8 Pergamon Wat. Sci. Tech. Vol. 33, No. 1, pp. 175-182. 1996. Copyright © 1996 lAWQ. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved. 0273-1223/96 $15'00 + 0'00 PII:S0273-1223(96)00170-9 EXPERIENCES WITH ON-LINE MEASUREMENTS AT A WASTEWATER TREATMENT PLANT FOR EXTENDED NITROGEN REMOVAL U. Nyberg, B. Andersson and H. Aspegren Malmo Water & Sewage Works, S-205 80 Malmo, Sweden ABSTRACT On-line sensors have been used at the Klagshamn treatment plant to measure the concentration of suspended solids in the raw wastewater, in the primary effluent and in the secondary effluent By means of the sensors the short-term performance of the primary and the secondary treatment have been evaluated. The sensors are important when the total discharges of pollutants from the plant are to be considered during storm water events. Parallel on-line measurements and laboratory analyses of grab samples have shown that the used sensor type, based on NIR technique, reflects the concentrations rather well. The on-line measurement in the raw wastewater is rather unreliable due to clogging which means that using this application for control can be questioned. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd. KEYWORDS By-pass; instrumentation; on-line; primary; clarification; secondary clarification; stormwater; suspended solids; wastewater treatment. INTRODUCTION The influent wastewater to a municipal wastewater treatment plant varies in time both in quantity and quality and contains both particulate and dissolved pollutants. The effluent wastewater however, contains mainly dissolved pollutants during normal operational conditions. During high flow rates in connection with rainfall the concentration of particulate pollutants in the raw wastewater as well as in the treated wastewater may increase significantly while the concentrations of dissolved pollutants are diluted by run-off water. Increased flow rates occasionally cause hydraulic overload of the activated sludge process resulting in sludge loss from the secondary clarifiers. It is often assumed that when sludge loss occurs the concentration of pollutants in the secondary effluent is higher than in the primary effluent (Schilling and Hartwig, 1990). This has also been confirmed by measurements at the Klagshamn wastewater treatment plant in Malmo. One way of decreasing the total discharge of pollutants during stonnwater is to use a controlled by-pass of primary effluent. The concentration of suspended solids (SS) in the primary effluent then also has to be considered in the short- tenn perspective. 175