The Walls Were Already Up. How Much That Plant Can Treat Depends on Which Tenants He Signs.
The owner of a four-storey commercial building asked me a reasonable question. He had poured the outer walls of his treatment plant room — the box only, no partitions of any kind, because he had stopped work to change the technology — and he wanted to know whether his plant would fit inside it before he committed any more concrete. Will it fit, yes or no.
I could not answer that, and the reason is the useful part. Whether a plant fits in a fixed box is not a property of the box. It is a property of the tenants he has not signed yet. So I answered a different question, the one that could actually be answered from what we knew: given this volume, how much wastewater can be treated in it — and the honest shape of that answer is not a number. It is a curve.
This is the second note off the same job. Yesterday's was about the specification sheet on his drawings and the dead regulation inside it. Two other posts sit near this one and it is worth saying what each of them does instead: designing the same plant at several wastewater strengths varies the strength and asks what plant you need, and working a design flow out of an owner's water bill is about where a flow number comes from when nobody has a meter. Both of those solve for the plant. This one is the inversion: the plant volume is already fixed in concrete, and the thing being solved for is capacity.
What a poured box actually gives you, which is less than the drawing says
His box measured 14.0 by 7.0 metres on the outside face, 150 mm walls, with a depth he recalled as around 2.1 metres. Those are the dimensions of a building, not of a plant, and three deductions sit between them and anything you can treat water in.
Inside the walls it is 13.70 by 6.70, so about 91.8 square metres of clear floor. The partitions have not been poured yet, but they will be, and at 150 mm each the chamber walls take something like 8 square metres of that floor away — so the net is closer to 83.8. Then the depth. You cannot fill a tank to its roof; freeboard is not optional, and on a box of this depth the side water depth we can actually design to is about 1.85 metres. Multiply it out and the working volume is roughly 155 cubic metres.
That is the real asset. Not 193 cubic metres of structure, not 91.8 square metres of floor — 155 cubic metres of water, and it is fixed, because once the slab is on an underground tank there are things you can never do again. Adding volume later is not an adjustment; it is a second structure. At the roughly ₱14,000 per cubic metre of tank capacity we see for a 3,000 psi water-retaining mix in Cebu, volume bought before the pour is the cheapest thing on the job and volume bought after it is among the dearest. Your site will differ — that rate moves with access, ground conditions, water table and who builds it, as the civil works note sets out, and how the tank is poured matters as much as what it costs.
The answer is a curve, and the axis is strength
With the volume fixed, the arithmetic runs the other way. Instead of taking a daily flow and a strength and producing a required volume, you take the 155 cubic metres, hold the process requirements, and ask what daily flow the chambers can carry at each strength the building might produce. That is a short afternoon's work once the model exists, and it produced this, on his own effluent target of DAO 2016-08 Class C:
- At a weak domestic strength, around 250 mg/L of BOD: about 108 cubic metres a day.
- At the strength written on his consultant's plumbing sheet, 400 mg/L: about 97.
- Restaurant-led, around 550: about 79.
- At the stronger influent set in the owner's own photographed specification, 700: about 68.
- Heavy food, around 900: about 59.
The same concrete treats either 108 cubic metres a day or 59, a difference of almost two to one, and nothing about the box changes across that range. What changes is who is inside the building. I have never found a cleaner illustration of the point that the design is where the cost of an STP is decided, because here the design is finished in concrete and the cost of the decision lands on the owner's leasing department.
Every kitchen he signs spends capacity he cannot see
The ground floor of that building has fourteen leasable units and six of them are marked food and beverage on the architect's own plan. The owner talks about three or four restaurants. Those are not the same building.
A restaurant does two things to a treatment plant at once. It adds flow, which everybody expects, and it adds strength, which almost nobody prices. A kitchen's wastewater carries several times the organic load of an office's at the same volume, and it brings fats, oils and grease with it, which is a separate problem with its own failure mode and needs interception at source, on each kitchen line, outside the treatment tank rather than a chamber inside it. On a hard-water supply you then get a calcium soap crust in the interceptor that makes the problem look like a dosing failure when it is a water problem, which is why commercial kitchen softening sometimes turns up inside what started as a wastewater conversation.
So the sentence I gave the owner was this: your treatment capacity is a quantity you spend when you sign a lease, and nothing on the lease says so. A sixth kitchen does not arrive with a bill from the plant. It arrives as rent, and eighteen months later it arrives as a failed sample. If the tenancy mix is genuinely undecided — and his is; he said so himself — then the honest thing is to state the curve, let him see where his likely mix lands on it, and tell him which point on that curve the plant is being designed to and why.
The parameter that set the volume was not BOD
One thing worth flagging rather than re-arguing, because the post before this one makes the case in full: the aeration volume on this job is set by ammonia, not by organic load. The train barely moves across a BOD range of 500 to 800 on buildings like this one, because what governs is the media surface area the nitrifiers need. Nitrifiers are slow, and they are last in the oxygen queue, so they need residence time the carbon-eaters do not, and a design exercise that varies only BOD will miss the column that actually decides the plant.
