The Cheaper Quotation Was Not the Cheaper Plant

A building owner in Cebu put two quotations side by side on the table and asked me which one was cheaper. One was about a fifth lower on the bottom line. He had already made up his mind; he wanted me to agree with him.

I told him I could not answer that from the cover page, and neither could the other supplier. What is printed on a quotation is what the plant costs to buy. He was going to own it for twenty years.

That conversation happens more often than any other in our work, and it is usually the one that decides whether a client is still happy with us in year three. So this is how we cost a plant, and what we have learned about which parts of a wastewater treatment system are actually expensive.

We cost it per cubic metre, because that is the number he pays every month

We take everything the plant consumes in a month — power, sludge hauling, laboratory work, chemicals if any are needed, and the operator's time — and divide it by the cubic metres the plant actually treated in that month. Not the design capacity. The volume that really went through.

Two of our own figures are already public. On a plant treating to a reuse-grade standard with fixed-film biology and filtration, we were running at roughly ₱29 per cubic metre. On a job where the end use demanded an advanced oxidation train ahead of the reuse point, the same arithmetic came out near ₱90. We wrote both of those down in our notes on when water reuse actually pays.

Those are our numbers from our sites. Your building is not that building. The level of filtration you need, the volume you run, your tariff and your influent all move that figure, and they can move it a long way in either direction.

What is inside the number

Power is the first line, and on most plants it is the biggest. Sludge hauling is the second. Third-party laboratory testing is the third. Chemicals come fourth, and often they are zero. The operator's time is the one nobody puts in the spreadsheet and everybody pays.

In our quotations we always show sludge hauling and third-party laboratory as estimates, never as firm figures, because both depend on things outside the plant — how far the truck comes from, when it is available, what sampling schedule your permit and your pollution control officer settle on. Alkalinity correction and coagulant are shown as an estimate and marked if required, because they are only spent if the influent actually calls for them. We would rather a client find no surprise line in month four than find a low number in the quotation.

Sludge is the line people argue with us about most, usually because someone told them their plant would not produce any. Every biological plant produces sludge; the only question is where it accumulates and who takes it away. We wrote out where the sludge actually goes rather than repeat that claim.

The blower is the line that moves

Here is the arithmetic we do in front of the client, on his own tariff, because it is the one piece of the cost he can check himself.

A 2.2 kW blower running twenty-four hours uses about 53 kilowatt-hours a day. On a plant genuinely treating 30 cubic metres a day, that is about 1.8 kilowatt-hours per cubic metre before anything else is counted. Multiply by whatever you pay per kilowatt-hour and you have the dominant part of your running cost.

Now change one thing. Suppose the building is only sending 12 cubic metres a day into a plant sized for 30, and the blower still runs twenty-four hours because nobody changed anything at handover. The same 53 kilowatt-hours now spreads over 12 cubic metres — about 4.4 per cubic metre. The plant did not get worse. The loading did.

This is why we do not believe a per-cubic-metre figure quoted as a property of a technology. It is a property of the plant and the building it serves. It is also one of the practical reasons the number you declared on your discharge permit matters after commissioning and not just before it, and why a 5 cubic metre a day plant is harder to run than a 500.

Where capital money buys running money back

An equalisation tank is concrete, poured once. What it buys is a plant that receives its load at an even rate instead of in a lunchtime surge, so the blower is sized for an average rather than a peak and the clarifier is not asked to do something hydraulically impossible twice a day. It is the cheapest running-cost reduction we know, and it is the first thing value engineering deletes. What happens when it is missing usually shows up at the clarifier, which we described in the clarifier is not the problem, it is where the problem shows up.

A grease interceptor, sized properly and maintained, is the other one. Fat that reaches the aeration tank is paid for twice — once in the air needed to break it down, once in the sludge it puts in the truck. On hard water it is worse than fat, because the crust is calcium soap rather than grease, and no amount of dosing touches it. In kitchens where that is the pattern, the cheapest fix sits before the wastewater line entirely, in a softener on the hot water.

