Liquid purification or separation – With gas-liquid surface contact means – With separator
Reexamination Certificate
2003-02-24
2003-11-11
Prince, Fred G. (Department: 1724)
Liquid purification or separation
With gas-liquid surface contact means
With separator
C210S195300, C210S206000, C210S903000, C210S906000
Reexamination Certificate
active
06645374
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to membrane modules used to transfer a gas to or from a liquid and to a process using a membrane supported biofilm for treating wastewater to remove one or more of nitrogen, phosphorous, BOD and COD.
BACKGROUND OF THE INVENTION
Transferring gases to or from a liquid is most commonly practiced by providing a bubble diffuser in the liquid. As bubbles rise through the liquid, gases move across the boundary of the bubble driven by the relative partial pressures of the gas in the bubble and in the liquid. Such a process has serious drawbacks including high energy costs, difficulty in independently controlling mixing of the liquid, foaming on the liquid surface and lack of control over the gas released by the bubbles as they break at the liquid surface. Gas permeable membrane modules provide an alternate means for transferring a gas to or from a liquid and have been used in various reactor designs. Some examples are described below.
U.S. Pat. No. 4,181,604 (issued to Onishi et al. on Jan. 1, 1980), describes a module having several loops of hollow fibre membranes connected at both ends to a pipe at the bottom of a tank containing wastewater. The pipe carries a gas containing oxygen to the lumens of the membranes. Oxygen flows through the membranes to the wastewater and to an aerobic biofilm growing on the outer surface of the membranes. In U.S. Pat. No. 4,746,435 (issued to Onishi et al. on May 24, 1988), the same apparatus is used but the amount of oxygen containing gas is controlled to produce a biofilm having aerobic zones and anaerobic zones.
U.S. Pat. No. 4,416,993 (issued to McKeown on Nov. 22, 1983), describes a membrane module in the form of a hollow plate. The plates are made of a rigid frame wrapped in a porous “netting” made of PTFE laminated to a woven nylon fabric. The plates are attached to an overlapping strip which has an inlet port and an outlet port.
In “Bubble-Free Aeration Using Membranes: Mass Transfer Analysis” (
Journal of Membrane Science,
47 (1989) 91-106) and “Bubble-Free Aeration Using Membranes: Process Analysis” (
Journal Water Pollution Control Federation,
1988, Volume 60, Number 11, 1986-1992), Côté et al. describe the use of silicone rubber tubes to transfer oxygen to water without creating bubbles in the water. The apparatus for these studies includes a module having vertically oriented tubes suspended between an inlet header and an outlet header. The module is immersed in a tank containing water recirculated by a pump to provide a horizontal current in the tank.
U.S. Pat. No. 5,116,506 (issued to Williamson et al. on May 26, 1992) describes a reactor having a gas permeable membrane dividing the reactor into a gas compartment and a liquid compartment. The gas compartment is provided with oxygen and methane which diffuse through the membrane to support a biofilm layer in the liquid compartment. The membrane is made of a teflon and nylon laminate commonly known as Gore-tex (TM). In one embodiment, the membrane divides the reactor into lower and upper portions. In another embodiment, the gas compartment rotates within the liquid compartment.
In “Studies of a Membrane Aerated Bioreactor for Wastewater Treatment” (MBR 2—Jun. 2, 1999, Cranfield University), Semmens et al. describe a membrane module having microporous polypropylene hollow fibres stitched together to form a fabric. The fabric is mounted between a gas inlet header and a gas outlet header such that the fibres are oriented horizontally. The module is immersed in water in an open reactor with water recirculated by a pump to provide a horizontal current in the reactor.
Despite the variety of designs available, gas transfer membranes have not achieved widespread commercial success. Common criticisms of modules or reactors include (a) that membrane materials lack sufficient strength to be durable in hostile environments (b) that membrane surface area is inadequate, particularly for a tank of a fixed and pre-selected size, (c) that excessive movement of liquid is required which is costly to implement in large systems, (d) that biofilm growth on the membranes is difficult to prevent or maintain at a controlled thickness and (e) that even small leaks or defects in the membranes cause a significant loss of system capacity.
Gas transfer is used for a number of processes, one of which is wastewater treatment. Discharging wastewater containing large amounts of carbon (BOD or COD), nitrogen and phosphorous into a natural body of water causes eutrophication, algae blooms, pollution and health problems. Various processes have been developed to treat wastewater to remove some or all of the carbon, nitrogen and phosphorous, some of which will be summarized below.
Activated Sludge with Chemical Phosphorous Removal
In a typical activated sludge process, wastewater flows in series through an anoxic reactor, an aerobic reactor and a clarifier. Effluent from the clarifier is released to the environment. Activated sludge from the bottom of the clarifier is partially recycled to the anoxic reactor and partially wasted. Significant removal of nitrogen requires a significant rate of recycle to alternately nitrify and denitrify the wastewater.
Phosphorous is removed by dosing soluble metal salts, such as ferric chloride or aluminum sulphate, at one or more points in the process into the aerobic reactor to precipitate phosphate metal salts. The waste water, however, contains many different ions which create undesirable side reactions. As a result, and particularly where very low effluent total phosphorus levels are required, precipitating phosphorous may require the addition of 2-6 times the stoichiometric amount of the metal salt. Accordingly, these processes result in high chemical costs, high sludge production, and a high level of metallic impurities in the sludge.
Activated Sludge with Biological Phosphorous Removal
Activated sludge techniques can also be modified to use microorganisms to store the phosphates. For example, U.S. Pat. No. 4,867,883 discusses a process which attempts to encourage the selection and growth of Bio-P organisms which uptake phosphorus in excess of the amount normally needed for cell growth. Generally, the process consists of an anaerobic zone, an anoxic zone, an aerobic zone, and a clarifier. In the anaerobic zone, soluble BOD is assimilated and stored by the Bio-P organisms and phosphorus is released. Subsequently, in the anoxic and aerobic zones, the stored BOD is depleted and soluble phosphorous is taken-up in excess and stored as polyphosphates by the Bio-P organisms. In the clarifier, sludge containing phosphates settles out of the effluent. 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. The sludge recycle is done in multiple phases to ensure that nitrates are not recycled to the anaerobic zone, which would limit phosphorous release. The biological mechanism by which bacteria release phosphorous in the anaerobic section involves the uptake of easily assimilated organic compounds such as volatile fatty acids (VFA). Depending on the level of VFA in the raw wastewater, an extra anaerobic section may be added at the head of the process.
One problem with this process is that the settling characteristics of the sludge in the clarifier impose significant design limitations. For example, the process cannot operate at very high process solids levels or high sludge retention times, particularly when high removal rates of both nitrogen and phosphorous are required. As a result, the system is generally considered to be inefficient and there is a high generation rate of waste sludge. In some cases, sand filters are added to the tail of the process to help remove solids carryover from an overloaded clarifier and reduce the amount of phosphorous in the effluent.
Another problem with this process is that there is a buildup of phosphates in the system. The waste activated sludge contains Bio-P org
Behmann Henry
Cote Pierre
Husain Hidayat
Pedersen Steven
Phagoo Deonarine
Bereskin & Parr
Prince Fred G.
Zenon Environmental Inc.
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