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title: “Continuous Ammonia and COD Verification for Agricultural Reuse Under EU Class A: A Shanghai ChiMay Technical Guide”
date: 2026-07-20
perspective: Technical Deep-Dive
theme: Water Recycling & Circular Water Economy

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Continuous Ammonia and COD Verification for Agricultural Reuse Under EU Class A: A Shanghai ChiMay Technical Guide

The Short Version

• The EU 2025 Water Reuse Regulation (Regulation 2020/741, fully enforceable from June 2025) establishes four reuse quality classes, with Class A—the highest standard—permitting unrestricted irrigation of crops consumed raw, including leafy greens and berries. Class A requires E. coli ≤ 1 CFU/100 mL and places stringent limits on chemical indicators of organic and nitrogenous pollution.
• Ammonia nitrogen and chemical oxygen demand (COD) are the two most widely monitored chemical surrogates for verifying that reclaimed water meets Class A limits. They directly indicate how complete biological treatment was and how much residual organic load could support pathogen regrowth in distribution systems.
• Continuous in-line ammonia and COD sensors have reached the point where their accuracy is comparable to laboratory methods: ammonia nitrogen sensors achieve ±0.1 mg/L precision and UV-based COD sensors achieve ±2 mg/L accuracy, enabling real-time compliance verification instead of periodic grab-sample testing.
• Shanghai ChiMay’s Ammonia Nitrogen Sensor (ion-selective electrode technology, 0-1000 mg/L range) and COD Sensor (UV absorption, 0-500 mg/L range, zero reagent consumption) are deployed across agricultural reuse installations in Europe and Asia, providing continuous compliance-grade data streams.

The EU Class A Reuse Standard and Its Implications for Monitoring

The EU Water Reuse Regulation is the most prescriptive agricultural reuse framework in the world. Class A water, intended for irrigation of food crops consumed unprocessed, must meet limits that are effectively equivalent to drinking water quality for key chemical and microbiological parameters. For treatment plant operators, that means the water leaving the reuse polishing train has to be continuously verified so no excursion slips past the Class A boundary.

Two chemical parameters matter most in this verification:

• Ammonia nitrogen (NH₃-N): indicates the completeness of nitrification in the biological treatment stage. Residual ammonia above 1-2 mg/L suggests incomplete nitrification, which can lead to oxygen depletion in receiving waters and support the growth of ammonia-oxidizing organisms in distribution systems.
• Chemical Oxygen Demand (COD): measures the total oxidizable organic matter in the water. For Class A reuse, COD limits typically fall in the range of 20-50 mg/L, and excursions above this range indicate incomplete organic matter removal that could support bacterial regrowth in the reuse distribution network.

Ion-Selective Electrode Technology for Continuous Ammonia Monitoring

Traditional ammonia measurement relies on colorimetric methods (such as the salicylate or Nessler reactions) that require reagent addition, sample conditioning, and laboratory analysis. These methods produce accurate results but are unsuitable for continuous monitoring: they generate hazardous waste, consume reagents on every measurement cycle, and need operator intervention every 4-8 hours.

Ion-selective electrode (ISE) technology takes a fundamentally different approach. An ammonia-sensitive membrane—typically a gas-permeable hydrophobic polymer—separates the sample water from an internal electrolyte solution. Ammonia molecules diffuse through the membrane and alter the pH of the internal electrolyte, which an internal pH electrode measures. The resulting potential is proportional to the ammonia concentration in the sample.

Shanghai ChiMay’s Ammonia Nitrogen Sensor uses this ISE principle with a measurement range of 0-1000 mg/L NH₃-N and accuracy of ±0.1 mg/L in the 0-50 mg/L range most relevant to agricultural reuse monitoring. The sensor operates without reagents, produces no chemical waste, and provides a continuous output via Modbus RTU/TCP communication.

