They Said They Wanted to Reuse the Water. We Had to Ask What For.

A client tells us he wants to reuse the treated water. Almost every enquiry we get now has that sentence in it somewhere. It sounds like one requirement. It is four, and they are not close together. The plant that lets you water a garden and the plant that lets you flush toilets on the fourth floor are different plants, with different equipment, different running costs and different things that can go wrong on a Tuesday afternoon.

So before we draw anything, we ask what the water is actually going to be used for. Not because we are being difficult. Because that one answer decides more of the design than the flow figure does.

"Reuse" is four different plants

When we work through this with an owner we put the uses in the order we have found them to sit, from the easiest to satisfy to the hardest: gardening, general washing, cooling tower makeup, toilet flushing. We design to one of those rungs. We do not design to "reuse" as a word, and we write in the quotation which rung the plant buys, so nobody discovers eighteen months later that the reuse they had in mind was never in the scope.

The reason we are strict about this is not paperwork. Treating above the rung the client actually needs is money spent on quality that nobody will ever use, and it does not stop at the purchase — it shows up every month in power and chemicals. Treating below it is worse, because the plant passes its compliance test, the owner is told he has reuse water, and then the first thing it touches goes wrong.

Both of those mistakes are made at the drawing stage, which is where the cost of a plant is really decided.

The lowest rung is the one people over-treat

Garden and landscape irrigation is the easiest use to supply and the one we most often see specified far past what it needs. Secondary treatment and disinfection get you there. What matters is the things that hurt plants rather than the things that fail a lab test: chlorine residual carried into the irrigation line, and salinity if the raw water was already hard.

And here is the part that turns the usual logic upside down. The nitrate and phosphate you spent money removing are fertiliser to a garden. We have priced jobs where the client wanted full nutrient removal and then wanted to irrigate with the result. If the discharge permit requires those numbers, fine, they are not optional — but if the water is leaving through a sprinkler and not through an outfall, we say plainly that we are building a nutrient removal stage so it can be thrown at a lawn. Sometimes the answer is still yes, because the plant also has to be able to discharge. Often the answer is that the nutrient stage gets sized for the discharge case and is simply not run when the garden is taking the water.

Chlorine is the other one. A dosing set point that suits a discharge point is not automatically a set point that suits a hedge, and the fix is usually a contact time and a draw-off position, not another chemical. We would rather solve it with the tank layout, because we do not like designs that depend on a chemical staying available.

Washing down is judged by eye, not by a lab sheet

The second rung — floor washing, vehicle washing, external areas, back-of-house cleaning — is where the complaint stops being analytical. Nobody brings us a lab result. They say the water looked cloudy, or it left a mark on the tiles, or the wash-bay smelled at two in the afternoon.

So this rung is about solids and about storage, not about biology. Tertiary filtration after the clarifier, and a residual that survives sitting in a tank. The failure we see is carry-over: a plant whose clarifier is doing fine on average sends a slug of solids through on the morning peak, and because the reuse tank is downstream of everything, that slug is what someone hoses the floor with. If that sounds familiar, the diagnosis usually is not the clarifier itself — the clarifier is where the problem shows up, not where it starts.

On kitchen-heavy buildings there is one more thing we look at before anything else, which is the water coming in. If the incoming supply is hard, the grease side of the plant behaves differently and so does anything you wash with the reuse water — we wrote about the crust in the grease trap being soap rather than grease, and the same chemistry leaves a film on a wet floor. In a few of these buildings the honest answer was a softener on the incoming line rather than anything added to the treatment plant.

The cooling tower is where the treatment level jumps

This is the rung that gets underestimated most often, and it is the reason we ask the question at all.

A cooling tower does not care about BOD. It cares about what concentrates. It evaporates water and keeps the dissolved load behind, so whatever you send in is multiplied several times over inside the tower's circuit. Hardness scales the fill and the tubes. Phosphate feeds biofilm and interferes with the tower's own treatment programme. Ammonia is the one that catches people, because ammonia attacks the copper alloys that cooling circuits are full of, and a few milligrams per litre in the makeup becomes a real number once the tower has cycled it up.

So a plant feeding a cooling tower needs its nitrification to be genuinely reliable, not just good enough to pass. That is a design decision with a cost attached — more aeration volume, a longer sludge age, and the alkalinity to support it. The nutrient field guide has the readings we actually take on site for this, and the arithmetic behind the carbon needed if the nitrate has to come out too, which follows the ratio we divide before anyone buys methanol.

There is a second half to it that has nothing to do with the sewage side. Reuse water going to a tower still has to be conditioned for the tower, and that is a filtration and water-treatment question sitting downstream of the treatment plant. We have seen this split fall down the gap between two suppliers, each assuming the other owned it. When we quote a cooling-tower reuse job we state which side of that line we are on.

