Materials and Methods
Description of Field Experiments
The new mixing device was tested at two different sites. Pilot scale experiments (17,000 L) were performed at the Pepper’s Ferry wastewater treatment plant in Radford, Virginia. Tests under full-scale conditions (380,000 L) were conducted in Conway, South Carolina.
Full-Scale System, Conway, SC
Initial experimental setup and testing took place in early July 2006 and continued through August 2006. The testing site was located at the High Tech Turf Farm in Conway, South Carolina. At the turf farm, Class B biosolids were land-applied after being aerated in six cylindrical tanks, that each held approximately 380,000 L of sludge. Each of the tanks (Fisher Tank Company, Lexington, SC )was 10.4 m in diameter, 4.9 m tall and filled with ca. 1.3% TSS sludge from the Schwartz Wastewater Treatment Plant.
The tanks were each equipped with a 5 hp blower powering 26 coarse bubble diffusers. The diffusers were located 0.3 m above the tank floor in two rows. Each row, consisting of 13 diffusers, was positioned 2.7 m from the center (Fig. 4). For this research project, only one tank was used. The aeration tank was modified by retrofitting TotalMix into the tank. TotalMix was placed 0.3 m way from each diffuser towards the tank wall. Additionally TotalMix systems were installed perpendicular to the diffusers (Fig. 4). Every set of TotalMix systems was connected to a solenoid valve and programmed to release air bursts in sequence to maximize the turbulence.
Sampling beams were only placed on one half of the tank since the tank’s aeration was installed symmetrically. It could therefore be assumed that the aeration pattern on the other half of the tank was the same this way, the sampling time was decreased, which allowed for a comparison of data points from each beam. Measurements were taken on each sampling beam (Fig. 5) at three different depths (0.6, 1.8, and 3.7 m) and different widths (1.2, 4.3 and 7.3 m) alternating between the diffusers alone and the diffuser/TotalMix combination.
Pilot-Scale System, Radford, VA
To test different diffusers and wastewater suspensions, a pilot-scale system with a 17,000 L tank (3.7 m high) was set-up at the Pepper’s Ferry Wastewater Treatment Plant in Radford,VA. The tank was equipped with one TotalMix head placed in the center of the tank and one diffuser located ca. 0.6 m towards the tank wall (Fig. 6).
The wastewater types tested included activated sludge, thickened activated sludge, and treated effluent (used for clean water testing). The diffusers considered were ceramic and membrane diffusers. The activated sludge was taken from a clarifier; the thickened activated sludge was taken after a dissolved air flotation unit (DAF), and the water for the clean water tests was taken from the plant’s effluent.
The newly patented device is referred to as TotalMix and is a type of pneumatic mixing system (Fig.7). The programmable and intermittent high pressure mixing system has never been used in wastewater treatment applications.
The bottom plate of the disk-shaped mixing device has a 30.5 cm diameter and is 1.3 cm thick. The plate is separated into 6 equal wedges (50°) which are placed 5 cm. apart from each other (Fig. 8).
Six bolts hold the bottom plate to the metal top. The metal plate, with the standard 1.9 cm (¾ inch) pipe thread allows for an easy retrofit. Furthermore, the systems benefits include that the high pressure prevents the canals from clogging and keeps it clean. Also compared to standard diffusers, which are located about 15 cm above the tank bottom, the TotalMix head is placed about 2.5 cm above the tank bottom, which has the advantage that particles are kept in suspension and cannot accumulate below the diffusers.
The disk-shaped mixing device was located 2.5 cm above the basin floor and ca. 13 cm below the diffusers. Air was introduced under a pressure of about 2,800 hPa. The period of pressured air delivery was varied manually, but can also be changed through feedback control to optimize oxygen transfer and the interaction with a regular aeration system (ceramic or membrane diffusers). The piping from each frame was connected to a solenoid valve, which was operated by a programmable time-sequence controller (Fig. 9).
