Sand filters

Filtration is employed when the suspended soUds concentration is less than 100 mg/L and high effluent clarity is required. Finely dispersed suspended soUds require the addition of a coagulant prior to filtration. Filters most commonly used in wastewater treatment are a dual media (anthrafUt and sand) or a moving bed or continuous-backwash sand filter. Performance data for the tertiary filtration of municipal and industrial wastewater are shown in Table 10. 

Neither rapid sand nor mixed-media filters remove appreciable quantities of coUoidal particles without adequate pretreatment. Although it is widely beheved that filters are an effective barrier against unsafe water, the effluent may be as colored, as turbid, or as bacteriologicaHy unsafe as the water appHed. In contrast, slow sand filters requite no pretreatment, as the slow passage through the bed allows the particles to contact and attach to the schmut ecke. 

Filter Cleaners. Grease and oils from bathers can affect filtration. Degreasers are employed to clean diatomaceous earth (DE) and sand filters. They can be surfactant or enzyme based. 

Dyna Sand Filter. A filter that avoids batch backwashing for cleaning, the Dyna Sand Filter is available from Parkson Corporation. The bed is continuously cleaned and regenerated by recycling solids internally through an air-lift pipe and a sand washer. Thus a constant pressure drop is maintained across the bed, and the need for parallel fdters to low continued on-stream operation, as with conventional designs, is avoided. 

The growth of community water supply systems in the United States started in the early 1800s. By 1860, over 400, and by the turn of the century over 3000 major water systems had been built to serve major cities and towns. Many older plants were equipped with slow sand filters. In the mid 1890s, the Louisville Water Company introduced the technologies of coagulation with rapid sand filtration. 

Rotten Egg Smell a) Manganese green-sand filter up to 6 ppm HjS with pH not lower than 6.7... 

Rapid-sand filters force water through a 0.45-lm layer of sand (d=0.4-1.2mm) and work faster, needing a smaller area. They need frequent back-washing. 

Precipitation is often applied to the removal of most metals from wastewater including zinc, cadmium, chromium, copper, fluoride, lead, manganese, and mercury. Also, certain anionic species can be removed by precipitation, such as phosphate, sulfate, and fluoride. Note that in some cases, organic compounds may form organometallic complexes with metals, which could inhibit precipitation. Cyanide and other ions in the wastewater may also complex with metals, making treatment by precipitation less efficient. A cutaway view of a rapid sand filter that is most often used in a municipal treatment plant is illustrated in Figure 4. The design features of this filter have been relied upon for more than 60 years in municipal applications. 

Figure 5. Example of a sand filter eonfiguration. Table 1. Typieal removal effieieneies.

Porosity constitutes a important criterion in a description based on straining. Porosity is determined by the formula V /Vc, in which V c is the total or apparent volume limitated by the filter wall and is the free volume between the particles. The porosity of a filter layer changes as a function of the operation time of the filters. The grains become thicker because of the adherence of material removed from the water, whether by straining or by some other fixative mechanism of particles on the filtering sand. Simultaneously the interstices between the grains diminish in size. This effect assists the filtration process, in particular for slow sand filters, where a deposit is formed as a skin or layer of slime that has settled on the... 

It should be noted that the total loss of head of a filter bed is in inverse ratio to the depth of penetration of the matter in suspension. In a normal wastewater treatment plant, the water is brought onto a series of rapid sand filters and the impurities are removed by coagulation-flocculation-filtration. Backwashing is typically performed in the counterfiow mode, using air and water. One type of common filter is illustrated in Figure 6, consisting of closed horizontal pressurized filters. 

Belfort, Georges "Evaluation of a Rapid Sand Filter", Filtration Experiment, Rensselaer Polytechnic Institute, 1990. 

The water is then pumped through series operated sand filters, which provide the final stage of suspended solids removal and protect the garnualr activated carbon (GAC) filters from particulate contamination. Series operated GAC filters are then used to remove the dissolved creosote and pesticides from the water. To achieve compliance with specifications levels, water should be sampled and analyzed after leaving the first GAC filter. The second GAC filter normally serves as a guard bed. 

Insoluble suspended matter either picked up from the atmosphere or formed by deposition and corrosion within the system, together with slimes will, if not removed, cause blocking and abrasion problems. The build-up of such material can be controlled by side stream filtration, in which about 2-5 per cent of the circulating water flow is filtered continuously. A sand filter is commonly used for this type of duty. 

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