Zero liquid discharge (ZLD) solves a discharge problem by creating a concentrate problem. Water is recovered and reused, salts and organics are concentrated into a brine, and the emerging pollutants that were present at µg/L in the feed arrive at the evaporator at concentrations high enough to matter. Market analysts track a growing but still limited population of ZLD installations, concentrated in China, the United States, India and the Middle East, driven by discharge restrictions and water scarcity rather than by economics alone.
What makes ZLD operationally difficult is that the concentrate chemistry changes as the system runs, and the instrumentation that matters most is the one that sees those changes coming. COD is the most widely available candidate: cheap, continuous, and indicative of the organic load that carries much of the emerging pollutant risk.
Table of Contents
COD as an Emerging Pollutant Proxy
What COD Can and Cannot Tell You
COD measures the oxygen demand of oxidisable material in the sample. It does not identify anything. Pharmaceutical residues, personal care products and industrial chemicals are not quantified by a COD reading, and any monitoring programme that presents COD as a measurement of emerging pollutants is overstating what the instrument does.
What COD does provide is a real-time indicator of organic load that correlates with the concentration of organic micropollutants in many industrial waste streams, because both come from the same sources and much of the treatment that removes COD removes micropollutants with it. Where the relationship has been established on the site’s own data — COD against periodic LC-MS/MS results — COD trends give early warning of a load change and a basis for adjusting treatment. Where it has not been established, COD is a process parameter and nothing more.
The relationship also weakens as the concentration drops. At low µg/L levels the micropollutant fraction is a tiny part of the COD, and the correlation degrades accordingly. In concentrate streams, where concentrations are much higher, COD is a more useful indicator.
ChiMay COD sensors cover a 0–5,000 mg/L range (extended ranges available) with accuracy of ±5% of reading or ±10 mg/L, whichever is greater, response time under 2 minutes for process monitoring, and automatic compensation for chloride and other interferents — which matters in ZLD, where chloride loading is the norm.
ZLD Process Integration
Monitoring COD stage by stage shows where the organic load is being removed and where it is being concentrated. Indicative behaviour across a typical train:
| ZLD stage | COD reduction | Emerging pollutant removal |
|---|---|---|
| Pretreatment | 10–20% | Partial, mostly solids-associated |
| Biological treatment | 60–80% | Substantial for biodegradable compounds |
| Ultrafiltration | 5–15% | Limited; retains particulate-bound material |
| Nanofiltration | 15–25% | Moderate; size and charge dependent |
| Reverse osmosis | 20–35% | High for larger molecules |
| Evaporator / crystalliser | Remaining COD leaves with the concentrate or solids | Concentrated in the brine |
The pattern that matters operationally: RO and NF do not destroy organics, they move them into the concentrate. COD measurement on the concentrate side is what tells an operator that the brine is loading up, and it is the parameter that determines whether the evaporator will foul or the crystalliser will produce off-spec salt.
In practice COD data is used for four things: adjusting treatment parameters as influent load changes, triggering alarms when the concentrate load approaches a limit, tracking accumulation in the brine loop, and setting recovery targets that can be met without exceeding discharge or disposal limits.
Sensor Technologies for ZLD Applications
UV-Vis Spectrophotometric COD Sensors
UV-Vis COD measurement uses absorption at 254 nm, which correlates with aromatic and unsaturated organic content — the UV254 parameter is a well-established surrogate for organic load. Turbidity and temperature compensation are required for accuracy.
Typical specification: 0–500 mg/L standard range, extendable to 5,000 mg/L; ±5% of reading accuracy; response under 60 seconds; lamp life of two to three years with periodic cleaning.
The advantages are operational: no reagents, so no chemical handling, no reagent cost and no reagent disposal; continuous measurement with very little consumable burden; and a response fast enough for control rather than just reporting.
The limitation is interference. Chloride, nitrate and other UV-absorbing species bias the reading, and in a ZLD concentrate with high salinity, compensation is not optional.
Electrochemical COD Sensors
For high-salinity streams, electrochemical oxidation-based COD sensors are often the better fit: range to 10,000 mg/L, accuracy around ±3% of reading, and tolerance of TDS up to 150,000 mg/L, which suits concentrate streams where UV measurement struggles.
ChiMay supplies both approaches — UV-based sensors for standard wastewater duty and electrochemical sensors for high-salinity concentrate streams — so a ZLD site can match instrumentation to each stream rather than compromising across the whole train.
Process Control Applications
Real-Time Treatment Optimization
Representative control logic built on continuous COD:
- Biological stage: increase aeration when influent COD rises above the design band, extend hydraulic retention time, and watch nitrification rate as an indicator of inhibition
- Membrane stage: trigger inspection when COD rejection across the membrane falls below the expected value, reduce feed pressure, and increase crossflow velocity to control fouling
- Evaporator: reduce evaporation rate when concentrate COD climbs steeply, increase brine purge, and verify crystalliser performance
Load-Based Process Adjustment
Dynamic setpoints let a plant run to the load instead of to the worst case:
- Low load (COD well below design): reduce aeration energy, which is the single largest saving available
- Normal load: standard operating setpoints
- High load: increase treatment capacity before the effluent or the concentrate degrades
- Shock load: divert to equalisation and protect the biological stage
The purpose is not only energy saving. Keeping treatment efficiency steady through load variation is what prevents the excursions that a ZLD system, by definition, has nowhere to send.
What a ZLD Monitoring Programme Looks Like
Two common configurations:
Pharmaceutical ZLD (biological + membrane + thermal): COD sensors on the biological influent, membrane feed and concentrate, and evaporator feed; five-minute logging integrated to the control system; alarm management on threshold exceedance; periodic LC-MS/MS on the concentrate to confirm that the COD relationship still holds for the compounds of interest.
Textile ZLD (high colour, high COD): COD monitoring on the dyeing effluent, biological reactor, membrane stages and evaporator feed. Textile effluent typically runs 800–2,000 mg/L COD with complex chemistry, and COD trends against throughput are what tell an operator whether the plant is coping or accumulating.
Both configurations use the same logic: continuous measurement at the points where the load changes, with laboratory confirmation at intervals to validate the interpretation.
Economic Considerations
The economics of COD-based control in a ZLD plant come from four places: fewer laboratory analyses because sampling is triggered by data rather than by calendar, lower aeration and thermal energy consumption from load-following operation, longer membrane and evaporator life because fouling is caught earlier, and fewer off-spec events. Capital cost depends on the number of measurement points and the technology chosen; the recurring benefit is dominated by energy and membrane replacement, both of which are large line items in any ZLD plant.
Payback estimates are site-specific. A ZLD plant running far from its design point will see a faster return than one already operating efficiently.
Conclusion
COD sensors cannot measure emerging pollutants, and a monitoring programme built on that assumption will not survive technical scrutiny. What continuous COD measurement does is show organic load in real time, stage by stage, including the concentrate streams where emerging pollutants actually accumulate. Paired with periodic targeted analysis to validate the relationship, that gives ZLD operators the information they need to protect membranes and evaporators, control energy consumption, and keep the concentrate within its disposal limits. ChiMay’s COD sensor range covers both standard wastewater and high-salinity concentrate duty.
