On-line sensors in wastewater treatment plant help reduce costs

By David Laurier, Kristof De Neve

On-line analysers can solve the shortcomings of laboratory (off-line) analyses that are performed to check the quality of water and wastewater treatment if they achieve the same accuracy and reproducibility as off-line methods, and perform well in industrial environments.

Wastewater treatment quality is checked by analysing and comparing incoming and outgoing water using several parameters like chemical oxygen demand (COD), biological oxygen demand (BOD), total nitrogen (TN), total phosphorus (TP), suspended solids (SS) and sometimes heavy metals.

Heavy metals indicate toxicity so sometimes the toxicity parameter, which may include the presence of organic toxic substances, is also tested. These analyses are performed in the laboratory. The only parameter that is sometimes measured on-line is total organic carbon (TOC), which provides a measure for the organic load of the wastewater (as COD). The parameters COD, BOD, TN and TP are especially important to optimise the treatment process, because the optimum carbon/nitrogen ratio (for (de)nitrification process and nitrogen incorporation in biomass) and the optimum phosphorous content (nutrient for biomass) should be observed.

The choice of the sampling point and sample preconditioning is as important as choosing the analyser. The sampling point should meet the following conditions:
• the tubing to transport the sample should have sufficient diameter (against clogging);
• the transport of the sample should be fast enough enabling fast analysis;
• and the sample should be transported without any pumps.

All these requirements and advantages will be shown in the following case.

At a pharmaceutical plant in Belgium no nitrification and denitrification of wastewater is carried out. Instead, an external C-source is dosed in the influent to have an optimum N-incorporation in the biomass to decrease N-concentration in effluent. The dosing of the external C-source is controlled and optimised via the measurement of different parameters like TOC and toxicity in the influent, and TN, TP and TOC in the effluent.

TOC in the influent is measured at the outcome of the primary and secondary clarifier (near dosing point C-source) and this value determines the action of the local feedback controller for the dosing pump of the external C-source.

The quality of the external C-source needs to be controlled by detecting the effects of a toxic or less biodegradable C-source. If toxicity is detected, an alarm should help avoid the biosystems going down. At this plant the on-line toxicity analyser Ra-BOD® (see further) acts as an early warning system to prevent intoxication of the biomass and to protect the treatment process. In the case of only off-line toxicity analysis, the biomass could already have been affected before the results were ready.

Finally, the quality of treated wastewater is controlled by several on-line analysers, including TONI®, for TN, TOPHO®, for TP and a TOC analyser1. The P-signal is used to control the automatic dosing of FeCl3 in order to precipitate the excess of ortho-phosphate as FePO4. In this way, the P-concentration in the effluent will not exceed the norm.

The N- and TOC-signal of the effluent are also used to control the dosing of the external C-source and consequently the N-incorporation in the biomass. These values are used as inputs in a "fuzzy supervisory control system," which determines the action of the dosing pump. In this control system, both signals are evaluated and several possibilities are checked before any action occurs.

Author's note
President and CEO David Laurier, M. Sc., and Applications/R&D Manager Kristof De Neve, Ph.D. work at AppliTek Inc., based in Deinze, Belgium.