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Tideflex High Density Configuration

High Rate Mix of 4% Solids Solution

 

Mixing & Oxygenating Thickened Aerobic Waste Sludge

    The typical activated sludge treatment process generates waste sludge which is extracted through a clarification process. The clarification process thickens the collected waste biomass to concentrations ranging from 8,000 mg/L to 10,000 mg/L (0.8% to 1.0% solids). The waste solution is stored and can be further processed for solids reduction through aerobic and anaerobic treatment processes. For aerobic treatment processes, the volume of volatile suspended solids (VSS) that can be reduced is directly proportional to the temperature of the solution, the activated sludge age, and the amount of dissolved oxygen that can be provided to the process.

Effects on Oxygen Transfer Capacity for Thickened Sludge

    Reduction of the free-liquid portion of the waste sludge volume can be achieved through mechanical thickening and/or decanting within the storage vessels. These practices are utilized to reduce the required storage volumes, reduce hydraulic loading on dewatering equipment, and also reduce transfer volume for off-site disposal. Increasing the concentration of the waste sludge decreases it’s efficiency in converting the diffused air applied to dissolved oxygen within the solution. This reduction in efficiency is related to the reduction of the associated alpha value and it’s effect on the overall aeration coefficient (kLa).

    As a waste sludge solution increases in concentration it’s liquid characteristics begin to approach the characteristics associated with non-Newtonian fluids. The production of extracellular polymetric substances at higher solids concentrations will also have a reducing effect on the alpha value. With these higher viscous fluids the shearing force of the fluid increases drastically resulting in increased bubble coalescence and reducing the effective surface area to air mass ratio. Consult Tideflex Engineering for the appropriate alpha value to be applied relative to the solids concentration of the process.

The application of Ten States Standards and EPA recommended design values for keeping Waste Sludge aerobic

    The ‘Recommended Standards for Wastewater Facilities’ as published by the Great Lakes – Upper Mississippi Board of State Health & Environmental Managers (also referred to as Ten States Standards) states that a design value of 30 cfm per 1000 cuft liquid volume for waste sludge will be adequate to maintain aerobic conditions and provide sufficient mixing. Applying this design value broadly across all sludge applications is not advised and can produce undesirable results in field applications. First, the conditions when this design value was developed should be considered. The development period for this design value was the late 1950’s and early 1960’s when the majority of waste sludge process concentrations were in the range of 0.8% to 1.0% solids. The target residual dissolved oxygen concentration was just above 1.0 mg/L and the site elevations within the Ohio Valley area ranged from 10 to 200 feet above mean sea level. The associated alpha value for this sludge concentration range is 0.55. For current waste sludge processes the average concentration of sludge within the storage/treatment vessel ranges from 1.5% to 2.5% solids and the desired residual dissolved oxygen concentration is closer to 2.0 mg/L. The effects of higher site elevations will also significantly effect the oxygen transfer efficiency for the specific application. Therefore, applying a design rate of 30 cfm/1000 cuft may provide sufficient mixing of the process solution but be insufficient in maintaining the desired aerobic condition or VSS reduction levels.

Applying Oxygen Uptake Rates as the Design Method for Aerobic Sludge Processes

    Since the solids concentration, desired level of residual dissolved oxygen, and the site elevation all have a significant impact on the oxygen transfer capacity of the process, the design of an aerobic sludge holding and/or treatment system should be based on oxygen transfer then checked for mixing energy requirements. All activated sludge processes have an associated oxygen demand which is a time based value (lbs O2/hr). This is also referred to as the ‘Oxygen Uptake Rate’ of the process. This value can be converted to a dissolved oxygen concentration uptake rate in mg/L/hr (request from Tideflex Engineering the Technical Document "Oxygen Uptake Rate of Waste Sludge”). This uptake rate can be applied to the liquid volume of the process and produce a design Actual Oxygen Required (AOR) value.

Applying Mixing Energy Design to the Solids Concentration of the Process

    As the concentration of solids increases for the waste sludge process, the characteristics of the liquid will change. The viscosity of the fluid increases thus requiring more energy to achieve the desired turnover velocities. The Velocity Gradient mixing design method is recommended because it takes into account the viscosity of the liquid, the total geometry of the fluid body, and the relative energy that needs to be imposed on the fluid in relation to the rate of mixing produced (request from Tideflex Engineering the Technical Document “Determining the Velocity Gradient and Power Dissipation for Diffused Aeration Mixing Systems”).

Eliminating the increased risk of Maintenance associated with High Solids Processes

    Many of the waste sludge processes today utilize decanting or mechanical thickening to increase the solids concentration. Cycling blowers off and on for decanting (or power saving) subjects diffused aeration systems to high density sludge blankets that settle and compact around the submerged diffuser system. If ‘open-pore’ type diffusers (no backflow prevention mechanisms on the diffusers) are utilized then the entire piping system can become filled with sludge and clog within a few blower cycles. Clogging of an aeration system will result in un-proportional air distribution, inadequate mixing, inadequate oxygen transfer, and increased blower pressures costing more money for operation.

Utilizing the Tideflex ‘Check Valve’ Coarse Bubble Diffuser for a Maintenance Free System

    The Tideflex ‘Check Valve’ Coarse Bubble Diffuser is the most reliable and widely utilized diffuser for high solids applications. The elastomeric diffuser closes shut when the blower discharge is shut-off preventing liquids and solids from entering the interior of the diffuser and piping system.

US Patent No. 6,016,839 / 6,193,220 / 6,372,140 / 6,702,263
Canada Patent No. 2,366,252 / 2,385,902; United Kingdom Patent No. 2,326,603