The present invention relates generally to the field of treatment of wastewater, and more particularly to an improved system and method for treating wastewater containing contaminants which system and method offer a broad array of advantages over conventional activated sludge wastewater treatment systems, including smaller size, higher rates of operation, high oxygen transfer efficiency, lower operating costs, and a decreased level of excess sludge production.
With an increased awareness of problems with water quality, particularly those caused by the discharge of wastewater from industrial sources, has come a demand for improved equipment and methods to treat wastewater prior to discharging it into a sewer, as surface water, or to other destination for effluent discharge. While such treatment systems and methods are generally not required to produce potable water, they are increasingly required by law to enhance the quality of wastewater prior to discharging it as effluent. For industrial waste, this treatment process must typically remove certain type of pollutants such as organic contaminants, nitrogen and phosphorus, metals, and suspended solids.
The first wastewater treatment systems were of simple design, with a single container or tank being used for both treatment of the wastewater and the removal of solids from the wastewater, typically by allowing them to settle out. These early wastewater treatment systems were not aerated, and typically generated foul odors as a byproduct of the process utilized by these systems. Over time, these early wastewater treatment systems evolved into systems which use a popular type of wastewater treatment process referred to as the activated sludge wastewater treatment method.
The activated sludge wastewater treatment system and method use an aeration tank which is followed by a solid/liquid separator which acts as a secondary clarifier to remove separated solids from the liquid, which is discharged by the system. As its name suggests, the contents of the aeration tank are aerated and mixed to facilitate an aerobic reaction (a reaction taking place in the presence of oxygen) which is facilitated by the presence of activated sludge. This activated sludge, which is an accumulation of microorganism-rich residue contained in the solids which are separated from the liquid in the solid/liquid separator, is seeded into the incoming wastewater in the aeration tank. In conventional activated sludge wastewater treatment systems, the concentration of activated sludge solids is typically 2,000 to 5,000 milligrams per liter in the aeration tank.
The aerobic reaction which takes place in the aeration tank includes three types of phenomenaxe2x80x94absorption, adsorption, and biological digestion. Absorption takes place when a contaminant is absorbed into the cell wall of the bacteria contained in the activated sludge. Adsorption, on the other hand, is a surface phenomenon which takes place when there is an interaction between a contaminant and the surface of the activated sludge whereby the contaminant adheres to the surface of molecules of the bacteria. Any one of these three phenomena will result in contaminants reacting with the bacteria contained in the activated sludge. Biological digestion takes place when the bacteria contained in the activated sludge consume waste constituents contained in the wastewater. Biological digestion can occur after the material has been absorbed or adsorbed.
As mentioned above, the reaction is an aerobic reaction occurring in the presence of oxygen, which decreases both the amount of time required for the reaction to occur and the level of foul odors produced by the reaction. Typically, the aeration and mixing may be produced by injecting compressed air or oxygen into the mixture, typically through diffuser devices located near the bottom of the aerator tank. As the air bubbles to the surface of the mixture, the diffused air provides both oxygen to the mixture and a vigorous mixing action. The amount of material contained in the wastewater may be characterized by the xe2x80x9cchemical oxygen demandxe2x80x9d or COD of the material. A chemical oxygen demand of one pound indicates that the material contained in the wastewater requires one pound of oxygen to degrade.
Air may also be added by the churning action of mechanical mixers located near the surface of the mixture contained in the aeration tank. In still another variation, mixing of the contents of the aeration tank may be caused by hydraulic pumping in which liquid is pumped out of the tank and back in through nozzles causing highly efficient mixing of the contents of the aeration tank. In a still further variation, air nozzles may be arranged around the liquid nozzles to further stimulate the mixing and simultaneously provide oxygen to the mixture. Still further variations include processes known as extended aeration and contact stabilization, both of which omit the primary settling step, and high-purity oxygen aeration, which can substantially reduce both the aeration time and the size of the aeration tank.
