Filtration is the easiest way to remove solids from water and wastewater streams. In wastewater systems, filtration is used to lower suspended solids entering the publicly owned treatment works or water-reuse systems or to increase solids in sludge dewatering applications.

Filter choice begins after an organization determines the required level of filtration or filtered water quality; decides whether or not filtrate will be disposed or reused; identifies how much area is available; calculates the amount of capital available to buy, operate, and maintain the system; decides whether the system needs to be automated or manual; and considers any pertinent water conservation requirements.

The level of solids removal or filtered-water quality desired defines the type and dimension of filter essential; most are available with various automation levels and price ranges.

Proper filter size is determined by flow rate and the quantity, dimension, and type of solids to be removed. Particle sizes and filtration levels can be decided by filtering water and solids by way of various sizes of filter paper and measuring the total solids content material of filtered water.

Solids remaining in filtered water will be the identical measurement or smaller than the last filter paper used and will point out which filter media opening dimension to use. (A filter producer or laboratory can be hired to perform this procedure.) The quantity of solids in water or wastewater may be decided by a laboratory evaluation of total solids; flow might be decided from plant records or an in-line flow meter.

The next equation is used to calculate solids loadings (how quickly a filter will load with solids): solids (lb/h) = flow (gal/min) x total solids (ppm) x (8.34/60/1,000,000).

After solids loading, filter area, and filter media opening dimension have been determined, the period of time required before cleaning, or back-washing, must be calculated, and the destiny of filtrate waste must he determined.

In other words, after a filter is backwashed, where will wastewater go? Backwashing often requires up to 10% of the total filtered volume, a 10:1 ratio. For example, 379 m3 (a hundred,000 gal) of filtered water would generate 38 m3 (10,000 gal) of backwash that contains concentrated solid and should require additional treatment.

Some wastewater have to be pretreated before filtration. Oily wastewater, for instance, tends to agglomerate quickly between or over filter media and requires conditioning earlier than filtration. Fine-grade filtration systems are prone to fouling from biological progress, yeast, and other microorganisms; performing bench tests or consulting with filter producers about particular applications is recommended.

Y-Strainers

Y-strainers are the best, least expensive ($30 to $one hundred) filters, however they’ve a small filter area and require frequent maintenance. They typically are used on small pipes to protect such equipment as seals, pumps, and nozzles from solids buildup.

Bag Filters

Bag filters basically are larger Y-strainers. These filters are bags of selected pore sizes that are enclosed in a pressure housing with a removable cover for cleaning. They’re straightforward to make use of, come in variable filtering sizes, and are comparatively low in value, particularly for closing filtration or applications with low solids loadings. Bag filters don’t exceed 203 mm (eight in.) in diameter; for bigger diameters, an inline bank with a number of filters in a single housing is used.

The price of carbon or stainless steel bag filter housings varies according to dimension, from $one hundred for a fifty one-mm (2-in.) housing to $3000 for a 1219 mm (48-in.) housing. Filter bags range in value from $6 to $10 for pore sizes of 25 to 800 microns. Bags are available straight or pleated. Pleated bags provide added surface area however value more than twice as a lot as flat ones. The disadvantages of bag filters embody high upkeep necessities for systems which can be underdesigned or have high solids loads and the price of bag replacement.

Vacuum Filters

Vacuum filters use a slowly rotating vacuum (negative pressure) drum that pulls dirty liquid from the outside of the drum and media to the inside of the drum. The vacuum filter usually is packaged with a pre-batch therapy system to coagulate the wastestream earlier than filtration. The advantage of this filter is its ability to filter a variety of wastewaters. The disadvantages embrace sluggish system throughput and high costs associated with the filter media, which should be applied repeatedly to the batch pretreatment tank instantly before vacuum filtration. In addition, the filtering media increase total sludge volume as a lot as 60%. Because vacuum filter value normally includes the pretreatment system, a unit that processes 19 m3/d (5000 gal/d) can price $60,000.

Cyclone Separators

Cyclone separators are comparatively new. Because they have higher rotating speeds than vacuum filters, they create a significant amount of centrifugal energy to filter wastewater through numerous media sizes. The rotating filter makes use of a fixed-micronsize medium, akin to diatomaceous earth, to filter the wastestream. The benefits of cyclone separators embody their ability to handle variable throughputs, filter various particle sizes, and produce drier sludge cake than vacuum filters for about the identical price (around $60,00). heir drawbacks are similar to those of vacuum filters, namely high filter media prices and sludge volume.

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