Continuous sand filter
The principle of operation of the continuous sand filter is based on the countercurrent filtration. Raw water, which enters the filter via inlet connection at its top is evenly distributed within the filtration bed by a distributor and filtered during its upflow through the sand bed. Clean filtrate is discharged via upper outlet. The air lift pump lifts contaminated slurry from the filter bottom to a hydro pneumatic grit washer. Taking the advantage of the difference in water table levels between the filter and hydro pneumatic grit washer, the grit backwash process using the clean filtrate flowing into the flushing labyrinth at its bottom is continuously taking place concurrently with water filtration. While the separation of the sand grains in the turbulent slurry starts already in its upward movement in the air lift pump, the proper backwashing takes place in the flushing labyrinth. The reject water is removed by the reject water pipe to the outside, while the clean sand falls down to the filtration sand bed. As a result, there is a continuous downward circulation of the filtration bed with the concurrent self-correcting processes of the water treatment and sand backwashing.
- no automated valves, any controls or auxiliary fittings,
- compact design: as an open type device, this filter can be installed one next to another, saving space needed for installation,
- long lifetime of the filtration bed: in typical filter service conditions, loss of sand is as low as 0.2%,
- low energy consumption: electricity is needed only for the air compressor to supply the air lift pump with moderate amounts of compressed air to lift the contaminated sand,
- minimum maintenance: the simplicity of the process reduces the attention time needed to a few minutes daily,
- no shutoff breaks for rinsing: the concurrent with the filtration backwash process enables continuous operation of the filter without shutoffs,
- no need of separate flushing installation and pumps,
- moderate amount of reject water,
- effective and efficient process control: the filter remains continuously in an equilibrium state in terms of the suspended matter in the sand; the backwash speed follows the changes in the concentration of the suspended matter in raw water which, with the concurrent backwashing, makes it possible to maintain the suspension concentration in the filtration bed constant and to keep filtration effectiveness well controlled.