The present invention relates a coagulator, and more particularly it relates to a coagulator in which an inorganic coagulant such as an aluminum-based coagulant is added to raw water to coagulate and separate suspended solids in the raw water.
Coagulators are apparatuses used to treat water for use where water from a river or the like is treated so that it can be used as municipal or industrial water, and where waste water such as public sewage or factory waste water, is treated to meet regulatory limits. In particular, an upflow type coagulator is often employed because such a coagulator has high coagulating and filtering efficiencies and is easy to operate.
In an upflow coagulator, raw water to which a coagulant is added is passed upward through the interior of a packing medium accumulation layer formed by stacking small pieces of packing medium having a high void ratio to thereby flocculate and settle suspended solids in the water.
Referring now to FIG. 5, description will be given of a configuration and an operation of a conventional upflow type coagulator 10.
This typical conventional upflow type coagulator 10 comprises, as shown in FIG. 5, a raw water tank 12, a raw water pump 14 for pumping raw water from the raw water tank 12 to feed the raw water, coagulant adding facilities 16, a coagulant mixing tank 18 and a coagulation tank 20.
The coagulant adding facilities 16 include a turbidimeter 22 for measuring the turbidity of the raw water, a coagulant tank 24, a coagulant pump 28 for injecting the coagulant from the coagulant tank 24 into a raw water supply pipe 26 on the downstream side of the turbidimeter 22 so that a desired amount of the coagulant may be added to the raw water on the basis of a measured value of the turbidimeter 22.
The coagulant mixing tank 18 is a container fitted with a stirrer 29, in which the raw water bearing the coagulant is temporarily stored and stirred by the stirrer 29 to rapidly mix the raw water and the coagulant. The water bearing the coagulant is then sent via an inflow pipe 30 to the coagulation tank 20.
The coagulation tank 20 is a tank in which the suspended solids in the water which have been aggregated by the coagulant are coagulated, flocculated, filtered and separated. As shown in FIG. 6, the coagulation tank 20 consists of a water inflow zone 32, a packing medium accumulation zone 34, and a water collection zone 36 which are partitioned in sequence from the underside.
The packing medium accumulation zone 34 is partitioned by perforated outflow prevention plates 38, 40 which are provided in an upper part and a lower part of the zone. In this packing medium accumulation zone 34, many small packing media having a small specific gravity and a high void ratio are accommodated, and a packing medium accumulation layer 44 is formed below the upper outflow prevention plate 38 with the upflow of the water. As the small packing media for forming the packing medium accumulation layer 44, for example, there are employed short tube-shaped plastic small packing media 42 having a relatively small specific gravity as shown in FIG. 7.
The water collection zone 36 is a region for collecting the treated water which has been treated through the packing medium accumulation zone 34, and this zone 36 consists of a water collecting part 46 disposed immediately above the outflow prevention plate 38 of the packing medium accumulation zone 34, a water collection trough 48 for collecting the treated water overflowing from the upper end of the water collection part 46, and an outflow pipe 50, connected to the water collection trough 48, for sending the treated water to a treated water tank 52 (see FIG. 5).
The raw water leaving the coagulant mixing tank 18 flows via the inflow pipe 30 into the inflow zone 32. The inflow pipe 30 extends to the middle of the inflow zone 32, and has at its tip a downward opening. An inverted umbrella-shaped baffle plate 54 is provided under the opening of the inflow pipe 30 to change the direction of flow of the raw water from downward to upward. An alkaline agent injection pipe 56 is also connected to the inflow pipe 30 so as to inject an alkaline solution, if desired, for the control of the pH of the raw water.
In the lower part of the inflow zone 32, that is, below the baffle plate 54, there lies a hopper-shaped sludge storage zone 58 for storing the sludge, to the lowermost part of which is connected a sludge discharge pipe 60 for discharging the sludge.
Above the inflow zone 32 is disposed an air supply pipe 62 having a plurality of air nozzles for jetting air upward, so as to eject air fed by the air blower 64 to thereby stir and wash the packing media 42 of the packing medium accumulation zone 34.
In the coagulation tank 20, the water bearing the coagulant first flows into the inflow zone 32. In this inflow zone 32, among flocs formed as a result of coagulation of the suspended solids in the raw water, relatively large flocs are first settled and separated.
The water then flows into the packing medium accumulation zone 34, in which micro-flocs remaining in the water come into contact with the packing media and adhere onto the external surfaces of the packing media, or are captured in the interstices between the respective packing media 42 and are separated. Water flows upwardly through voids of the packing media 42 or flows through between the respective packing media, and is filtered through the floc layer formed in the voids or between the packing media, while simultaneously the micro-flocs in the raw water are captured by the floc layer.
Flocs which have adhered onto the packing media 42 or have been captured between the packing media 42 gradually grow due to contact with the subsequent micro-flocs or the like, resulting in flocs of increased diameter. Then, accordingly as the flocs having a higher settlement velocity than the upward flow rate of the raw water become formed, these flocs are dislodged from the packing media 42 by the flow of the water, settle against the flow of the water, retained in the sludge storage zone 58, and then discharged via the sludge discharge pipe 60.
