Industrial Water Reuse: Quality Classes, Treatment Train and Economics

Regulatory Framework: EU Regulation 2020/741

In the European Union, Regulation (EU) 2020/741 sets minimum requirements for water reuse in agricultural irrigation. Although it addresses reclaimed municipal wastewater, its quality classes have become a common reference point for industrial reuse concepts as well, especially where water leaves the site or comes into contact with crops, people or the environment.

The regulation defines four reclaimed water quality classes (A to D). Each class is linked to permitted crop categories and irrigation methods, and each requires a risk management plan covering hazards, barriers and monitoring.

For purely internal industrial uses (cooling towers, washing, process water), requirements are usually defined by the site operator, equipment manufacturers and the competent authority. In practice, the Class A parameters for TSS and turbidity are often used as a robust baseline.

Defining the Target Quality for Each End Use

A reuse concept should start with the end user, not with the treatment technology. Different applications tolerate very different water qualities:

• Cooling water make-up: low TSS and turbidity to avoid deposits, limits for hardness, chloride and nutrients to control scaling, corrosion and biofouling.
• Cleaning and washing water: low solids and safe microbiology; requirements depend on whether there is product contact.
• Irrigation of green areas or crops: microbiological class according to crop and irrigation method, plus salinity and sodium limits.
• Process make-up water: often the strictest requirements; frequently needs partial desalination by reverse osmosis.

The detailed requirements for irrigation are covered in our article on effluent quality for irrigation reuse.

The Typical Treatment Train

Most industrial reuse schemes build on an existing biological treatment stage and add several barriers:

Why Filtration Is the Key Barrier

Filtration is the step that makes every downstream barrier work. Suspended solids shield microorganisms from UV light, consume disinfectant and cause fouling in membranes and heat exchangers.

• UV disinfection: reliable UV dose delivery requires low turbidity and high UV transmittance. Particles above a few micrometres can protect bacteria from irradiation.
• Chlorination: solids increase chlorine demand and the formation of by-products.
• Reverse osmosis: a low silt density index (SDI) is essential; tertiary filtration reduces the load on any downstream fine filtration or ultrafiltration.

Pile cloth media filtration is well suited for this task because it achieves low filtrate TSS at high hydraulic loading on a compact footprint, with backwash water fractions typically in the range of 0.5–2 % of the filtrate flow.

Monitoring and Risk Management

Reclaimed water must be safe at the point of use, not only at the plant outlet. A robust monitoring concept combines online measurements with laboratory analyses:

• Online: turbidity after filtration, UV transmittance, UV dose or residual chlorine, flow and storage tank levels.
• Laboratory: E. coli and other indicator organisms, BOD5, TSS, salinity, nutrients, depending on end use.
• Barrier logic: automatic diversion or recirculation if turbidity or disinfection parameters leave the permitted range.
• Documentation: a risk management plan describing hazards, barriers, critical limits and corrective actions.

Economics: When Does Reuse Pay Off?

The business case depends on the avoided costs: fresh water purchase, groundwater fees, wastewater discharge charges and, increasingly, the risk of supply restrictions during droughts. Treatment costs for filtration and UV disinfection are usually moderate; desalination by reverse osmosis increases energy demand and creates a concentrate stream that must be managed.

A structured approach to costs, savings and risks is described in our article on building a defensible water reuse business case. Operating cost items are broken down in operating cost per cubic metre.