Mesure du débit des eaux usées ou fortement chargées en matières solides en vue de déterminer les flux de pollution

Abstract

The quantity of pollution due to industrial or urban effluent over a given time T is given by P = S T 0 q c dt, where q and c are instantaneous polluant discharge and concentration respectively. Where continous concentration measurement is impracticable, the only theoretically correct procedure is by taking a sample at a discharge proportional to that of the effluent. A common alternative is to take a set number of samples at constant discharge and to multiply the average concentration found for each by the volume flow during the smapling time. The aim in every case is to record discharge. No special methods are available for polluted water discharge measurement. Standard methods are applied, there fore, and are briefly discussed from the point of view of possible effects of pollution or of how the instrumentation is installed on measurement accuracy and sensitivity. Vane-type water meters or discharge measurement orifice plates or nozzles are hardly suitable for pipes containing highly sediment-laden or corrosive water. Dilution methods and electro-magnetic flow measurement instruments are less sensitive to effluent quality, and acoustic instruments seem to hold great promise. Finally, pump running time measurement should not be disregarded altogether, in spite of its not particularly accurate results. Measurements by electro-magnetic pickup are preferable to current meters for free-surface flow. Broad-crested weirs or Venturi flumes producing a hydraulic jump are preferable to sharp-crested weirs, but are more difficult to set up. Dilution methods call for precautions in the choice of a suitable tracer, and it is not always easy to achieve the right mixture. The use of radio-active tracers enables discharge to be recorded. With chemical tracers, the present trend is towards constant-rate injection and the constitution of average samples from which both flow volume and pollution concentration can be established. There is a systematic error with this method which, however, remains within acceptable limits if the discharge does not vary too much. Present research subjects include the use of electro-magnetic pickups and the behaviour of various tracers in polluted water. It is proposed to design special equipment for the use of radio-active tracers.