Key technical characteristics for reuse monitoring:

• Response time: T90 response within 120 seconds, enabling detection of ammonia breakthrough events in near-real time.
• Temperature compensation: Automatic compensation from 0-50°C using an integrated temperature sensor.
• pH dependency: Ammonia ISE measurement is pH-dependent because the ammonia-ammonium equilibrium shifts with pH. The sensor must be paired with a pH measurement for accurate speciation, which Shanghai ChiMay addresses through its multi-parameter sensor or a standalone pH electrode.
• Interference management: Potassium ions (K⁺) can interfere with ammonia ISE measurement at high concentrations. Shanghai ChiMay’s membrane formulation achieves a K⁺/NH₄⁺ selectivity ratio better than 100:1, ensuring reliable ammonia measurement even in waters with elevated potassium from agricultural fertilizer runoff.

UV Absorption Technology for Continuous COD Monitoring

COD measurement by the standard laboratory method (dichromate oxidation at 150°C for two hours) is time-consuming, generates hazardous chromium waste, and cannot provide real-time data. UV absorption spectroscopy offers a reagent-free alternative that correlates absorbance at 254 nm with the concentration of aromatic and conjugated organic compounds in the water.

UV absorption does not measure the exact same chemical parameter as dichromate COD, but the correlation between UV254 absorbance and COD is strong enough for continuous monitoring in most wastewater and reuse applications, with typical correlation coefficients (R²) of 0.92-0.98.

Shanghai ChiMay’s COD Sensor uses a dual-beam UV absorption design with a measurement range of 0-500 mg/L COD and accuracy of ±2 mg/L. The dual-beam configuration compensates for lamp aging and window fouling by continuously referencing a clean beam path against the measurement beam, maintaining accuracy over extended deployment periods.

Key technical characteristics for agricultural reuse monitoring:

• No reagent consumption: Unlike laboratory COD methods, the UV sensor requires zero chemicals, eliminating supply-chain risk and hazardous waste disposal.
• Response time: Near-instantaneous measurement with a T90 response under 30 seconds, enabling detection of organic load spikes that would be invisible to periodic grab sampling.
• Fouling management: An integrated compressed air cleaning system or optional wiper mechanism keeps the optical window clear in waters with moderate suspended solids loading.
• Site-specific correlation: A one-time side-by-side comparison of UV sensor readings with laboratory COD results establishes a site-specific correlation factor that converts the UV output to COD-equivalent values.

Sensor Placement Strategy for Agricultural Reuse Monitoring

A typical agricultural reuse treatment train with continuous ammonia and COD monitoring includes these measurement points:

• After secondary biological treatment (nitrification): The ammonia sensor verifies complete nitrification before the water enters the polishing stage. A rising ammonia reading signals nitrification failure and triggers process investigation before non-compliant water reaches the reuse distribution.
• After filtration (sand or membrane): The COD sensor confirms that organic matter has been reduced below the Class A threshold. If COD exceeds the alarm setpoint, the water is automatically diverted away from the reuse storage tank.
• In the reuse distribution network: Both ammonia and COD sensors at the distribution network outlet provide continuous verification that the water reaching the agricultural end user meets Class A limits.
• Return flow from agricultural drainage: Monitoring ammonia and COD in drainage water returning from irrigated fields helps operators assess whether the applied reuse water is contributing to nutrient loading in receiving waters.

Comparing Continuous Sensors to Grab Sampling

The transition from grab sampling to continuous monitoring for ammonia and COD in agricultural reuse delivers measurable improvements:

• Detection speed: Continuous sensors detect parameter excursions within 2-5 minutes, compared with 4-24 hours for grab sampling with laboratory turnaround.
• Data density: Continuous monitoring produces 288 data points per day (at 5-minute intervals), compared with 1-4 data points per day from grab sampling. That density enables trend analysis, early warning, and automated compliance reporting.
• Operating cost: After the initial capital investment, continuous sensors cost approximately USD 3,000-5,000 per year in maintenance and consumables per parameter, compared with USD 15,000-30,000 per year in laboratory fees for grab sampling at four-times-daily frequency.
• Audit defensibility: Continuous data with tamper-evident timestamps provides stronger compliance evidence during regulatory audits than grab samples, which are subject to chain-of-custody challenges.

Shanghai ChiMay supports agricultural reuse operators with application engineering to design the optimal sensor placement strategy, establish site-specific correlations, and integrate continuous data into SCADA systems for automated compliance reporting.