A water plant is a chemical building with a wet floor
A water or wastewater treatment plant is largely made of concrete: basins, channels, clarifiers, digesters. The steel buildings on the site are small and specific — a chemical store, a dosing house, a blower house, a control building, a sludge handling shed.
Those small buildings carry a disproportionate share of the plant's corrosion problems, because that is where the chemicals are.
The chemical store is the hardest room on site
Chlorine, sodium hypochlorite, ferric chloride, alum, lime and polymers all end up stored and dosed in the same area. Some are oxidising, some are acidic, some are alkaline, and all of them attack steel when they are wet.
Where a store holds more than one chemical, the separation between them is a safety requirement and also a corrosion requirement: a spill of one chemical onto the residue of another produces something more aggressive than either.
Bunds, separate compartments and a floor with falls to a collection point are the physical expression of that. For the building, the steel should sit on plinths and the coating should be selected for the specific chemicals rather than for "chemical resistant".
Dosing areas are wet continuously
Dosing pumps leak a little, day after day. Sampling points drip. The floor is washed. The result is an environment like a small chemical plant, in a room the size of a garage.
Ventilation is the other half: hypochlorite releases chlorine, and an enclosed dosing room without extraction becomes a hazardous space rather than a working one.
Blowers are noisy and they vibrate
A blower house for aeration or backwash contains machines running continuously. They are noisy, they vibrate, and they need air for cooling.
Acoustic treatment and ventilation both want wall area, and both add weight. It is worth deciding on the noise limit before the wall construction is chosen, because a blower house that fails a noise limit is expensive to fix afterwards.
Humidity is high everywhere near water
Even the buildings that do not handle chemicals sit above basins and channels. The air is saturated most of the year, and any steel surface below the dew point will be wet.
That affects the control building and the electrical rooms as much as the sheds: a switchroom with a saturated atmosphere and a steel envelope needs a vapour barrier and dehumidification, not just a coating.
Small buildings are the ones that get designed last
On a treatment plant the civil works dominate the programme and the buildings arrive as a package near the end. By then the pipe routes, the cable routes and the access roads are fixed, and the buildings are fitted into what is left.
That is where a small steel building is genuinely useful: it can be adapted to the space that remains, provided the chemical and ventilation requirements were defined early. If they were not, the building is adapted to the space and not to the process.
What I want before drawings
- The chemical list with concentrations and quantities stored.
- The separation required between chemicals, and the bund arrangement.
- The ventilation requirement for each room.
- The noise limit for blower and pump houses.
- Which buildings need to be dehumidified or air conditioned.
- The wind speed for the site in km/h from the applicable local code, not a round number.
Treatment plant buildings are small, cheap and easy to get wrong. The two questions that prevent most of that are what is stored and what leaves through the wall.
Written by Johnny Joe, who advises owners and project managers in Africa and South America on steel buildings. If this decision is live on a project, send the layout and the constraint that worries you and I will tell you what it changes.
Related: corrosion class as a selection input · separating a chemical store · a small technical building · a floor added inside a shed
