Coagulant Dosing Room Design: PAC Storage, Make-Up and Feed Layout

Good coagulant dosing room design follows the chemical in one direction: delivery, storage, make-up, day tank, metering pump, injection point. No backtracking, and no crossing paths between people, forklifts and hoses. Get that sequence right on the floor plan and corrosion protection, ventilation, drainage and maintenance access fall into place much more easily. PAC, or polyaluminium chloride, is the most common coagulant in water and wastewater treatment. Its solution is acidic and corrosive, which drives many of the choices below. This guide is for designers working on water and wastewater plants and for operators planning to rebuild an existing chemical room.

For how the system components work together, see how a PAC dosing system works. Here we focus on coagulant dosing room design itself: the room and the equipment layout.

The process flow in words

A coagulant dosing room design drawing, read along the chemical path, looks like this:

1. Trucks arrive at the unloading area. Bagged solid PAC goes into the store, liquid PAC is pumped into a bulk tank.
2. Solid PAC is carried to the make-up tank and dissolved in water under mixing. Liquid PAC is transferred to a dilution or day tank.
3. Finished solution flows by gravity or is pumped to the day tank, which feeds the metering pumps.
4. Duty and standby metering pumps draw from the day tank. Each discharge has a back pressure valve, pressure relief valve and pulsation dampener.
5. Feed lines carry solution to the mixing chamber, static mixer or the head of the flocculation tank.

Show level gauges, overflows, drains and flushing water lines for every tank on the drawing. These are the lines most often forgotten in coagulant dosing room design, and the ones operators miss most.

Storage in coagulant dosing room design

Solid and liquid deliveries need different storage. Bagged PAC absorbs moisture, so the store must be dry, with bags on pallets rather than on the floor. Liquid PAC tanks sit inside a bund sized to hold a leak from the largest tank.

Place the store next to the delivery door and close to the make-up area to keep handling short. Size storage on consumption and delivery reliability; remote sites with irregular deliveries need more on hand. If bags are handled manually, leave room for a lifting beam or hoist. Carrying bags all day means more torn bags and more dust.

Make-up tank and day tank

In coagulant dosing room design, the make-up tank dissolves solid PAC and the day tank holds finished solution. Keeping them separate gives the pumps a stable concentration.

The make-up tank needs a mixer and acid-resistant construction, such as lined concrete, PE or FRP. Give make-up water its own line and valve, ideally with a water meter, so operators can hit the target strength.

Set the day tank outlet above the tank floor to leave a settling zone, so undissolved solids and debris stay out of the pump. Every tank needs a level gauge, overflow and drain piped to the trench, not onto the floor.

Metering pump area in a coagulant dosing room design

Put the pumps close to the day tank and below its lowest level for flooded suction. That avoids most priming trouble. Duty and standby pumps are normal practice, and the back pressure and relief valves are not optional.

Leave enough space in front of each pump to remove the head and change a diaphragm. Add a drip tray or trench under the pumps so spilled chemical has somewhere to go. Browse our diaphragm metering pumps, or for small plants consider a packaged dosing skid to cut site assembly.

Feed line routing

A coagulant dosing room design should run feed lines in a trench or on wall supports rather than buried, so leaks are visible. The pipe must resist PAC solution; see PAC dosing line material. On long runs, add drains at low points and vents at high points.

Floors, drainage, ventilation and safety

Any coagulant dosing room design needs a corrosion-resistant, non-slip floor sloped to a drainage trench. The trench and sump need the same protection, and walls are usually lined up to a skirting height.

Opening bags of solid PAC raises dust, so fit local extraction above the make-up area plus general room ventilation. Operators should wear goggles and acid-resistant gloves when handling the chemical, and the room needs an eyewash and safety shower. Keep electrical panels away from the make-up tank. Dust and acid mist attack electrical parts, so use a separate control room or at least a partition.

Mistakes that are hard to fix later

The first coagulant dosing room design mistake is putting PAC and polymer side by side. Polymer solution makes floors extremely slippery, so give each chemical its own area and piping. The second is frost: outdoor lines and tanks in cold regions need insulation and heat tracing. The third is no space for expansion, which leaves nowhere to add a pump when flow grows. For general room layout ideas, see chemical dosing room design for wastewater plants.

Conclusion

Effective coagulant dosing room design follows the chemical: store by the delivery door, separate make-up and day tanks, flooded suction with duty and standby pumps, and feed lines that are easy to inspect. Plan corrosion protection, drainage, dust control and electrical separation at layout stage and you avoid costly rework. Send us your plant flow, dose rate and delivery form, and contact our engineers to size the pumps and dosing system.

FAQ

Solid or liquid PAC for a small plant?

Liquid PAC is easier for small plants. There is no make-up step and little dust. Large users far from suppliers often save on transport and storage with solid PAC.

Can one tank do both make-up and storage?

On a very small system, yes, but dosing must pause or switch to a standby tank during make-up. For continuous duty, use two tanks.

Does a PAC room need explosion-proof design?

Usually not, but it does need corrosion protection, ventilation and dust control. Follow the project risk assessment and local codes in any coagulant dosing room design.

Can the pump sit on top of the tank?

It is not recommended. A tank-top pump relies on suction lift, and PAC solution tends to release gas and crystallize in the suction line. Flooded suction is more reliable.

Can room drainage go straight to the sewer?

No. Chemical drainage should return to the plant's own treatment process or equalization tank and be treated to local discharge limits.