Bed Expansion

Under the upflow conditions of backwashing, the resins will expand their volume (see Figures 1a-1c). Such expansion allows the re-grading of the resin, fines removal and avoids channeling during the subsequent service cycle. At the same time, accumulated particulate contamination is removed.

For an efficient backwash a uniform bead resin requires less flow to expand to the same height as a conventional polydispersed resin of the same average particle size. Due to the smaller and more uniform bead size of the AQUAPRO 500-3 resins, an expansion of around 60-80% is normally sufficient to remove particulate matter from the resin bed, thereby reducing the backwash flow rate required even further. Due to the absence of fine beads in the uniform resin, the   risk of resin loss during backwashing is reduced.

The pressure drop across a resin bed can vary depending on a number of factors. These include resin type, bead size, interstitial space (bed voidage), flow rate, temperature and bed contamination. The data in Figures 2a, 2b and 2c relates the pressure drop per unit bed depth to both flow velocity and water temperature for AQUAPRO 500-1, AQUAPRO 500-2 and AQUAPRO 500-3 resins. Depending on the degree of bed classification, the smaller beads    in conventional polydispersed resins may fill the interstitial spaces between the larger beads, thereby increasing the head loss. Compared to conventional resins, uniform beads have a higher voidage which compensates for the smaller mean bead diameter, resulting in similar head loss characteristics for AQUAPRO 500-Series resins as for conventional resins.

Resin operation in the sodium cycle

The exchange capacity of the resins when used in the sodium cycle depends upon the following:

• allowable residual hardness in the treated water (leakage)
• analytical characteristics of the raw water
• operating conditions, such as regeneration level

The level of hardness leakage is shown in Figures 3a-3c as a function of raw water total dissolved solids (TDS expressed as salinity in ppm CaCO3) and at different resin regeneration levels.

Concentrations of ionic species in water may be expressed in different units in different countries. The following table gives the conversion factors for commonly encountered units to milliequivalents/litre (meq/L) and ppm CaCO3.

The methodology for designing a co- or counter-flow plant is to determine the resin operating capacity based on one reference set of operating conditions and then to apply correction factors for the specific conditions of the design. The reference conditions are:

• Linear flow of 12 m/hr (5 gpm/ft2) or 16 bed volumes/hr
• Temperature 20°C (68°F)
• 500 ppm TDS feed
• 75 cm (30″) resin bed depth
• 10% NaCl regenerant at 25 minutes contact time
• Capacity TH endpoint of 3% (15 ppm CaCO3) for co-flow operation

Before proceeding with the design, consideration should be made of the particular conditions applying to the softener (e.g. temperature, oxidants) which may impact the choice of resin (see resin descriptions above). A description of how to make a design in given below:

Co-flow design

1. From Figures 3a-3c, determine the level of regenerant required for the particular water feed TDS to give an acceptable hardness
2. Use Figure 4 to determine the resin operating capacity at that level of regeneration.

To design at other conditions, correction factors should be applied to the operating capacity curve as described below:

1. Correct the operating capacity for feed water TDS using Figure
2. Correct the operating capacity for feed temperature using Figure
3. Correct the operating capacity for %Na/TH in feed using Figure
4. Correct the operating capacity for TH end-point (if desired) using Figure 8.

From the calculated resin operating capacity above, apply capacity safety factors (if desired) and determine the resin volume required to produce the desired plant throughput. Design of the vessel dimensions is described as follows:

1. Choose a vessel dimension to give a service flow rate between 5 and 50 m/hour (2-20 gpm/ft2). With an increase in flow rate there is an increase in hardness leakage, which may be contained within certain limits by reducing service exchange This operating capacity correction is given in Figure 9. Correct the operating capacity for flow rate and adjust the resin volume accordingly.
2. The resin bed height correction is given in Figure Leakage and capacity data presented here are based on resin bed depths of 75 cm (30″), the minimum depth recommended. Average leakage for the run is lower for deeper beds due to continually improving water during exhaustion. The capacity correction factors are shown for up to 300 cm (10 ft) beds.

Modification of the vessel dimensions should be made by iteration using Figures 9 and 10 until the final design is obtained.

Counter-current design

Leakage data presented in Figures 3a-3c are based on co-current operation. In designing counter-flow softening systems, leakages are very low (expect <1 ppm CaCO3), so Figures 3a-3c are not used.

The operating capacities for counter-flow can be taken as the same as for co-flow, so Figures 4-10 above can be applied using the same methodology. Note that Figure 8 is not relevant for counter-flow operation. In general, maximum salt efficiency is obtained at lower regeneration levels, while maximum capacity results from higher levels. The designer must balance these considerations.

In addition to the information given above, some further guidelines on design and operation of a softener unit are included below:

Concentration of regenerant solution

Other operating conditions being equal, the highest exchange capacity values are attained by utilizing 10-12% solutions of sodium chloride.

Regeneration contact time

It is advisable to adopt a contact time of at least 30 minutes. When particular characteristics of the raw water to be decalcified impose the use of regeneration levels higher than 200 g/L (12.5 lbs/ft3) of NaCl (100% basis), a longer contact time is advisable (about 45 minutes).

Rinse procedure

The excess regenerant and regeneration derivative products are removed by rinsing with raw water after flow of the regenerant solution through the exchanger. In conditions of efficient column drainage, the rinse requirements of raw water are 3-6 BV.

Calcium-magnesium ratio

The reported data are based on a calcium-magnesium ration of 2/1 (67% of total hardness as calcium). As percent calcium increases from 67 to 100%, capacity decreases 5%. As calcium decreases from 67 to 33%, capacity increases by 5%, with hardness leakage increasing 15 to 20%. Below 33% calcium, both capacity and leakage increase at a faster rate.