In this manner, suspended solids in the raw water are separated from the water and settled in the sludge storage zone 58 by the agglomeration of the suspended solid flocs, the filtration of raw water through the layer of flocs, the separation and settlement of the agglomerated flocs, and the like. On the other hand, raw water thus treated flows out from the water collection zone 36 into the treated water tank 52.
When the packing medium accumulation layer 44 of the packing medium accumulation zone 34 clogs, air jets through the air nozzles of the air supply pipe 62 to stir and wash the packing medium accumulation layer 44.
This upflow type coagulator enables treatment to be performed at a high speed because the density of the coagulated flocs which have become thickly agglomerated is high as is settlement speed. Accordingly, the facilities become compact, so that facility installation area can be reduced, the amounts of chemical agents can be decreased, and the treatment and disposal of the generated sludge can be simplified.
While upflow type coagulators have many advantages, for the purpose of further heightening its treatment efficiency, various improvements have been made.
For example, it has been attempted to prolong water treatment time between the respective wash treatments by the use of packing media having a large size in place of the small packing media which constitute the packing medium accumulation layer, but this results in a problem that the turbidity of the treated water increases. Conversely, when the packing media having smaller sizes are used in order to decrease the turbidity of the treated water, the wash frequency of the packing medium accumulation layer increases, so that there arises a problem that the raw water treatment time is shortened.
Thus, an object of the present invention is to provide a coagulator which can decrease the frequency of the media wash operation while the turbidity of treated water is maintained at a low level.
A coagulator in accordance with the present invention has a first coagulation part. This first coagulation part possesses at least one packing medium accumulation layer through which raw water is passed at a superficial velocity higher than that of a packing medium accumulation layer in a subsequent coagulation and sedimentation part. Accordingly, primary treated water subjected to coagulation in the packing medium accumulation layer in the upstream coagulation part can be allowed to flow into the subsequent coagulation and sedimentation part.
The superficial velocity (a velocity obtained by dividing a water passage flow rate by a sectional area of the packing medium accumulation layer) in the packing medium accumulation layer in the first coagulation part should be higher than the superficial velocity (e.g., 150 to 800 m/day, preferably 300 to 500 m/day) in the packing medium accumulation layer in a downstream coagulation and sedimentation part. For example, the former velocity can be about twice that of the latter velocity.
Small packing media which constitute the packing medium accumulation layers of the upstream coagulation part and the downstream coagulation and sedimentation part have high void ratios. No particular restriction is put on the shape, material and kind of the small packing media, so long as they can be accumulated to form the packing medium accumulation layer, but these small packing media preferably should have voids which functions as water passages at a high void ratio of 60% or more and have such large surface areas that the surface area per m3 of the accumulated packing media is 200 cm2 or more, preferably 300 cm2 or more.
For example, as the small packing media, there may be preferably used plastic tubes having a diameter of about 4 mm and a length of about 4 mm, hollow spheres having many holes on their surfaces, a Tellerette packing or the like, but they are not particularly restrictive.
The small packing media which constitute the packing medium accumulation layer of the upstream coagulation part may be similar, in shape and size, to the small packing media which constitute the packing medium accumulation layer of the downstream coagulation and sedimentation part. Alternatively, at least one of the shape and size of the small packing media which constitute the packing medium accumulation layer of the downstream coagulation part may be different from that of the small packing media which constitute the packing medium accumulation layer of the coagulation and sedimentation part.
Suitably, the small packing media which constitute the packing medium accumulation layer of the downstream coagulation and sedimentation part has the same shape as the small packing media which constitute the packing medium accumulation layer of the upstream coagulation part and has sizes smaller than that of the small packing media which constitute the packing medium accumulation layer of the upstream coagulation part.
With regard to the raw water which can be treated in the coagulator of the present invention, its source and quality are not limited, and for example, water having a turbidity of several degrees to 2000 degrees is applicable. In this specification, raw water refers to water which is introduced into the coagulator, and includes water such as river water, well water, lake water, and swamp water, as well as waste water which is introduced into the coagulator.
No restriction is imposed on the coagulant to be added to the raw water, as long as it has a coagulating effect for the suspended solids in the raw water. The coagulant may preferably be, for example, aluminum salts such as aluminum sulfate and polyaluminum chloride.
If a ratio of coagulated flocs having diameters more 100 xcexcm in the raw water (added with the coagulant) which is passed through the packing medium accumulation layer increases, the viscosity of the flocs which adhere to the packing media and grow thereon increases, so that the separability of the flocs from the packing media and the dewaterability of collected sludge deteriorate inconveniently. Therefore, the ratio of the flocs having diameters more than 100 xcexcm should better be as low as possible, and it is necessary to control the dosage level of the coagulant so that this ratio does not exceed 5% or so. The dosage level of the coagulant depends on its kind, the quality of the raw water and the like, and hence is previously set to a preferred value by means of experiments or the like.
