The Drawing Took Longer Than the Build. Here Is What We Were Deciding.

A client asked me once why the drawing took two weeks when the plant itself went up in a fortnight. It is a fair question, and the honest answer is that the fortnight was never the part I worried about. Pouring tanks, setting blowers, running pipe — that work is real, but it is the easy part. The drawing is where we spend the client's money. Once the ink is dry, most of what the plant will cost to build and to run for the next fifteen years is already decided. So we take our time on the page and we move fast on the ground.

These are field notes on how we actually size and specify a sewage treatment plant — not a method anyone should copy line for line, because every site hands us a different problem, but the order we think in and the places we have learned to slow down.

The first thing we decide is not the technology. It is the end goal.

Before we size a single tank we ask what the water is for after we have cleaned it. If the answer is basic compliance — discharge to the receiving water and nothing more — then we design to the discharge class and we stop there. If the client plans to reuse the water, we treat only to the level that reuse actually needs. Gardening and general washing sit at one rung. Cooling-tower make-up sits higher. Toilet flushing higher again. Each rung is a different tertiary train and a different bill.

We are deliberate about this because treating above the rung the client will use is money spent on quality nobody touches. Someone who only wants to water a lawn does not need water polished for a cooling tower. We would rather write the smaller design and tell the client plainly what it does and does not buy, so that nobody discovers a year later that the reuse they pictured was never in the scope. When reuse does pay, it pays on arithmetic most people do not run first, and when it does not, we say so.

The number we least often have is the influent.

Here is the uncomfortable part of designing plants in this market. We ask every client for a lab result on their raw wastewater — the strength of the thing we are being asked to treat — and almost nobody has one. Nine times in ten there is no influent analysis at all. So we assume it. There is no other way to draw a plant for a building that has not opened yet.

And assuming has a price the client pays in concrete. Every assumption we make, we have to make safely, and safe means bigger. If we are not certain how strong the wastewater will be, we design for the stronger case, because a plant sized for water dirtier than it turns out to be still passes — it just cost more than it needed to. A plant sized for water cleaner than it turns out to be fails, and then we are back on site. So uncertainty does not disappear when there is no data. It gets bought, in tank volume and steel, by the owner.

That is why we push for samples even though we rarely get them: more information means a tighter design, and a tighter design is a smaller, cheaper plant. When a client can give us even a few grab samples composited across an operating day, we can stop designing for the worst case we can imagine and start designing for the case in front of us. The strength of the wastewater also depends heavily on what industry it comes from — a canteen, a dressing plant and a dormitory do not produce the same water — so where we have no lab sheet, the industry and the headcount are the next best thing to guess from.

We size to a band, not to a single number.

Even when we are given a figure, we do not design to it as if it were exact. If a client tells us the BOD is 300, we will usually draw the plant to hold 350 or 400. Real wastewater is not a steady number — it swings across the day, across the week, across a kitchen deep-clean or a shift change. A design that only works at the average will fail on the peak. So we build in headroom and we hold the peaks back with equalisation rather than pretending they are not there. One drawing decision made carelessly here — running two very different waste streams into one line, for instance — can hand a plant a problem it never needed.

Flow gets the same treatment as strength. The number a client declares is not just an engineering input; on the discharge permit it becomes the ceiling they are held to. We would rather have that conversation on the page than after the plant is running.

The calculations are the part I worry about least.

People assume the hard part of design is the mathematics — the loading rates, the oxygen demand, the sludge age. In fairness, that used to be true, back when we did not have the tools we have now. Today the arithmetic is close to a solved problem. We check our own numbers against software and, honestly, against a chatbot — I tell clients to check our design with ChatGPT and I mean it. If the sizing is right, a second opinion only confirms it.

What the arithmetic cannot do is choose the right equipment, and it cannot commission the plant. Those are the two places the real value sits, and neither is a formula. Choosing the correct specification for a blower or a pump, and then knowing what to do next based on the results we are getting on site, the smell we are smelling on site — that judgement is worth far more than the loading calculation, and it is the part that took us years to earn. It is also, not by accident, the part a good drawing protects and a bad one exposes, because an operator can carry a plant but cannot carry a bad design.

