Potable Water Reuse: Treatment Trains, Risk Frameworks and Regulatory Approaches

The case for potable reuse

Conventional water resource planning treats wastewater as a waste stream to be managed, not a resource to be recovered. Potable reuse reframes this: highly treated reclaimed water — produced from secondary-treated municipal wastewater and subjected to advanced treatment including microfiltration (MF), reverse osmosis (RO), advanced oxidation (UV/H₂O₂), and stabilisation — can meet or exceed the quality of conventionally treated surface water.

NEWater and the operational benchmark

Singapore's NEWater programme, operational since 2003, remains the global reference for full-scale DPR. Four plants produce a combined 228,000 m³/d from secondary-treated wastewater, contributing approximately 40% of Singapore's total water demand. The treatment train — UF + RO + UV — achieves 12-log removal credit for bacteria, 14-log for viruses (Cryptosporidium) and 6-log for Giardia through the multiple-barrier concept. A fifth plant, at Changi, will add 148,000 m³/d by 2025, pushing NEWater's share to over 50% of national demand.

California's risk framework: the 12-17 rule

California's 2023 Potable Reuse regulations (Title 22) established log-reduction value (LRV) targets of 12-log for Cryptosporidium, 10-log for Giardia and 12-log for enteric viruses for DPR entering drinking water treatment plants. Treatment trains must demonstrate specified LRVs for each pathogen group, with credit assigned to each unit process (MF: up to 3-log for protozoa; RO: up to 2-log for all pathogens; UV/AOP: up to 6-log for virus inactivation). The framework explicitly allows for treatment-train optimization — different combinations of barriers achieving the same overall LRV target.