1. Field of the Invention
The invention relates to liquid waste treatment operations. In particular, it relates to improved liquid waste treatment oxidation operations.
2. Description of the Prior Art
Biotreatment of industrial and municipal wastewater is commonly practiced in the art. By this approach, continuously fed, toxic chemicals, which are typically dissolved in water, are consumed by living, typically aerobic organisms, and are converted into environmentally safe substances. Similarly, the term "bioremediation" is used to refer to the destruction of toxic chemicals that are both soluble and insoluble in water. Insoluble materials are typically stirred into suspension, from either the bottom or top of a body of water, to make them accessible to such organisms.
For aerobic biotreatment processes to succeed, sufficient oxygen must be available to the organisms for them to survive and multiply. For this purpose, oxygen may be supplied to the liquid waste at a variety of purity levels, ranging from air to pure oxygen. If oxygen or air enrichment are used, most of the injected oxygen must dissolve for the process to be economically feasible, since the cost of oxygen is typically higher than any other element of operating cost.
In response to the need for improved oxygen dissolution technology for liquid waste treatment, a so-called MIXFLO.RTM. system has been developed by Societa Italiana Acetilene & Derivati (SIAD) for use in both closed and open tanks, as well as in lagoons. In the MIXFLO.RTM. system, oxygen is dissolved in a two stage process. In the first stage, liquid waste is pumped from a lagoon or holding tank and is pressurized to between 2 and 4 atm. Pure oxygen is then dispersed in the liquid, and the resulting two-phase mixture is passed through a pipeline contactor in which typically about 60% of the injected oxygen dissolves. At an elevated operating pressure, the solubility of oxygen in liquid, e.g. water, increases substantially. Therefore, the rate at which oxygen dissolves is increased, and the contactor length necessary to dissolve the oxygen is minimized. The amount of water pumped also decreases, as the pressure increases, because a given water volume has a greater dissolved oxygen capacity at elevated pressures.
In the second stage, the oxygen/water dispersion is re-injected into the lagoon or holding tank using a conventional liquid/liquid eductor which (1) dissipates the pumping energy into the oxygen/water mixture, forming a fine bubble dispersion, and (2) ingests unoxygenated water, mixes it with the oxygenated water, and then discharges the resulting mixture into a lagoon or holding tank. For this purpose, a typical ratio of unoxygenated water to oxygenated water employed is 3:1.
Diluting oxygenated water with unoxygenated water within the eductor has two advantages. First, the dissolved oxygen level obtained in the pipeline contactor is significantly greater than the saturation oxygen concentration at atmospheric pressure. When the oxygenated water is diluted, the dissolved oxygen level decreases to below atmospheric pressure saturation. Therefore, oxygen that is dissolved within the pipeline contactor does not come out of solution upon being discharged into the lagoon or holding tank. Secondly, oxygen not dissolved in the pipeline contactor is well distributed by the large volume of water passing through the eductor. As a result, the frequency of undesired bubble coalescence downstream of the eductor is low.
Typically, 75% of the oxygen that is not dissolved within the pipeline contactor dissolves within the lagoon or holding tank because of the enhanced dispersion action of the eductors. The first and second stages of the MIXFLO.RTM. system, therefore, together result in the dissolution of 90% of the injected oxygen.
The application of the MIXFLO.RTM. system has been extended to bioremediation processes carried out at Superfund and Resource Conservation and Recovery Act (RCRA) waste sites. Such activities differ from the liquid waste biotreatment referred to above in several respects as discussed below.
Bioremediation is often carried out in a batch lagoon, with nothing being added to, or subtracted from, the lagoon until the entire body of water is cleaned, and the insoluble toxic chemicals present therein are destroyed. As contaminants are destroyed, the contents of the body of water changes, so that the mass transfer characteristics of the lagoon contents vary significantly with time.
The solids level encountered in bioremediation processes is usually much higher than in liquid waste treatment processes because the soil at the base of the lagoon is generally contaminated with organics and is treated simultaneously with the liquid portion of the lagoon.
The contaminants are often highly toxic in bioremediation processes, and, at high concentration levels, such as those contained in the soil, such contaminants can become toxic to the organisms used in the treatment process. Therefore, the solids suspension levels applicable in bioremediation processes must be carefully controlled.
The liquid encountered in such bioremediation processing often is found to contain oily substances that are not soluble in water. Such oils must be precluded from entering the oxygen supply piping.
While the MIXFLO.RTM. system is a highly desirable approach for carrying out bioremediation activities, it is appreciated that further improvements in the art are needed in response to the various factors and needs specific to bioremediation as outlined above. Some of such needs will be understood to pertain also with respect to wastewater biotreatment processing.
It is an object of the invention, therefore, to provide an improved system for bioremediation processing.
It is another object of the invention to provide a treatment system response to specific needs of bioremediation operations.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.