The conditions which are thus provided in the aeration tank promote the growth of the microorganisms introduced in the activated sludge with the resultant reaction removing contaminants from the wastewater. In conventional activated sludge technology, a predetermined period of time related to the strength of the wastewater and treatment objectives is required for the mixture to react in the aeration tank in a batch flow process. This time is required to allow the bacteria in the aeration tank to react with the contaminants contained in the wastewater, with much of the material being oxidized by the microorganisms. Generally, in conventional activated sludge processes, the contaminants are completely digested in the aeration tank.
The mixture is then allowed to flow from the aeration tank into the solid/liquid separator, which can be any of a number of different mechanical devices, all of which are well known in the art. The solid/liquid separator may be as simple as a secondary clarifier, which allows activated sludge to settle out by gravity. The clean liquid then overflows from the clarifier and it is discharged as secondary effluent, while the activated sludge may be separated out in a settling tank. The bacteria will tend to clump together and settle to the bottom of the settling tank, from which the activated sludge may be pumped out.
Some of the activated sludge will be recirculated back into the aeration tank, with this sludge being referred to as xe2x80x9creturn activated sludgexe2x80x9d or RAS. The microorganisms contained in the return activated sludge are thus well acclimated to the environment in the aeration tank. The remaining activated sludge is treated and disposed of in a conventional solids processing technique which is well known to those skilled in the art. This sludge is referred to as xe2x80x9cwaste activated sludgexe2x80x9d or WAS. In conventional activated sludge technology, the waste activated sludge may amount to as much as seventy percent of the sludge recovered in the solid/liquid separator.
The amount of excess activated sludge which is generated by an activated sludge waste treatment system may be controlled by a term referred to as xe2x80x9csolid retention timexe2x80x9d or SRT, which is the amount of time an average particle of solid material remains in the waste processing system. The solid retention time is inversely proportional to the relative volume of excess activated sludge which must be disposed of. Conventional extended activated sludge waste processing systems (designed for surface water discharge of effluent) have a solid retention time of approximately twenty days.
The excess solids produced may be determined by the yield of the activated sludge process multiplied by the mass of the contaminants removed. The yield may be measured in units of pounds of xe2x80x9cchemical oxygen demandxe2x80x9d or COD, which is a term commonly used to measure the amount of contaminants which are removed. Conventional extended activated sludge waste treatment systems produce a yield of approximately 0.25 pounds of xe2x80x9ctotal suspended solidsxe2x80x9d or TSS of excess activated sludge per pound of chemical oxygen demand of yield. Less conservatively operated systems can produce yields of 0.7 pounds of total suspended solids per pound of chemical oxygen demand removed.
The waste activated sludge is typically accumulated, and may be further biologically processed and/or dewatered prior to its ultimate disposal. When stabilized, the waste activated sludge does not have an offensive odor, and can be handled without the requirement for special procedures. While the waste activated sludge thus does not constitute a health hazard, it will be appreciated that its removal does constitute a significant part of the cost of the wastewater treatment process. Similarly, while the waste activated sludge systems and methods currently known in the art present an advantageous way to remove contaminants from wastewater, they do, however, present a number of disadvantages as well.
First, presently known waste activated sludge systems are relatively large in size and therefore present a significant cost to initially purchase and install. In addition, the size of such presently known systems also mandates a relatively large amount of space in which to install them. In addition, another limitation of presently known waste activated sludge systems is that they can only be operated at a relatively low rate in order to achieve treatment objectives, with higher rates of operation resulting in reduced effectiveness of the systems. Also, as noted above, presently known waste activated sludge systems produce a substantial amount of waste activated sludge which must be disposed of. This high disposal burden presents both storage problems and a significant cost to transport the sludge away for disposal.
