes as dimensioned. Equipment as listed. And then the numbers that follow from those two things but were not written anywhere on the sheet: the loading rate each tank is actually carrying, the contact time at the stated flow, the surface loading on the clarifier, the air the aeration tank would need.

Clients are sometimes surprised by this section, which is the point. A drawing states dimensions. It rarely states what those dimensions imply. The person paying for the plant has usually never been shown the implied numbers, only the picture.

There is a commercial reason for it too, and we would rather say it plainly. If we silently corrected the design and priced the corrected version, our number would sit beside a rival who priced the drawing exactly as issued, and ours would look dearer for no visible reason. We have watched that happen. The cheaper quotation is not always the cheaper plant, but nobody can see that unless the two things being compared are laid out separately.

Then the problems, with the calculation beside each one

Section two is not a list of objections. An objection without arithmetic is an opinion, and on a drawing somebody else was paid to produce, an opinion is worth nothing. Every item here carries the calculation that produced it.

The same faults recur, and almost none of them are arithmetic. The influent basis is first. A strength appears on the sheet — BOD 300 is the usual one — and nobody can say where the number came from. Often there is no sample at all, because almost nobody has an influent result when the plant has not been built yet. That is not a scandal. It is the normal condition. But a design drawn to one unprovenanced number, with no band around it, has a cost consequence the owner has not been shown, and the parameters that matter differ by industry enough that the guess matters.

Second is the receiving water. A great many drawings specify a treated quality without establishing which class of water body the plant discharges to, and the two classes in front of most commercial buildings are not close: ammonia as nitrogen at 4 against 3, phosphate as phosphorus at 4 against 1.5. A river can be reclassified without the plant changing at all, and at larger flows the class is worth millions. Establishing it is our work, not the client's, and we do it at our own cost before we price anything — the compliance basis has to be settled before the chambers can be.

Third, and most often, the plant is sized on BOD alone. BOD is the parameter that passes. Ammonia and phosphate are the two that fail, and they do not scale with organic strength, so a tank volume derived from BOD says nothing about whether the nitrifiers will have the time and conditions they need. We check the ammonia, nitrate and phosphate duties separately, against the nutrient field guide we work from, and that is where most corrected designs grow.

Then the quieter ones. Equalisation absent, or set at a percentage of daily flow rather than sized from the building's own pattern — and the one tank that treats nothing is the cheapest thing to build now and the dearest to add later. Process and domestic lines combined at a manhole, which creates an ammonia problem nobody needed. A kitchen upstream with hard water, where the crust in the interceptor is soap rather than grease and the real fix is a softener on the kitchen supply. Six freestanding tanks with 150 mm between them, which cannot be formed, and which shared cross walls answer at a third less footprint. A bought-in mechanical screen where a formed concrete chamber would do the same work with no motor to support.

And one we find often enough that it is worth naming: a primary settling tank. We do not include primary settling in our designs by default, and we do not carry it across from somebody else's drawing unquestioned. On domestic sewage at commercial-building scale it buys a modest share of the organic load, costs a chamber, and hands the owner a second sludge stream to haul. If it is there for a reason the drawing does not state, we ask. If there is no reason, it comes out and the saving is shown.

The arithmetic half of all this used to be the expensive part of a review. It is not any more. Calculations are close to commoditised now, which is why we tell clients to put our own design through an AI and check it. A review that only survives because the reader cannot do the sums is not a review.

Then the same technology, drawn so that it works

Section three is the correction, and the rule we hold ourselves to is that the technology does not change here. If the drawing says extended aeration, section three is extended aeration done properly. Corrected chamber volumes. Aeration sized on the nitrogen duty rather than the carbon one, with turn-down, because a machine that is loud enough to be switched off at night will be switched off at night. Our standard rectangular clarifier, poured with every other chamber, because the clarifier is where other people's problems show up rather than where they start. FBBR-style fixed-film media where a building's occupancy will climb slowly and the population needs somewhere to hold on.

Section four says why ours is a better version of theirs, and it says it as a list of differences with the consequence of each, not as a verdict. Same technology, same site, same flow: here is what moved, here is the number it moved by, here is what it buys. A client who has been handed two drawings and a judgement has to take somebody's word. A client who has been handed two drawings and the differences between them can decide for himself.

Then, separately, how we would have solved it if nobody had named a technology

Section five is the one clients remember, and it is also the one we are most careful with. It answers a question nobody asked: if the technology in your title block had never been specified, what would we have drawn?

