Water Reuse
Tertiary polishing to reuse quality, membrane and reverse osmosis pretreatment, and closed industrial loops — producing water good enough to use again without a filtration stage in between.
The economics of reuse are the economics of pretreatment
Reuse schemes rarely fail on the reuse itself. They fail on what has to be built in front of it — the filtration, the membrane replacement, the cleaning chemicals and the operator attention that a poorly clarified feed makes inevitable.
High-rate clarification changes that arithmetic by delivering a feed clean enough that the next stage can be smaller, or in some cases removed. Magnetite ballasting in particular produces effluent below 2 mg/L suspended solids and below 1 NTU turbidity, with high UV transmittance — a specification that would normally require a filter.
Tertiary polishing for non-potable reuse
Irrigation, industrial makeup, dust suppression, toilet flushing and construction water all need reliable removal of suspended solids, phosphorus and a good part of the pathogen load. A high-rate clarification stage on secondary effluent achieves that in one step, and the resulting high UV transmittance reduces the size and energy demand of the ultraviolet disinfection that usually follows.
Membrane and reverse osmosis pretreatment
Membrane life and cleaning frequency are governed by what reaches the membrane. Clarification that consistently delivers low turbidity and a low silt density index extends element life, lengthens the interval between clean-in-place cycles and reduces antiscalant demand. On brackish and seawater RO, this is generally the cheapest place in the whole scheme to spend money.
Closed industrial loops
Cooling tower blowdown, wash water, quench water and process rinse streams can often be clarified and returned rather than discharged. The driver is usually the cost of raw water intake or of discharge consent, and the compactness of a high-rate unit means it can be sited next to the process rather than at the plant boundary.
Filter and membrane backwash recovery
Backwash streams are 2–5 % of throughput and are concentrated rather than dirty in the ordinary sense. Clarifying them and returning the supernatant to the head of the works recovers most of that water; a unit with integrated thickening also produces a sludge concentrated enough to go straight to dewatering.
| Design parameter | Typical range |
|---|---|
| Feed | Secondary or tertiary effluent |
| Rise rate | 20–50 m/h |
| Product suspended solids | < 2–5 mg/L |
| Product turbidity | < 1–2 NTU |
| Product total phosphorus | < 0.05–0.3 mg/L |
| UV transmittance | > 75 % |
| Silt density index for RO feed | typically < 3–5 |
| Backwash recovery | 2–5 % of plant throughput |
| Downstream disinfection | UV or chlorine, sized on the improved transmittance |
Reuse standards vary by jurisdiction and by end use. The design basis is set from the applicable reuse standard, not from these ranges.
Reuse quality is defined by what the water is used for, and the difference between irrigation quality and membrane feed quality is a different flow sheet, not a different setpoint. Tell us the end use and the standard it has to meet and the design follows from that.
Planning a reuse scheme?
Tell us the source, the end use and the standard it must meet. We will set out what clarification can deliver and what still has to sit downstream of it.