We specify for the day the honeymoon ends.

When we pick equipment we are not choosing what works on commissioning day. We are choosing what still works in year three, when the enthusiasm has worn off and whoever operates the plant is doing the minimum. We design around after-sales support — we try not to put anything into a plant that will give us, or the client, a headache later.

Dosing pumps are the honest example of a problem we are still working on. They fail faster than anything else we install — some last barely a year — and part of the reason, I suspect, is that they are often left out in the open when they should be sheltered. We have not fully solved it. I would rather write that here than pretend our specification is finished, because the client who reads it will keep a dosing pump running longer than the client who assumes it is maintenance-free. The same instinct is why we do not build in proprietary media or chemicals that lock a client to one supplier — a plant that only we can feed is a plant that has traded one problem for another. That thinking runs right through how we handle maintenance and operation, and it is one of the four things that separate how we work as a company from a catalogue sale.

Some things we have simply standardised.

Not every decision is fresh on every job. The clarifier is one we settled a long time ago and now carry to every underground tank we build. It is rectangular, poured as part of the tank with every other chamber, not circular — a round clarifier wastes the space around it, and even drinking-water plants use rectangular ones. Instead of a fabricated hopper we cast a mass-concrete cone on the flat slab, all four faces falling inward to a central sump whose floor is the structural slab itself. Nothing drops below the foundation line. If concrete can do the job, we would rather not introduce a separate insert to fail later.

What we do not put in an underground clarifier is a weir plate or a scum board. On an above-ground tank they earn their place. Underground, once the cover slab is on, neither can be levelled or skimmed without a person entering a confined space — and a fitting that can only be serviced by confined-space entry is a fitting that will not be serviced. So we draw a submerged draw-off through the cross wall instead, and a sludge pump in the sump, and we leave out the parts that look right on paper but cannot be reached. This is the kind of choice that never shows up in a headline price and decides how the plant ages. It is also why the clarifier is usually where a problem shows up rather than where it starts.

What the design costs, and what we tell the client before they sign.

We are willing to put numbers on the page, with the caveat that every one of them moves with the site. As a rough benchmark we price a tank at around ₱14,000 per cubic metre of capacity in a 3,000-psi mix, which is more than enough for the duty. On one recent job a 70,000-litre three-chamber tank with a heavier top slab came to about ₱1.2 million. Your site will differ — the level of treatment, the volume, the ground conditions and the reuse rung all move the figure — which is exactly why we build a price estimator and a design generator that let an owner see the shape of the number before they ever talk to us. Running cost separates the same way: a plant leaning on FBBR-style fixed-film media can sit near ₱29 per cubic metre where an advanced-oxidation train runs closer to ₱90, and the cheaper quotation is often not the cheaper plant.

Whatever we assume, we write it down where the client can see it before they commit. The risks and the assumptions belong in the quotation and the presentation, not in our heads — a client has the right to know every one of them before they sign, including which reuse rung the design buys and which parameters we sized against. If we assumed an influent strength because there was no sample, that assumption is on the page. It is the same honesty we bring to the fact that we both design and build, which creates a conflict we would rather name than hide. There are a few questions worth asking any designer before you sign, and most of them are really asking whether the assumptions were disclosed.

Where this stops being transferable.

None of this is a recipe. The band we design a canteen to would starve a plant sized for a dormitory, and the clarifier detail we standardised for underground tanks is the wrong answer above ground. A design that reuses to cooling-tower quality is a different plant from one that only waters a garden, even for the same building and the same flow. The reason we can size confidently is not that we have a universal formula — it is that we have built and commissioned enough of these to know where our own assumptions usually sit, and we have watched enough of them from the smallest packages to full plants across Cebu to know how far a guess can be trusted. Your site is not our site. What we can offer is that the way we arrived at our numbers is visible, and you are welcome to test it.

If you are sizing a plant now, or holding a design you are not sure about, book a free 15-minute consultation and we will walk through what your site actually needs — and, just as usefully, what it does not.