Characteristics of UV Treatment

• Short contact time of 1-10 seconds. Ozone and chlorine require 10-50 minutes, necessitating large reaction tanks. Ozonation can be run on a flow-through basis.

• Destroys most viruses and bacteria without chemical additives. The destruction of Giardia lamblia, however, requires prefiltration. Leaves no residual disinfection potential in the water so that, for water entering a distribution system, light chlorination is still needed to provide prolonged disinfection.

• Low overall installation costs. Ozone generators are expensive. Chlorine metering systems are not especially expensive, but large reaction tanks and safety systems are high cost items.

• Not influenced by pH or temperature. Chlorination and ozonation work best at lower pH (chlorine because it is in the HOCl form; ozone because it decomposes more rapidly at higher pH). Chlorination and ozonation both require longer contact time at lower temperatures.

• No toxic residues. It adds nothing to the water unless some organics are present that photoreact to form toxic compounds. The formation of THMs or other DBPs is minimal.

Membrane Filtration Water Treatment

Membrane filters are being used to treat groundwater, surface water, and reclaimed wastewater. Membrane filtration is a physical separation process that removes unwanted substances from water without utilizing chemical reactions that can lead to undesirable byproducts. The range of membrane filters available is shown in Figure 6.5, along with common substances that can be removed by filtering. Although membranes sometimes serve as a stand-alone treatment, they are more often combined with other treatment technologies. For example, currently available microfiltration (MF) and ultrafiltration (UF) membranes are not very effective in removing dissolved organic carbon, some synthetic organic compounds or THM precursors. Their performance is improved by adding powdered adsorbent material to the wastewater flow. Contaminants that might pass through the filters are adsorbed to the larger adsorbent particles and rejected by the filters.

Filter membranes are made of organic and inorganic materials. Organic membrane filters are made from several different organic polymer films, normally formed as a thin film supported on a woven or nonwoven fabric. Inorganic membrane filters are made from ceramics, glass, or carbon. They generally consist of porous supporting layer on which a thin microporous layer is chemically deposited. Inorganic membranes resist higher pressures, a wide pH range, and more extreme temperatures than do organic membranes. Their main disadvantages are greater weight and expense.

Filters can be fabricated to remove substances as small as dissolved ions. They are useful for removing total dissolved solids (TDS), nitrate sulfate, radium, iron, manganese, DBP precursors, bacteria, viruses, and other pathogens from water without adding chemicals. It must also be recognized that there will be imperfections in manufactured membrane filters through which contaminants may pass. This is of particular concern with pathogens. Therefore, filters must never be regarded as having 100% rejection for any size range. Furthermore, filters do not protect water from reinfection after it has entered a distribution system, so it is common to add chlorine or another residual disinfectant at the end of the treatment chain for this purpose. Because most organic matter has already been removed, end-of-treatment chlorination does not generate significant disinfection byproducts.

Unlike coarser filters operating in a "normal" mode, where all of the water passes through the filter surface, membrane filters operate in a cross-flow mode. In cross-flow filtration, the feed or influent stream is separated into two separate effluent streams. The pressurized feed water flows parallel to the membrane filter surface and some of the water diffuses through the filter. The remaining feed stream continues parallel to the membrane to exit the system without passing through the membrane surface. Filtered contaminants remain in the feed stream water, increasing in concentration until the feed stream exits the filter unit. The filtered water is called the permeate effluent, and the exiting feed stream water is called the concentrate effluent. Crossflow filtration provides a self-cleaning effect that allows continuous flushing away of contaminants, which, in "normal" filtration, would plug filters of small pore size very quickly.

Depending on the nominal size of the pores engineered into the membrane, cross-flow filters are used in filtering applications classified as reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF), listed in order of increasing pore size range. The pore sizes in these membranes are so small that significant pressure is required to force water through them; the smaller the pore size, the higher the required pressure. Figure 6.5 illustrates some of the uses for different membrane filter types.

FIGURE 6.5 Comparison of filter processes and size ranges.

Reverse Osmosis (RO)

RO, sometimes called hyperfiltration, was the first cross-flow membrane separation process to be widely used for water treatment. It requires operating pressures of 150 to 1200 psi. It removes up to 99% of ions and most dissolved organic compounds. RO can meet most drinking water standards with a single-pass system. Although it might not be the most economical approach, using RO in multiple-pass systems allows the most stringent drinking water standards to be met. For example, rejection of 99.9% of viruses, bacteria, and pyrogens is achievable with a double-pass system.

Nanofiltration (NF)

NF membranes can separate organic compounds with molecular weights as small as 250 Da.* It will also separate most divalent ions and is effective for softening water (removing Ca2+ and Mg2+). It allows greater water flow-through at lower operating pressure (60 to 300 psi) than RO.

* Da stands for dalton. 1 Da = 1 molecular weight unit. For example, the molecular weight of chloroform (CHCl3) is 119. The mass of one chloroform molecule is 119 Da. Chloroform will not be separated by nanofiltration, which does not reject molecules smaller than 250 Da.

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