Gravel filter

Figure 20-14 shows the protection of a gravel filter in the treatment of raw water. The 200-m internal surface was coated with 300 jlm of tar-pitch epoxy resin. Long-term experiments indicated that at = -0.83 V, no cathodic blisters were formed, but were formed at more negative potentials (see Section 5.2.1.4). TiPt anodes 400 and 1100 mm in length, 12 mm in diameter, and with an active surface of 0.11 m were installed [29]. 

Figure 26.33 shows the gravel filter and cobblestone components of the biotic barrier and their placement in the landfill system. The proposed 1-m thickness for a biotic barrier should effectively prevent penetration by all but the smallest insects. Note that the biotic barrier also serves as the surface water collection/drainage layer. Biotic barriers used in nuclear caps may be up to 14 ft thick... 

In the operation of a downflow-type saturator, water is introduced at the top of the saturator tank (there is an air gap to avoid the possibility of a cross-connection) and the level is regulated with a float-operated controller valve on the water supply line. The water then trickles down through the bed of sodium fluoride solids the solution is clarified in the sand and gravel filter bed and ends up as a clear, saturated solution at the bottom of the tank where it is withdrawn by the feeder. The fluoride-water solution pump withdraws the solution and delivers it into the water system at a desired dosage rate. Operator attention is required only to ensure that an adequate quantity of sodium fluoride solids is kept in the saturator and that the saturator is kept in a reasonably clean condition. 

Bottomhole gravel filters and metal frlters with dense screens were at first employed in an attempt to stop sand entry into the well and avoid work interruptions. This problem was frnally solved by cementing these sands through coking, a method that was first tried in 1972. 

Fig. 1 schematically shows a cooling water circuit with preparation of the make-up water. Instead of the settling basin a gravel filter with backwash system may be used. A large basin or pond with spray equipment may be substituted for the relatively expensive cooling tower. 

Several types of aggregate-bed filters are available which provide in-depth filtration. Both gravel and particle-bed filters have been developed for removal of dry particulates but have not been used extensively. Filters have also been developed using a porous ceramic or porous metal filter surface. 

The arrangement and distribution of anodes in gravel and activated charcoal filters is different. Cathodic protection of activated charcoal filters is basically feasible but requires a large number of electrodes and high protection current densities that are twice those for gravel bed filters, so that an electrically insulating layer can be deposited on the steel wall. 

Pressure filters are worth noting. These are usually set up in the form of steel cylinders positioned vertically. Another variation consists of using horizontal filtration groups. This has the drawback that the surface loading is variable in the different layers of the filter bed moreover, it increases with greater penetration in the filter bed (the infiltration velocity is lowest at the level of the horizontal diameter of the cylinder). The filter bottom usually consists of a number of screens or mesh sieves that decrease in size from top to bottom or, as an alternative, perforated plates supporting gravel similar to that used in the filter bottoms of an open filter system. 

Size the filter for 30-inch bed depth and gravel underbed, with a service flow rate of 2 to 5 gpm/sq ft. Backwash typically every 24 hours for 10 min at 12 to 14 gpm/ sq ft to achieve 35 to 40% bed expansion. 

The most common filter pack material is quartz (silica). Quartz is relatively inert, readily available, and workable therefore, it is preferred to replace formation materials removed from the borehole. The grain size of the filter material (i.e., sand or gravel) should be chosen based on the characteristics of the formation to be monitored and the slot size of the screen. Sand and gravel are available in various uniform sizes to accommodate different monitoring environments. 

An unknown amount of the substances described above were lost in the oil and gravel separator and by flushing of the fine filters. An attempt to make a full mass balance failed. 

---