SEQUENTIAL DISINFECTANT TREATMENT FOR WASTE WATER
DESCRIPTION OF THE INVENTION The present invention is generally related to a method and apparatus for treating wastewater and, more specifically, to a method and apparatus for treating wastewater to reduce bacterial contamination thereof through a sequential dosing of a stream or column of wastewater with a single disinfecting agent such as chlorine. The pollution attributable to the discharge of wastewater into water densities such as rivers, lakes, estuaries and larger water densities have long been recognized as a significant public health problem due to the presence of bacteria and microorganisms within them. viruses that present threats to humans and other animal life. Untreated wastewater typically carries fecal matter, which includes coliform bacteria, which presents a significant health risk. Chlorination is a well-known method for treating wastewater to reduce the levels of bacteria and viral microorganisms to recognized and acceptably low levels. The chlorine solution in the water forms a mixture of hypochlorite (HC10) and hydrochloric acid (HCl). The latter decomposes completely in hydrogen ions and chlorine, while the
. , «, Í« -lt, fc, first it only partially decomposes in hydrogen and hypochlo- prate ions, in relation to the pH of the water. In any case, chlorine effectively destroys and kills viral microorganisms and bacteria present in wastewater. Chlorine also, unfortunately, reacts with other substances typically contained in wastewater to form, over time, other compounds. For example, chlorine reacts with ammonia to form chloramines. In this way, with the passage of time, chlorine becomes depleted of residual water or, put another way, the chlorine demand depends on time. The amount of active chlorine present in the wastewater at any given time is referred to in the art as "free chlorine residue", or simply "residual". The residual can be determined as the difference between the demand for the time of determination and the total chlorine dose introduced into the wastewater. In stage one drainage water treatment plants, a common method for disinfecting wastewater is by injecting chlorine gas and / or liquid into the water as it enters the "contact tank". In such a disinfection process, it is important to provide a sufficient dose of chlorine to make "contact" with an effluent wastewater density for a sufficient period of "contact time" to effect disinfection of the water
l? a i A * ¿i - _ i - - • * - * »* up to an acceptably low level of contamination. The dose and contact time should be sufficient to achieve the desired level of disinfection without a large excess of residual chlorine in the treated water. For example, a typical public health standard 5 may require disinfection of wastewater so that there are no more than 5000 coliform colonies per millimeter ("cfu / ml") in treated water with a residual chlorine level of approximately 0.1 parts per million (ppm) A typical design for a "contact tank" is
10 shows in Figures 1 and 2. In said contact tank, the waste water is disinfected by providing a single initial dose of chlorine in the water as it enters the contact tank. As shown in Figure 2, chlorine is typically injected through, or through the entire area in cut
Cross section of a moving wastewater column that enters the contact tank in a single dosing location. The chlorine dose is transported with the water in a sealed flow manner and disinfects the water as it passes through the tank. A typical problem with
20 said disinfection process of a single dose is that the chlorine has a short half-life in the water. In this way, the initial dose of chlorine must be carefully selected to be of sufficient magnitude to ensure that the free chlorine residue will be sufficient during the time of passage to
25 through the contact tank to disinfect the water to a
The minimum desired level of contaminants without an unacceptable level of residual chlorine remaining at the end of the treatment process A system that employs the single-point dose chlorination process found in US Patent 4,019,983 (for Mandt) issued April 26, 1977. Mandt discloses a method for disinfecting residual liquid as a sewage effluent by means of a single dose of a disinfecting agent, Mandt particularly shows a method wherein the agent Disinfectant and residual liquid pass through a turbulent flow zone for mixing purposes A significant problem with the traditional single point chlorination process can be attributed to the application of chlorine only at the contact tank inlet , since it is very difficult to accurately judge the single dose of adequate chlorine that should be added to the wastewater.The appropriate dose should be adjusted according to the The amount of bacteria in the wastewater is reduced, as is the amount of effluent in that wastewater. A further problem is due to the extended residence time of each portion of the wastewater flow in the contact tank, typically around 30 minutes for a tank of approximately 70 meters in length. Because chlorine is
l LlA, Í M. * ..t > ~ - »-» - t _. jaj a-A, -. "..." _.... ^^ ^ .. ^ .--. _ _. "JJ, J injected into the waste water at a single point, results in the aforementioned" plug flow ". As each "seal" or segment of wastewater flows through the contact tank, the chlorine reacts with the inherent organic material and bacteria in the surrounding wastewater and with any effluent in that wastewater. Those reactions reduce the residual of free chlorine available for the treatment of harmful microorganisms and can, in some circumstances, reduce the residual of free chlorine to an insufficient level for an effective treatment. Thus, the single dose of suitable chlorine at the entrance of the contact tank must be varied to accommodate the flow velocity, the length of the flow path through the contact tank, the load of viral and bacterial contaminants (bacterial load). usually being used as the standard), the concentration of effluent in the wastewater, and the inherent chlorine demand attributable to the organisms in the wastewater. The failure to properly regulate the chlorine dose can result in either an unacceptably high level of microorganism colonies at the end of the treatment process or an unacceptably high level of residual chlorine. Since the above result is more undesirable, the conventional approach is to introduce a dose of excessive chlorine, resulting in excessive residual chlorine but at least
