Pharmaceutical Water Treatment — Purified Water by RO and EDI

Pharmacopoeias define water grades by use, not by the equipment that makes them. Drinking water is the starting point. Purified water is used for cleaning, granulation, coating and non-sterile formulations. Water for injection is used for parenterals and carries tighter limits on endotoxin and microbial contamination. Laboratories and sterile preparation areas may need their own polishing stage.

The definitions, test methods and limits differ between the USP, EP, JP and ChP, and they are revised periodically. This page describes typical engineering practice. The applicable pharmacopoeia, its current edition and the local regulatory requirements govern the final specification. The grade should be fixed before the system is sized, because it determines the treatment train, the loop design, the sampling plan and the documentation scope.

Targets should be taken from the pharmacopoeia that applies to the product and the market, not from a generic table. As engineering guidance, purified water is commonly specified at conductivity below 1.3 µS/cm at 25 °C, TOC below 0.5 mg/L, and a microbial action level of 100 CFU/mL. Water for injection adds an endotoxin limit. These figures are widely cited, but the current text and the local authority expectations must be confirmed before they are written into a specification.

The instrument set matters as much as the numbers. Conductivity must be temperature-compensated and installed in a flowing sample line. TOC analysers need defined calibration and a limit of detection below the target. Microbial monitoring depends on validated sampling, incubation and trending, not on a single grab sample. Alert and action levels are normally set below the compendial limit so that a trend is visible before the specification is breached.

Pretreatment is driven by the feed analysis and by the requirements of the RO and EDI stages. Municipal water usually needs multimedia filtration for turbidity, activated carbon or bisulphite for chlorine removal, and softening for hardness. Surface water may need coagulation, clarification or ultrafiltration. Well water may carry iron, manganese, hardness or silica. Chlorine must be removed before the RO because it oxidises polyamide membranes, and hardness must be controlled before the EDI because scaling damages the resin and the module.

Pretreatment also protects the microbial state of the train. Granular activated carbon can support bacterial growth if it is not backwashed and sanitized on schedule, and softener resin can become a biofilm site. Sanitization of pretreatment, filter replacement intervals and sampling points should be defined at the design stage rather than added after the first positive count.

Size on daily demand plus peak draw, not on the nominal output of the RO skid. Add formulation, cleaning and sampling volumes, apply a diversity factor, and check that the storage tank covers the interval between sanitization or regeneration cycles. A loop serving many intermittent use points needs a larger tank and a higher recirculation flow than the same daily volume concentrated in one shift. Build in redundancy where a shutdown stops production: duplicate feed pumps, a spare EDI module or a second loop branch.

Maintenance follows the same logic: keep the pretreatment ahead of the membranes, keep the loop hygienic, and trend the instruments instead of reacting to alarms. Sanitization frequency is set by the validated schedule and by microbial trends; filter and resin replacement by differential pressure and test results; EDI performance by product resistivity, current and pressure drop. A monthly review of conductivity, TOC, microbial counts and sanitization records detects drift before it becomes a deviation.