Aeration – The First step of Organic Water Treatment
The first operation in a water treatment process is usually aeration. The purpose here is to remove any undesirable dissolved gases such as CO2 or H2S, which are more commonly found in ground water and arise from biological degradation of organic matter. The air or oxygen that is added converts these into more manageable forms. H2S even in the smallest of concentrations adds an unpleasant taste and odor to the water and excess CO2 causes corrosion to metallic parts of the plant and also affects the chemical efficiency.
Following aeration, the raw water is then passed through a long open channel and into flash mixers. The plant inlet flow rate is usually measured between the aerator and the mixer using either weirs or flumes.
Flow rate can be measured by ultrasonic open channel flowmeters or magnetic flowmeters. At Bauda, inlet flow is measured by insertion type magnetic flowmeter at the aerator inlet. In the inlet mixing tank, flocculants are added, so that they are quickly dispersed through the inlet flow to create a homogeneous mixture. This helps to prevent the formation of hydroxides that reduce the effectiveness of the process. The correct amount of flocculants and chemicals is added by full feedback control, but is also influenced by using sensors in the outlet channel for aluminum content as well as turbidity.
Sedimentation (Clarification) and Filtration
Following the addition of reagents and flocs, the mixture passes into large tanks or ponds. This is the sedimentation (or clarification as these terms are used interchangeably) process, where gravity is used to separate the particulate matter from the water. In some plants the clarifier and flocculator may be combined, but hydraulically they are best kept separate, with a proportion of the flocs being returned to the inlet to reduce the addition of new chemicals.
The sediment that gathers at the base consists of three types of particles. The largest are the calcium carbonate particles with a specific gravity of 2.5 and a size between 10 and 20mm. Next comes the fine silica, clay and sand grains less than 1mm in size and finally there are the alum flocs with a specific gravity around 1.3 and sizes down to almost the micron range.
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