I Both Design and Build Sewage Treatment Plants. Here Is the Conflict That Creates.

A client sent me three bids side by side last year and asked a fair question: why is one of these almost half the price of the other two, for the same building?

I looked at the three proposals. Same number of residential units. Same estimated flow written on the cover page. Three completely different plants underneath.

The cheapest one had an aeration tank a little over half the size of mine. That was the whole difference. Not better equipment, not smarter engineering, not a leaner company. A smaller tank, quoted against the same building, by a company that would also be the one to build it and then walk away from it.

This is the thing I want to write about, and I am aware of the awkward position it puts me in. Our company designs sewage treatment plants and our company builds them. The conflict I am about to describe is one I sit inside every working day.

Where the conflict actually lives

People assume the risk in an STP contract is in the equipment list. It is not. By the time you are comparing blower brands you are already past the decision that matters.

The decision that matters is the design basis: what flow the plant is sized for, what organic load is assumed in that flow, which discharge parameters it must meet, and what safety margin is carried against a bad day. Every peso in the quotation follows from those four things.

And in most projects in this country, the person who writes the design basis is the same person who profits from it. That is the conflict. It does not require anybody to be dishonest. It only requires the ordinary human tendency to make an assumption that happens to be convenient.

It distorts in two directions, and I have seen both.

Downward, to win the job. If you are bidding against three other companies and the client is comparing on price alone, the fastest way to be cheapest is not to negotiate harder with your fabricator. It is to assume a lower design flow, or a lower influent BOD, or a shorter retention time. Nobody in the tender room can see you do it. The tank simply comes out smaller and the price comes out lower and you win.

Upward, to pad the contract. The opposite also happens, usually where the client has no technical person at all and the seller has a large catalogue. Stages get added that this water does not need. A tertiary process appears for a discharge that is going to a sewer line. Instrumentation goes in that nobody on site will ever read.

Both distortions come from the same root: the only person who knows whether the numbers are honest is the person being paid according to those numbers.

The arithmetic that the tender table hides

Let me show you the size of what is being decided, because it is not a detail.

Take a residential building at roughly 150 cubic metres a day of sewage, at a typical influent BOD of about 300 milligrams per litre. That is 45 kilograms of BOD arriving at the plant every day. That number is not an opinion. It is the food the bacteria have to eat.

Now, how much aeration volume do you need to eat it? That depends on how hard you decide to work the bacteria — the food-to-microorganism ratio — and how thick a population you intend to carry. Design it at an F/M of about 0.15 with mixed liquor around 3,500 milligrams per litre, which is a comfortable extended-aeration range, and you need roughly 86 cubic metres of aeration volume. That is about fourteen hours of retention.

Design the same building at an F/M near 0.26 and you need about 50 cubic metres. Eight hours of retention. Thirty-six cubic metres of concrete and steel disappear from the quotation.

Both plants can be made to pass a lab test on a good day with a careful operator and a steady load. Only one of them survives a full function hall on a Saturday, a two-week stretch where sludge could not be hauled, or a cleaning chemical going down a drain at midnight. The second plant has no margin. It is not a cheaper plant. It is the same plant with the safety factor sold off. If you want an independent figure to hold the bids against, our estimator will size and price one for your flow.

And here is the part that should bother you: none of that appears anywhere in a bid comparison table. The tender document says 150 cubic metres a day on both. The difference is buried in an assumption neither bidder was asked to state.

The building I was called to look at afterwards

I was asked once to review a plant a few years after it was built, in a commercial building whose occupancy had grown the way commercial buildings do. Ammonia was failing. The owner assumed the plant had aged, or the operator had gotten lazy, and had already replaced the operator twice. That pattern almost always means something else.

The plant had not aged. It had been correct for a building that no longer existed, and it had been correct with nothing to spare on the day it was commissioned. The design basis had never been written down anywhere the owner could find it. There was no line in the contract saying what flow it was built for, so there was no moment where anyone could say: you have now passed the number this plant was sold against.

The owner had no way of knowing that. He had bought a plant, not a design basis. He had a quotation that listed tanks and pumps and a lump sum. Nobody had ever handed him the four numbers that would have told him what he was actually buying.

Fixing it cost more than the original margin the low bid had saved him, which is the usual ending to this story.

So what do I do about it, since I am on the wrong side of it?

I do not think the answer is that design-build is corrupt and should be avoided. I have watched the split model fail badly too. When one company designs and another builds, and the plant does not perform, you get the worst meeting in this industry: the designer says it was built wrong, the builder says it was designed wrong, and the client sits between two invoices with a plant that still does not work. Single accountability is worth a lot. When it is our design and our construction, there is nobody for me to blame, and that is a healthy pressure to be under.

What I do not accept is the version where the client has no independent way to check anything. So we work like this.

The design basis goes to the client in writing before price is discussed at all — the design flow, the assumed influent load per parameter, the target effluent per parameter, the aeration volume and retention time, the assumed mixed liquor concentration, the expected sludge production and blower duty. Not as a technical appendix nobody reads. As the thing the quotation is answering. That is what we send when somebody asks us for a proposal.

Those numbers then go into the contract. If a plant is sold as capable of a certain flow at a certain load, that belongs in the agreement, alongside acceptance based on actual laboratory results after commissioning and a stated stabilisation period, not on a delivery receipt.

And we invite the check. Take our design basis to a consultant who is not selling you equipment. Take it to a professor. Take it to whoever built the last plant you were happy with. I have told clients to run our sizing calculations through an AI model and come back with the arguments — I would rather answer hard questions in a meeting than answer for a failed plant two years later.

An honest design survives being checked. That is the entire test.

What to ask for, whoever you buy from

If you are about to sign for a sewage treatment plant, you do not need to become an engineer. You need to make the assumptions visible, because assumptions that stay invisible are where your money leaks.

Ask every bidder to state, on one page, the design flow and the influent load per parameter their price is based on. Then put those pages next to each other. You will very quickly see which company is cheaper because it is efficient and which is cheaper because it assumed a smaller problem.

Ask what happens when the building exceeds that flow — not whether it will, but what the plant does when it does. A plant with FBBR-style fixed-film media in the aeration tank behaves very differently under overload than a plain suspended-growth tank, because a large part of the biomass is anchored on the media and does not wash out when the plant is pushed. That is a design choice with a cost, and you are entitled to know whether it was made and why.

Ask what the plant is expected to cost to run per cubic metre, and ask each bidder to show the electricity and sludge assumptions behind that figure. The cheapest plant to buy and the cheapest plant to own are frequently not the same plant.

Ask who signs the acceptance test, what parameters it covers, and how many months after commissioning it happens.

And if the project is large enough that a wrong answer will hurt, pay somebody independent to write the specification, then tender the specification. A separate design fee looks like an added cost. Against thirty-six cubic metres of aeration volume quietly removed from a bid, it is nothing.

Where I land

I am not going to tell you never to let one company design and build your plant, because that is what we do, and I believe we do it well. The rule I actually believe is narrower and harder to argue with.

Never let the company that profits from the numbers be the only party that has ever looked at them.

If your supplier will not put the design basis in writing, or gets uncomfortable when you say you want a second opinion on the sizing, you have learned something more useful about that company than any reference list would have told you. The tank size is the product. Everything else is fittings.

If you have bids on the table right now and you cannot tell why one is cheaper, send me the design basis pages and I will walk you through what the differences actually mean for your building. You can book a free 15-minute consultation with us. We are one of the wastewater companies in the Philippines that will happily tell you a competitor's number is the correct one, if it is.