Aerobic digestion has long been the process of choice for small to medium size facilities (up to 5 mgd). Traditionally, digesters have been designed based on detention time of 20 to 30 days with little attention given to process considerations. With the advent of these new regulations, the easy avenue to Class B sludge is to hold the sludge for a minimum of 60 days at 15°C. However, this results in the need to double tank volumes or requires that plants increase the solids content of the feed solids.
More and more facilities are opting for the latter solution and are installing belt thickeners or drum thickeners to increase the solids content of the digesters to 4% to 6%. While this brings the treatment plant into compliance under the new 60-day detention requirements, it creates a whole series of new problems if the digester is not specifically designed to handle thickened sludge. In particular, air requirements can be significantly underestimated, as the thickened sludge has a very high oxygen uptake rate in the first ten days of digestion. Mixing also is a major challenge, given the high viscosity of the sludge.
With correct process design, process professionals can move away from simply designing on 60-day detention and look at meeting the alternative requirements of the 503 Class B regulations. Class B sludge requirements also can be met in the following ways: a) reducing the pathogens below 2,000,000 colony forming units and b) either reducing volatile solids by 38% or having a standard oxygen uptake rate (S.O.U.R.) of less than 1.5 mg. O2/hr/gm T.S.S. A correctly designed controlled aerobic digestion system can meet these alternative requirements with a detention time of 30 days or less. To design such a process, however, careful consideration must be given to all of the following parameters.
When thickened sludges are fed to the aerobic digester, the oxygen uptake rate in the first part of the digester is extremely high, with as much as 70% of the total oxygen requirement needed in the first ten days. The aeration system must be designed so that there is enough built-in flexibility to ensure that the oxygen profile in the basin matches the oxygen demand. If this is not achieved, then anaerobic conditions will occur in parts of the digester and odors and ammonia release will be the result. When calculating air requirements, 2 lbs O2/lb V.S.S. should be allowed, and sufficient air should then be provided to ensure that 2 mg/l of oxygen is maintained during aeration. This will enable nitrification to take place.
Care also should be taken when relating clean water transfer efficiency to thickened sludge applications. Traditional oxygen transfer efficiency factors should be downgraded by at least 20% to allow for the extremely viscous material into which the oxygen is being transferred. The selection of aeration equipment also is vital, since it is likely that fine bubble aeration will not provide sufficient mixing and will have a tendency to clog in this environment.
Enviroquip in Austin, Texas, has pioneered the development of a full range of high shear, non-clog aeration systems for this specific application to ensure that aeration and mixing requirements are met. Inexpensive, adjustable, above-water orifices allow for profiling the air requirements to meet demand and non-clog diffusers ensure that there is no degradation of performance over time, while shear tubes and draft tubes maintain basin mixing.
The viscosity of undigested waste activated sludge is much higher than water and changes with sludge concentration and temperature. Undigested sludge at 6% solids has four times the viscosity of digested sludge. Thus during the critical first stage of digestion when oxygen demand is highest (the sludge is at its most viscous) intense mixing is essential to ensure that oxygen transfer occurs.
Aeration combined with shear tubes and draft tubes has proven most efficient in providing the type of rapid mixing and shearing action that is necessary. To optimize mixing and oxygen transfer and to reduce heat losses, deep tanks with liquid depths greater than 20 ft., should be used wherever possible.
The best tank configuration to ensure class B compliance is three or more basins. The first basin will require most of the oxygen and will achieve the major portion of the volatile solids reduction. The remaining basins will further reduce volatile solids but will also serve as pathogen reduction basins. Care must be taken to keep the second and third basins free of raw sludge to avoid contamination and the introduction of new pathogens. Air requirements for basins two and three combined will be between 30% and 50% of the total air requirement. Care should be taken, however, to ensure mixing requirements are met.
Nitrification/De-nitrification, Digester pH
As mentioned earlier, sufficient oxygen should be provided to achieve nitrification of the sludge. If, however, nitrification is allowed to continue unchecked, then a pH drop will occur in the digester. If this drop is not controlled, upset conditions will occur. Provisions should be made to allow the air to be cycled on and off or alternatively an anoxic zone should be included in the design, so that de-nitrification can take place. By incorporating this de-nitrification step into the process, up to 50% of the alkalinity can be recovered as nitrates are converted to nitrogen, and oxygen will be released, thus reducing oxygen requirements by as much as 17%.
To achieve optimum performance, digester pH must be maintained in the 6.8 to 8 range, with the air being turned off if the pH drops below 6.8 and increased if the pH rises above 8.
A critical element of aerobic digestion is temperature. When the temperature drops to 10°C or lower, biological activity is severely reduced and volatile solids reductions of as low as 20% are the norm. Above 37°C, nitrification will be inhibited due to the adverse effects of high temperature on nitrifying bacteria. So for optimum digester performance, temperatures should be maintained between 20°C and 35°C year-round. Where winter temperatures are low, covers should be provided on the first stage digester. Enviroquip's airbeam cover, incorporating the latest aluminum cover technology, as well as its aeration system have been specifically designed for this application.
When thickening waste activated sludges mechanically, it is usually possible to achieve greater than 7% to 8% solids. However, if this is fed continuously to the aerobic digester, then it is likely that any aeration system will have difficulty meeting the oxygen demand given the viscosity and high concentration. Therefore, the design of the sludge handling system should allow the flexibility to bypass the thickener if necessary.
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