In the case of using an aluminum based inorganic coagulant, its dosage level, which may vary depending on the turbidity of the raw water, may usually be in the range of 0.1 to 0.001, and preferably 0.05 to 0.005 in terms of an ALT ratio [aluminum (AL) dosage level/turbidity]. By employing the ALT ratio in this range, the suspended solids in the raw water are so coagulated as to form micro-flocs of the suspended solids having dimensions of 100 xcexcm or less, preferably several xcexcm to several tens xcexcm.
When the upstream coagulation part is installed and when the superficial velocity in the packing medium accumulation layer of the upstream coagulation part is regulated to be higher than the superficial velocity in the packing medium accumulation layer of the downstream coagulation and sedimentation part, the micro-flocs introduced to the upstream coagulation part grow into the coarse flocs having large sizes and most of the coagulated coarse flocs are discharged from the packing medium accumulation layer without stagnating in the packing medium accumulation layer. In the downstream coagulation and sedimentation part, therefore, the thus discharged coagulated coarse flocs are allowed to settle as much as possible by weight and then removed, and the remaining micro-flocs alone are introduced into the packing medium accumulation layer of the downstream coagulation and sedimentation part and they grow into the coarse flocs therein to be captured by the packing medium accumulation layer.
That is to say, in the upstream coagulation part, the micro-flocs produced by the addition of the coagulant to the raw water can be coarsened, and the thus coarsened coagulated flocs are settled and separated early in the downstream coagulation and sedimentation part to reduce the amount of the flocs per unit amount of the raw water which flow into the packing medium accumulation layer of the downstream coagulation and sedimentation part, whereby the occurrence of the clogging of the packing medium accumulation layer in the downstream coagulation and sedimentation part can be decreased and hence a packing medium wash interval can be prolonged.
In particular, when the small packing media constituting the packing medium accumulation layer in the downstream coagulation and sedimentation part are formed so as to have the same shape as,the small packing media constituting the packing medium accumulation layer in the upstream coagulation part and so as to have smaller sizes than the small packing media constituting the packing medium accumulation layer in the upstream coagulation part, the turbidity of the treated water can be decreased. Furthermore, in the upstream coagulation part, the micro-flocs are coarsened, but the coarsened flocs are discharged as much as possible from the packing medium accumulation layer without stagnating in this layer. On the other hand, in the packing medium accumulation layer in the downstream coagulation and sedimentation part, the micro-flocs which have not been coarsened in the upstream coagulation part are securely captured. As described above, the sharing of functions is clarified so that the effect of the present invention can be improved.
Additionally, in the present invention, most micro-flocs grow into large size coagulated flocs in the upstream coagulation part, and, when these flow into the downstream coagulation and sedimentation part, they are settled and separated at an early stage. As a result, the amount of coagulated micro-flocs which flow into the packing medium accumulation layer in the downstream coagulation and sedimentation part decreases, thereby permitting the use of smaller packing media compared with conventional media, without shortening the medium wash interval of the packing medium accumulation layer in the downstream coagulation and sedimentation part.
The use of the smaller packing media in the downstream coagulation and sedimentation part enables the coagulated flocs to be densely held in the packing medium accumulation layer, and the contact chance of the coagulated flocs and the subsequent micro-flocs and the like can be increased to reduce the turbidity of the treated water.
The packing medium accumulation layer in the upstream coagulation part and the packing medium accumulation layer in the downstream coagulation and sedimentation part may be disposed in turn from bottom to top in one treatment tank so that the raw water and the primary treated water (raw water created in the upstream coagulation part) may flow upwardly. In this case, the two packing medium accumulation layers are partitioned by a perforated partition plate having such an opening that the small packing media constituting the packing medium accumulation layers may not be mixed with each other.
Furthermore, the packing medium accumulation layer in the upstream coagulation part and the packing medium accumulation layer in the downstream coagulation and sedimentation part may be disposed in parallel in one treatment tank so that the raw water may flow through the packing medium accumulation layer in the upstream coagulation part in the manner of an upward or downward flow and the primary treated water may flow through the packing medium accumulation layer in the downstream coagulation and sedimentation part in the manner of the upward flow.
In addition, the packing medium accumulation layer in the upstream coagulation part and the packing medium accumulation layer in the downstream coagulation and sedimentation part may be disposed in different treatment tanks, respectively, so that the raw water may flow through the packing medium accumulation layer in the upstream coagulation part in the manner of an upward or downward flow.
Suitably, there are disposed a turbidity detecting part for measuring the turbidity of the raw water, and a dosing control part in which the amount of an aluminum salt to be dosed by a dosing part is controlled in the range of 0.1 to 0.001 in terms of an ALT ratio on the basis of turbidity values as determined by the turbidity detecting part.
Moreover, an additional constitution may be provided in which when the turbidity of the treated water exceeds a predetermined turbidity level on the basis of turbidity values of the treated water as determined in the turbidity detecting part, the packing medium accumulation layer in the downstream coagulation and sedimentation part can be automatically washed.