Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2002-04-30
2002-11-26
Barry, Chester T. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S620000, C210S195300, C210S196000, C210S723000
Reexamination Certificate
active
06485645
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to waste water treatment, and more particularly to a process for removing phosphorous from waste water involving biological processes and a membrane filter.
BACKGROUND OF THE INVENTION
BOD, nitrates and phosphates released into the environment cause eutrophication and algae blooming resulting in serious pollution and health problems. Waste water treatment processes attempt to remove BOD, nitrates and phosphates to produce an acceptable effluent.
Conventional processes for removing phosphates from waste water include chemical precipitation and biological methods. In chemical precipitation methods, soluble salts, such as ferrous/ferric chloride or aluminum sulphate, are added to the waste water to form insoluble phosphate metal salts. The waste water, however, contains many different ions which create undesirable side reactions with the precipitants. As a result, and particularly where very low effluent total phosphorus levels are required, these processes may require the addition of 5-6 times the stoichiometric amount of chemicals required to remove the phosphates. Accordingly, these processes result in high chemical costs, high sludge production, and a high level of metallic impurities in the sludge.
In contrast, biological methods use microorganisms to digest the phosphates. For example, U.S. Pat. No. 4,867,883 discusses a process which attempts to create conditions which encourage the selection and growth of Bio-P organisms, a strain of bacteria which have the ability-to uptake phosphorus in excess of the amount normally needed for cell growth. The amount of phosphorus removal that can be achieved is directly proportional to the amount of Bio-P organisms in the system. Generally, the process consists of an anaerobic zone, an anoxic zone, an aerobic zone, a clarifier, and a variety of recycles to interconnect the various zones. In a preferred embodiment of the process, there is a denitrified recycle from the anoxic zone to the anaerobic zone, a nitrified recycle from the aerobic zone to the anoxic zone, and an activated sludge recycle from the clarifier to the anoxic zone. In the anaerobic zone, there is BOD assimilation and phosphorus release. Subsequently, in the anoxic and aerobic zones, there is phosphorus uptake. In the clarifier, sludge containing phosphates settles out of the effluent. In some cases, sand filters are employed to try to further reduce the amount of phosphates in the effluent.
One problem with the US '883 process is that there can be a build up of phosphates in the system. At the end of the process, a portion of the recycled activated sludge is wasted and is subsequently treated, typically by aerobic or anaerobic digestion processes. This results in a release of phosphorus taken up in the process. This phosphorus is then returned back to the process in the form of digester supernatant. Consequently, this reduces the efficiency of phosphorus removal in the process and results in higher levels of phosphorus in the effluent. A partial solution to this problem is to employ a side stream process called ‘Phos-Pho Strip’ as described in U.S. Pat. No. 3,654,147. In this process, the activated sludge, which has a high concentration of phosphorus, passes from the clarifier to a phosphorus stripper. In the stripper, phosphorus is released into the filtrate stream by either: creating anaerobic conditions; adjusting the pH; or extended aeration. The resulting phosphate-rich filtrate stream passes to a chemical precipitator. The phosphate-free effluent stream is added to the main effluent stream, the waste stream from the precipitator containing the phosphates is discarded, and the phosphate-depleted activated sludge is returned to the main process.
Another disadvantage with the process in US '883 is that significant design limitations are imposed by the settling characteristics of the sludge in the clarifier. For example, the process cannot operate at very high process solids levels or high sludge retention times. As a result, the system is generally considered to be inefficient and there is a high generation rate of waste sludge.
A second type of biological treatment is referred to as a membrane bioreactor which can be combined with chemical precipitation techniques. In a simple example, precipitating chemicals are added to an aerobic tank containing or connected to a membrane filter. As above, however, dosages of precipitating chemicals substantially in excess of the stoichiometric amount of phosphates are required to achieve low levels of phosphates in the effluent. This results in excessive sludge generation and the presence of metallic precipitates which increase the rate of membrane fouling or force the operator to operate the system at an inefficient low sludge retention time.
Also relevant to the present invention is U.S. Pat. No. 5,658,458 which discloses a treatment for activated sludge involving the separation of trash and inerts. Generally, the process consists of a screen which removes relatively large pieces of ‘trash’ and a hydrocyclone which uses a centrifugal force to separate the organics from the inerts. The activated sludge is recycled back to the system and the trash and inerts are discarded.
SUMMARY OF THE INVENTION
It is an object of the present invention to remove phosphorous from waste water. In some aspects, the invention provides a process for treating water to remove phosphorous and nitrogen. The process includes three zones, an anaerobic zone, an anoxic zone and an aerobic zone. In the anaerobic zone, an anaerobic mixed liquor has organisms which release phosphorous into the anaerobic mixed liquor and store volatile fatty acids from the anaerobic mixed liquor. In the anoxic zone, an anoxic mixed liquor has organisms which metabolize stored volatile fatty acids, uptake phosphorous and denitrify the anoxic mixed liquor. In the aerobic zone, an aerobic mixed liquor has organisms which metabolize stored volatile fatty acids, uptake phosphorous and nitrify the aerobic mixed liquor.
Water to be treated flows first into the anaerobic zone to join the anaerobic mixed liquor. Anaerobic mixed liquor flows to the anoxic zone to join the anoxic mixed liquor. Anoxic mixed liquor flows both back to the anaerobic zone to join the anaerobic mixed liquor and to the aerobic zone to join the aerobic mixed liquor. The aerobic mixed liquor flows to the anoxic zone to join the anoxic mixed liquor and also contacts the feed side of a membrane filter. The membrane filter treats the aerobic mixed liquor to produce a treated effluent lean in phosphorous, nitrogen, BOD, suspended solids and organisms at a permeate side of the membrane filter and a liquid rich in rejected solids and organisms.
Some or all of the material rejected by the membrane filter is removed from the process. This may be done by locating the membrane filter outside of the aerobic zone and directly removing the liquid rich in rejected solids and organisms from the retentate or feed side of the membrane filter. Alternatively, the membrane filter may be located in the aerobic zone so that the material rejected by the membrane filter mixes with the aerobic mixed liquor. The material rejected by the membrane filter is then removed by removing aerobic mixed liquor. Further alternatively, the liquid rich in material rejected by the membrane filter may be recycled to the anoxic or aerobic zones. The material rejected by the membrane filter is then removed by removing aerobic mixed liquor. Combinations of the first and third methods described above may also be used.
The steps described above are performed substantially continuously and substantially simultaneously. In the anaerobic zone, fermentive bacteria convert BOD into volatile fatty acids. Bio-P organisms use the volatile fatty acids as a carbon source. In doing so, they release phosphorus into the liquor, and store volatile fatty acids for later use. The stored carbon compounds may come from volatile fatty acids produced in the anaerobic zone or from materials produced external to th
Husain Hidayat
Koch Frederic
Phagoo Deonarine
Barry Chester T.
Bereskin & Parr
Zenon Environmental Inc
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