The performance of a wastewater treatment plant is understood through the regular and accurate monitoring of a few core parameters. Chief among these are COD (Chemical Oxygen Demand) and TSS (Total Suspended Solids). Together, these two indicators offer the most practical way to assess both process efficiency and discharge quality.
What is COD?
COD expresses the amount of oxygen required to chemically oxidise the organic pollutants in water and is usually measured in mg/L. A high COD value indicates an increased organic load being discharged into the receiving environment. As the organic load rises, oxygen consumption in the receiving environment rises too, creating a risk for aquatic life.
COD is also used to monitor the efficiency of the treatment process. The difference between the inlet and outlet COD values reveals how effectively biological treatment is operating.
What is TSS?
TSS expresses the amount of suspended solids that do not dissolve in water, in mg/L. These substances can consist of sludge particles and undissolved organic and inorganic particulates. A high TSS value signals a deterioration in treatment efficiency or problems in the decantation or filtration stages.
TSS can be tracked both under laboratory conditions and through online measurement. Online measurement allows sudden deteriorations to be noticed without waiting for laboratory results.
Why are they monitored together?
COD and TSS are complementary indicators. An increase in undissolved matter (high TSS) generally also raises the total organic load (COD). When tracked together:
- It is possible to distinguish whether the problem is within the process or a sudden load at the inlet.
- Trends in treatment efficiency can be seen early.
- The risk of non-compliance with discharge limits can be assessed in advance.
For this reason, COD and TSS are inseparable parts of a healthy wastewater treatment plant scorecard.
Limits and interpretation
Discharge limits vary depending on the type of facility, the discharge point and the applicable regulation. For a facility operating near legal limits, even small fluctuations carry the risk of exceedance. Therefore, it is not enough to monitor only the value at the limit; the trend and variability of the data must also be monitored.
Trend analysis helps to distinguish seasonal effects, sudden loads from industrial discharges and process-driven deviations.
Why is online monitoring challenging?
Measuring COD and TSS is technically more demanding than parameters such as pH or flow. The calibration of analysers, the sampling arrangement and maintenance periods are critical for accurate measurement. Nevertheless, online COD and TSS solutions offer the advantage of continuous data and early warning.
The critical point is ensuring the measuring device operates correctly. Calibration delays and sampling line issues directly affect the reliability of the data. For this reason, tracking calibration and maintenance is as important as the measurement itself.
Monitoring COD and TSS with Aqualon
Aqualon Wastewater brings all monitored parameters, including COD and TSS, into a single dashboard. With second-level data, hourly and daily trend charts and audit-ready reports, it makes your treatment performance continuously visible. Alarms are raised on limit exceedance, calibration delay or data interruption, and notifications are sent to the relevant groups.
The on-site cabinet buffers measurement data locally; even if the internet is lost, data is not lost and is transmitted to the Ministry automatically once the connection is restored. This makes COD and TSS monitoring not only a legal obligation but also a data source for continuous improvement.
See the Wastewater module page for the parameter list, legal limits and reporting features.