I. Field of the Invention
This invention relates generally to wastewater treatment systems which biologically treat wastewater and, more particularly, to biological treatment systems which use floating filter media to biologically treat wastewater.
II. Description of Prior Art
In high density aquaculture systems used for the production of aquatic animals such as catfish, tilapia, alligators, and clams, and in other water systems which generate wastewater, it is necessary to remove suspended solids, optimize nitrification and to reduce biochemical oxygen demand (BOD) of the wastewater. In all cases, treated water is eventually returned to the ecosystem, whether the return is to a lake or stream or back to a controlled aquaculture system. Varying load and flow conditions make operation of waste treatment systems difficult, in that the timing of treatment steps is dependent upon such changing conditions.
In the operation of any wastewater treatment system, it is desirable to waste as little water as possible and to maximize the concentration of waste sludge accumulated by the system. This is especially true in high density aquaculture systems, where constant recirculation of water is necessary, and where it is desirable to minimize the amount of water wasted in the treatment process. Water loss also becomes critically important when the waste water volume approaches the volume of the aquaculture system being treated, such as in aquaria used in pet stores and research labs. The principal problems with high water loss in any aquaculture system are: 1) the costs of treating high volumes of the backwashed waters, 2) in warm water aquaculture systems, heat losses due to release of the backwash water, 3) increased water demands and pretreatment costs, and 4) high capital costs for pumps and other equipment whose size is dictated by peak water demands.
Most prior art systems accomplish treatment using various traditional treatment components, such as aeration basins, filters and clarifier units, with each component having its own treatment and energy consumption limitations. A system which efficiently combines the features of separate component systems would have greater advantages over those presently in use.
The only known filters which overcome most of these problems are the biofilters disclosed in U.S. Pat. Nos. 5,126,042 and 5,232,586, both issued to the inventor herein, Dr. Ronald F. Malone (hereinafter "Malone I" and "Malone II", respectively). Malone I uses a tank having sidewalls which are inwardly sloping toward the bottom, wherein a floating media pack is caused to form near the top of the tank when it is filled with liquid during filtration. A high-speed, propeller-type agitator is employed to fluidize and expand the media pack and break up the filtered matter prior to backwashing the system. A drain line is opened near the bottom of the tank to allow accumulated sludge to leave the tank, and an outlet line is provided immediately above the media pack.
Malone II employs a tank having an upper filtration chamber and a lower expansion chamber fluidically connected to each other by a constricted passageway. An inlet line supplies water to the tank through the lower chamber, while a floating media pack forms within the upper chamber during filtration. As in Malone I, an outlet line is connected to the tank above the media pack and delivers filtered water back to the aquatic environment. Contrary to Malone I, however, backwashing is accomplished by the displacement and expansion of the media pack through the constricted passageway. The turbulence of this expansion causes the filtered matter and sludge to fall toward a drain line located at the bottom of the tank.
While these devices are well-suited to accomplishing the objectives stated in those patents, there are some aquaculture environments which require more careful management of the biofloc remaining on the media after backwashing. Biofiltering depends heavily on the gradual formation of a film of biofloc on the individual elements of the media pack. Highly turbulent backwashing methods can shear away this film and lessen the effectiveness of the filtration. Therefore, it would be desirable to have a biofilter which avoids excessive damage to the media biofloc, but which still satisfies the goals of removing suspended solids, minimizing water loss during backwashing, reducing biochemical oxygen demand (BOD), and providing nitrification for the aquaculture subject to filtration.