Reverse Osmosis (RO)

Reverse Osmosis (RO) is a water purification process in which pressure is applied to a feed solution to force water through a semi-permeable membrane while dissolved salts, dissolved organics, bacteria, and viruses are rejected and concentrated in a reject (concentrate) stream. The membrane pore size is approximately 0.0001 µm — small enough to reject dissolved sodium and chloride ions and large enough to allow water molecules through. Typical operating pressures are 8–15 bar for brackish water, 50–80 bar for seawater, and 30–60 bar for digestate centrate.

In waste-processing applications, RO appears in two principal roles. Process water polishing: producing demineralised water for boiler feed, scrubber make-up, or laboratory use, replacing the standard ion-exchange route in plants where regeneration chemicals are restricted. Digestate liquid fraction concentration: producing a high-quality water permeate suitable for in-plant reuse or surface discharge, and a concentrated nutrient-rich reject that can be sold as liquid fertiliser or further evaporated. Modern membrane systems achieve 70–85% water recovery on digestate centrate, depending on pre-treatment quality.

RO systems have specific design constraints. Feed water must be pre-treated to remove suspended solids (by ultrafiltration or multimedia filters), iron and manganese (by oxidation and filtration), and chlorine (by activated carbon) — failure to do so causes irreversible membrane fouling within months. Membrane life is typically 3–5 years at 8–12% replacement per year. Specific energy consumption is 2–5 kWh per m³ of permeate produced, and chemical-cleaning cycles every 1–3 months require sequenced acid wash (for scale) and alkali wash (for organic fouling). For digestate applications, the reject stream — at 3–5× the salt concentration of the feed — creates its own disposal challenge. The pathway from raw centrate to discharge-quality permeate is typically pre-filtration → ultrafiltration → RO → permeate polishing, with capex of ₹2–5 crore for a 5 TPD CBG plant. Trade-offs versus alternatives like multi-effect evaporation are capex (RO is lower) versus reject volume (RO produces 4–7× more reject mass than MEE) versus operating cost (RO is cheaper per m³ unless waste heat is freely available for evaporation).