Compressor, Controller, and Valves
The same rotary contact cooled compressor, UNI 15TAS (Ingersoll-Rand Company, Davidson, NC) was used for the pilot-scale as well as for the full-scale experiments. The valve controlling the air burst times at both field sites was connected to an Allen-Breadely PICO/GFX-70 controller (Rockwell Automation Global Headquaters, Milwaukee, WI) that included PicoSoft Pro Software. Four 2-way solenoid valves (MAC valves,Wixom, MI) were used for the full-scale tank and one of those valves was used for the pilot-scale tank
Oxygen transport is a complex process depending on a variety of factors, amongst others, the type of diffusers. To evaluate the most efficient set-up, we tested a membrane diffuser and a ceramic diffuser.
FlexLine Nonbuoyant Tubular Diffuser (Siemens, Waukesha,WI)
The FlexLine Nonbuoyant Tubuluar Diffuser (Fig. 10) increases the bubble surface contact area with water by producing microfine bubbles. A bigger surface contact area results in an increase in oxygen transfer efficiencies and ultimately in lower air volume requirements. Higher transfer efficiencies will lower energy costs and improve effluent quality. While other diffusers emit a narrow column of air, the FlexLine diffuser produces a broad envelope of bubbles that greatly increases transfer efficiency and improves mixing.
The FlexLine diffuser is open at both ends, so the water can fill the tube completely when airflow is off. The filled tube eliminates buoyancy and bounce that weakens joints and causes leakage. Also, with no airflow, the membrane contracts and seals off the distribution chamber, eliminating a chance of backflow. On the other hand, when airflow is on, the sleeve inflates around the exterior of the support tube and creates air distribution over the entire membrane surface. As a result, a larger perforated surface area greatly increases transfer efficiency. The biggest advantage of the FlexLine diffusers is the length. They are 60 cm long and have 8400 bubble producing ‘I’ slits. This set-up allows flow ranges up to 200 L/min, while still maintaining high oxygen transfer efficiencies and keeping the headloss low.
Ceramic Fine Bubble Diffuser (Diffused Gas Technologies, Inc., Lebanon,OH) Ceramic fine bubble diffusers (Fig. 11) are commonly used in environments which require aeration in extreme duty applications. The Ceramic Dome diffuser introduces the gas between the dome and its base. The gas permeates throughout the porous labyrinth of the dome and migrates through the minute passages of the dense ceramic matrix structure. When the gas reaches the surface of the dome, it creates a surface tension between the gas and the liquid. A minute bubble is formed once the surface tension is overcome. The ceramic diffuser operates at flow ranges up to 70 L/min.
1. Different Aeration Systems
2. Evaluation of Aeration Systems
2.1. Theoretical Background
2.2. Oxygen Transfer Rate (OTR)
Material and Methods
1. Description of Field Experiments
1.1. Full-Scale System, Conway, SC
1.2. Pilot-Scale System, Radford, VA
2.1. TotalMix System
2.1.2. Pilot-Scale Tank
2.2. Compressor and Controller
2.3.1. FlexLine Nonbuoyant Tubular Diffuser
2.3.2. Ceramic Fine Bubble Diffuser
2.3.3. Coarse Bubble Diffuser
2.4. Analytical Equipment
2.4.1. YSI 6820 and 650MSD
2.4.2. Advanced Hach LDO® Process Dissolved Oxygen Probe
4. Statistical Analysis
1. Aeration Efficiency
1.1. Pilot-Scale Tank (17,000 L)
1.1.1. Thickened Activated Sludge
126.96.36.199. Total and Soluble Chemical Oxygen Demand (TCOD, SCOD)
188.8.131.52. DO in Thickened Activated Sludge
1.1.2. Activated Sludge
184.108.40.206. DO Concentration in Activated Sludge
1.2. Summary of Test in Pilot Scale Tank
1.3. Full Scale Tank (380,000 L)
1.3.1. Activated Sludge
220.127.116.11. Valve time evaluation
18.104.22.168. DO Concentration
2. Energy Conservation
2.1. DO Concentration
2.2. Energy Consumption
2.3. Aeration Efficiency
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