DYNASAND FILTER
Introduction:
The Dynasand Filter is a unique filtration system, that is used as a polish for the final effluent. It continuously filters water from the DMP Wastewater System suspensions to excellent quality while it continuously cleans the filter media. The influent suspension is introduced into the filter through the feed nozzle. The flow enters a series of vertical tubes which protrude into the filter. These riser tubes discharge beneath the inlet distribution hood which allows even distribution of the influent into the sand bed. The influent waste is filtered upward through the sand and the clean filtrate exits the unit over the filtrate weir at the top of the tank.
For the continuous cleaning at the media, a small supply of air is continuously introduced into the airlift which is centrally located at the bottom of the tank. This airlift, which is encased in the airlift protective housing, extends to the top of the unit. The filtered solids and sand are pulled into the airlift and conveyed to the top of the unit. The agitation and the turbulence within the airlift cleanses the sand of the filtered solids. These solids and sand exit the airlift into the central compartment/washer section. Within this section, the sand is further washed and the solids and sand are separated. The clean sand, having fallen through the washer, is distributed evenly over the sand bed. The washer/central compartment section is isolated from the filtrate except where the sand exits at the bottom of the washer. The blow up of the washer shows that a small amount of filtrate flows up through the sand washer assembly and acts as the reject water. The reject water traps all of the separated solids and this stream flows over the reject weir and out of the filter through the reject nozzle.
The operation of the Dynasand Filter is continuous. There is no shutdown for batch backwashing and the rejected solids stream is continuous. This means that the Dynasand Filter can take higher solids loadings than conventional filters. In some applications the filter can filter up to 500 ppm suspended solids for short periods of time, whereas conventional gravity filters cannot handle more than 10-30 ppm without extensive backwashing. There are no separate backwash water storage tanks, pump, complicated valving, and instruments and controls.
Reject Weirs and Reject Rate:
A certain amount of reject water is needed to wash the filtered suspended solids off the sand. The amount of reject rate needed to thoroughly wash the sand depends on the application. Varying the height of the reject weir will vary the amount of reject water.
The reject weir are plastic plates shipped loose. You should receive approximately 6 plates which are various heights and are designed to slide into a slot in the central compartment.
Because the reject rate is an important parameter in filter operation, be sure to provide a means to measure the rate. In most cases the rate may be very small and accessibility to the reject line may be sufficient to measure with a bucket and stop watch. Other types of volumetric measurements with time are also acceptable. Calibrated flow meters may be hindered as the reject may contain high concentrations of suspended solids.
Sand Rate:
The sand rate is controlled by the volume of compressed air rate. The compressed air rate is set by the flow control valve on the air flow meter and will remain the same as long as the valve setting is the same and the pressure upstream to the valve is constant. To ensure constant pressure a pressure control valve is also included on the panel. The pressure control valve can be set on 35 psi and will then give a downstream pressure (as long as the upstream pressure is always higher than 35 psi).
The sand rate can be varied and can be increased to compensate for higher suspended solids concentrations in the feed. There is however a limit to the capacity of the sand washer separator.
Sand Procurement and Filling:
Both the size distribution and the quality of the sand is of importance when selecting a filter sand. With our submittal drawings we specify the recommended sand size in the form of ES (effective size) and UC (uniformity coefficient).
If you elect to obtain the filter sand on your own you should also be aware that there are a few other things in addition to size which are [of importance.
· The sand should be high in silica (greater than 98% preferably) and have a low acid solubility which generally also would indicate that the sand is resistant against abrasion.
· The sand should be fairly round. Angular sand with sharp edges (characteristic of crushed sand) is generally not recommended as filter sand. "Flat" sand should also be avoided.
· The amount of real fine material (say of a size less than 80% of the effective size) should be at a minimum.
Sand Specifications:
· Effective size, shape 0.8 - 0.85 mm spherical (.86-.90) average specifications
· Uniformity coefficient 1.5 (1.57) average
· Silica content 99% (98%) average
· Low acid solubility
The sand can be delivered either in 100 lb. bags (for ease of handling) or in bulk. If the sand demand is less than 20 tons it usually makes sense to use bagged sand. The filter can then be changed using a fork-lift and a pallet to lift the sand to the top of the filter where the sandbag is opened.
Our recommended amount of sand includes 10 - 20% safety margin, as sand bags sometimes rupture, etc. The filter should only be filled to the recommended level and the rest of the sand be kept as backup. During the filling you can easily check the sand level in the filter by using a rod (check at least six different places).
* Read manufacturer instruction manual before filling.
Note: Care should be taken to ensure that pieces of the bags or other foreign objects do not fall into the filter.