1.10.3 Water filtration
The final clearing up of water is achieved by filtration, i.e. water passing through fine-grain layer of filtration material (quartz sand, haydite, ground up anthracite, made foam polisterol, etc.).
Filters are differed by speed of filtration as quick (V=5-10m/hour), super quick (V=25-100 m/hour), and slow (V=0.1-0.1 m/hour). Quick filters are used during clearing of coagulated water, slow – during water clearing without coagulants, super quick can work with water coagulation or without it.
Filters can be opened (without pressure) and closed (with pressure) by their construction. Quick filters are built mainly as opened in form of reinforced concrete chamber, loaded with filtration material (fig. 1.54). Filters can have homogeneous filtration material or to be from different materials: double- or multi-layer filters. Thickness of filtration loading is taken in the range of 0.7-2 m, diameter of grains 0.5-2 mm. The supporting layer from gravel is poured between net and filtration mass to prevent filtration material washing.
Quick filters during the work are contaminated and, that’s why, they need regular washing, which is conducted by water inverse flow 1-2 times per day.
As a rule, quartz sand, rarer anthracite, haydite, made foam polisterol are used for filter loading. In the last case, loading is lighter than water and, that’s why, such filters are named as filters with floating loading (FFL). Most often this filters work with ascending water flow, and washing is conducted by water flow from top to bottom.
Parameters of filtration loading of quick filters and mode of their work during reagent water clearing are appointed according to pointing of СНиП 2.04.02-84. In the process of exploitation, properties of filtration materials, content and height of loading layers are specified.
Fig. 1.54. Scheme of quick filter:
a-during filtration; b-during washing; 1-water service; 2-side pocket; 3-drainage; 4-clean water taking; 5-pipeline of washing water service; 6-trays; 7-taking of washing water.
Unlike quick filters, during the water filtration in slow filters hanged up substances settle on the loading surface and in its top layer the filtration membrane is created, that ensures high degree of water clearing up without coagulation and assists a persorption of the bacterial contamination main mass. Absence of reagent economy during slow filters usage considerably simplifies their exploitation, but low filtration speeds require considerable capital charges and, that’s why, these filters are used only at not big stations (up to 1000m3/day). Only upper layer of loading is cleared due to degree of contamination in slow filters, and this is more difficult than washing of quick filters. Slow filters can be of two types by method of loading regeneration :
with removing of contaminated upper layer;
with washing of contaminated layer in filter through its mechanical loosing and hydraulic contamination removing.
In the working mode in slow filters the water layer above the loading should be 1.5 m, loading layer – 1.3 m (including 0.85 – sand with grain size 0.3-2mm and 0.45m – gravel with grain size 2-40mm).
Super quick filters are used mostly for partial water clearing up. They are produced as closed of vertical or horizontal type by their contruction. Filtration in such filters is conducted under the pressure and pressure looses tends to 10m.
Except of filters for water clearing up, the contact clearing up devices are also used, in which the processes of flakes creation and filtration are connected. Principle of clearing up devices’ work consists in those, that during water filtration the adsorption of colloidal particles and particles hanged up on grains of filtration materials pass through grain loading layer. Water motion in contact clearing up devices is from top to the bottom. Speed of filtration – 4-5.5m/hour. Loading of clearing up devises from sand with grain size 0.7-5mm and gravel with grain size 5-40mm. Height of filtration loading 2.3-2.7m, gravel 0.4-0.6m.
Uninterrupted previous water chlorination is recommended for increasing of reliability of contact clearing up devices work and ensuring of clearness. Contact clearing up devices practically don’t differ from quick filters by construction .