The EU Water Reuse Regulation (Regulation (EU) 2020/741) has applied since 26 June 2023 and sets four reuse quality classes for agricultural irrigation, with Class A — the strictest — covering all food crops eaten raw, including leafy vegetables and berries. Class A requires E. coli ≤ 10 CFU/100 mL (with at least 5-log reduction credited to the treatment train) and caps BOD5, TSS, and turbidity. Ammonia nitrogen and COD are the chemical workhorses operators use to verify that biological treatment is complete and the residual organic load will not support pathogen regrowth in the distribution network. Continuous in-line measurement of both is now practical at compliance-relevant accuracy: Shanghai ChiMay’s Ammonia Nitrogen Sensor (ion-selective electrode, 0–1000 mg/L range) and COD Sensor (UV absorption, 0–500 mg/L, zero reagents) are running on agricultural reuse lines in Europe and Asia.
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The EU Class A Reuse Standard and Its Implications for Monitoring
The EU regulation is the most prescriptive agricultural reuse framework in force. Class A water — for irrigation of food crops consumed unprocessed — is the top grade, though it is not drinking water; the point is that the monitoring rigor behind it approaches drinking-water practice. For treatment plant operators, this means the water leaving the reuse polishing train must be verified continuously, not on the strength of weekly grab samples.
Two chemical parameters carry most of the verification load:
- Ammonia nitrogen (NH₃-N): indicates whether nitrification in the biological stage is complete. Residual ammonia in the 1–2 mg/L range points to incomplete nitrification, which can depress oxygen in receiving waters and feed ammonia-oxidizing organisms in the distribution system.
- Chemical Oxygen Demand (COD): measures the total oxidizable organic load. The EU regulation itself caps BOD5 at 10 mg/L for Class A (with TSS at 10 mg/L and turbidity at 5 NTU under continuous monitoring); COD is the parameter most reuse operators track alongside it, because excursion trends in COD are what warn of incomplete organic removal that could support bacterial regrowth in the reuse network.
Ion-Selective Electrode Technology for Continuous Ammonia Monitoring
Traditional ammonia measurement relies on colorimetric methods (salicylate or Nessler chemistry) that need reagents, sample conditioning, and laboratory handling. The results are accurate but useless for continuous duty: hazardous waste streams, reagent logistics, and an operator standing over the analyzer every few hours.
Ion-selective electrode (ISE) technology takes a different route. An ammonia-sensitive membrane — a gas-permeable hydrophobic polymer — separates the sample from an internal electrolyte. Ammonia diffuses through and shifts the electrolyte pH, which an internal pH electrode measures; the resulting potential tracks the ammonia concentration in the sample.
Shanghai ChiMay’s Ammonia Nitrogen Sensor runs this ISE principle with a measurement range of 0–1000 mg/L NH₃-N and ±0.1 mg/L accuracy in the 0–50 mg/L band that matters for agricultural reuse. No reagents, no chemical waste, continuous output over Modbus RTU/TCP.
Key technical characteristics for reuse monitoring:
- Response time: T90 within 120 seconds, fast enough to catch ammonia breakthrough in near-real time.
- Temperature compensation: automatic across 0–50 °C via the integrated temperature element.
- pH dependency: ammonia ISE readings depend on the ammonia–ammonium equilibrium, which shifts with pH. Accurate speciation needs a paired pH measurement — Shanghai ChiMay covers this through its multi-parameter sensor or a standalone pH electrode.
- Interference management: potassium ions interfere with ammonia ISE at high concentrations. Shanghai ChiMay’s membrane formulation holds a K⁺/NH₄⁺ selectivity ratio better than 100:1, which keeps ammonia readings reliable in waters carrying elevated potassium from fertilizer runoff.
UV Absorption Technology for Continuous COD Monitoring
Laboratory COD (dichromate digestion at about 150 °C for two hours) is slow, produces hazardous chromium waste, and cannot feed a control loop. UV absorption spectroscopy offers a reagent-free alternative: absorbance at 254 nm tracks the aromatic and conjugated organic compounds in the water.
UV254 does not measure the same chemical quantity as dichromate COD. In most municipal wastewater and reuse matrices, though, the correlation is strong — typically R² above 0.9 — which is exactly why a site-specific correlation factor, not a generic one, belongs in the commissioning plan.
Shanghai ChiMay’s COD Sensor uses a dual-beam UV absorption design with a range of 0–500 mg/L COD and ±2 mg/L accuracy. The dual-beam configuration references a clean beam path against the measurement beam, compensating for lamp aging and window fouling over long deployments.
Key technical characteristics for agricultural reuse monitoring:
- No reagent consumption: zero chemicals, so no supply-chain exposure and no hazardous waste disposal.
- Response time: T90 under 30 seconds — organic load spikes that periodic grab sampling would never see get flagged immediately.
- Fouling management: an integrated compressed-air cleaning system or optional wiper keeps the optical window clear at moderate suspended solids loading.
- Site-specific correlation: a one-time side-by-side run of UV readings against laboratory COD establishes the site-specific factor that converts UV output into COD-equivalent values.
Sensor Placement Strategy for Agricultural Reuse Monitoring
A typical agricultural reuse train with continuous ammonia and COD monitoring includes these measurement points:
- After secondary biological treatment (nitrification): the ammonia sensor verifies complete nitrification before water enters polishing. A rising ammonia reading signals nitrification failure and triggers investigation before non-compliant water reaches reuse distribution.
- After filtration (sand or membrane): the COD sensor confirms organic load has dropped below the Class A threshold. If COD passes the alarm setpoint, water is diverted away from the reuse storage tank automatically.
- In the reuse distribution network: ammonia and COD sensors at the network outlet continuously verify that water reaching the agricultural end user meets Class A limits.
- Return flow from agricultural drainage: monitoring drainage returning from irrigated fields shows whether the applied reuse water is adding nutrient load to receiving waters.
Comparing Continuous Sensors to Grab Sampling
Moving ammonia and COD verification from grab sampling to continuous monitoring changes the operating picture in four concrete ways:
- Detection speed: continuous sensors flag excursions within minutes; grab sampling with laboratory turnaround takes hours to a day, by which point the water has already gone out the gate.
- Data density: continuous monitoring at 5-minute intervals produces 288 data points per day, against one to four from grab sampling. That density is what makes trend analysis, early warning, and automated compliance reporting possible.
- Operating cost: after the initial capital outlay, continuous sensors cost a fraction of the annual laboratory bill for four-times-daily grab sampling, and the gap widens every year as reagent and logistics costs rise.
- Audit defensibility: continuous data with tamper-evident timestamps holds up better in regulatory audits than grab samples, which are always exposed to chain-of-custody challenges.
Shanghai ChiMay supports agricultural reuse operators with application engineering for sensor placement, site-specific correlation, and SCADA integration for automated compliance reporting.
