The following is a summation of ESI technology as it relates to sludge digestion in oxidation ponds. The discussion particularly pertains to ponds that serve as complete treatment units, without additional unit operations such as primary clarifiers (excluding grit removal), other biological secondary treatment, or complete mix aeration in the pond.
Oxidation ponds often have severe sludge accumulation zones, particularly in the front half of the pond, or nearer to the raw influent feed point. As time passes, this accumulation spreads, but remains particularly heavy and deep near the influent end.
Basic Biochemical Considerations
An oxidation pond is by definition a mixed-process unit. It will have anaerobic zones (where sludge accumulates and in the bottom of the pond elsewhere), facultative zones, and aerobic zones (particularly near the water-air interface, or in the vicinity of surface aerators).
Removal of BOD occurs through 2 modes: physical and biological. Physical treatment occurs through gravity settling of solids. The effluent SS in the final discharge of the plant is the only sludge “wasted” from the plant. All other solids settle and persist in the lagoon over time, or they are slowly mineralized to CO2 and water.
Soluble BOD must be consumed through bacterial action rather than through physical treatment. Soluble substrate will not settle via gravity. Bacteria are the primary agents that perform consumption of soluble BOD. Many different species of bacteria (including myriad anaerobic, facultative, and aerobic species) are present in the pond at any given time.
A common trait of all bacteria, regardless of species, is the presence of a cell wall. Bacteria range in size from 1 to 5 microns, and are enveloped by a semi-permeable cell wall. This cell wall allows passage of small, soluble substrate, but excludes the direct passage of compounds, larger than 500 unit’s molecular weight.
The influent organic load to the oxidation pond includes settleable solids, suspended solids, colloidal organic waste, and simple soluble organic waste. Of these, only the simple soluble substrate is directly available to bacteria as a food source that can pass through the cell wall. All other classes of substrate must first be solubilized into smaller monomers before it can become a bacterial food source.
Colloidal BOD must be solubilized to its constituent monomers (building blocks) before bacteria can consume it. This means that removal of BOD from colloidal substrate takes longer than for simple, soluble BOD. At some times, depending on the particular condition of your treatment plant, the colloidal BOD might not get solubilised at all. Instead, in that it is too large to be consumed by bacteria, yet too small to effectively settle in the pond, it may pass through to the final effluent of the plant, making the final BOD higher than desired.
Another problem presented by colloidal BOD is that its presence is a detrimental to effective sludge dewatering. When sludge is being concentrated, water is trapped in the colloidal matrix, which makes the dewatering process less efficient.
Finally, large amounts of colloidal BOD make oxygen transfer less effective. When colloidal materials are a significant fraction of the biomass, the colloidal material may tend to occlude and blind the mixed liquor particles. This increases the distance that oxygen must travel to reach the interior of bacterial cells (where it is actually utilized). So significant oxygen is wasted or poorly utilized in waste streams that contain large amounts of colloidal material.
While domestic waste does not contain excess amounts of colloidal material (as would be found in certain industrial waste streams) colloidal material may make up 25% or more of the organic load on the pond. As such, it makes overall BOD reduction less efficient than it need be, particularly in terms of rate of removal.
Effect of Influent Solids on Sludge Build Up
For domestic sewage, numerous studies have confirmed that sludge build up is more strongly influenced by the solids load to the plant than by any other factor. For an equal weight of solids compared to soluble BOD, assuming that both are removed during treatment to an equal extent (meaning absent in the plant effluent), the sludge accumulation due to solids load is two to three times that due to soluble BOD load. To illustrate this situation, consider the following example using equal loads and removals of glucose and cellulose.
Glucose is simple, soluble sugar. Cellulose is a polymer consisting of glucose subunits (monomers). For this discussion, assume that one kg of each is added to an oxidation pond.
Bacteria easily consume the glucose, since glucose is soluble and low in molecular weight (C6H12O6). As the bacteria metabolize the sugar, it will be largely converted to H2O, CO2, plus some new bacterial cells. Only about 0.3 kg of new bacteria will be made for every kg of glucose consumed. Therefore, when the pond removes 1 kg of glucose, 0.3 kg of sludge is generated. Even this 0.3 kg will enter death phase, and endogenous decay will further reduce the mass.
In contrast, cellulose is a polymer that is too large to be directly consumed by bacteria. It is also relatively slow to be hydrolyzed or solubilised. When 1 kg of cellulose is fed to the oxidation pond, assuming that it is not passed through the effluent as unintentional wasting, it simply settles and contributes to accumulated sludge. Since none of the cellulose mass enters the bacteria, none is converted to CO2 and H2O. Instead, 1 kg load is removed by settling, resulting in one more kg of accumulation.
In this example, the removal in a treatment plant of equal weights (1 kg) of cellulose and glucose produce 1 kg and 0.3 kg of sludge, respectively. This is the mechanism behind the finding that solids load to a treatment plant influences sludge production more so than soluble loads to a treatment plant.
Why Existing Bacteria Do Not Solubilize Sludge at a Sufficient Rate
Many existing bacteria in the oxidation pond are fully capable of producing hydrolytic enzymes that would solubilize domestic sludge. However, the capability to perform a function is not the same as specifically expressing that function.
