Equalization Tank Design
Simply a well - mixed vessel with fluctuating input flow rates and / or concentration with fairly constant output flow rates and/or concentrations. Processes for waste treatment work best with uniform conditions. Shocks to the bioprocesses in the form of sudden changes in concentrations of nutrients can cause upsets. If the concentrations or flow rates of the waste vary greatly, dosages for treatment must be constantly be readjusted. Consider sedimentation. If the input flow increases suddenly, the settling patterns will be upset to lower collection efficiency. Equalization dampens fluctuations. Flow equalization can improve performance of subsequent steps significantly. Often the rest of the plant can be designed with smaller equipment (less capital investment) because of this improvement in performance. Equalization allows reactions in the equalization tank. There may be aeration both to keep the fluid from becoming anaerobic and smelly and to biodegrade some of the organic compounds present. More important for industrial wastes that can have wide swings in pH is the reaction of acids with bases because otherwise each would have to be neutralized with costs for equipment and reagents. Early in the process, usually following the initial step of collecting debris from the input stream from the sewer.
Some common ways of incorporating flow equalization are shown in the sketches:
The top method is called in-line. The side-line shown above it has equalization in the surge tank with overflow when flow rates are high into the equalization tank. This method makes sense when there is a combined sewer system for both wastewater and storm water. Much of the time the surge tank is adequate. When there are heavy rains, this overflows into the equalization tank to be proportioned into the main stream over a period of time. The pump shown in blue is usually several pumps because one may be down for maintenance. The pumps may have constant flow, either off or on. One pump may have variable speed that is fairly expensive. The system must have some sort of logical control to decide which pumps are on and to add back the surplus wastewater at rates that keep the rest of the plant operating with nearly constant conditions. Sludges from the various sedimentation units are digested anaerobically to reduce their organic matter and to improve handling and drying. The supernatant from treated sludge is rich in organic matter and goes back for more treatment. The equalization step is a good place to add back this supernatant.
Design of Equalization Units
Equalization vessels often have sloping sides to keep the head more constant as volume changes.
A common method for combining aeration and mixing is to have a mixer at the surface to splash the liquid into the air. Because the level is changing, this mixer must be mounted on floats. The aeration in the equalization vessel will reduce the BOD of the wastewater by 10 to 20 %. A graphical method is used to specify the volume of an equalization vessel. A plot of flow rate versus time gets a horizontal line for the average flow rate. All flows above this line are integrated to get the fluctuating volume for the vessel. This is shown in the next sketch.
The vessel volume should equal the average flow multiplied by the time period, and then add on the fluctuating volume. Note that when the flow variation is small, the fluctuating volume is small so that the required vessel size is close to that based on average flow. When the variation is great, the equalization vessel is quite a bit larger than that based on the average flow.
Storage is considered a necessary control alternative for wet weather flows because of the high volume and variability associated with stormwater and combined sewer overflows. Some of the most critical problems currently facing wastewater management agencies are the control of Infiltration/Inflow (I/I)-induced sanitary sewer overflows (SSOs) and treatment facilities, including hydraulic overloading and disruptions of biological and other plant processes. As agencies examine alternative approaches to controlling I/I problems, storage facilities are increasingly being planned, designed, and constructed for the control of wet weather flows in sanitary sewer systems. While storage facilities have commonly been constructed at wastewater treatment facilities (where they are referred to as equalization basins) to maximize the processing of wastewater generated during wet weather while protecting the plant processes from hydraulic overloading or biological disruption, the use of storage at upstream locations within the collection system has only recently begun to gain acceptance. Collection system storage facilities are now being recognized as providing much of the benefit to treatment facilities realized with equalization basins, with the additional benefit of controlling SSOs and basement flooding while minimizing or eliminating the need to construct relief sewers.
