Wastewater Treatment FILTRATION:
 

PROCESS: There are several different types of filtration processes that are able to be employed in wastewater treatment:
Filtration: single (sand), dual (anthracite and sand), and multi-media (such as anthracite coal, sand, garnet, pea gravel). Typically, plain filtration (without upstream chemical addition) reduces suspended solids concentration from the range of 20 to 30 mg/L down to 5 to 10 mg/L; with chemical flocculants, to zero to 3 mg/L; with 2 to 6 gal/min/sq ft hydraulic loadings for gravity filters.

PURPOSE: Filtration is employed to polish secondary treatment process effluent, usually prior, to the disinfection process; or to provide a highly clarified water for reuse.

DESIGN: During the design phase, the filter influent characteristics of suspended solids concentration, particle size and their relative numbers, type of particles, and the flow rate are considered among other parameters. The filter is designed and constructed based on the type, size, relative number, & electrical charge of particle to be removed; allowable headloss; the filtration rate, in gpm/sq ft desired; filter media type, porosity, shape, and density; and media type depth(s). Single media filter beds are usually of sand; dual media of a layer of anthracite or activated carbon over a layer of sand; multimedia of a layer of anthracite or activated carbon over a layer of sand, which is on top of a layer of garnet. There are many types of these different media combinations, in addition to the several newer types of filters utilizing newer technology.

The filter media is classified by it's effective size (refers to the size of the sieve opening which allows passage of 10% of the media, and holds 90% on the media); specific gravity (what its mass is in ratio to water); hardness (strength); uniformity coefficient (ratio of 60% of the particle diameters and 10% of the media weight <smaller number means particles are closer to the same size>)

Filtration units that I have seen or operated are usually 30 to 36" in media depth. I found that grease/oil may coat the media, reducing its density (making it "lighter") and allowing the media to leave the filter during the backwash cycle. In water treatment facilities, the flow rate is usually constant, in wastewater facilities the flow rate is quite variable, which makes the unit more difficult to operate. The flow rate surges may force particles deep into the media, break them up and even force them through the filters. Most facilities have their backwash water sent to a backwash surge tank, where the water is metered back into the plant headworks to reduce hydraulic and solids shock loading. The most often cited critical design criteria is to design the filtration process based on the maximum 4-hr flowrate at the highest expected loading solids rate (suspended solids concentration). There are three types of filter backwash aids: hydraulic jets, air scour, and mechanical rakes.

OPERATION:
Suspended solids are reduced; affected by the influent water quality, type and number of particles to be removed; filter type; upstream process type and efficiency; & filter operation. Proper chemical dosage and upstream process control minimizes filtration problems and cost to operate. Desirable to have some particle penetration into the filter media, but not to break through; desire "bed loading" to maximize filter run length. The strength of the particle floc is important so that it does not brake up when it enters the filter media. Generally speaking, the floc strength increases with an increasing activated sludge MCRT. "Strong" floc is usually filtered or "strained" out in the top layer of anthracite, with the "weaker" floc biologically, chemically or physically adsorbed by the media. Dual media: desire to have most strong floc solids in anthracite coal layer (on top) with sand (lower layer) to polish out finer particles and prevent break through. In the dual media or multimedia filters, the carbon layer does more work removing strong floc, and the sand layer removing more of the "weaker" floc particles. As filter loads up with pounds of particles, increases "head loss"; (caused by resistance to water flowing through the filter voids that are filling with turbidity causing particles). We want to backwash filter at a point just before break through (filter clogged up and turbidity/suspended solids broke through) and to coincide with filter head loss time, to maximize best operational cost and design. Usually best to have one backwash per 24 hour period of time. Backwashing is very important, usually a filter bed expansion of 25% for the carbon layer for maximum media collisions and therefore cleaning of the media by scouring. The removal of the biologically or chemical/physical adsorbed material is enhanced by a backwash aid, such as an air scour sequence. Backwash to clean filter, using 2 to 4 % of filtered water to do so (above 5% investigate, may have a problem...some backwash up to 8% of filtered water volume though). As the temperature of the backwash water increases, it is less dense, and therefore requires an increase in the backwash water flow rate to produce the same results. An increase in temperature from 10 degrees C to 30 degrees C, may mean up to a 30% increase in backwash water to suspend the sand media, and provide the energy for effective media collisions and the resulting scouring of the filtered material from the media. Important records: headloss at start of filter run; length of filter run; quantity of water filtered; headloss at the start of the time of backwash. An slow increase over time, of the filter headloss value, at the time it is being placed back into service, means that you may have an insufficient backwash, that the media is "loading up". Corrective action after examination of records: may wish to try increasing the backwash air scour time, backwash water quantity, backwash length of time, as appropriate.

INTERACTION:
Must control upstream processes: primary sedimentation, the secondary treatment process; any upstream coagulation, flocculation, step prior to filtration. Must control filter operational parameters of coagulant dosage and flocculation, filtration rate, rate of head loss, filter run time, filter effluent turbidity (suspended solids), & proper backwash. Do not want larger particles to hit filters as they coat the filter surface and cause short filter runs and can also breakup and increase filter effluent turbidity. The downstream disinfection process is better served not having to disinfect large/numerous particles! Filter backwash water is best when the backwash water and the solids it is carrying, are not "shock loaded" hydraulically or solids loading rate, back to the plant headworks for treatment.

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