Ifef "f? TiuntlÉ-» - • < -MttnHn 'n - - -. - *. > .. «~. ^ .. - -», ^. I. »! Complying with the public health standards required However, such an approach requires a relatively higher chlorine generating capacity, with attention to the greater capital and operating expenses Finally, as mentioned previously, chlorine shows a short half-life, thus, when using the flow of chlorine. Sealing of the wastewater to "transport" a single dose of chlorine along a total length of the flow path through the contact tank during the residence time of the prolonged tank is ineffective due to the resulting deterioration of the chlorine concentration The problems associated with single-dose processes are further compounded by variations in wastewater composition.This may be particularly true in Hong Kong and other places where the entire drainage system uses seawater or other water. It is water that exhibits organic content and highly variable bacterial load. Because seawater has exceptional seasonal variations in composition, including their inherent organic content and their bacterial load, the continuous obtaining of both the desired bacterial count norm and the standard of residual chlorine level in the treated water is very difficult. The inability to comply with both standards is unacceptable where a water permit or other environmental regulation establishes specific limits for both bacterial counts and residual chlorine levels. As mentioned above, typical treatment processes apply a high dose of chlorine to the individual initial injection site to ensure adequate disinfection throughout the residual water flow path. However, the resulting chlorine residual in the treated water is unacceptably high from an environmental point of view because it harms fish and other marine life, which prevents commercial harvests that produce marine life and may also have an adverse effect on tourism that depends on fishing. Technologies other than chlorine dosages, such as ultraviolet and ozone treatments, have been developed to disinfect wastewater. These alternatives are unacceptable in many places, however, due. operating and capital expenses higher than those associated with the conventional chlorination technique described above. These technologies can also present safety and environmental problems not associated with the conventional chlorination technique. An example of an alternative system is included in the North American Patejite 4,690,764 (for Okumura et al.) Issued on September 1, 1987. The Okumura patent
t & Aíá &? t ?. .... to ylsA &. * _ ,. «•» * »* - * - describes an aerator that includes a steam generator to inject a gas, such as oxygen or ozone, and a liquid in a mixed state from a nozzle to treat wastewater. The Okumura patent further describes the use of multiple aeration nozzles in separate locations in a tank but does not teach the application of said system for chlorination techniques. In addition, the Okumura patent does not teach a general disinfectant reduction in relation to a single-dose system. Additional systems employing multiple nozzles in a system include British Patent 1,263,915 (for addleton) issued on February 16, 1972 and PCT Application Publication No. WO98 / 51404 (for Life Technologies, Inc.) published on November 19, 1998. The Waddleton patent describes a method and apparatus for inhibiting the growth of sludge in paper machines including the injection of a sludge inhibitor substance at multiple dose points within the machine. However, the addleton patent is related to a continuous treatment of recycled fluids within the paper shredding machine which does not indicate that said system can be applied to the wastewater treatment. The Life Technologies application, while describing a fluid system that has multiple nozzles in separate locations, deals with the continuous preparation of
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formulations of a cell culture medium and buffered saline solutions of selected groups of media concentrates. The multiple nozzles allow the introduction of different concentrates of media to mix with an adverse chemical reaction. However, the Life application
Technologies does not describe the use of multiple nozzles that are used in wastewater treatment. In addition, the Life Technologies application does not teach the use of multiple nozzles to deliver multiple doses of the same medium in separate locations. The present invention includes a method and apparatus for secondarily dosing wastewater flowing along a path, such as in a contact tank, in multiple lumps along the length of the flow path. The invention provides the advantages of admixing doses of disinfectant at separate points along the flow path so that an effective level of disinfectant can be maintained across the length of the flow path to achieve reduction. desired amount of viral and bacterial microorganisms while a lower total amount of disinfectant than that required with a traditional single-dose technique is employed, thus providing an effective treatment process while reducing the level of residual chlorine at the end of the treatment.