A bacterium requires material and energy in order to grow and reproduce. Organic substrate (food) is converted to intermediate storage compounds. These compounds are then used to drive synthesis of required cell components.
Synthesis of solubilising enzymes required material and energy. Therefore, all else being equal, when a cell is producing solubilising enzymes, it will not reproduce as rapidly, as it would in the absence of this production. Because of this, bacteria have evolved biochemical control pathways that initiate production of these hydrolytic enzymes ONLY when these enzymes are needed. Otherwise, production of an unneeded material would be a severe disadvantage in competitive mixed-species population dynamics.
- Most bacteria do not produce solubilizing enzymes until these enzymes are needed – which means when the soluble food source is exhausted and the culture is starving or dying.
- Bacteria prefer growing as rapidly as possible. This is how they survive – by reproducing faster, their chances of survival are maximized.
- Since production of solubilizing enzymes takes material and energy away from reproduction, bacteria that produce solubilizing enzymes reproduce more slowly than bacteria that do not produce these enzymes. Bacteria have control systems that shut off production of solubilising enzymes when they are not needed. This allows bacteria to reproduce at a maximum rate.
- Even when variants begin to produce enzyme when it is not needed, these variants will not persist in the mixed culture. Because they are making hydrolytic enzymes while other bacteria are not, they will reproduce much more slowly than their less wasteful competitors. Such bacteria will not be a significant fraction of the biomass. Therefore, in oxidation ponds, where soluble food is constantly fed via the raw influent, the existing bacterial population does not produce solubilising enzymes to a significant extent.
The ESI Approach for Enhancing Solubilization Rate
The ESI system is designed to provide maximum solubilization capability that circumvents this problem. The ESI system involves the following components, all of which must be used together as designed by ESI to bring useful solubilizing enzymes production to the treatment plant:
- Bacteria capable of high rate solubilizing-enzyme production
- A bacterial mix capable of providing sufficient diversity of enzyme production (enzymes are by definition specific to certain chemical functional groups, and many different functional groups are found in domestic sewage)
- Safe, natural, non-pathogenic bacteria that are not engineered or modified in any way.
- Nutrients that provide 12 generations of growth to the ESI bacteria before they are discharged to the plant (the starting bacterial culture in each batch reproduces 2 to the 12th Power in the bioreactor before enzymes synthesis begins).
- Recognition compounds that help direct the ESI bacteria to produce the right mix of proteases, lipases, amylases, and cellulases (solubilizing enzymes)
- Side-stream bioreactor equipped with aeration and temperature control to keep these ingredients in a controlled aqueous reaction
- Continuing the reaction until the culture is “starved”, with a total reaction of time in a “best window” of 48 to 72 hours
At the end of the batch cycle, the result is a bioreactor full of solubilizing enzymes produced by dead or dying bacteria that had been starved of nutrients for an extended time.
Real Effect of ESI Solubilization
It is critically important to stress that ESI solubilization technology is an aid to the existing biomass of the treatment plant. The ESI technology does NOT replace the existing biomass in any way.
ESI technology for sludge reduction is really an addition of enzymes produced by bacteria. The bacteria used by ESI in its side stream bioreactors are forced into death phase metabolism by prolonged aerobic starvation conditions. When a batch is finished and dosed to the plant, the ESI bacteria themselves are reproducing slowly if at all, and have no chance of becoming dominant in the biomass. The active agent is not the ESI bacteria, but the enzymes that they produce.
The existing bacteria in the treatment plant are the beneficiaries of the solubilization. Improved solubilization converts colloidal and solid organic material into simple soluble food that the existing plant bacteria can easily digest.
Specific Benefits of ESI System
Use The following benefits result from increased solubilization due to ESI System use:
Colloidal and particulate organic material is solubilized into simple, soluble substrate that the existing plant bacteria can readily consume. By speeding this conversion, more of the material is mineralized, which means converted to CO2 and H2O.
More Efficient Oxygen Transfer:
With greater solubilization, the particle size distribution of the floc is smaller, interference of colloidal material is reduced, and oxygen transfer is enhanced since the transfer pathway is shortened.
With reduction of a colloidal matrix surrounding floc particles, dewatering is much improved. Early on in a treatment program, the additional activity may result in “fluffy” sludge that floats nearer the pond surface than it originally did. But over a few months time, this sludge begins to sink back down. The final sludge will in fact compact better and be much more stabilized.
The main improvement in discharge quality is really a result of increase hydraulic retention time. Use of ESI technology will reduce sludge build-up without need for intrusive, expensive dredging and disposal.
Sustainable Sludge Management Program
ESI technology is designed to be the cornerstone of sustainable sludge maintenance programs in oxidation ponds (operating the pond without dredging). Proper and regular use will decrease existing sludge (even sludge that is 10 or 20 years old) and it will digest and stabilize incoming sludge. In some cases, oxidation ponds also have seasonal algal blooms, which contribute to sludge accumulations as die-off occurs. When this is the case, ESI Algae Reduction technology should be jointly applied with the sludge reduction technology.
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