Particulate and Turbidity Removal Technologies
Small Systems Treatment Technologies Research
Source water may contain turbidity (muddiness), particles, and organic material. Federal and state laws require all surface water systems and systems under the influence of surface water to filter their water. The primary water treatment technology uses filtration techniques.
Filtration is the removal of particulates, and thus some contaminants, by water flowing through a porous medium. Filtration is the most practical treatment process. Filtration techniques are categorized as:
Conventional Filtration: consists of:
- adding coagulant chemicals,
- flash mixing (a millisecond blending),
- coagulation/flocculation (forming solid particles),
- sedimentation (the settling of a floating material), and
Direct Filtration: is similar to conventional filtration but without the sedimentation step. It is also known as "dead-end filtration."
Packaged Filtration: is often used to treat small community water supplies, as well as supplies in recreational areas, state parks, construction sites, ski areas, and military installations. It is similar to conventional filtration. The unit is mounted on a frame for simple hook-up of pipes and services. It is most widely used removal of turbidity, color, and coliform organisms.
Packaged Slow Sand Filtration: is a process where untreated water seeps slowly down through a layer of fine sand, then through a layer of gravel, and ultimately collects in a system of under drains. A complex biological layer, called schmutzdecke, forms on the surface of the sand filter, trapping small particles. It also helps decompose organic material in the water.
Diatomaceous Earth (DE): are the fossilized remains of diatoms, a type of hard-shelled algae. These filters consist of a pre-coat layer of DE, about 1/8-inch thick, supported by a septum or filter element. The process using DE is used to directly treat:
- raw water supplies; or
- more muddy water sources, if chemically coagulated.
Bag Filtration: uses bag filters that are disposable, non ridged, replaceable fabric units contained (either singly in series, parallel or grouped together in multiples) within one vessel. The vessels are usually made of stainless steel for corrosion resistance, strength, cleaning, and disinfection. Water flows from the inside of the bag to the outside. If the pore size of the bag filter is smaller than the microbe, removal will occur. Bag filtration is generally not recommended as a single filtration method to remove parasites. However, for small systems that have very high quality source water, bag filters may serve as an effective single barrier against parasites. Bag filters can be used as a pretreatment step before cartridge filtration to remove large particles and high levels of muddiness to improve parasite removal.
Cartridge Filtration: is a technology suitable for removing microbes and reducing muddiness. These filters are easy to operate and maintain, making them effective for treating low-turbidity water. These filter systems are operationally simple. They are not automated. They can require relatively large operating budgets. They can become contaminated relatively quickly and must be replaced with new units. A disinfectant may be recommended to prevent microbial contamination. Cartridge filters are commonly used without costly chemical additions. Cartridge filters are designed for protozoan, parasite, or oocysts capture. At the same time, these filters permit bacteria, viruses, and fine colloids to pass through, depending on the pore size.
Packaged Filtration: A majority of EPA small systems research has focused on the evaluation of packaged filtration technologies that are most useful to small system operators. EPA has evaluated various bag, cartridge, and membrane filters. Depending on the quality of the source water, EPA suggests a series of filters, such as sand or multimedia filters followed by bag or cartridge filtration, to increase particulate removal and extend the life of the secondary filter.
Membrane Filtration: is a pressure-driven separation process. Any particulate matter larger than 1 micrometer in diameter is rejected by the membrane. The membrane has a measurable removal capability for a target organism. This can be verified through the use of a direct integrity test. There are four categories of pressure-driven membrane processes:
Microfiltration and ultrafiltration membranes act as strainers. It is similar to the bag and cartridge filters but with smaller pore sizes. Nanofiltration and reverse osmosis membranes actually block contaminants dissolved in water down to the molecular level.
Ultrafiltration – This membrane filtration process typically uses hollow-fiber membranes with pore sizes ranging from 0.01 to 0.05 micrometer (nominally 0.01 micrometers). These systems are effective for removing pathogens from water. Small systems find this filtration affordable. Systems may be designed to operate in a single pass or in a recirculation mode.
Reverse Osmosis (RO) –Contamination is removed by the use of a membrane. However, reverse osmosis uses hydraulic pressure to oppose the liquid osmotic pressure across a semi-permeable membrane, forcing the water from the concentrated solution side to the dilute solution side. Thus, the membrane allows the passage of the water but not the dissolved solids (solutes). Because the membrane is nonporous, the water does not travel through pores, but rather dissolves into the membrane, diffuses across, and then becomes demineralized water. Reverse osmosis can effectively remove nearly all contaminants from water including:
- arsenic (III),
- arsenic (V),
- chromium (VI),
- natural organic substances,
- pesticides, and
- microbiological contaminants.