One embodiment of the invention includes a method of "graduated multipoint dosing" (GMPD), whereby a disinfectant such as chlorine is injected at a plurality of separate points along the length and an elongated flow path through a treatment vessel as a contact tank. Preferably, the disinfectant can be injected into a stream of wastewater that moves along the flow path at about the same time as the plurality of points so that an effective level of disinfectant can be maintained within the custom stream. that travels along the flow path. Doses of disinfectant are also preferably injected proportionally, as by a relative volume, in the plurality of points. The dose of disinfectant is preferably the highest at the upper injection point, upstream, since the microorganism load, the effluent concentration and the organic disinfectant demand inherent in the wastewater will be the highest before any treatment. Downstream of the first injection point, the above constituents of the wastewater will be relatively smaller in magnitude, requiring a lower dose of disinfectant to maintain an effective residual level of disinfectant. As used herein, an "effective level" of disinfectant means an effective level of disinfectant to kill or otherwise
I Lat ^ t, »u-udiii;,. It is believed that at least one viral or bacterial microorganism present in the waste water is substantially harmless. If more than two injection points are used, the dose required to be injected at the third, fourth and each subsequent injection point will usually be less than the dose injected at the injection point upstream thereof and greater than the dose used at an injection point downstream thereof, so that the doses are graded. Another embodiment of the invention includes an apparatus for injecting doses of disinfectant into a plurality of separate points along a flow path through a container such as a contact tank. The apparatus is configured with an inlet to receive a stream of waste water and at least two separate nozzles or groups of injectors at dosing locations along the flow path to inject doses of disinfectant into the waste water streams before that the current leaves the container through an outlet. A source of disinfectant is provided, as a conduit arrangement, which can take the form of a manifold, to supply disinfectant from the source to the injectors. Preferably, a flow control device is associated with each injector or group of injectors for a single dosing site so that the dose of
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Disinfectant supplied to the flow path for each injector or group of injectors can be selected and varied. A controller may also be provided to initiate the flow control devices to adjust the dose level of disinfectant supplied by each injector, and one or more detectors provided to detect conditions in the waste water at one or more locations to provide signals from response for which the controller can initiate the flow control devices to vary the disinfectant flow at each dosing site. When using the method and apparatus according to the invention, it is desirable to employ a dosage liquid or diluent, as a carrier for disinfectant from the source to the injection sites, and preferably in conjunction with the GMPD technique previously described. When using a dosing liquid and injecting disinfectant carried by it into the waste water flow path at a plurality of points instead of using conventional single point injection does not require an amount of disinfectant in excess as required by the technique conventional, and in fact it could be smaller. Instead of using the entire volume of wastewater in a shutter flow as a transport vehicle to transport the disinfectant through the length of the flow path, the vehicle
it comprises only much less volume of dosing liquid. Because a smaller volume (usually at least several orders of magnitude) of the dosing liquid transfers the disinfectant to the waste water, the inherent demand for disinfectant of the liquid dosing vehicle is much lower than that of the waste water stream. conventionally employed. In this way, a more active disinfectant can be transferred from the source of the wastewater stream to
10 disinfect it. Consequently, when using GMPD, substantially less disinfectant can be used to effectively reduce the concentration of microorganisms (such as, for example, a coliform bacterial count) while also reducing the residual of
15 disinfectant at the end of the flow path. The apparatus of the present invention can be added to a contact tank to the existing wastewater treatment or other container at a relatively nominal cost of capital compared to that required for
20 to implement an ultraviolet or ozone disinfectant technique, and substantially without increasing the operating costs after its installation, compared to the injection of conventional single-point disinfectant. The disinfectant used with the invention can
25 understand any effective chemical agent to reduce
