Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2003-03-27
2004-10-26
Prince, Fred G. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S614000, C210S623000, C210S629000, C210S631000, C210S906000
Reexamination Certificate
active
06808629
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for purifying wastewater, this process being of the “biological activated-sludge” type and its subject being the simultaneous removal of the nitrogen-based pollution and of the phosphorus-based pollution irrespective of the variations in daily pollutant loads to be treated, these variations possibly being periodical.
BACKGROUND OF THE INVENTION
It is known that in wastewater treatment plants, variations in pollutant loads may arise especially:
from a specific and cyclic pollutant activity connected to the purification network (for example the wine-making industry);
from rainfall in conjunction with a purification network configuration that promotes the decantation of the materials in suspension in the pipework, during dry weather (low load) in this case, the increase in the flows of wastewater transported during wet weather (high load) results in cleaning of the pipework, reflected in a temporary increase (of about 24 to 48 hours) in the pollutant load received by the treatment plant;
from the seasons (summer-winter).
A review of the current state of the art regarding the treatment of nitrogen-based pollution and also the treatment of phosphorus-based pollution by biological purification plants of the activated-sludge type will be given hereinbelow. Reference may also be made to the Mémento Technique de l'Eau [Technical Water Guide], 9th Edition, 1989, Volume 2, Chapter 24, edited by Degremont.
In this description, reference will be made to
FIGS. 1
to
8
b
which diagrammatically represent embodiments of the processes according to this current state of the art.
1) Treatment of Nitrogen
It is known that, in its general principle, the biological treatment of nitrogen involves two steps:
the first step, performed in an “aerobic” zone, constitutes the nitrification step during which the reduced forms of nitrogen (organic and mineral) are oxidized in the form of nitrates by means of autotrophic bacteria. During this step, the bacteria used must have available firstly a source of mineral carbon normally present in the effluent to be treated, in the form of carbonates, and secondly a source of oxygen supplied by mechanical aeration means;
the second step, performed in an “anoxic” zone, constitutes the denitrification step during which the nitrates formed during the first step are reduced to nitrogen gas, which is released into the atmosphere. This step uses heterotrophic bacteria, which must have available firstly a source of organic carbon naturally present in the effluent to be treated, and secondly a source of oxygen consisting of the oxygen chemically bonded to the nitrogen of nitrates, thereby allowing these nitrates to be reduced to nitrogen gas.
At the present time, there are two techniques for carrying out the process mentioned above. These two techniques differ only as regards the second step, i.e. the use of the anoxic zone. The first of these techniques uses two treatment tanks (
FIG. 1
) and the second uses only one tank (
FIGS. 2
a
and
2
b
).
a) Two-Tank Technique
As seen in
FIG. 1
, each step of the treatment mentioned above is performed in a separate tank:
an aerobic tank in which the first step (nitrification) is performed;
an anoxic tank in which the second step (denitrification) is carried out.
FIG. 1
clearly shows the manner in which this known treatment is performed.
The raw water, i.e. the effluent to be treated, supplies the carbon required by the heterotrophic bacteria performing the denitrification in the anoxic tank, the mass of bacteria required, contained in the activated sludge, is supplied continuously by means of an internal recycling of the activated sludge from the aerobic tank to the anoxic tank.
The oxygen required for the aeration in the aerobic tank is provided by any mechanical aeration means (turbomixer, brushes, supercharger, etc.). The capacity of the aeration means (QO
2
) is characterized by their hourly power expressed as kilograms of oxygen per hour (kgO
2
/h). This hourly power is calculated according to the following principle:
QO
2
=daily amount of oxygen/daily running time.
The daily amount of oxygen is the amount required to treat all the carbon-based and nitrogen-based pollution, the daily running time of the aeration means generally being about 20 hours per day. With this treatment technique, the hourly aeration power is about {fraction (1/20)} of the daily amount of oxygen to be supplied.
b) One-Tank Technique
In this known technique, illustrated by
FIGS. 2
a
and
2
b
, the two steps of nitrification and denitrification proceed in a single tank with alternation of the aerobic and anoxic phases. Thus, the same biological tank serves alternately as the aerobic tank and as the anoxic tank so as to perform all of the nitrogen treatment process.
FIG. 2
a
shows the operating mode of the single tank in the aerobic phase (first step—nitrification) and
FIG. 2
b
shows the operating mode of this tank in the anoxic phase (second step—denitrification).
The raw water, i.e. the effluent to be treated, supplies the carbon required by the heterotrophic bacteria that perform the denitrification in the anoxic phase (second step).
The oxygen required for the aeration during running in the aerobic phase is supplied by any mechanical aeration means, as in the case of FIG.
1
. The power of the aeration means (QO
2
) is determined in the same way as indicated above in the case of the treatment in two tanks.
The daily running time of the aeration means incorporates the time required for the anoxic phases (no aeration), this being directly proportional to the amount of nitrogen to be treated. Consequently, the duration of the aeration over a 24-hour cycle is much shorter in the case of this one-tank technique compared with the two-tank technique, for the same pollutant load to be treated. In general, this aeration time is about 12 hours per day.
In this technique, the hourly aeration power is about {fraction (1/12)} of the daily amount of oxygen to be supplied.
A comparison of the costs of the two nitrogen treatment techniques reviewed above is summarized below.
In these two techniques, the total volume of the biological aeration tank (cumulative volume of the two aerobic and anoxic tanks in the first technique or total volume of the tank operating alternately according to the second technique) will be the same for an identical pollution load to be treated. The essential differences between the two techniques are the following:
the constructional arrangements of the biological stage including the two-tank configuration are more expensive than the one-tank configuration;
the power of the aeration means, which is about 70% higher in the one-tank technique than that required in the two-tank technique.
Since the two treatment techniques are of equivalent acknowledged efficacy, insofar as the rules of the art are complied with, the choice of one or other of these techniques is usually made on the basis of economic considerations. Specifically, it is commonly accepted that for small purification plants, the one-tank technique requires less investment than the two-tank technique. This advantage decreases as the treatment capacity increases, and is totally reversed in the range of large treatment units, the cost of the aeration systems then becoming predominant over the constraints imposed by the two-tank configuration.
2) Treatment of Phosphorus
In the current state of the art, the treatment of phosphorus is performed via two routes, a physicochemical route and a biological route, respectively.
a) Physicochemical Treatment Route
The implementation of this treatment is illustrated by FIG.
3
. It consists in injecting a metal salt (for example FeCl
3
) in an amount which is suitable for the required level of treatment, so as to precipitate the phosphorus to be removed in the form of a metal phosphate. As may be seen in
FIG. 3
, the reagent may be injected into several points of the biological system: into the aerobic tank or between the aerobic tank and the clarifie
Demain Daniel
Dupont Frederic
Huber Olivier
Wouters-Wasiak Katia
Connolly Bove & Lodge & Hutz LLP
Degremont
Prince Fred G.
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