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Direct Filtration

Figure 1: Direct filtration.
Figure 1: Direct filtration.

Direct filtration is a used for the treatment of good quality water supplies. It involves the addition of coagulant, rapid mix, flocculation and filtration. The major difference relative to conventional treatment is the absence of a separation process, such as sedimentation or flotation, between coagulant addition and filtration. Direct filtration can be preceded by pre-oxidation, may be accompanied by powdered activated carbon (PAC) addition, and in some cases followed by granular activated carbon (GAC) adsorption. Thus, it may be concurrent with other processes.

The same basic general physical chemical principles described in conventional treatment apply to direct filtration. Low coagulant dosages and high-intensity, short duration flocculation in a tank or in the media pores are used in direct filtration to promote the formation of a pinpoint sized floc which can penetrate the filter depth maximizing the filter bed¿s storage capacity.

Because the type of flocculation process typically used in direct filtration is not as efficient as conventional treatment in forming floc, variable water turbidity and bacterial levels constitute problems for maintaining good filter effluent quality. Thus, direct filtration is primarily used for the treatment of good quality sources characterized by turbidity of less than 5 to 15 NTU, color of less than 20 to 40 units, and low concentrations of algae iron and manganese. For water supplies that are consistently very low in turbidity and color, the flocculation tank is sometimes omitted and the process is then referred to as in-line filtration.

Direct filtration has several advantages compared to conventional treatment: (1) lower chemical costs due to lower coagulant dosages used in direct filtration, (2) lower capital costs as the sedimentation (and sometimes the flocculation) tank is not needed, and (3) lower operation and maintenance costs as the sedimentation (and sometimes the flocculation) tank need not to be powered or maintained.

There are also disadvantages to direct filtration, including: (1) it cannot handle water supplies that are high in turbidity and/or color, (2) short response time for operators to adjust treatment to changes in source water quality, and (3) less detention time for controlling seasonal taste and odor problems.

In direct filtration, instrumentation is very important for the automatic process control. The monitoring of source and finished water is also crucial to warn operators to changes and consent adjustments to the treatment process. A fail-safe shutdown mode is often included in design of direct filtration facilities to prevents the distribution of treated water that does not meet treatment goals.

Water quality parameters such as pH, temperature, and alkalinity may dictate effectiveness of direct filtration. The pH affects the speciation of the coagulant as well as its solubility, the speciation of the contaminants, and the filterability of particles. Temperature also impacts the process because it affects the viscosity of the water. At lower temperature waters can decrease the hydrolysis and precipitation kinetics. Some of the alternative coagulants such as polyaluminum chloride can be advantageous over the traditional aluminum and iron salts in low temperature conditions as these coagulants are already hydrolyzed, and therefore temperature tends to have less effect on the coagulation process.

Parameters used to characterize the direct filtration process include filter loading rate, filter run time, and head loss. The filter loading rate is a measure of the filter production per unit area and is typically expressed in gpm/sf. Typical direct filtration loading rates range from 2 to 8 gpm/sf; however, filter loading rates greater than 10 gpm/sf have been used in some installations. This can be a critical parameter because it determines the water velocity through the filter bed and can impact the depth to which particles pass through the media. The filter run time describes the length of time between filter backwashes during which a filter is in production mode. Filter performance, particularly with regard to particulate contaminants, is often poorest immediately following a backwash. As the filter run time increases and the concentration of solids in the media increases, the filtration process often performs better with regard to particulate contaminant removal. Head loss is the pressure drop that occurs when water flow through the filter media. Its development during the filtration run gives an indication of how quickly the filter is approaching the terminal head loss and the end of the run.

Residuals generated by the direct filtration process include coagulation solids (sludge) and spent backwash. The amount of residuals that is generated in direct filtration is significantly less than in conventional treatment. This is a consequence of the lower coagulant dosages that are used in direct filtration.

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