Ax ^ concentrations of bacterial and viral microorganisms present in the wastewater to be treated. Chlorine in various forms is a preferred disinfectant, and may include, by way of example only, chlorine in its liquid or gaseous form, sodium hypochlorite (NaOCl), disinfectants including a chlorine component, and mixtures of the foregoing. Other disinfectants as are known in the art and which are suitable for use with the multipoint injection technique according to the method and apparatus of the invention may also be employed. The disinfectant source may comprise a pressurized container, a container from which disinfectant may be pumped, a generator to provide disinfectant in the gaseous form or other sources as are known in the art. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an elevated view of a contact tank and a chlorine injection line according to the configuration of the prior art; Figure 2 is a cross-sectional view, taken through line 2-2 of Figure 1, of a portion of the contact tank of Figure 1 looking along the passage therethrough in a direction of flow of wastewater; Figure 3 is an elevated view of a first embodiment of the apparatus of the present invention; Y
Figure 4 is an elevated view of a second embodiment of the apparatus of the present invention. Referring to Figure 3, an exemplary contact tank 10 is depicted with a sinuous passage 15 defining a flow path therethrough. Other possible configurations for the contact tank are, of course, readily apparent to those with ordinary skill in the art. A stream of wastewater enters the contact tank 10 at an inlet 20 and flows along the passage 15 as represented by the arrows shown therein to exit the contact tank 10 at the outlet 25. The waste water that Contains effluent can be provided from any source, such as, for example, drinking water or salty drainage water from any sanitary sewer, rainwater drainage (runoff), wastewater from processing plants or factories, and wastewater from culture of farm or processing, including without limitation animal waste from reserved livestock pens and other breeding facilities, eg, pigs, chickens and other livestock. Other sources of wastewater are, of course, possible. A disinfectant, preferably chlorine, is provided from any suitable source. Chlorine can, for example, be gaseous chlorine, liquid chlorine or sodium hypochlorite (NaOCl) The source can be pressurized
- »- á, j .. S? to assist the flow of the disinfectant, optionally by mixing with a dosing liquid, through a duct arrangement such as a manifold 35 to each of the plurality of injectors 40 in communication with the passage 15. A plurality of injectors 40 can be grouped at any given place in passage 15 and accommodated to introduce disinfectant into the waste water stream through a large cross-sectional portion thereof, as an arrangement known in the art and previously described and depicted with respect to Figure 2. A dosing liquid can be provided from any desired source, such as waste water or another liquid suitable for transporting chlorine. The dosing liquid may, alternatively, be potable water and / or salt water, for example. Referring to Figure 3, the source of dosing liquid in this case is the waste water stream itself, a portion of which is diverted from the primary flow path through the contact tank via the dosing line 45, which conveys the flow diverted to the pump 50, by adding the disinfectant in the flow of dosing liquid at the collection end of the pump 50 in a substantially measured proportion that can be controlled by a measuring device, which can take the form of a valve 55. A dedicated mixing chamber can be used
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for mixing the disinfectant and the dosing liquid or, as shown, a pump 50 can be used as a mixing chamber. The dosing liquid with the disinfectant included, or in some embodiments a liquid disinfectant alone, is transported to the injectors 40 through the branches of the conduit arrangement or the manifold 35, each branch having a control device associated therewith. of flow, which can take the form of a valve 60, to regulate the flow to one or more injectors fed by that particular branch. The valves 60 can be used to regulate the amount of disinfectant, or dosage liquid that carries the disinfectant, to its associated injectors, and to provide, or graduate, the relative flow of disinfectant to each injector 40. Preferably, each valve is controlled from separate way. While the valves 60 may comprise manually operated valves, it is preferable that the valves are remotely operated valves, such as servo valves, and it is also preferred that a controller 65 can be operatively coupled to each valve 60 (and, optionally, as shown in the line). dotted to valve 55) so that the flow of disinfectant to each injector can be remotely, separately and, if desired, automatically regulated in response to operator input or conditions
detected in the wastewater flow stream in passage 15. Controller 65 may comprise, for example, a dedicated programmable logic controller, or a properly programmed personal computer. As mentioned above, the controller 65 can also be operatively connected to one or more detector devices 70 placed in communication with the passage 15, detector devices 70 are used to verify, for example, residual disinfectant, bacterial content, content organic or other parameters having utility for determining a dose of disinfectant required to be added to the waste water stream at each location of each injector or group of injectors 40 at each separate dosing site along the passage 15. Detector devices 70 preferably they are placed upstream of each associated dosing site, to verify the effects of the known disinfectant in the flow stream at an immediately upstream dosing site. In the example shown in Figure 3, four dosing sites designated 1-4 are located in the contact tank 10 along the passage 15. The dosing sites can be substantial and equally spaced along the passageway 15. , as shown in Figure 3, or non-uniform separations may be employed. As will be appreciated for those experts in