Which means the capacity curve above has a second axis I did not plot for him, because it would have obscured the argument rather than sharpened it. Nitrogen varies with the tenancy mix too, and not in step with BOD. Where that lands you is a different set of constraints — the nitrate limit downstream of the ammonia you have just solved, whether the water carries enough carbon to destroy that nitrate, and the limits themselves, which are set out in full on our compliance page and which moved for ammonia and phosphate but not for nitrate. Our ammonia removal page and the nutrient field guide cover that ground for anybody already holding a failed certificate.
Hybrid was what made it fit, and that was a volume decision
At the design point we settled on, a pure fixed-film aeration stage wanted about 43 cubic metres. A hybrid arrangement — suspended biomass and FBBR-style fixed-film media in the same tank, so the carbon load is carried in suspension and the nitrifiers are held on the media — wanted about 32. Eleven cubic metres, in a box with 155 of them, is the difference between a plant that fits with margin and a plant that fits exactly.
I want to be careful about how that reads. We did not choose the technology because it was elegant. We chose it because the volume was already fixed and that choice bought back floor area we could put somewhere useful. On a greenfield site where the tank is still a drawing, the same comparison would be made on operating cost and on what the plant is like to run rather than on cubic metres, and it might well come out differently. A retrofit and a new build are two different jobs, and a half-poured box is closer to a retrofit than anybody wants it to be.
The margin was placed, not left over
The chamber schedule came to about 160 cubic metres built against a process minimum of 117 — a two-row layout, each row 3.275 metres wide, with the flow doubling back on itself. That leaves something over 43 cubic metres of margin, and the temptation is to call it spare and move on. Spare volume in a sewage plant is not a virtue; it is an unspent decision. So each cubic metre of it was put somewhere on purpose:
- The aeration tank got about 36 per cent over its minimum, because ammonia is the parameter that decides this plant and the one most likely to be worse than assumed.
- Equalisation was taken out to roughly fifteen hours, which is not a buffer for a daily peak — it is a buffer for a wedding. The third floor of that building is an events venue, and an events venue does not have a shape of day, it has a shape of Saturday.
- Sludge holding went to about nineteen days, which roughly halves the number of desludging trips. Everything you take out of a plant has to go somewhere, and the volume that decides how often a truck comes is bought once in concrete and paid for monthly otherwise. A cone reading, not a calendar, is what should call that truck.
A clarifier in an underground tank has its own constraints, and a shallow box tightens them considerably — that is a note of its own and I will write it separately. The short version is that the depth you are given changes which clarifier details are even available to you.
What an owner can do with this before he pours
If you are holding a box and a question, four things are worth doing in this order, and none of them needs us.
Measure the box yourself, inside the walls, and measure the depth to the underside of the slab above. Every number in this note is an arithmetic consequence of those two measurements, and an owner working from the architect's outside dimensions will believe he has about a quarter more plant than he has. Then write down the tenancy mix you are actually likely to sign, not the one in the leasing brochure, and count the kitchens. Then ask whoever is designing your plant for the capacity at your mix and at one mix stronger, as two numbers, because a single number hides exactly the thing you need to see. And last, ask which point on that curve the plant is being designed to, in writing, on page one — the same way the end goal of the plant belongs on page one.
Our STP price estimator and wastewater design generator will both take a flow and a strength and give you a first shape for free, and the parameters that matter by industry are worth reading before deciding what your own mix produces. If you have no influent result at all, almost nobody does, and that is not the obstacle it looks like.
Where this does not transfer
The method travels. The curve does not, and I would not want anyone reading five numbers off this page and applying them to their own box.
They are tied to one depth, one effluent class and one process train. A deeper box changes the oxygen transfer and therefore the air, and a shallower one changes which clarifier arrangement is possible at all. A different discharge class moves every chamber: which class your outfall actually discharges to is the first thing to establish and it is our work rather than the owner's. A building with reuse in the scope needs treated-water storage sized off the use pattern, which is volume that has to come out of the same box. And a plant designed close to the edge of its box is a plant with no room for the thing nobody predicted — a blower that has to be enclosed for noise, a vent that has to be routed properly, a dewatering unit a specification demands late. Who will be standing in front of the plant in three years is a design input too, and a tight plant is an unforgiving one.
If the walls are already up on your own site, that is not a problem — it is just a constraint, and constraints are easier to design to than guesses. What we do on the wastewater side is set out on our main page, the plants we have finished are mostly within a couple of hours of Cordova, wastewater treatment in Cebu and what an STP contractor should be expected to carry cover how we work, and operation and maintenance is the right starting point if your plant is already running and capacity is the question. If you want the box looked at properly, getting an STP and our consultancy work set out what that involves, and the proposal request form is the quickest way to put the numbers in front of us.