Wet weather flow storage facilities may be constructed inline or offline. These facilities can be constructed inland and upstream, on the shoreline, or in the receiving water. In addition to storage, other functions of these facilities may include sedimentation (and associated toxics removal), dry-weather flow equalization, flood protection, flow attenuation to enhance receiving stream assimilation, and hazardous material spills capture. The development of new and improved concepts for wet weather flow treatment and control is covered in NRMRL's Wet Weather Flow Research Plan. This plan predominantly addresses the structural aspects of stormwater and wastewater storage facilities. Conventional facilities are usually concrete or earthen basins or impoundments. During the last two years alone, hundreds of lives were lost, thousands of square miles of ecosystems were destroyed or devastated, and $50 billion of property damage was caused by the failure of these types of facilities due to excessive stormwater. Besides these conventional wet weather flow related facilities, there are as many as 500,000 earthen-diked impoundments (pits, ponds and lagoons) in the United States containing potentially hazardous wastes. This total includes small waste ponds at minor chemical manufacturing plants as well as mile-square tailing lagoons at mines, smelters, and phosphoric acid plants. A common element between these similar systems is that a seemingly secure earthen impoundment may suddenly fail with provocation as slight as a heavy rain. These failures have resulted in the loss of life, caused the uncontrolled release of contaminants, polluted waterways, killed aquatic species, adversely impacted drinking water systems, generated public and political concerns, and despoiled scenic areas.
Earthen impoundments, levees and dams generally consist of an embankment (with an upstream and downstream slope) and a base foundation. Such systems are constructed of a variety of soils; frequently, with the materials that were at hand. The resulting structures may be susceptible to sudden failure. Additionally, seepage may also reduce slope strength and contribute to failure, because it brings fluid to the downstream slope and thereby adds weight, increases the load, and decreases apparent cohesiveness and effective normal force. Failure through the base must be considered, particularly when the soil beneath the dam is softer than the slope-forming soil. In addition, unless adequate freeboard (embankment height above the maximum expected fluid level) is provided, failure can occur through overtopping. Earthen impoundments are subject to overtopping and failure through the collection of rainwater, run-off or other uncontrolled inflow. Overflows are generally the result of insufficient capacity due either to insufficient freeboard or structural problems that reduce the effective capacity. No matter what the cause, impoundment overflows of hazardous waste-scan pose significant environmental and public health risks through the contamination of soils, ground water, and surface water, as well as the potential to spread the contamination into areas not currently impacted by the impoundment.
Because this problem involves thousands of earthen impoundments and dams, and tens of thousands of levee-miles (six thousand in northern California alone) throughout the country, an inexpensive and simple technique for monitoring the stability of earthen dams is needed. The high cost of conventional methods of monitoring dam stability usually rules them out as candidates for evaluating failure potential of small waste dams. It seems unlikely that conventional methods of inspection, which require expensive instrumentation and trained geo-technical engineers, can meet the need. Under this impetus, EPA ORD developed a prototypical acoustic emission monitoring system -- based on the phenomenon that soils emit sounds under stress. Initial research results demonstrated that the resulting acoustical signals, when properly amplified and quantified, can be valuable guides in evaluating the stability of earthen impoundments. Recent advances in signal processing, computer hardware and software, and telemetry now provide a cost-effective platform from which these initial research efforts can be developed into an accurate, reliable and inexpensive system for monitoring the stability of earthen structures. Such systems will provide real-time data regarding the structural condition of remote earthen structures. This is especially critical during emergency situations since it would enable responsible officials to deploy limited resources to reinforce the most critical areas and avert catastrophic failures.
Equalization in the Activated Sludge Process - An Example:
*Activated Sludge - No Action Alternative:
For the Activated Sludge/No Action alternative (No Action alternative), the SBIWTP (South Bay International Wastewater Treatment Plant) would have the same activated sludge secondary treatment as selected in the 1994 Final EIS. This alternative assumes that Mexico will manage the wastewater flows to provide a constant flow of 25 mgd (1,095 L/s) to the SBIWTP; thus, the constant flow through both primary and secondary treatment would be 25 mgd (1,095 L/s). Mexico would be responsible for peak flows above 25 mgd (1,095 L/s). Construction and operation of these facilities were approved in the 1994 Final EIS and ROD for the SBIWTP project. The proposed new activated sludge and related facilities are sized to treat an average monthly organic loading of 370 milligrams per liter (mg/L) 5day biochemical oxygen demand (BOD5), 350 mg/L total suspended solids (TSS), and an average flow of 25 mgd (1,095 L/s). BOD5 and TSS would be reduced to 19 mg/L each in the effluent from this alternative.