" { ? The technique, the number of dosing sites and the number of dosing outlets (eg injectors 40) at each dosing site can be varied according to the residual water flow rate, in the dose desired disinfectant, the concentration of disinfectant in a dosing liquid, and the demand for disinfectant from the wastewater flow. The injectors 40 at each dosing location are preferably positioned to effectively disperse the disinfectant into the waste water through the entire cross section of the passageway 15 at the dosing site. It is also contemplated that the mixing devices may be employed at a dosing site to effect a more rapid dispersion of the disinfectant into the wastewater stream. Referring to Figure 4 of the drawings, a second embodiment of the apparatus of the present invention is shown. The elements of the second modality corresponding to those described with respect to the first modality, for clarity, are identified by the same reference numbers. In addition to the elements of the first embodiment, the second embodiment includes one or more pre-treatment tanks or units 90, which can be used to remove large solids, including particles, from the wastewater stream prior to treatment in tank 10 of Contact. Known techniques for said removal
"< * > - *" include classification, sedimentation, and filtration, said pre-treatment tanks 90 can improve the effectiveness of the treatment in the contract tank 10 by reducing the presence of solid wastes carrying microorganisms and providing a stream of water more easily treatable waste without materials which, over time, could otherwise accumulate in passage 15, coat, clog or even damage the injectors 40, and inhibit dispersion of disinfectant in the wastewater stream.One or more The post-treatment tank or units 100 may also be used to receive wastewater discharged from the outlet 25 of the contact tank 10. For example, a tank 100 may comprise a sedimentation tank The following example illustrates aspects of the invention, although the invention should not be limited by this example.A flow of 500 cubic meters per hour of wastewater is introduced into a contact tank of 7 0 meters (passage length) A chlorine dose of approximately 10 kilograms per hour is introduced into the flow of dosing fluid of approximately 3.6 cubic meters per hour. The dosing fluid can be potable water or seawater. The chlorine-containing dose stream is introduced proportionally (by volume) and at the same time, the waste water stream in a plurality of (in this example, four) dosing locations along the
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length of the contact tank passage. In this example, the dose flow of 3.5 cubic meters per hour, containing 10 kilograms per hour of chlorine, is introduced proportionally into the wastewater stream through the GMPD at the various dosing sites. Referring to the following Table 1 together with the dosing sites shown in Figures 3 and 4, about 50% of the dosing liquid is introduced into Place 1, about 28% of the dosing liquid is introduced into Place 2, approximately 14% of the dosing liquid is introduced in the Place 3, and approximately 8% of the dosage liquid is introduced in the Place 4. Also as it was mentioned, a fifth place of dosage can be used, the use of which, of course, provoke it that the percentages of the dosing liquid introduced in the other dosing sites vary. Optionally, a Location 5 can only be used intermittently, such as when the detection devices are used in passage 15 and particularly detect concentrations of problematic remaining microorganisms after the waste water stream passes to places 1-4. The effect of each of these doses of disinfectant, in terms of the reduction in compliant and residual chlorine levels, is verified by taking regular samples of residual water at the end of the "leg" of the tank downstream from the place of
Yes. ~, dosing. The coliform beads are determined by a traditional agar plate assay. The residual chlorine levels can be verified using a standard DPD colorimetric assay or by an amperometric method. During the test period, the chlorine concentration in the dose flow can be varied as well as the volume of flow itself. As will be appreciated by those skilled in the art, the assay techniques used to determine coliform beads and residual chlorine levels allow the GMPD technique to be optimized for the contact tank and wastewater conditions.
Having thus described the invention, it should be understood that the invention should not be limited to the particular details set forth in the foregoing description or in the claims, since many obvious variations thereof are possible without departing from the spirit or scope thereof. .
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