Waste water treatment using activated sludge process

Details Waste water treatment using activated sludge process Waste water treatment using activated sludge process

2001-05-22

2003-07-08

Barry, Chester T. (Department: 1724)

Liquid purification or separation

Processes

Including controlling process in response to a sensed condition

C210S744000, C210S104000, C210S138000, C210S143000, C210S197000

Reexamination Certificate

active

06589428

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to improvements to the treatment of wastewater using activated sludge processes. It relates more particularly to a process and to a plant for controlling the retention time of the sludge undergoing clarification in a process for the treatment of wastewater by activated sludge comprising a recirculation of the activated sludge from the clarification stage to the aeration stage.
BACKGROUND OF THE INVENTION
It is known that in such activated sludge processes, the recirculation of the sludge is a significant component in the overall operation of purification plants. The role of the secondary clarifiers consists in separating the sludge from the treated water, and to do this a sludge-concentrating stage is essential in the structure. This concentrating phase requires a retention time imposed on the sludge in the clarifier, and, if this retention time is not controlled, the process drifts towards malfunction logic.
Too long a retention time firstly results in anoxia conditions, with an immediate effect of potential denitrification and the appearance of froth on the surface of the clarifiers. Secondly, the sludge comes into anaerobic conditions, the repercussions of which are often disastrous on the water system and the treatment of the sludge since the development of filamentous microorganisms is favoured, which causes, over the entire plant, foaming problems and problems of poor flock settling (a rise in the Mohlman index). The risk of sludge egress is t hen increased in the event of a hydraulic surge. In addition, anaerobic conditions are completely incompatible with biological dephosphatizing processes, and in this case the release of phosphorous into the clarifier results in the discharges being immediately off-specification. These events are thus characteristic of a major malfunction of the water system. In parallel, and from a direct economic standpoint, the sludge treatment is not optimized when the flock-settling properties are poor, whatever the treatment system involved. The operating times of the equipment of the sludge treatment system are lengthened, the solids contents are reduced and, immediately, the volumes of sludge extracted are increased for the same mass of treated dry matter.
In the clarification stage, the retention time of the sludge must therefore remain limited, with the risk of causing malfunctions in the water treatment. This limit is about 2 hours in the case of activated sludge processes operating with prolonged aeration. It is less in the case of moderate or high load conditions.
On the other hand, too short or variable a retention time for sludge under clarification may also be a limiting factor in the case of sludge treatment systems. This is because it gives rise to a sludge concentration not suitable for certain thickening and dehydrating structures when the sludge is extracted from the recirculation line. This is generally the case for small-sized and medium-sized plants equipped with structures for thickening by draining. The dilution of the sludge entering this type of equipment results in insufficient capture rates or in the need for excessive dosing with flocculating polymer, resulting in risks of blockage. variations in load on these conditioning apparatuses often result in malfunctions, such as creep or blockage, which are connected with the modifications in the operating conditions predefined during the initial set-up operations. The minimum permitted concentration on the draining screens or tables is about 6 g/l. The variations in loads withstood by this type of equipment are about 10 to 20%. These conditions on the concentration, which corresponds to the concentration encountered in the sludge well, are directly connected with a constraint on the minimum retention time in the clarification stage. This minimum retention time itself depends on the sludge settleability properties.
Maintaining and controlling a mean retention time of the sludge undergoing clarification are therefore of prime importance for ensuring quality, reliability and economy of the treatment. This is indeed the technical problem that the present invention aims to solve.
In the technology of water treatment with regard to the activated-sludge recirculation function, two types of recirculation in a purification plant may be identified:
recirculation which returns the sludge from the clarifier into the aeration basin: the objective is to recycle part of the biological sludge so as not to impoverish the purifying mass of the aeration basin;
recirculation of mixed liquors which exist in the plants having an anoxia basin and an aeration basin; they recover the sludge from the aeration basin rich in nitrates in order to bring it into the anoxia region so that the denitrification can reduce the contents of the discharges.
The process forming the subject-matter of the present invention relates to the recirculation of activated sludge from the clarifier into the aeration basin. Given that the various arrangements for the activated sludge to be recirculated from the clarifier into the aeration basin form part of the conventional technical knowledge of those skilled in the art in the water treatment field, they will not be described. The reader may refer for this purpose to Mémento Technique de l'Eau, Ninth Edition (1989).
The general principle of managing the recirculation of activated sludge in water treatment will now be explained.
The objectives of management suitable for sludge recirculation must satisfy the criteria below:
to prevent the sludge in the clarifier from undergoing anaerobiosis in order to reduce any risk of malfunction;
to adapt the operating mode of the plant to the conditions encountered in operation (sludge masses in the system, sludge-settling properties, rain showers, etc.), so as in particular to prevent sludge bed egress;
to manage the distribution of the total mass of sludge between the biological basin and the clarifier in order to make the biological treatment reliable.
Reference is made to
FIG. 1
of the appended drawings, which is a diagram illustrating the construction of the sludge flow balance and in this diagram the aeration basin is represented by 1 and the clarifier by 2.
The amount of recirculation of the sludge from the clarifier 2 into the aeration basin 1 is by definition the ratio of the rate of recirculation to the throughput of the plant:
&tgr;
rec
=Q
r
/Q
t
where &tgr;
rec
denotes the rate of recirculation (0<&tgr;
rec
<1)
Q
r
denotes the rate of recirculation (m
3
/h) and
Q
t
denotes the throughput (m
3
/h).
This amount of recirculation is defined by the conditions encountered on the site. There must be an overall balance between the sludge flows into and out of the clarifier:
(
Q
t
+Q
r
)×
C
as
=Q
r
×C
r
where C
as
is the sludge concentration (g/l) in the aeration basin and
C
r
is the sludge concentration (g/l) in the recirculation line.
The storage term is neglected here, just like the treated-water discharge outflow.
This equation is used to calculate the degree of recirculation &tgr;
rec
:
 &tgr;
rec
=Q
r
/Q
t
=C
as
/(
C
r
−C
as
)
C
as
is an operational parameter that can be measured directly in the aeration basin. As regards C
r
, this must be controlled so as to meet the requirements on the retention time of the sludge in the clarifier since it has been seen, empirically, that the sludge retention time is linked to the Mohlman index, representative of the sludge-settling and thickening properties, and to the sludge concentrations in the aeration basin and in the recirculation line. The following correlation has been established between these variables:
RT
/60=(
C
r
MI
/1000)
3
−(
C
as
MI
/1000)
3
where RT is the retention time (in minutes) of the sludge undergoing clarification and
MI is the Mohlman index (ml/g).
This formula is the reference relationship for the automated recirculation management logic tool. It is based on a formula of the same type published by ATV (ATV Standard, A 131. (19

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