Wastewater Treatment – Clarifying the Choices

Companies should consider such factors as space and capital requirement, desired filtrate quality, and level of automation when selecting water and wastewater filters.

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

Filter selection begins after a company determines the required level of filtration or filtered water quality; decides whether filtrate will be disposed or reused; identifies how much space is available; calculates the amount of capital available to purchase, operate, and maintain the system; decides whether the system should be automatic or manual; and considers any pertinent water conservation requirements.

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

Proper filter size is determined by flow rate and the amount, size, and type of solids to be removed. Particle sizes and filtration levels can be determined by filtering water and solids through varying sizes of filter paper and measuring the total solids content of filtered water.

Solids remaining in filtered water will be the same size or smaller than the last filter paper used and will indicate which filter media opening size to use. (A filter manufacturer or laboratory can be hired to perform this procedure.) The number of solids in water or wastewater can be determined by a laboratory analysis of total solids; flow can be determined from plant records or an in-line flow meter.

The following 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 size have been determined, the amount of time required before cleaning, or back-washing, should be calculated, and the fate of filtrate waste must be 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 (100,000 gals) of filtered water would generate 38 m3 (10,000 gals) of backwash that contains concentrated solid and may require additional treatment.

Some Wastewater treatment must be pretreated before filtration. Oily wastewater, for example, tends to agglomerate quickly between or over filter media and requires conditioning before filtration. Fine-grade filtration systems are susceptible to fouling from biological growth, yeast, and other microorganisms; performing bench tests or consulting with filter manufacturers about specific applications is recommended.

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