Patent Application
20220098067
The invention is concerned with water or wastewater treatment in which membrane filtration is used, and to reduction of fouling of such membranes.
During filtration of water or wastewater via membranes, in a water or wastewater treatment process, membrane pores may become clogged by particulate matter, organic material, or microorganisms. In a particular plant, once a membrane becomes clogged to the extent that hydraulic capacity is significantly compromised, the system is taken offline and cleaned using a variety of chemicals. This slows down process flow and is a costly maintenance procedure.
It is known that ozone added to water or wastewater in a treatment process can reduce clogging of membranes in that ozone effectively attacks microorganisms, oxidizes organic material, and aids in flocculation of suspended particulate material. However, ozone has been suggested for use in water and wastewater treatment only at very low levels, typically below 2 mg/l. In many cases the ozone was used to treat wastewater prior to or along with biological processes. Although typically not recognized in prior art relating to ozone use, dissolved ozone can attack polymeric filter membranes causing deterioration. Depending on the type of materials used, some polymeric membranes may experience chemical attack by ozone at concentrations as little as 0.5 mg/l.
The following patents and publications have disclosed uses of ozone in treatment of water or wastewater: U.S. Pat. Nos. 5,645,727, 6,027,642, 6,464,877, 6,755,977, 7,309,432, 7,578,939, 7,695,622, 7,867,397, 7,875,179, 8,062,525, 8,628,174, 8,968,569, 9,028,695, 9,352,989, 9,375,663, 10,576,427, 2009/0321354, 2010/0084338, 2013/0126433, 2016/0023166, 2017/0182465, 2017/0313598, 2019/0046930 and 2020/0147556.
Pursuant to the invention ozone gas is added directly to membrane influent to prevent fouling, in a system for treating water or wastewater. Ceramic membranes are used, as they are inert to the action of ozone. The ozone treatment is applied in a zone or stage of treatment following any biological processes, or in absence of biological processes, since the ozone will kill microorganisms. This can be in treating secondary effluent (post-biological treatment) in a municipal wastewater treatment plant, a combined sewer overflow or sanitary sewer overflow, treatment of surface water, industrial wastewater or ground water. The invention takes a novel approach of proactively preventing membrane clogging, through the addition of ozone directly into the membrane system feedwater.
It has been discovered that the higher the ozone concentration, the more membrane fouling is reduced. Historically, ozone concentrations when added to a membrane feed stream has been intentionally kept low, perhaps <2.0 mg/l. In fact, ozone concentration must be minimal to avoid degradation of polymeric based membranes. However, with newer available membrane materials, particularly silicon carbide, higher ozone concentrations than what has been previously used are now possible.
Ozone dosages just over 2 mg/l (as applied) have proven effective at reducing membrane fouling. With the invention it has been discovered that every increase in ozone concentration reduces the impact of membrane fouling even more. At ozone dosages of 5 mg/l or higher, considerable reduction in membrane fouling is observed. However, even higher dosages between 10-15 mg/l have proven to be considerably effective, resulting in an increase in membrane flux as much as 50% in some cases. Even higher performance is realized when ozone concentrations above 15 mg/l are utilized. In some cases, operation at very high ozone concentrations can eliminate off-line membrane cleaning altogether.
An important consideration when trying to operate at higher ozone dosage rates is the gas transfer system itself. Complete ozone dissolution in feed streams is critical to ensure proper gas transfer while minimizing economic impact. If an inefficient gas transfer system is utilized, it becomes expensive to utilize higher ozone dosages in a membrane system since more ozone is generated than required. Traditional ozone gas transfer systems consist of direct injection devices, such as venturi nozzles, followed by some type of in-line mixing. Contact vessels under pressure are also used to transfer ozone, but consume more space and energy.
Novel gas transfer devices, such as the ROTURI manufactured by UP2E!, increase the gas mass transfer efficiency while operating at atmospheric pressure. The increased transfer efficiency is due to the rotation of liquid within the feed pipe which increases the available surface area for mass transfer to take place. UPZE!'s sonotrodes can be used for further enhancement. By increasing the mass transfer efficiency, it is possible to achieve higher ozone dosage concentrations more efficiently.
The use of ozone addition to membrane feed water is most effective when the membranes are decoupled from a biological process, such as a membrane bioreactor, as ozone is quite effective at killing microorganisms. Suitable feed streams for ozone addition prior to membrane treatment include tertiary treatment in a municipal wastewater treatment plant, combined sewer overflow, sanitary system overflow, industrial wastewater surface water, and even ground water.
In the event membranes become clogged even with the addition of ozone to the membrane system feed water, the clogging effects can be reversed by adding ozone to the membrane backwash at similar concentrations. When used as a cleaning agent by direct injection into the membrane backwash it may be beneficial to apply even higher concentrations, such as >25 mg/l, to clean the membranes from severe clogging issues. Such a process will clean the membrane pores of any clogging, thus restoring membrane hydraulic capacity.
Another important benefit of ozone addition as described above is pathogen removal, especially viruses. Viruses are smaller pathogens than bacteria, which can be removed by ceramic membranes of minimum pore size, i.e. 0.1 to 0.5 micron. The ozone can kill pathogens smaller than 0.1 micron diameter.
It is thus an object of the invention to reduce or eliminate fouling of ceramic membranes in a water or wastewater treatment system, while also increasing pathogen removal, in a stage or zone without biological treatment, using high concentrations of ozone efficiently injected into solution in the membrane system feedwater. Other objects and features of the invention will be apparent from the following description of a preferred embodiment.
In the drawings,
A clarifier effluent exits the clarifier as the clarified effluent 12. At this point biological treatment of the clarified liquid has been completed, and tertiary treatment, in the illustrated example, occurs in a membrane zone 14 which can be a series of tanks. The micropore membranes produce a highly clarified permeate as indicated at 16, removing considerable amount of particulate matter, some very fine, from the liquid. The membranes can also remove much of the bacteria present in the clarified liquid.
Pursuant to the invention the feedwater 12 to the membrane separators is injected with ozone, indicated at 18. As explained above, this ozone gas transfer produces a relatively high applied concentration of ozone to the liquid, greater than 2 mg/l, and preferably at least 5 mg/l. The concentration can be greater than 10 mg/l, or even above 15 mg/l, resulting in greater and greater effectiveness in reducing or eliminating fouling of the membranes. These are doses as applied to the liquid. Residual concentrations after application become lower with time, depending on organic concentration in the influent, biological constituents and site specific conditions. The ozone may be essentially fully reacted when the liquid reaches the membranes, or in some cases may still be at 90% or more of the applied dose. A benefit of the ozone injection, in addition to clogging reduction, is removal of pathogens including viruses.
The gas transfer device employed in the system of the invention preferably is a high efficiency, atmospheric pressure system in which the feedwater flowing through a pipe is rotated as the ozone is injected, producing increased efficiency due to the liquid rotation, which increases available surface efficacy area for mass transfer to take place. For example, a preferred gas transfer system is that referenced above, ROTURI system UP2E! (Eliquo Technologies).
As explained above, with the ozone injection at a relatively high concentration, and with ozone treatment decoupled from any biological process, membrane fouling is very substantially reduced, and in some cases can even be eliminated, so that the membranes operate more efficiently and over a longer period of time without servicing.
The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to these preferred embodiments will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims.
This application claims benefit of provisional application Ser. No. 63/083,752, filed Sep. 25, 2020.
| Number | Date | Country | |
|---|---|---|---|
| 63083752 | Sep 2020 | US |