It is written as a column, not as a recommendation. We do not recommend systems. We put them beside each other on equal footing with the real differences shown and let the owner choose, which is why we once priced the same building three ways and the technology was not what decided it. What usually decides section five is not the process at all. It is the end goal, and the end goal is a question about the building rather than about treatment: a permit, a use for the water inside the premises, or both. Above the governing goal you are buying quality nobody uses, and if reuse is wanted, which rung of reuse changes the plant more than the choice of biology does.

Sometimes section five and section three come out nearly identical, and we say so. That is a good outcome for the original designer and we do not disguise it.

The price is section six, and that is on purpose

Four technical sections come before any number. By the time the commercial page arrives, the reader already knows which plant he is pricing and why it differs from the one he sent us, so the number has something to attach to. A price on page one is a number with no design behind it.

Three things go in that section besides the figure. The civil works, priced the three ways we always price them — we build it, he builds it, or his contractor builds it under our supervision at a stated fee — because on a reviewed design the civil scope is the part most likely to already be committed elsewhere. Tank concrete sits around ₱14,000 per cubic metre of capacity at a 3,000 psi mix, and for scale, a 70,000-litre three-chamber tank was about ₱1.2 million. Your own site will differ: price moves with filtration level, volume and site conditions, so the estimator and the design generator are there to put you in the right range before anyone quotes.

Second, the risks and assumptions, stated before signing rather than after. A reviewed design carries more of them than a fresh one, because we are inheriting decisions we did not make. We mark what is load-bearing, and we do not dress a problem we have already solved inside the scope as a liability — but anything still open and still the owner's to decide goes in front of him with numbers attached. These are the questions worth asking before signing whoever drew the plant.

Third, commissioning. On a reviewed design this matters more than usual, because if the plant is built to somebody else's drawing and then fails its first test, the argument about whose fault that is will be unwinnable in either direction. So commissioning completes on a result, not on a date, and the document says who schedules the sample.

What we will not do with a drawing somebody else was paid for

We do not tell the client his designer was incompetent. Almost every drawing we read is the work of someone who was given less information than we were given — no influent sample, no established receiving class, sometimes no stated end goal. Those are information problems wearing the costume of engineering problems, and a reviewer who arrives after the information has been collected is in an unfairly good position.

We do not quietly reprice. Nothing is deleted from a drawing in our quote without being named, with the reason and the saving beside it.

We do not take over the other engineer's role by default. If the owner has read all six sections and still wants the plant built as originally drawn, we will build it as originally drawn, and we will say in writing which parameters we stand behind and which we do not. We both design and build, and that creates a conflict we would rather name than have discovered — a review that always concludes the reviewer should get the whole job is a sales document. Being a contractor on somebody else's design is a legitimate scope and it is priced as one.

And we do not put our construction method into drawings handed to the owner's contractor. He gets the what — dimensions, mix, waterstop and kicker spec, the fill-and-mark test. The pour sequence stays ours. That is scope hygiene, not secrecy, and it cuts both ways: we also will not ask another designer to hand over his.

Where this does not transfer

Six sections are proportionate to a large contract and absurd on a small one. A modest plant gets one simple price and a short note, not a review document — and small plants have their own difficulty, since a 5 cubic metre a day plant is harder to run than a 500. If you want the fast version, tell us what the building is and we will keep the answer the size of the job.

It assumes an owner who will read. Some will not, and for them section four ends up being the whole document, which is a worse way to decide.

It assumes a drawing complete enough to check. A single-page concept sketch cannot be reviewed; it can only be replaced, and we say that rather than pretending to audit it.

And it rests on where we sit. Our corrected designs lean on concrete because we have our own fabrication and civil network in Cebu and local concrete and labour prices make a poured chamber cheap against a bought machine — you can see what that produces in our completed projects. A reviewer importing equipment into a different market would honestly correct the same drawing in a different direction. A failing existing plant is a different job again, where the constraint is what is already in the ground rather than what is on the sheet, and the answer usually lives in operation as much as in design. There is also the honest limit on all of it: a good operator can carry a plant but cannot carry a bad design, which is the reason we read the drawing at all.

Once in a while we read a drawing we cannot fault. Then the answer is one section long and says so, and we quote the build. That has happened, and it is not a bad day.

If you are holding a design and a price and cannot tell whether they match, send us the drawing and book a free 15-minute consultation. We will tell you what the sheet actually implies, whether or not it turns into work for us — that is also what our consultations are for, and what a wastewater treatment company ought to be able to do in a quarter of an hour.