Patent Application
20120234752
This invention relates to methods of treating wastewater in wastewater treatment plants. More specifically, the invention relates to nitrate removal from wastewater streams in plants using trickling filters.
Excessive nitrates in wastewater streams are related to a variety of problems. Some literature has shown that high levels of nitrate in water are associated with adverse health effects. The nitrate outflow onto shallow continental shelves can produce undesirable near-shore algae blooms. Nitrate's role as a plant nutrient can likewise cause undesirable plant growth in other water bodies such as ponds and lagoons. In the United States and Europe, legislation now specifies a maximum permissible nitrate and/or total nitrogen level in water for drinking or industrial discharge. Maximum legal nitrate levels in drinking water are currently 10 mg/liter (NO3) in the United States. In the United States, Federal and State Agencies regulate nitrate concentrations in wastewater discharges and groundwater in an effort to reduce impact to the nation's water supply.
Conventional secondary wastewater treatment plants are generally designed to primarily reduce carbon and ammonia concentrations via biological treatment. Nitrogen removal is accomplished by converting ammonia contained in the mixed waste stream to nitrites and nitrates, in the presence of oxygen and known as an aerobic nitrifying stage. Ammonia conversion to nitrite is carried out by microbes known as Nitrosomonas, while the conversion of nitrite to nitrate is accomplished by Nitrobacters. Nitrate conversion to nitrogen gas occurs in an anoxic denitrifying stage that takes place in a suspended growth environment and is devoid of dissolved oxygen. Nitrogen, carbon dioxide and water is produced, with the gas being vented from the system.
Nitrification rates can be optimized by regulating interdependent waste stream parameters such as temperature, dissolved oxygen levels (D.O.), pH, solids retention time (SRT), ammonia concentration and BOD/TKN ratio (Total Kjeldahl Nitrogen, or TKN, is organic nitrogen plus the nitrogen from ammonia and ammonium). Higher temperatures and higher dissolved oxygen levels tend to promote increased nitrification rates, as does a pH level in the 7.0 to 8.0 range. Sludge retention times of from 3½ to 5 days dramatically increase nitrification efficiency, after which time efficiencies tend to remain constant.
Prior art techniques for removing nitrogen compounds from wastewater have either been ineffective or too expensive. What is needed is an inexpensive, effective method for removing nitrogen compounds such as ammonia from wastewater.
Accordingly, in one aspect, the invention provides a method of improving nitrification in a wastewater treatment plant, the method comprising the steps of a) providing a trickling filter effluent water basin having biological filter media placed on a support platform positioned at least three feet above a bottom of the basin; and (b) contacting the filter media with the wastewater.
In additional aspects, the invention provides a trickling filter effluent water basin comprising: a) a retaining wall; b) a support platform to support filter media positioned at a height at least 3 feet above a bottom of the basin; and c) an outlet pipe in fluid communication with the storage space, wherein the trickling filter effluent water basin is a substantially closed system with the exception of an outlet pipe for effluent out of the basin.
The invention provides a cost-effective solution to the above described problem. By modifying existing trickling filter tanks or building new tanks with an elevated support platform for biological media, removal of nitrogen can be substantially improved without any additional effort. Additionally, the invention can be combined with the inventions disclosed in U.S. Pat. No. 7,238,286, fully incorporated herein by reference, for the purpose of providing overflow relief to wastewater treatment plants. In contrast to the inventions described in this above-referenced patent, the ventilation ports in embodiments of this invention are substantially removed or minimized in size, so that the water basin in some embodiments may not contain one or more ventilation ports defined in the retaining wall. It is thought that additional ventilation negatively affects the reduction in nitrification achieved by having a closed plenum.
As will be understood by one skilled in the art, embodiments of the invention can be used for both municipal and industrial wastewater treatment systems.
The invention is further illustrated by the following drawings in which:
As used herein, including the specification and the claims, all numerical references are to be read as including the term “about”. Any numerical range is intended to subsume each and every number contained within the range, as in “at least 30%” will include 31%, 32%, and the like, and the range “10% to 30% will include 10%, 11%, 12%, etc. and all numbers in between.
