Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2001-08-30
2003-09-09
Drodge, Joseph (Department: 1723)
Liquid purification or separation
Processes
Treatment by living organism
C210S614000, C210S617000, C210S630000, C210S631000, C210S636000, C210S096200, C210S143000, C210S221200, C210S321690
Reexamination Certificate
active
06616843
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a 371 of PCT/CA99/01205, which claims priority from Canadian Patent Application No. 2,256,989; filed Dec. 18, 1998 and U.S. Provisional Patent Application No. 60/133,249; filed May 6 1999. The disclosures of Canadian Patent Application No. 2,256,989 and U.S. Provisional Patent Application No. 60/133,249 are incorporated herein by this reference to them.
FIELD OF THE INVENTION
This invention relates to a process and apparatus for treating municipal, industrial, agricultural or other wastewater feeds in a submerged membrane bioreactor. Specifically, the invention concerns the removal of ammonia and total nitrogen in a single tank submerged membrane bioreactor through the processes of nitrification and denitrification.
BACKGROUND OF THE INVENTION
Excessive nitrogen is often introduced into the environment through the discharge of municipal, industrial, agricultural and other wastewaters. Such wastewater feeds contain organic nitrogen primarily in the form of proteins and ammonia. During wastewater treatment, ammonia and other sources of total nitrogen are partially oxidized to produce nitrites and nitrates which can be hazardous to the x environment. For example, discharging excessive amounts of total nitrogen into rivers and lakes contributes to their eutrophication which is characterized by frequent algal blooms and reduced levels of free oxygen available to plants and fish. Recently, many jurisdictions have started to regulate the amounts of total nitrogen and ammonia that may be discharged from wastewater treatment plants.
To remove ammonia and total nitrogen from wastewater, bacteria in a bioreactor are used to alternately nitrify and denitrify the water. During nitrification, a two step process converts ammonia into nitrites and nitrates. Firstly, bacteria primarily of the genus Nitrosomonas oxidize the ammonia and convert it to nitrite. Secondly, bacteria primarily of the genus Nitrobacter oxidize the nitrite and convert it to nitrate. Since oxidation occurs in both steps of the nitrification process, an aerobic environment is required. During denitrification, nitrate is converted to ammonia and nitrogen gas by other bacteria (which could include the bacteria mentioned above after they have switched functions) and fungi which remove oxygen from the nitrate. Anoxic conditions are needed to encourage the growth of such bacteria. After denitrification, the nitrogen gas leaves the process and joins the atmosphere and the ammonia is treated by further nitrification and denitrification steps. By cycling back and forth between nitrification and denitrification steps, or aerobic and anoxic conditions in the bioreactor, nitrogen continuously leaves the water reducing the levels of total nitrogen and ammonia in the wastewater.
Submerged membrane bioreactors have been used to remove biological oxygen demand (BOD) and suspended solids from wastewater streams but they are difficult to use under anoxic conditions. A membrane bioreactor typically comprises a plurality of ultraporous or microporous membranes submerged in a tank of wastewater with suction applied to one side of the membranes. Clean water permeates through the membrane walls but bacteria and suspended solids are rejected by the membranes and remain in the tank to be biologically treated. To prevent pollutants in the tank from rapidly fouling the pores of the membranes, the membranes are typically kept awash in air bubbles. Without aeration, the membranes would quickly foul and lose their permeability, but the aeration prevents anoxic conditions and denitrification from occurring in the membrane bioreactor.
Multistage systems have been used to provide both aerobic and anoxic conditions for nitrogen treatment. In these systems, an aerated membrane bioreactor is used to provide an aerobic environment suitable for digesting BOD and nitrifying ammonia. A second bioreactor, without a membrane, provides an anoxic environment for denitrifcation and partially treated wastewater (mixed liquor) continually flows between the two tanks. These systems are undesirable in many applications because the cost and space required for the second tank and mixed liquor transfer apparatus is prohibitive.
Nitrogen has also been treated in commercial applications in a single tank membrane bioreactor where the oxygen content in the bioreactor is varied to produce alternating aerobic and anoxic conditions. In these systems, permeation through the membranes typically stops during the anoxic phase to avoid membrane fouling in the absence of membrane aeration. The loss of permeation during the anoxic phase significantly reduces the daily yield of filtered permeate through the membranes and is a disadvantage of these systems.
Japanese Patent Application No. HE I 5-220346 discusses a system combining the single tank and multistage systems described above. The system has an anoxic tank for denitrification and a membrane bioreactor for nitrification and digesting BOD with mixed liquor recirculating between them. Filtered effluent is permeated through submerged membranes filters in the membrane bioreactor. If the rate of denitrification in the anoxic tank is insufficient, the air supply to the membrane bioreactor is discontinued, to bring the tank into anoxic conditions. However, when the air supply to the membrane bioreactor is disconnected, permeation through the submerged membrane filter is also stopped to prevent membrane fouling. Again the average yield of the system is compromised.
Japanese Patent Application HI 6-181645 describes a single tank membrane bioreactor for treating nitrogen which is cycled between aerobic and anoxic conditions by alternately turning an air supply to the membranes off and on. In one embodiment, an attempt was made to continue to permeate effluent through the membranes during the anoxic phase. Each anoxic phase was started with a minimum level of mixed liquor in the bioreactor and the level of mixed liquor was allowed to rise to a maximum level by the end of the anoxic phase so that a reduced amount of permeate could be withdrawn during the anoxic phase. The yield of the system was reduced because permeate was not withdrawn at the full rate during the anoxic phase and yet it was found that the membranes still fouled quickly. In another mode of operation, aeration was again used during an aerobic period and then stopped to allow an anoxic phase to begin. Permeation through the membranes was stopped at the beginning of the anoxic phase but resumed after the mixed liquor had settled to the bottom of the bioreactor. With this method, the time needed to allow the mixed liquor to settle undesirably reduces the frequency at which the reactor can be switched between anoxic and aerobic phases and the system yield is still reduced because permeation stops during the settling period.
European Patent Application EP 0 695 722 A1 shows in its second embodiment a single tank membrane bioreactor also containing vinylidene chloride fillers. The bioreactor is continuously aerated. A layer of microoigarisms on the vinylidene chloride fillers is dispersed and broken into a fine floc every 12 hours to consume oxygen so as to create anaerobic conditions in the bioreactor.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a simple and cost effective membrane bioreactor that produces a high yield of water having acceptable levels of nitrogenous compounds. In particular, it is an object of the invention to provide a singe tank membrane bioreactor which can be cycled between primarily anoxic and aerobic conditions without stopping or reducing the rate of permeation during the anoxic phase.
To accomplish these and other objects, the present invention is directed at an apparatus for treating feed water with pollutants containing nitrogen in a bioreactor having:
(a) a tank for holding mixed liquor to be treated;
(b) a feed water inlet to the tank;
(c) at least one membrane having a first side in fluid communication with mixed liquor in the tank and a second side in fluid commun
Behmann Henry
Buisson Herve
Husain Hidayat
Payraudeau Michele
Alexander Andrew
Drodge Joseph
Omnium de Traitement et de Valorisation
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