Flushing puts the water back inside the building

Toilet flushing is the top rung in our order, and not for a technical reason you would guess from the numbers. The numbers are not extreme. The difficulty is that the water goes back inside the building, where people see it and smell it, and where it sits.

Three things have to hold. It has to be visually clean every single time, because a tinted bowl on the third floor becomes a complaint the same morning. It has to hold a disinfectant residual through storage, since flush demand is nothing at three in the morning and everything at eight, so the water can sit in a roof tank for hours. And the plumbing has to be separate and marked, which is a decision for the building's designers rather than for us, and is cheap while the building is being built and expensive afterwards.

We have been asked more than once whether the water is safe, usually phrased more bluntly than that, and we answer it honestly. For flushing, the treatment we would put in is secondary treatment, tertiary filtration, and disinfection we can hold — and where a client wants more certainty than that we use our own advanced oxidation, which for us means ozone catalysis, with a membrane stage behind it if the case really calls for one. That is the top of the ladder and it is priced like it.

The reuse decision changes the tanks, not just the equipment

The part that gets missed in the equipment discussion is that reuse changes the concrete, and concrete is the expensive half of most of these jobs. You now need a treated-water storage tank, and its size comes from the use pattern, not from the daily flow. A garden takes its water in roughly an hour in the morning. A cooling tower draws all day. Flushing follows the building's occupancy curve. Those three give you three different storage volumes for the same plant, and at roughly fourteen thousand pesos per cubic metre of tank capacity in our own recent civil works, that is not a rounding error.

It also changes where the tanks go, which is worth settling early because a chamber added late is the most expensive chamber on the job. We lay these out as one blocked structure with shared walls rather than separate tanks, for reasons we went through in the post on how the civil works decides the footprint. A reuse tank bolted on after the slab is poured usually means breaking something.

We keep the discharge line no matter what

Every reuse plant we design still has a compliant discharge route, sized and permitted as though the reuse did not exist.

The reason is ordinary. It rains for a week and the garden wants nothing. The building is at a third of occupancy and flush demand is small. The tower is off for maintenance. On those days the plant is still producing water and it has to go somewhere, and it has to go there legally, at the quality the receiving water's class requires. An owner who treated reuse as a way to avoid the discharge permit has bought himself a problem in the first wet season — the class of the water body you discharge to is what sets the limits, and reuse does not move it.

Practically this means the reuse quality target is the stricter of the two duties — the rung the client wants, and the discharge limits — parameter by parameter. Not one or the other.

What this costs to run

The operating cost is the part owners feel, and it tracks the rung. A fixed-film train of the kind we build most often runs at around twenty-nine pesos per cubic metre in the numbers we have published before. Put advanced oxidation in the line and it is nearer ninety. Both of those are from real contracts we have sent out, and both come with the same caution: your site will differ. Level of filtration, volume, incoming water quality and site conditions all move it, and the gap between those two figures is exactly why we want to know which rung before we price anything.

If you want to see roughly where your own building lands, the price estimator and the design generator are on the site, and the parameters that matter for your industry are listed on the industry parameters page. What those tools cannot do is choose the rung for you. That is a business decision about what the water is worth to you, and it belongs to the owner, not to us.

We also keep the running cost honest against the purchase price, because the two trade against each other — the cheaper quotation is often not the cheaper plant, and reuse is one of the places that shows up fastest.

Where this stops being transferable

All of the above is what has worked for us, on our jobs, in this climate. It will not all carry.

Our incoming water in parts of Cebu is hard, which is why softening comes up in these conversations more than it might elsewhere. A building at a fifth of its design occupancy behaves nothing like one that is full, and reuse on a half-empty building means a storage tank that goes stale rather than one that turns over. The technology on the drawing matters less than whether the plant is being run at the load it was designed for, and a plant that is comfortably meeting a discharge limit is not automatically one rung away from reuse — sometimes it is three. And none of it survives an operator who was not trained on the reuse side, which is the part of the job that outlasts us.

What we would say is only this: decide the use first, in plain words, before anybody draws a tank. We have never regretted asking that question early, and we have been called in to fix the result of it being asked late more times than we would like. You can see the kinds of plants this has produced on our completed projects, and how we scope a new one on get an STP or through a consultation. We work across Cebu and the wider Philippines, and the way we size and specify a plant is set out in our notes on the drawing stage. The arithmetic of whether reuse pays at all, separately from how you build for it, is in the earlier post on when reuse pays.

If you are weighing this up for your own building and want to talk it through with someone who will tell you which rung you actually need, book a free 15-minute consultation.