Accordingly, in some embodiments the invention provides substantially improved nitrification in wastewater treatment plants utilizing trickling filter effluent water basins. As used herein, the term “nitrification” means conversion of ammonia to nitrate. While not being bound by any theory, it is thought that the trickling filter of the invention gets its air through a down-draft convection air current created by the influent water being placed on the top of the filter by the rotary distributors. With limited or no ventilation portals, as the water flows down through the trickling filter media, it forces the air inside the large plenum up, as it has no place else to move. As the next arm of the rotary distributor passes by it is thought to create a down draft or vacuum, which forces fresh air down into the plenum. Because the air in the plenum is continually re-circulated in this manner, the temperature of the biological media stays consistent with the temp of the wastewater inflow. Since the temperature of the wastewater generally does not go below 55 degrees Fahrenheit (the same temperature as ground water) cold air that inhibits nitrification is never brought into the filter, even when the outside temperature drops well below freezing. As the air is re-circulated through the filter the microorganisms that are beneficial to nitrification removal are re-introduced into the filter, creating a stronger population of microorganisms in the trickling filter media.
Accordingly, a trickling filter of the invention is a substantially closed system with the exception of an outlet pipe for effluent out of the basin. As used herein, the term “substantially closed” means that there are minimal (very small in size) or no ventilation ports in the side walls of the filter tank. It is thought that ventilation ports will decrease the downdraft effect of air circulation through the plenum, diminishing the improvement in nitrification as ventilation ports increase in size.
In all figures, like numerals refer to like features having the same described function.
The terms “trickling filter effluent water basin”, “trickling filter tank” and “trickling filter” are used herein interchangeably and refer to a wastewater treatment tank in which water flows downward over biological media to remove soluble organic contaminants in the water, as is known in the art. Such tanks are used in both municipal and industrial wastewater treatment systems. As will be understood by one skilled in the art, any of the embodiments described herein can be used in either setting.
Referring now to
Stay rods 56 support the distributor arms 48 and turnbuckles 58 on the stay rods 56 permit adjusting and leveling of the distributor arms 48 to produce an even distribution of wastewater over the filter media 40. The speed of the distributor arms 48 is controlled with the speed retarder orifice 62 or with other means such as a mechanical driver.
Wastewater flows through the inlet pipe 42 and is pumped up through the center well 54 of the distributor 44, through the distributor arms 48 and over the filter media 40 via outlet orifices 64 which control the flow of water to the filter media 40. Outlet orifices 64 are adjustable to provide an even distribution of wastewater to each square foot of filter media 40. Splash plates 60 on the distributor arms 48 distribute the flow from the outlet orifices 64 evenly over the filter media 40.
An arm dump gate 72 drains the distributor arms 48 and controls filter flies along the filter retaining wall 38. The dump gate 72 is also used for flushing the distributor arms 48 to remove accumulated debris that could block the outlet orifices 64.
In trickling filter systems with rock media, the media rests on a bottom 73 of the tank on a support grill 74 which lays directly on top of the underdrainage system 76. The support grill 74 holds the filter media 40 in place and keeps it out of the underdrainage system 76. The underdrainage system 76 is a network of pipes made of clay, plastic or other material directly beneath the media support grill 74. As will be understood by one skilled in the art, the underdrainage system in a trickling filter tank with rock media cannot store water, nor can it be adapted to do so. After the wastewater flows over the filter media 40, it falls to the underdrainage system 76 where it is conveyed to an underdrain channel 78 via a sloped floor 80. The underdrain channel 78 drains filter effluent to an outlet box 82, where it is collected before it flows to the next step in the wastewater treatment process. An outlet valve 84 in outlet box 82 is used for maintenance and is placed at the outlet pipe 86. In rock media systems, the outlet valve is normally in the open position but is closed when it becomes necessary to backwash the filter, such as when it becomes clogged.
After wastewater flows over the plastic filter media 90, it falls to the floor where it is conveyed to an underdrain channel 78 via the sloped floor 80. The underdrain channel 78 drains filter effluent to an outlet box 82, where it may be collected before it flows to the next step in the wastewater treatment process. An outlet valve 84 at the outlet pipe 86 is used for maintenance purposes.
