Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2000-06-02
2001-09-18
Barry, Chester T. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S621000, C210S623000, C210S626000
Reexamination Certificate
active
06290850
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a novel method of monitoring the aeration systems of biological basins of a wastewater treatment plant, this method having more particularly the objective of monitoring the efficiency of the aeration system. The invention applies to the case in which the aeration of the basin takes place by on/off sequences of the aeration system and it makes it possible in particular to take into account the case in which constant flows of biomass and of pollution enter and leave the basin during the monitoring, thus allowing regulation of the aerobic biological systems, based on various physical measurements which have the advantage of being rapidly obtainable, the method according to the invention thus allowing in-line regulation.
According to the invention and as will be explained below, these measurements relate to (FIG.
1
):
the concentration of dissolved oxygen in the aeration basin;
the throughput Qt, through the plant; and
the rate Qr of recirculation of the mixed liquor from the clarifier to the aeration basin.
BACKGROUND OF THE INVENTION
As a consequence of the European directives, the purification techniques currently employed are aiming to limit discharges into the natural environment of untreated wastewater, stations having to treat all collected water apart from that resulting from exceptional downpours. The common characteristic of these treatment stations is that of carrying out purification by activated sludge, comprising a treatment basin in which oxygen is transferred to the “water/biomass” mixture. This transfer is necessary for the development of the purifying biomass.
Furthermore, increased reliability is demanded by the European directives in question and therefore requires purification stations to be strictly monitored and to be maintained in the best possible state of operation, with a maximum available purifying capacity (that is to say with a biomass “in the top of its form” and with equipment in a perfect state of maintenance and operation).
One of the major causes of malfunction of purification stations is the deficiency or insufficiency of the oxygen supply; this oxygenation deficit is manifested long term by the appearance of anaerobiosis phenomena, resulting in the development of filamentary bacteria in the biomass and then the appearance of foams, the phenomenon of performance degradation being all the more rapid as it is autoaccelerated.
The efficiency of the aeration system is usually measured, before the purification station is commissioned, using in vitro measurement techniques which absolutely do not take into account the influence of the biomass on the oxygen transfer. This is because the transfer coefficient (which in this case we will call the standard transfer coefficient) is usually measured in clarified water and in the presence of sodium bisulphite—a reducing agent for oxygen; it is thus possible to control gas-liquid transfer, but to the exclusion of any biological phenomenon. On the other hand, when the biomass is “installed” in the aeration basin, the said biomass operates as a biological oxygen pump and it is then possible to obtain a corrected transfer coefficient &agr;k
L
a, where &agr; represents a correction coefficient having a value of between 0.5 and 0.9, which corrected transfer coefficient takes into account the biological operation, the differences in viscosity between the mixed liquor and the clarified water, and the differences in hydrodynamics resulting from operation in the presence of biomass. Once the station is running, tricky measurement techniques have to be used to determine the amount of oxygen transferred to the water/biomass mixture, while still obtaining only instantaneous values, giving, for example, no information about a possible drift in the efficiency of the aeration system.
The systems according to the prior art which are usually employed for monitoring and/or controlling the operation of wastewater purification stations are of various types:
use of a delay, which acts on the frequency and the duration of the aeration periods;
detection of the oxydoreduction-potential or oxygen-concentration thresholds in the aeration basin: thus, GB-A-2 184 110 describes an apparatus for the treatment of effluents in which means are used for monitoring the concentration of dissolved oxygen in the biological reactor as well as means for measuring the rate of reduction of the dissolved-oxygen concentration in the absence of aeration, and the information obtained is used to send an alarm signal when this reduction is below a predetermined set value; likewise, EP-A-0 260 187 relates to a biological treatment process for the purpose of optimising the management of the aeration, consisting in continuously measuring the dissolved-oxygen concentration during the aeration step, the concentration values thus obtained making it possible to compute the oxygen demand and to adjust the aeration;
monitoring systems based on the derivative of the variation in the oxydoreduction potential as a function of time (FR-A-2 724 646).
BRIEF DESCRIPTION OF THE INVENTION
None of these systems solves the technical problem solved by the present invention, that is to say the establishment, during periods of aeration and of non-aeration, of the material balance between oxygen supplied, oxygen consumed and oxygen discharged with the effluent during the aeration phase and of the material balance between oxygen consumed and oxygen discharged with the effluent during the non-aeration phase, respectively, for this purpose of diagnosing any change in the performance of a station and therefore of anticipating possible incidents, the ultimate stage of their performance in this field being to trigger an alarm when the capacity limit of the plant is reached. Experience acquired shows the advantage of using preventative management, something which constitutes one of the objectives of the present invention. discharged with the effluent during the non-aeration phase, respectively, for the purpose of diagnosing any change in the performance of a station and therefore of anticipating possible incidents, the ultimate stage of their performance in this field being to trigger an alarm when the capacity limit of the plant is reached. Experience acquired shows the advantage of using preventative management, something which constitutes one of the objectives of the present invention.
Another objective of the invention is to measure in situ the amount of oxygen transferred to a biological reactor, thereby making it possible to take into account the influence of the biomass and of the hydraulics on the amount of oxygen transferred.
Preventative management of the operation of a biological purification station presupposes that the efficiency of the aeration system is regularly monitored. Such monitoring makes it possible to show that there is a progressive reduction in the efficiency of the apparatus, which phenomenon requires planning of any maintenance work and, periodically, the use of remedial measures such as, for example, increasing the number of aeration turbines or their speed of rotation. The monitoring method forming the subject of the invention has therefore been developed for the purpose of ensuring that the efficiency of the aeration system is monitored sufficiently frequently.
REFERENCES:
patent: 3342727 (1967-09-01), Bringle
patent: 4280910 (1981-07-01), Baumann
patent: 4818408 (1989-04-01), Hamamoto
patent: 6007721 (1999-12-01), Payraudeau et al.
patent: 0260187 (1988-03-01), None
patent: 2184110 (1987-06-01), None
Caulet Philippe
Chatellier Patrice
Barry Chester T.
Connolly Bove Lodge & Hutz
Prince Fred
Suez-Lyonnaise des Eaux
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