Disposal and reuse of wastewater from raw municipal sewage is problematic. Stringent wastewater treatment requirements have been promulgated to protect human health, particularly in those areas having limited water supply or dense populations. For example, Title 22 of the California Administrative Code establishes stringent water reuse criteria where human contact is likely to occur with treated wastewater.
Typically, wastewater is disinfected by chlorination or ultraviolet irradiation where the treated wastewater is discharged to inland surface waters. Disinfection of this type typically achieves complete destruction of pathogenic bacteria and substantial deactivation of viruses, but does not provide complete virus destruction. Viruses have been detected in secondary effluents.
Title 22 of the California Administrative Code is directed to tertiary treatment requirements. Viral monitoring is not specified in Title 22 because viruses typically occur in low concentrations in treated wastewater. Viral monitoring is expensive. Viral assays require special expertise. Laboratory procedures usually are off line and time consuming. Analytical costs are high. Therefore, instead of imposing measurements of viral concentrations, Title 22 sets forth a tertiary treatment system that consists of chemical coagulation, sedimentation, filtration, and disinfection where the public may exposed to the treated wastewater, as in a recreational impoundment.
Under the provisions of Title 22, turbidity of the treated effluent normally cannot exceed an average operating value of 2 NTU after final filtration, and cannot exceed 5 turbidity units more than 5 percent of the time during any 24 hour period. Chlorination after this level of treatment typically insures effective virus destruction sufficient for the protection of public health. Direct filtration with chemical addition is allowed as an alternative to the complete treatment systems specified in Title 22 where it has been demonstrated that the results of the two treatment systems are comparable and meet the appropriate criteria.
It has been determined that disinfection rates typically correlate well with wastewater particle size distributions and that the ability to inactivate an individual wastewater particle is a function of the size of the particle. Direct tertiary filtration alone usually does not enhance the rate of disinfection unless the particle size distribution of the settled wastewater is modified. Tertiary filtration systems that operate to remove larger size particles should safely reduce the long contact times and high chlorine dosages typically employed in wastewater reclamation processes. Accordingly, granular filtration media is almost universally required as a part of wastewater reclamation. Granular filtration is somewhat time consuming and can be the limiting factor for a wastewater treatment system.
Masuda et al. U.S. Pat. No. 5,248,415 discloses an upward flow filtration apparatus that is said to be useful as a tertiary filter for wastewater treatment systems and to operate at a relatively high flow rate. An embodiment of the subject matter described in the Masuda patent is represented in FIG. 24 generally at 28 and is labelled Prior Art.
The filtration media described in the Masuda patent comprise a plurality of crimpy fibrous lumps. The fibrous lumps are disposed in the upward flow filtration apparatus 28 between first and second perforated panels 36 and 38, respectively. The wastewater flows in an upward direction through the fibrous lumps and suspended matter is captured by the individual fibrous lumps.
The first perforated panel 36 is immovably mounted within the apparatus and the second perforated panel 38 is movably mounted within the apparatus and spaced below the first perforated panel. The lower movable perforated panel 38, or bottom plate, is raised to compress the fibrous lumps to eliminate air gaps and to form a dense filter layer. The wastewater passes upwardly through the movable bottom plate and the filter layer and exits the top immovable plate 36. Fine solid materials in the upward flow are said to progressively adhere to the filter layer from the lower portion to the upper portion thereof in that order. With progressive filtration, resistance to filtration is increased. The movable bottom plate is lowered from time to time and is said to define a cleaning chamber when the filtration performance is reduced and it becomes necessary to clean the fibrous lumps.
However, the apparatus described in the Masuda et al. patent with the movable bottom plate has some difficulties associated with it. A ram or screw 40 for moving the bottom plate passes through the wastewater, the media, and the top stationary plate 36. The screw decreases the amount of room available for the media and potentially causes some channeling through the media in the region of the screw. The media are constructed of a loose fiber and can become entangled in the screw as it turns. A seal is used where the screw passes through the top plate, further complicating the operation of the device.
When the bottom plate is moved to compress the media, the lower layers of media become compressed. The compressed filter media in the lower portion of the bed is then the first portion of the media to contact the wastewater since the filter is operated in an upflow mode. The filter clogs up fast because large particles and fines are both trapped by the compressed initial layers of the filter media. The entire unit is shut down, usually before the upper filter layers are fully loaded, and the filter media is washed before the next cycle is begun.
The filter media is washed by moving the bottom plate downwardly away from the media to define a cleaning chamber. However, the flow rate of the wash water makes it difficult to achieve separation between the media and to obtain efficient cleaning. The Masuda device has typically required washing of the filter media on a frequent basis at a full flow rate of wash water equivalent to the flow rate of wastewater. Thus the overall efficiency of the apparatus described in the Masuda patent is greatly reduced.
It would be desirable to develop a filtration system suitable for tertiary wastewater treatment that substantially reduced or eliminated at least some of the problems associated with the Masuda device and yet provided a high rate filtration system as a suitable alternative to granular filtration media.