In addition, a substantial amount of energy is required in order to produce sufficient oxygen to drive the reaction in the aeration tank. These costs result either from electrical energy required to produce compressed air, or from the cost of purchasing oxygen which may be dispensed into the aeration tank. Another problem of conventional waste activated sludge systems is that they have only limited capacity to deal with spills; for example, if a substantial amount of milk is spilled at a dairy, a conventional waste activated sludge system will either have to be radically oversized to deal with the spill, or it will simply not be able to cope with a large volume spill such as this. Finally, operating a waste activated sludge system is a relatively complex process with a number of variables; it would be preferable to have a simpler system if possible.
It is accordingly the primary objective of the present invention that it provide an improved waste processing system which is smaller and therefore less expensive to initially purchase and install than a conventional waste activated sludge system. It is a closely related objective of the waste processing system of the present invention that it be capable of operating at a higher rate than comparable conventional systems to deal with increased loads of contaminants, or to produce higher quality effluent, or both. It is an additional objective of the waste processing system of the present invention that it produce a greatly reduced amount of sludge, thereby substantially reducing the cost of operation of the system.
It is still another objective of the waste processing system of the present invention that it require less energy to provide oxygen to the reaction to further reduce operating costs, and that it do so without adversely affecting either the reaction itself or the amount of time required to react the materials in the waste processing system. It is a further objective of the waste processing system of the present invention that it be capable of handling high-volume spills of highly concentrated biomaterials, thereby being capable of fully and effectively treating such spills without disrupting the normal operation of the waste processing system. It is a still further objective of the waste processing system of the present invention that it present a simplified operating process which is easy to operate and which presents relatively few potential problems in its day-to-day operation.
The waste treatment system of the present invention must also be of construction which is both durable and long lasting, and it should also require little or no maintenance to be provided by the user throughtout its operating lifetime. In order to enhance the market appeal of the waste treatment system of the present invention, it should also be of inexpensive construction to thereby afford it the broadest possible market. Finally, it is also an objective that all of the aforesaid advantages and objectives be achieved without incurring any substantial relative disadvantage.
The disadvantages and limitations of the background art discussed above are overcome by the present invention. With this invention, instead of having a single tank in which activated sludge reaction takes place with subsequent solids processing occurring in other tanks, two tanks having very different environments and different purposes are utilized in a waste processing system in order to achieve all of the objectives mentioned above without incurring a single significant disadvantage. These tanks are a contact tank in which an initial reaction directed to removing the contaminants from the solution occurs, and a digester tank having a very high concentration of activated sludge solids in which essentially all of the organic contaminants are digested. Located between the contact tank and the digester tank is a solid/liquid separator, which may be of virtually any conventional design.
The process utilized by the waste processing system of the present invention operates on a novel principle whereby the objective of treatment in the contact tank is not to digest most or all of the organic contaminants in the wastewater, but rather to facilitate the removal of most or all of the contaminants from suspension in the wastewater by using the microorganisms contained in the activated sludge. Thus, by a combination of absorption, adsorption, and digestion, or by precipitation, the activated sludge treatment occurring in the contact tank is focused on the removal of the contaminants from suspension in the wastewater so that they can be separated out in the solid/liquid separator. Thus, wastewater is introduced to the contact tank together with an appropriate amount of waste activated sludge.
The concentration of activated sludge solids in the mixture contained in the contact tank can vary approximately in 200 to 10,000 milligrams per liter, with a level of approximately 2,000 milligrams per liter being typical. For a given level of contaminants in wastewater, the concentration of activated sludge solids in the mixture in the contact tank is substantially lower than the concentration required in a conventional activated sludge aeration tank. Typically, the concentration of activated sludge solids in the contact tank is between approximately ten and fifty percent of the level required in an aeration tank to attain the same level of contaminant removal from similar quality wastewater.
The practical effect of this is that the loading rate of the contact tank is between approximately two and ten times higher than the rates which are achievable in a conventional activated sludge aeration tank. What this means in plain language is that the contact tank of the present invention is capable of removing a substantially higher level of contaminants from wastewater than has previously been possible in conventional activated sludge aeration tank waste treatment systems.
