Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Reexamination Certificate
1999-07-22
2001-04-03
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
C210S713000, C210S714000, C210S727000, C210S738000
Reexamination Certificate
active
06210588
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the physico-chemical treatment of effluent, especially of surface water intended for consumption.
BACKGROUND OF THE INVENTION
It is known that the use of physico-chemical processes is common to most treatments applied to various types of water and that these treatments essentially consist of:
clarification of surface water for consumption or for industry;
clarification of municipal sewage, storm water or industrial waste water;
decarbonization;
removal of phosphates;
etc.
These types of physico-chemical treatments always comprise the following successive steps:
coagulation: a step of neutralization of the colloids using a metal salt, generally a trivalent iron or aluminum compound, in order to form a microflock. This coagulation step may be carried out in one or more steps;
flocculation: a step of agglomeration and growth of the microflock. This agglomeration step takes place by virtue of the addition of a polyelectrolyte (or polymer) downstream of the coagulation step;
settling: a step of separation of the flock from the interstitial water, causing the formation of sludge on the one hand, and-of clarified water on the other hand.
Over the last thirty years or so, the state of the art relating to such a physico-chemical treatment has evolved considerably as a result of the appearance of two technologies:
flocculation with a contacting mass, which has allowed the quality of the flocks to be improved, the volume of the reactors to be reduced and the clarification to be improved. This is because the microflocks of the coagulation have a greater chance of agglomerating and of growing as the reaction medium contains a high density of particles: the rate of flock formation is proportional to the number of free particles in the suspension;
lamellar settling, carried out by introducing inclined plates or tubes in the settling tanks. This technology has made it possible to reduce the size of the settling tanks by from 50 to 70%.
The current technological trend is towards improving the flocculation conditions, which are key in determining the quality of the treated water and in obtaining high settling velocities.
At the present time, modern settling tanks use two types of contacting masses in the flocculation reactor:
1. recirculated presettled sludge: an example of this technique is described in FR-A-2,553,082;
2. fine ballasts, such as microsand: an example of the use of this technique is described in FR-P-1,411,792 and in FR-A-2,627,704.
The present invention relates to improvements made to apparatus employing recirculated presettled sludge, these improvements being such that the apparatus, while still maintaining their specificities and their advantages, operate at much greater settling velocities.
Before describing the improvements, the advantages and disadvantages of this known technique, using a sludge contacting mas, will be explained.
FIG. 1
of the appended drawings shows diagrammatically a physico-chemical treatment plant employing this technique. This figure shows diagrammatically, at A, the coagulation reactor, at B, the flocculator and, at C, the settling tank. These are plants well known to those skilled in the art and, under these conditions, they will not be described.
Thus, as may be seen in this
FIG. 1
, the contacting mass in the flocculation reactor B consists of the recirculation of part of the sludge which has settled in C, this recycled part of the sludge being introduced into the flocculator B via the pipe
16
and the recirculating pump
18
. The recirculated sludge volume represents between 0.5 and 4% of the treated volume. The recirculated sludge, the raw water and the polyelectrolyte are brought into contact with each other in a highly turbulent zone, the polymer, as may be seen in
FIG. 1
, generally being injected near the propeller
10
of the flocculator B. The excess, concentrated sludge is extracted and removed.
The advantages of this flocculation technique using recirculated presettled sludge as the contacting mass are the following:
1—the contacting mass is generated by the process, and is therefore available without any quantity limitation, depending on the requirements of the process;
2—in the flocculation reactor B, the mass of sludge due to the recirculation is very high compared to the suspended matter brought in by the raw water. As a result, the system is insensitive both to significant increases and significant decreases in the amount of suspended matter in the raw water;
3—the contacting mass presents a very high specific surface area or spatial occupation because of its expanded structure and its low relative density; by way of example, 1 gram of flocculated sludge in one liter (average concentration in the reactor) occupies, after settling for approximately 5 minutes, a volume equal to 100 ml. This very high specific surface area or spatial occupation considerably increases the probability of contact between the flocks and the very fine particles, coagulated colloids and micro-organisms, and therefore the probability of “trapping” this suspended matter very efficiently;
4—because of the continuous recirculation of ever-reflocculated sludge, the latter densifies. Thus, the extracted sludge is highly concentrated (two to ten times more concentrated than the sludge in most apparatuses);
5—this technique makes it possible to achieve relatively high treatment rates. Thus, when clarifying river water, the announced rates through the lamellar modules of the settling tank are between 10 and 25 m
3
/m
2
.h, which corresponds to settling velocities UD (the flow rate/raft area ratio of the settling tank) of 6 to 15 m/h. These velocities are in fact limited by the limiting mass flux (Fml) of the flocculated suspension expressed in kg of suspended matter flowing per m
2
of settling tank raft and per hour (kg/m
2
/h).
The mass flux is the limiting factor which determines the limiting theoretical settling value Udl. This value is also related to the concentration CR of the sludge in the reactor B, expressed in kg/m
3
:
Fml=CR×Udl
i.e.: Udl=Fml/CR.
If the settling velocity UD applied to the settling tank C is equal to or greater than Udl, there is clogging. The lamellar settling tank is effective as a finisher, but it is incapable of retaining a sludge bed.
It is therefore necessary to check that the mass flux applied to the settling tank is less than Fml or that the settling velocity UD applied to the settling tank is less than Udl.
For example, in the case of river-water clarification, the limiting mass flux is generally less than or about 20 kg/m
3
.h. The concentration CR required for good flocculation is approximately 1 kg/m
3
. The limiting settling velocity Udl is then 20m/h, hence the velocity UD applied to the settling tank is less than 15 m/h for safety reasons;
6—often such apparatus are followed by a filtration system. This is the case with river-water clarification. The filtration is characterized by the quality of the filtered water and by the filtration cycle time (the period of operation after which the maximum caking of the filter is achieved, requiring the latter to be washed). In the case of settling tanks with sludge recirculation, the filtration cycle time is generally greater than 24 h.
In order to determine the ability of the clarified water to be filtered using correct filtration times, tests representative of the clarified-water filter-ability are carried out. Thus, one of the tests that can be used consists in measuring the time necessary to filter 250 cm
3
of clarified water on a 5 &mgr;m membrane under a vacuum of 8×10
4
Pa. The water will be more easily filtered in a shorter time. In the case of a settling tank operating with a settling velocity UD equal to 15 m/h, the filterability is approximately 30 seconds;
7—increasing the settling velocities UD is possible, but at the cost of increasing the dose of polyelectrolyte. However, an excess of polyelectrolyte reduces the filterability of clarified water (increase in the time of
Connolly Bove Lodge & Hutz
Degremont
Hruskoci Peter A.
LandOfFree
Method for the physico-chemical treatment of effluents in... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for the physico-chemical treatment of effluents in..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for the physico-chemical treatment of effluents in... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2511919