The height of the original retaining wall 38 may be extended with the addition of an extension wall 92 made of steel or other suitable material, for the purpose of adding additional media or providing a wind break to the media. A standard hydraulic flow-driven distributor (not shown) or a mechanical drive type of distributor 44 is used to control the speed of the distributor arms 48. Piers 94 made of concrete or other suitable material support cross-beams 96 made of concrete or other suitable material, which in turn support the plastic media 90 at a height of about one to three feet above the floor of the tank. In the prior art, the retaining wall 38 typically contains one or more ventilation ports (not shown) or forced air blowers 108 below the level of the filter media 90, to allow air to flow to the filter media 90. In both the old rock media tank shown in
In some preferred embodiments, the trickling filter has a wind wall (not shown) of at least three feet in height. In some embodiments, the wind wall is 4 feet, 5 feet, 6 feet or 7 feet or more in height, height being measured from the top of the biological media.
The trickling filter tank of the invention is optionally further modified with the addition of a flow control box 100. The flow control box 100 extends the height of the prior art outlet box 82 to a height near the height of the support platform cross beams 96 containing the media 90. The flow control box 100 contains a flow control assembly to set the elevation of water retained in the tank when storage is needed, such as a flow weir 126 or other suitable structure in combination with a flow restriction valve or gate 104, or a weir gate (as described in U.S. Pat. No. 7,238,286), as will be understood by one skilled in the art. When flow control is also used, a flow weir is adjustably sized at the maximum depth of the tank that will be used for storage, such that when full, the tank still contains sufficient air space 103 below the media 90 for ventilation. The size of the air space can be anywhere from a few inches to a foot or more, depending on the type of media used and the amount of aeration required, as will be determined by one skilled in the art. Aeration of the media can also be accomplished by other methods, such as aeration pipes through the media from the top of the tank (not shown). The flow restriction valve 104 is disposed adjacent to the flow weir 126. During normal operation of the tank, water flows over the filter media 90 and falls to the floor 80 where it is conveyed to an underdrain channel 78 via a sloped floor 80. The flow restriction valve 104, which regulates flow of water out of the tank, is in the open position and water flows through the underdrain channel 78, through the flow control box 100, into the outlet pipe 86 and to the next stage of processing in the wastewater treatment plant.
In additional embodiments, the trickling filter of the invention is built as a new tank, incorporating all of the modifications described above, including elevating the support platform for the filter media, and optionally, increasing the height of the flow control box and providing a flow control assembly such as a flow weir in combination with a flow restriction valve.
As will be understood by one skilled in the art, the plant operator of a trickling filter system must manage and take into account certain factors that affect the nitrification process. These factors include, for example, the efficiency of upstream treatment units and processes; the amount of dissolved oxygen in the water; the temperature, alkalinity and pH of the water, cBOD removal, toxic compounds, wet weather conditions, and overall facility design. Temperature, dissolved oxygen, alkalinity, pH and toxic compounds must be monitored regularly to make sure that conditions are optimal for the proper functioning of the biological media. For the purpose of the invention, it is assumed that these parameters are optimized for a trickling filter of the invention in the same manner as they would be optimized for any standard (without the invention) trickling filter system.
The invention is further illustrated by the following example.
A trickling filter was modified at a Pennsylvania wastewater treatment plant by providing a raised platform at 6 feet above ground level for support and storage of media. This platform created a plenum (air space) below the platform. The filter was substantially closed, except for the effluent opening. A wind wall of 6 feet above the level of the media was also built on the tank. The following measurements (Tables 1, 2 and 3) on effluent were made before and after the modifications.
The filters consistently achieved ammonia nitrogen removal well below 2 mg/l and 90% of the time below 1 mg/l, a level which is considered full nitrification.
The trickling filters of the invention can improve average monthly nitrification levels at least 30%, 40%, 50%, and 60% or more, as compared to nitrification levels in a trickling filter without the invention.
Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the invention may be made without departing from the invention as defined in the appended claims.
