Water Reuse Glossary

Advanced Oxidation Process (AOP) — Treatment process generating hydroxyl radicals to destroy refractory organics that membranes cannot remove cost-effectively. Common configurations: UV/H₂O₂ for 1,4-dioxane and NDMA; O₃/H₂O₂ for taste, odor, and pharmaceuticals; UV/O₃ for PFAS precursors.

Antiscalant — Chemical additive — typically polyacrylate, phosphonate, or maleic copolymer — that prevents mineral scaling on membranes and heat-transfer surfaces. Dose accuracy is directly proportional to membrane life; ±1% is the field standard for reuse-grade RO.

Aquifer Recharge — Injection or infiltration of treated water into a groundwater aquifer, often as the environmental buffer in an indirect potable reuse scheme.

BOD (Biochemical Oxygen Demand) — Mass of dissolved oxygen consumed by microorganisms in decomposing organic matter. Used as a surrogate for biodegradable organic load; the standard benchmark for secondary treatment effluent quality.

Brine Concentrator — Thermal evaporation system — typically MVC (Mechanical Vapor Compression) or MED (Multi-Effect Distillation) — used to concentrate brines ahead of crystallization in a ZLD train.

CEC (Contaminants of Emerging Concern) — Substances such as pharmaceuticals, PFAS, personal care products, and endocrine disruptors increasingly regulated in reuse contexts. Drive the specification of advanced oxidation and ion exchange polishing.

CIP (Clean-in-Place) — Periodic chemical cleaning of membranes or process equipment without disassembly. CIP frequency is a direct indicator of upstream chemistry control.

COD (Chemical Oxygen Demand) — Total oxygen required to oxidize all organics in water — biodegradable plus refractory. Used together with BOD to characterize treatability.

Cycles of Concentration — Ratio of dissolved solids in cooling tower water to dissolved solids in the make-up. The single most important lever on cooling tower water efficiency; on reclaimed make-up, typically 4–8 versus 3–5 on potable.

DAF (Dissolved Air Flotation) — Solids removal process using fine air bubbles to float suspended solids and oils to the surface for skimming. Used in primary and tertiary stages of reuse trains.

Dechlorination — Removal of free chlorine — typically with sodium bisulfite (SBS) — ahead of polyamide RO membranes, which are irreversibly damaged by chlorine residual.

DPR (Direct Potable Reuse) — Reclaimed water introduced directly into a potable distribution system or upstream of a drinking water treatment plant, without an environmental buffer. Requires a fully validated multi-barrier train.

EDR (Electrodialysis Reversal) — Membrane desalination using alternating electric fields, robust to fouling and effective on high-silica feeds where pressure-driven RO struggles.

Equalization — Buffering of flow and load variability ahead of biological treatment. The most underrated capex item in a reuse train and the source of stability for everything downstream.

FGD (Flue Gas Desulfurization) Wastewater — High-chloride, high-TDS effluent from coal-fired power scrubbers; a major driver of ZLD adoption due to tightening discharge limits on chloride, selenium, and mercury.

Flocculation — Aggregation of destabilized particles into larger flocs for removal by settling or flotation. Driven by polymer chemistry, mixing G-value, and residence time.

IPR (Indirect Potable Reuse) — Reclaimed water blended with an environmental buffer — aquifer or surface water — before potable treatment. Lower regulatory threshold than DPR while delivering similar end-water quality.

Ion Exchange (IX) — Removal of specific ions — hardness, nitrate, boron, perchlorate, fluoride — by resin exchange. Requires regeneration chemistry, which itself becomes a brine to manage.

LRV (Log Reduction Value) — Logarithmic measure of pathogen removal across a barrier; 4-log equals 99.99% removal. Reuse regulations are increasingly specified as LRV requirements rather than residual disinfectant concentrations.

LSI (Langelier Saturation Index) — Index of calcium carbonate scaling tendency in cooling water. The first-order benchmark used to set the operating envelope on a cooling reuse program.

MBR (Membrane Bioreactor) — Activated sludge combined with ultrafiltration membrane separation. Produces effluent of much higher quality than CAS (BOD <2 mg/L, turbidity <0.2 NTU) and eliminates the need for tertiary filtration. The dominant secondary technology for reuse-grade water.

MLD (Minimal Liquid Discharge) — Reuse strategy where most effluent is recovered and a small brine stream is discharged. Often the more economic answer when true ZLD is not regulatorily required.

Multi-Barrier Treatment — Use of multiple independent treatment steps to achieve redundant pathogen and contaminant control. A regulatory requirement for IPR and DPR; an engineering best practice everywhere else.

NEWater — Singapore's branded reclaimed water program, a global benchmark for direct potable reuse. Treatment train: MBR → RO → UV. Meets up to 40% of national water demand and targets 55% by 2060.

NDMA (N-Nitrosodimethylamine) — Carcinogenic disinfection byproduct associated with chloramination. Controlled by UV/AOP in modern reuse trains.

PAA (Peracetic Acid) — Strong disinfectant producing minimal regulated disinfection byproducts; increasingly used for reuse discharge where DBPs are a concern. Typical dose 1–5 mg/L.

PFAS — Per- and polyfluoroalkyl substances — persistent CECs requiring advanced treatment (GAC, ion exchange, AOP). Increasingly regulated in reuse loops.

Reclaimed Water — Wastewater treated to a quality fit for a defined reuse application. Class and regulatory framework determine the treatment intensity required.

Reverse Osmosis (RO) — Pressure-driven membrane process rejecting dissolved salts and most organics. Single-pass for non-potable reuse; double-pass for IPR, DPR, UPW, and boiler feed. The workhorse of advanced reuse treatment.

SBS (Sodium Bisulfite) — The standard dechlorination chemical injected ahead of RO membranes; typically 3–5 mg/L per mg/L of residual chlorine.

SDI (Silt Density Index) — Measure of fouling potential of RO feedwater. SDI <3 is the typical specification for RO feed; SDI <5 is the practical ceiling.

TDS (Total Dissolved Solids) — Mass of dissolved inorganic and organic compounds in water. Drives membrane sizing, antiscalant chemistry, and the practical ceiling on cycles of concentration.

TOC (Total Organic Carbon) — Mass of carbon in dissolved and suspended organics; the key indicator for biological stability of reuse water in distribution.

Tertiary Treatment — Polishing of secondary effluent, typically including filtration and disinfection. Largely redundant when secondary is MBR.

Title 22 — California regulation defining four classes (A–D) of recycled water for non-potable reuse. The reference framework for non-potable reuse in the United States.

UF (Ultrafiltration) — Membrane process removing particulates, bacteria, and most viruses. Used as RO pretreatment and as a barrier in multi-barrier disinfection.

UPW (Ultrapure Water) — Highest purity water grade, required for semiconductor manufacturing and increasingly for green hydrogen electrolyzers. Specifications down to part-per-billion levels of contaminants.

WRAP (Water Reuse Action Plan) — U.S. EPA federal strategy for accelerating reuse, with explicit focus on data centers, industrial manufacturing, and energy production.

ZLD (Zero Liquid Discharge) — Wastewater strategy producing no liquid effluent; all recovered water returns to process, all residue is solidified. The most demanding and most regulated position on the reuse continuum.