ASTRASAND® For continuous purification of water

Continuous purification of water
ASTRASAND® is a continuous sandfilter, developed for physical-chemical and/or biological treatment of different types of water, such as process, waste, surface, cooling and ground water. ASTRASAND® works as a continuous-process, in which the filter bed is cleaned during the filtration operation, and as a result process interruptions are eliminated. ASTRASAND® is self-cleansing.

Process characteristics

  • Suitable for physical-chemical as well as biological treatment and even for a combination of both.
  • Continuous cleaning allows for continuous operation; high filtrate efficiency under varying process conditions.
  • Easily built into larger treatment systems; compact installation with small footprint.
  • High efficiency because of uniformity of the sand circulation rate.
  • High availability; low maintenance due to lack of moving parts.
  • Low operational costs.

Applications
ASTRASAND® filtration can be utilised for many types of application, and with unit capacity varying from only a few, to several thousands m3/hour.
Examples are:

  • Biological (de)nitrification and polishing of communal and industrial waste water.
  • Recovery of waste water, when combined with polishing, membrane filtration and disinfection with UV light.
  • Aerobic biological purification of ground water for MAC and PAC degradation.
  • Filtration of flocculated contaminants in surface water as a pre-treatment for demineralised water production.
  • Wash water treatment in fresh and process water production.
  • Biological filtration of cooling water.
  • Biofiltration for BOD and heavy metal removal.

How ASTRASAND® works

The polluted water is transported into the filter by means of the feed pipeline (1). The water enters the filter bed (4) through the supply pipe (2) and the distributors (3). The water is purified as it flows through the filter in an upward direction. The filtrate is discharged from the upper part of the filter (5). The filter bed is moving downward as the water flows up. The dirty sand (6) is continuously abstracted from the sand bed and washed, after which it is released back on the top of the sand bed (7). The sand circulation is based on the airlift principle, forcing a mixture of dirty sand and water upward through a central pipeline (8). The intensive scouring movements separate the impurities from the sand particles. At the top of the pipeline the air is released, and the dirty water is discharged (9). The sand then settles in the washer. The sand washer (10) at the top of the filtration tank washes the sand with a small amount of clean filtrate. This removes the last traces of pollutant from the sand. Flow is achieved through a difference in level between the filtrate (11) and the rinse water (9).

ASTRAMETER®

Process monitoring and regulation
The plant can be equipped with a universal monitoring system to monitor the uniformity of sand circulation, and thus the smooth operation of the process: ASTRAMETER®. The process can be fully optimised for specific applications by applying an advanced regulation system: ASTRACONTROL®.

Process monitoring
An optimal continuous sand filtration process with the ASTRASAND® filter is ensured if the sand circulation rate is correctly tuned. Therefore the sand circulation rate across the filter bed surface has to be uniform. A manual control of the sand circulation rate is easy but requires frequent operator attention. An automatic measurement has been developed and is introduced as: ASTRAMETER®

The ASTRAMETER® measures the sand circulation rate at 2 or 4 points across the filter bed surface with a time interval of 5 - 60 minutes. Values are calculated and registered. An alarm is generated if calculated values exceed the pre-set alarm value.

Options
The ASTRAMETER® can be supplied with a local computer including LCD display. An integrated printer can be supplied to print out measured values and settings. The ASTRAMETER® can also be supplied without local computer. Signals are then transferred and interpreted in an external control system. The necessary software is available.

Process control with ASTRACONTROL®
In order to optimise the filtration process, even with fluctuating process conditions, the sand circulation rate can be controlled automatically (in function of the process characteristics) with a newly developed control system: ASTRACONTROL®.

An optimal ASTRASAND® continuous filtration process is ensured if the sand circulation rate is correctly tuned. The sand circulation rate and therefore the filtration process is to a great extent self-regulating. Due to the relationship between head loss and sand circulation rate, an increasing hydraulic load automatically results in higher sand circulation rates without manually changing the compressed air flow. However, in some cases it could be useful to amplify the self regulation function by introducing an automatic control of the air flow to the air lift e.g:

  • applications with swift and strong variations in process conditions applications with very strict effluent criteria;
  • biological processes in which best performance is guaranteed if as much as possible biomass can be stored in the filterbed.

ASTRACONTROL®

Process components
The ASTRACONTROL® system consists of a local process computer and continuous measurement systems for head loss, feed flow and water temperature. All the relevant process data (sand particle size, filter bed height) are incorporated in the process computer. Using calibrated process algorithms the data are converted into a continuous adjustment of the air flow to the air lift. The control system can be operated as a stand-alone process control unit, but can also be incorporated in a central control system.

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