*SBIWTP with Activated Sludge Secondary Treatment:
This alternative comprises activated sludge secondary treatment at the SBIWTP to accommodate an average flow of 25 mgd (1,095 L/s) with options for treating peak flows. The first option involves the construction of a flow equalization basin to accommodate peak flows up to 50 mgd (2,190 L/s). The second option under this alternative involves an increase in the capacity of the secondary facility at the SBIWTP to treat peak flows up to 50 mgd (2,190 L/s).
*Activated Sludge with Flow Equalization Basin:
This option would result in an average flow of 25 mgd (1,095 L/s) into the SBIWTP with a flow equalization basin to accommodate peak flow storage and subsequent off-peak discharge to the secondary activated sludge facility. A flow equalization basin capable of storing advanced-primary-treated peak flows greater than 25 mgd (1,095 L/s) would be constructed for this alternative. A storage volume of 7 million gallons (MG) would be required. Accordingly, the average flow through both the advanced primary and secondary portions of the plant would be 25 mgd (1,095 L/s). Flow through the advanced primary portion of the plant is projected to follow the identified daily flow variations with a low flow from 3.5 mgd (153 L/s) to a peak flow of 50 mgd (2,190 L/s). Before this variable flow enters the secondary facility, it will be equalized by the basin to a steady rate of 25 mgd (1,095 L/s). The flow equalization basin would be located within the existing footprint of the SBIWTP. Other than the flow equalization basin, construction and operation of these facilities were addressed in the 1994 Final EIS and ROD. (A smaller flow equalization basin sized at 5.5 mg, however, was considered as part of the 1997 Final Interim Operation SEIS.) These proposed new activated sludge and related facilities are sized to treat a monthly average organic loading of 370 mg/L BOD5 and 350 mg/L TSS, and an average flow of 25 mgd (1,095 L/s). The equalization basin facilities are designed to equalize flows to a constant 25 mgd (1,095 L/s). The activated sludge facilities are designed to provide an effluent quality of 19 mg/L BOD5 and 19 mg/L TSS.
*Activated Sludge with Expanded Capacity:
For this alternative, the secondary facility would be sized to treat peak flows up to 50 mgd (2,190 L/s). The number of secondary clarifiers would be doubled from 8 to 16 to accommodate these peaks. Thus, an average flow of 25 mgd (1,095 L/s) with peak flows up to 50 mgd (2,190 L/s) will be treated by both the advanced primary and secondary facilities. The proposed new facilities would be located on the existing footprint of the SBIWTP and on a portion of the Hofer site. Construction and operation of these facilities were addressed in the 1994 Final EIS and ROD. These proposed new activated sludge and related facilities are sized to treat an average monthly organic loading of 370 mg/L BOD5, 350 mg/L TSS, and an average flow of 25 mgd (1,095 L/s). These facilities are designed to treat peak flows of 50 mgd (2,190 L/s). The activated sludge facilities would be designed to provide an effluent quality of 19 mg/L BOD5 and 19 mg/L TSS.
*SBIWTP with Ponds Secondary or Secondary-Equivalent Treatment:
This alternative includes two treatment pond options capable of treating a 25-mgd (1,095 L/s) average flow with peaks up to 50 mgd (2,190 L/s). In this alternative, conventional primary treatment, as opposed to advanced primary treatment, would be provided at the SBIWTP to optimize the pond processes. In conventional primary treatment, settling would occur without chemicals to assist that process. The primary effluent would be the influent to the pond systems. The wastewater would be treated in the pond systems to a secondary or secondary-equivalent level. One option under this alternative is a Completely Mixed Aerated (CMA) system at the Hofer site. The second pond treatment option is the Advanced Integrated Pond System (AIPS) at the Spooner's Mesa site.