In the preferred embodiment, the contact tank has both mixing and aeration in order to stimulate the rapid removal of contaminants from the wastewater. The mixing and aeration may be achieved by use of air or oxygen pumped into the contact tank, or by mechanical mixing, or both. Alternately, hydraulic mixing may be used instead of mechanical mixing, together with the introduction of air to provide oxygen to facilitate the reaction. The process configuration can also be used in an aerobic mode, an anaerobic mode (a reaction occurring in the absence of oxygen), an anoxic mode (a reaction occurring in the absence of oxygen but with other electron donors such as nitrates, nitrites, or sulfates), or in any combination of these modes occurring consecutively where aeration can be continuous, intermittent, or not used at all. Mixing and aeration conditions can be varied to facilitate desired physical, chemical, and biological reactions.
It should be noted that the contact tank of the present invention is capable of operating in batch mode, continuous mode, or semi-continuous mode, all with equally high quality results. Following the reaction of the wastewater and the waste activated sludge in the contact tank, a high level of the contaminants contained in the wastewater have either been absorbed by, adsorbed to, or digested by the microorganisms contained in the activated sludge, or precipitated into the mixed solution. Thus, when the wastewater is directed into the solid/liquid separator, the microorganisms together with a high percentage of the contaminants are removed from the wastewater, with the cleansed water being discharged from the solid/liquid separator as effluent.
In another significant differentiation from previously noted aeration tank waste treatment technology, all of the particulates which are separated in the solid/liquid separator are directed into the digester tank. As will be appreciated following a brief discussion of the operation of the digester tank, the production of particulates by the waste processing system of the present invention is so much lower than that of previously known aeration tank waste treatment systems that it is not necessary to continuously divert a large portion of the solids produced by the solid/liquid separator.
The concentration of activated sludge solids in the digester tank is quite high, generally being on the order of 5,000 to 100,000 milligrams per liter, with 40,000 milligrams per liter being typical. In fact, the only limitation on the concentration of activated sludge solids in the digester tank is that the mixture must be of sufficient viscosity as to be amenable to being pumped. In the preferred embodiment, whenever the concentration of activated sludge in the digester tank is so high that the mixture may not be easily pumped, it may be watered down somewhat by directing a sufficient quantity of wastewater into the digester tank so as to improve the pumpability of the mixture.
In the preferred embodiment for an aerobic reaction, the digester tank also has both mixing and aeration in order to stimulate the rapid digestion of contaminants contained in the activated waste mixture. The mixing and aeration may be achieved by use of air or oxygen pumped into the digester tank, or by mechanical mixing, or both. Alternately, hydraulic mixing may be used instead of mechanical mixing, together with the introduction of air to provide oxygen to facilitate digestion. The mixing and aeration conditions may be varied to facilitate desired physical, chemical, and biological reactions. Like the contact tank, the digester tank can be operated in an aerobic mode, an anaerobic mode, an anoxic mode, or in any combination of these modes occurring consecutively where aeration can be continuous, intermittent, or not used.
In many embodiments of the present invention, the size of the digester tank will be substantially larger than the size of the contact tank. While this is not necessary per se, it is generally the case due to the fact that the increased efficiency of the reaction process in the contact tank makes it possible to have a contact tank which is substantially smaller than the aeration tanks used by previously known activated waste treatment processes. In addition, since it is an objective of the improved waste processing system of the present invention to create an environment in the digester tank to take the digestion process to the maximum level possible, and to ultimately reduce solid waste to the minimum level possible, having a larger digester tank is advantageous.
The completion of the digestion process in the digester tank thereby tremendously reduces the volume of excess activated sludge solids which must be disposed of. This reduction is produced by the large xe2x80x9csolid retention timexe2x80x9d or SRT, which is the amount of time an average particle of solid material remains in the waste processing system. The solid retention time is inversely proportional to the volume of excess activated sludge solids which must be disposed of. The waste processing system of the present invention has a solid retention time in excess of 200 days, and has produced excess activated sludge yields of less than 0.05 pounds of total suspended solids per pound of chemical oxygen demand removed. In addition, the waste processing system of the present invention also has a higher oxygen transfer efficiency, resulting in additional savings in operating costs.
