This invention relates in general to the treatment of wastewater and deals more particularly with a method and apparatus for use in a new lagoon or basin or for upgrading existing lagoon systems in a manner to enhance the wastewater treatment.
Lagoon systems for treatment of wastewater have long been in use and have achieved considerable popularity, especially in areas where land is readily available. A lagoon system typically involves use of an earthen basin in which the wastewater is contained. The organic wastes are converted to biological solids, either by operating the system as a simple stabilization pond or by using low rate partial mix aeration. The biological solids eventually settle and are retained on the bottom of the lagoon.
Aerated lagoon systems are simple and economically advantageous because expensive equipment is not required and there is no need for highly trained personnel to operate the facility. However, substantial amounts of land are required because of the need to detain the wastewater in the lagoon for an extended period to achieve significant levels of treatment. Also, the overall capacity or treatment level is limited, as virtually no flexibility is available in the treatment process.
By way of example, a typical lagoon system may require 15 to 30 days detention time to remove most carbonaceous BOD and oxidize ammonia during warm weather. In the upper midwest and other relatively cold climates, the lagoon temperature in the winter is too cold for nitrification to be carried out. With the increased emphasis that is being placed on the nitrification of ammonia, and with regulatory requirements being gradually expanded to require nitrification for all systems, the basic lagoon technology is severely handicapped due to its inability to consistently nitrify ammonia, particularly in cold climates. Many small municipalities have significant investment in an existing lagoon system and lack the financial capability to construct more advanced treatment facilities such as an activated sludge plant that is capable of meeting the regulatory requirements for nitrification and/or denitrification. Further, the costs of training operating personnel and maintaining more sophisticated systems are often beyond the capability of rural water districts and small municipalities.
The present invention is directed to a method and apparatus for treating wastewater that makes use of a new or existing lagoon facility and involves operating the lagoon in a manner to treat wastewater using more advanced techniques that allow nitrification and/or denitrification.
In accordance with the invention, the performance of a new or existing lagoon system is enhanced by providing baffles or added earthen berms that create a complete mix bioreactor zone at the front end of the basin or lagoon, or elsewhere if desired. The complete mix zone is operated using a low rate activated sludge process that involves complete mixing of the wastewater using only a small portion of the existing lagoon with a much shorter detention time than the original lagoon, or a shorter detention time than typical lagoon practice in the case of a new lagoon application.
A suitable aeration system is installed in the complete mix zone and may include floating air supply laterals from which air diffusers are suspended near the bottom of the lagoon. This type of aeration system can be installed without requiring de-watering of the basin and can accommodate uneven basin floors. Also installed in the complete mix zone is one or more bioconcentration modules which are preferably suspended from the floating air laterals or from floats or in another fashion. The bioconcentration modules do not function as clarifiers but instead provide settling chambers that are open at the top and bottom. The bioconcentration modules are internal to the bioreactor zone where aeration and mixing occur. Solids are concentrated in the bioconcentration chambers and drop by gravity out through the bottom of the settling chamber, thus returning them into the active bioreactor zone. This automatic return of solids maintains sufficient bacteria in the complete mix bioreactor zone to sustain a relatively high rate of biological activity. The solids are returned and are remixed in the complete mix zone by the aeration system and are circulated throughout the zone to maintain the proper active biomass for a complete mix process in the bioreactor zone.
Biomass concentrations in the bioreactor zone increase after start up with a very large percentage of all solids returned by the bioconcentration module at first (i.e., only a small amount of solids initially escape over the weir or other device used to control the discharge from the module). Continued operation of the system results in an increased level of biomass in the bioreactor zone until an equilibrium state of solids growth and return is reached, at which time escape of solids over the weir or other control device is higher.
Biomass equilibrium is thus achieved in the complete mix zone, and excess solids then simply pass into the rest of the lagoon where they eventually settle and are subject to the normal lagoon treatment process. In most systems, the detention time in the complete mix zone is 1-2 days as compared to 15-30 days in the original lagoon operated conventionally. When biological equilibrium is reached, the MLSS level is typically 1000 mg/l to 5000 mg/l. in the bioreactor zone.
The complete mix zone is typically operated as a low rate activated sludge process with an F/M ratio between 0.05 and 0.30. The system normally operates with a sludge age between 40 and 50 days. Sufficient sludge age is provided to effect complete nitrification because heat is conserved and maintained even in cold weather conditions. The relatively short detention time of 1-2 days maintains the temperature in the complete mix bioreactor zone high enough to accommodate biological nitrification.
Rather than monitoring and managing the sludge concentrations with separate clarifiers and controlled sludge recirculation, as occurs in extended aeration, conventional, or high rate activated sludge processes, the complete mix zone is simply allowed to reach a natural biomass equilibrium condition. Biomass from the bioconcentration modules is retained in the complete mix bioreactor zone. Excess solids from the system are discharged into the second or polishing lagoon with the bioreactor effluent where they are subjected to stabilization pond treatment with or without aeration.
The benefits achieved by the present invention are significant both economically and because of the enhanced wastewater treatment. The cost of upgrading an existing lagoon or constructing a new lagoon with advanced treatment is modest because use can be made of the existing basins, existing pumping and hydraulics, existing sludge disposal, a baffle, an aeration system, and the necessary settling modules as the primary upgrade modules. The treatment flexibility and capability are improved markedly in the upgrade because there is a high degree of carbonaceous and BOD removal and nitrification can be effected as well as denitrification as an option. Because biomass equilibrium is reached naturally in the mixed bioreactor zone using the bioconcentration modules, a wide range of effluents and a wide range of design conditions can be accommodated. At the same time, the basic simplicity of the lagoon system is retained and there is no need for a major increase in the training level or technical abilities of operating personnel.
Other and further objects of the invention, together with the features of novelty appurtenant thereto, will appear in the course of the following description.