The 6-station DBT base unit contains many of the components of the system, including jar support base, sample illumination lamp, mixer drive motor, cooling fan, tachometer, and controls.
The DBT is modular in design. One or more of three removable modules (mixer, baffle/dosing, recycle injection) are placed onto the base unit as required, and removed after use.
The use of one-litre (rather than two-litre) sample sizes helps avoid errors in dosage calculations, makes jar handling easier, and requires less sample volume (often an important consideration in off-site work). The ECE jar configuration, with improved sampling system, is used.
Removable mixer module
The entire mixer / drive gear assembly can be removed from the base and sample jars as a single unit, simply by lifting it off. This eliminates the need for individually lifting and securing each mixer when installing or removing jars. It also avoids the common problem of water remaining on the mixer shafts, being deposited in the shaft bearings, and causing the bearings to seize. The paddles are also easier to clean, resulting in less potential for cross-contamination between runs.
The mixer module includes aluminum housing, paddle assemblies, and main drive shaft with miter gears and coupling to the drive motor in the base unit.
High-intensity mixing
Without baffle
With baffle
The extended high-end mixing intensity of the DBT system allows more accurate simulation of the critical “flash mixing” phase commonly used in modern coagulation processes. To prevent vortexing and reduce air entrainment resulting from these high mixing intensities, a baffle system is used. The baffle assembly (a single unit, also used to hold the chemical dosing syringes) can be easily and quickly inserted into the jars, and lifted out after completion of the coagulation phase.
Baffle / dosing module
The chemical dosing system used with the DBT allows accurate, simultaneous, and consistent addition of coagulant or other chemicals to all jars. This eliminates errors resulting from differing coagulant addition procedures or mixing times. Shown below is the base unit with the mixing and baffle/dosing modules in place, and chemical dosing syringes installed.
The dosing/baffle module is normally removed after addition of chemicals, to give easier access to jars and to allow other attachments such as the recycle injection module to be used.
Recycle injection module
A major source of leaks, breakage, awkward handling, and other problems in previous DAF batch tester designs is the principle of injecting the recycle water through a connection at the base of the jar. The DBT system avoids this problem entirely by injecting the recycle stream from the top of the jar, so that no sub-surface, through-wall connections are involved and it is not necessary to use special jars.
The injection systems for all six stations are contained in a single, easily-removable assembly which is simply placed on the base when recycle injection is required, and lifted off immediately afterwards. This makes the initial jar setup and chemical addition, as well as later observation and manipulation, much easier. The removable recycle injection assembly means that the DBT can be used for evaluation of conventional (non-DAF) processes with maximum convenience, easy access to the sample jars, and no compromises.
The microbubbles required for the flotation process are formed in the injection nozzles at the ends of the recycle tubes. In the injection nozzle, a carefully-sized orifice controls the recycle flow rate, and the supersaturated air comes out of solution when the pressure is reduced at this point. An impingement surface at the correct distance from the discharge orifice breaks up the air bubbles to the sizes needed for optimum flotation. The exterior housing of the nozzle serves the important function of destroying the very high velocity of the recycle stream leaving the orifice, so that floc is not broken up as the recycle stream mixes with the jar contents.
Shown below is the base unit with the recycle injection module installed, shortly after injection of recycle.
Proper illumination is essential for accurate, reliable, and consistent observation of the samples being tested. The DBT incorporates a high-efficiency, high-intensity, dual-tube fluorescent lighting system for very intense illumination of the samples. Part of the DBT base structure is a solid (black) vertical surface behind the jars, to provide a consistent and uniform background for observation of samples. A white plastic curtain sheet is also provided for insertion behind the jars for those occasions when a white background would be better (such as when evaluating color removal). The excellent illumination and close spacing of the sample jars facilitates documentation of test results by photographic means.
An internal cooling fan greatly reduces the possibility of misleading results caused by the convection currents that would otherwise result from heating of the jar bases by the lamps. The fan operates automatically whenever the lamps are on.
Recycle saturator and air compressor
The DBT system comes with an 8 L recycle saturator pressure vessel, complete with quick-connect fittings for air and water, pressure gauge, relief valve, and all necessary hoses and tubing. A small portable air compressor is also included.
Control panel
The main DBT control panel, on the right side of the base unit, includes:
accurate digital tachometer for mixer speed indication
five preset mixer speeds plus continuously variable control
power, mixer, and light switches.
Behind the control panel are the regulated DC power supply, cooling fan, and electronic circuit boards.
Recycle injection controls
Recycle injection is carried out using controls on the recycle injection module. The module is automatically supplied with 16 volt DC power when placed on the base unit.
A selector switch provides three different modes of recycle injection. All six stations can be operated independently of each other; injection times for each station can be set independently but started at the same time using a single start switch; or all six stations can be operated together from one station setting. This provides maximum flexibility, ease of operation, and convenience.
Each station has its own controls, located on the recycle injection module directly above the station.
Recycle injection time is adjustable from zero to 10 seconds, corresponding to a recycle injection ratio of up to about 20%. Each station also has its own on/off and start switches.
Carrying cases
The DBT system has been designed to be readily portable, and can be carried by one person. The system is packed in two birch plywood carry cases.
The main case contains the components necessary to carry out conventional (non-DAF) jar test procedures - base unit, baffle/dosing module, mixer module, and jars. This case also accommodates the recycle module, when needed.
The DAF case contains the additional equipment needed for DAF testing - the recycle saturator vessel and (if desired) the air compressor. There is also space for miscellaneous items such as hoses, tubing, syringes, and stock chemical bottles.
Downloads
Here is some additional downloadable information on the DBT system (left click to view the files; right click and select Save Target to save to disk). All files are in Adobe Acrobat (.PDF) format. To access the files, you will need Acrobat Reader; if you don’t have it, you can download it free.
