Wastewater clarification methods and apparatus

#The invention relates to treatment of wastewater from residential, business, industrial, or other sources.

#Wastewaters obtained from various sources can contain bulk solid materials (sticks, rags, sand, and other materials that can be screened or filtered therefrom), smaller suspended solid materials that cannot be easily removed by screening or filtration, degradable organic compounds (sometimes designated biological oxygen demand or “BOD”), pathogenic microorganisms (e.g., bacteria, viruses, spores, cysts, and protists), and undesirable nutrients (e.g., nitrogen- or phosphorous-containing compounds that can encourage nuisance growth of algae or weeds at the wastewater discharge site). Discharge of these materials into the environment with the wastewater can have undesirable health and environmental consequences. As a result, wastewater discharges are closely regulated by state and national governments.

#Many methods exist for removing undesirable components from wastewater prior to its discharge. Typically, a wastewater treatment method might include one or more treatment steps in which settleable and floatable solids materials are removed, one or more (aerobic and/or anaerobic) microbial degradation steps in which most suspended solids and BOD are removed or captured in biological solids or slimes produced in the degradation step, and one or more further treatment steps in which wastewater is clarified, disinfected, or further purified.

#The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.

#FIG. 1 is a diagram illustrating steps of a method of clarifying wastewater in accordance with the present invention.

#FIG. 2 is a schematic representation of one embodiment of a wastewater clarifier in accordance with the present invention.

#The invention relates to an improved method of operating a wastewater clarifier. Prior clarifiers have the drawback that they often do not exhibit sufficient clarification, particularly when the solids content of the incoming wastewater (e.g., as assessed by measuring total suspended solids, or “TSS”) is relatively low. Thus, wastewater streams having TSS values less than about 1700 milligrams per liter (assessed on a dry weight basis, i.e., 1700 milligrams of dry material per liter of wastewater) often cannot be efficiently clarified using prior clarifiers, and the inefficiency of prior clarifiers is exacerbated as TSS values decrease, especially as the TSS value falls below about 1 gram per liter.

#Wastewater effluent streams from wastewater sludge biological reactors that are commonly used can have TSS values of 1700 milligrams per liter or less, depending on the type of biological reactor, the solids content of the wastewater fed to the biological reactor, the temperature at which the biological reactor is operated, non-solids components of the biological reactor feed stream, and other operating parameters. By way of example, fixed-film-type digesters are commonly used in wastewater treatment systems. The TSS value for an effluent stream from a fixed-film-type digester can be in the range 50 to 1000 milligrams per liter or more, depending on the concentration of BOD in the wastewater. When such effluent streams are provided to prior clarifiers, the effluent from the clarifier often retains an undesirable turbidity, owing to insufficient efficiency of the clarifier for removing small and fine particles (i.e., the TSS value is not decreased as much as desired).

#The invention is based on the discovery that the efficiency of a clarifier can be significantly improved by combining the wastewater fed to a clarifier with biological solids that settle in the clarifier (i.e., recycled biological solids). In this manner, a greater proportion of solids that are suspended in the wastewater can be removed in the clarifier, resulting in a less turbid (i.e., lower TSS, more highly clarified) effluent stream. Without being bound by any particular theory of operation, it is believed that mixing recycled settled biological solids obtained from the clarifier with wastewater fed to the clarifier increases the concentration of solids entering the clarifier, so that the frequency of particle-particle interactions among solids in the clarifier inlet and recycle streams increases and small and fine particles more readily agglomerate to form easily-settled particles.