The processing in the contact tank is maintained by pumping sufficient activated sludge from the digester tank into the contact tank. As mentioned above, the concentration of activated sludge required in the mixture contained in the contact tank is substantially lower than the concentration required by previously known aeration tank waste treatment systems. The activated sludge contained in the digester tank is monitored to periodically check the concentration of bacteria contained therein. Since essentially all of the digestible matter is consumed in the digester tank, the level of indigestible matter will eventually build up to the point where it reduces the concentration of the bacteria in the digester tank to an unsatisfactory level. It is at this point that excess activated sludge will typically be removed from the digester tank.
In alternative embodiments of the waste processing system of the present invention, a number of other features may be utilized. It may be desirable to provide conditioning agents to enhance or optimize the processing in the contact tank, the solid/liquid separator, or the digester tank. For example, it may be desirable to introduce a conditioning agent into the contact tank in order to maintain an optimal level of pH to facilitate the reaction of the bacteria in the contact tank. This may be accomplished by measuring the pH in the contact tank and pumping a sufficient amount of a first conditioning agent into the contact tank in order to optimize the pH in the contact tank.
With some kinds of solid/liquid separators, it may be desirable to introduce a conditioning agent to enhance separation of the solids from the liquid in the separator. For example, if a dissolved air floatation separator or DAF is used, it may be desirable to use a chemical flocculent. Dissolved air floatation separators, which use air bubbles to entrain clumps of solids contained in the liquid, may use a flocculent material which causes the solids to initially aggregate. Such flocculent are typically polymers such as a cationic emulsion polymer, which has large organic molecules which, when dispersed in the wastewater prior to its introduction into the dissolved air floatation separator, facilitates bridging between bacterial solids through an electrical charge phenomenon.
In addition, it may be useful to provide supplements to the contact digester tank, either using nutrients or additional microorganisms to enhance the environment in the digester tank. The addition of a second type of microorganism which will selectively consume primary microorganisms which have died may be useful. Nutrients which may be added to increase the efficiency of digestion include nitrogen, which may comprise fertilizer, urea, or ammonium chloride. Precipitation agents may be added to either tank to remove chemical species, such metals or nutrients.
Finally, it may be desirable to add enzymes or surfactants to the contact tank or the digester tank or both to improve the reaction process to remove contaminants from the wastewater. The addition of surfactants may help the absorption or adsorption processes. The addition of enzymes may enable a modification of the structure of contaminants making them more amenable to the reaction process. For example, the addition of an enzyme may make it easier to remove a food oil from wastewater by solubilizing the oil.
It may therefore be seen that the present invention teaches an improved waste processing system which is smaller and therefore less expensive to initially purchase and install than a conventional waste activated sludge system. The waste processing system of the present invention is also capable of operating at a higher rate than comparable conventional systems to deal with increased loads of contaminants, or of producing higher quality effluent, or both. The waste processing system of the present invention also produces a greatly reduced amount of sludge, thereby substantially reducing the cost of operation of the system.
The waste processing system of the present invention requires less energy to provide oxygen for the reaction, thereby further reducing operating costs, and it does so without adversely affecting either the reaction itself or the amount of time required to react the materials in the waste processing system. The waste processing system of the present invention is also capable of handling high-volume spills of highly concentrated biomaterials, and is thereby capable of fully and effectively treating such spills without disrupting the normal operation of the waste processing system. The waste processing system of the present invention presents a simplified operating process which is easy to operate and which presents virtually no significant problems in its day-to-day operation.
The waste processing system of the present invention is of a construction which is both durable and long lasting, and which will require little or no maintenance to be provided by the user throughout its operating lifetime. The waste processing system of the present invention is also of inexpensive construction to enhance its market appeal and to thereby afford it the broadest possible market. Finally, all of the aforesaid advantages and objectives are achieved by the waste processing system of the present invention without incurring any substantial relative disadvantage.