FBBR-style fixed-film media in the aeration tank is capital too. What it buys is biomass that is not travelling to the clarifier, so the clarifier has less to carry and the plant holds on better through a bad week. We first retrofitted it on a food plant that would not stabilise, and we have used it since.

And there is one we often argue against. A duty-standby blower is real insurance — a failed compliance result plus a re-test is a known cost, and on a large plant the arithmetic makes the second blower obvious. On a small plant with a spare unit sitting in the store and a technician an hour away, we would rather the client kept the money. We set that out when we wrote about the blower that stopped for three days. Every component in our quotations has to justify itself against a financial benefit, and sometimes the honest answer is that it does not.

Where we deliberately do not spend

We design to the client's end goal and no further. If the goal is compliance, we reach compliance. If the goal is reuse, we treat to the rung of reuse that will actually be used — gardening, general washing, cooling tower make-up, toilet flushing — and we say in the document which rung the design buys. Treating above the rung is quality nobody consumes, paid for every month in power and consumables. That is the same argument we made about design deciding the cost long before construction starts.

The receiving water class works the same way. At small flows the class barely changes the bill; at large flows it changes it by millions, which we worked through in at 5 cubic metres a day the class barely matters. Establishing the class is our job, not the client's, and the limits it lands you under are set out on our DENR discharge standards page. What the class costs you depends on which parameters your industry actually carries, which is what the parameters by industry list is for, and on whether you end up needing ammonia, nitrate or phosphate removal — the three that carry most of the running cost when they are needed. Our nutrient field guide has the readings we take before we decide.

The small plant is the one that reads badly

A contract we sent for a 5 cubic metre a day system came to ₱850,000. Per cubic metre over its life it will always read worse than a large plant, and there is no design trick that changes it. The blower, the panel, the monthly operator visit and the quarterly laboratory bill do not shrink in proportion to flow. A plant a hundred times larger does not cost a hundred times more to run.

That price is what we sent for that site. Yours will differ with the level of filtration, the volume, the ground conditions and what is already built. If you want an early figure to think with before you talk to anyone, our STP price estimator and the design generator will give you one in a few minutes, and you can put them alongside the plants on our completed projects page.

The one number we will not put in front of you

We do not present a ten-year cost of ownership. We have tried; it makes the decision harder rather than clearer. It asks a person to accept a projection about electricity tariffs and hauling rates a decade out, and it buries the only figure he can actually verify — what the plant costs him this month, per cubic metre, against his own bill.

If a supplier shows you a ten-year total, the useful question is not whether the total is right. It is which lines are in it, which are estimates, and which were left out. That is also the spirit of the four questions we would ask before signing, and part of why we are open about the conflict created by both designing and building.

What does not transfer

Our figures are what our plants did on our sites, with our tariffs, our hauling distances and our operators. Change the influent and the whole arithmetic moves — and almost nobody has an influent result to give us, which we wrote about in we ask for the influent result, almost nobody has one. Change the operator and it moves again; a man who reads the cone every morning and wastes sludge on what he sees will beat a better-specified plant run by somebody who wastes on a calendar. A good operator cannot rescue a bad design, but he changes the monthly bill more than most equipment choices do, which is part of why turnover shows up as a plant problem. Keeping that consistent is most of what our maintenance and operation work is.

So take our numbers as ours. Do the blower arithmetic on your own tariff and your own real flow. If the answer is far from ours in either direction, that is information about your site, not an error.

If you want us to look at what your plant costs you to run, or to cost one you have not built yet, book a free 15-minute consultation. We will do the per-cubic-metre arithmetic with you on the call. If you would rather send drawings and figures first, the get an STP page and our consultations page are the two routes in, and if you are here in Cebu, our wastewater treatment in Cebu page has how we work locally.