Chemical precipitation can be used to treat wastewater in ponds when environmental conditions are unsuitable for biological treatment, such as during periods of low temperature or reduced solar radiation. The main purpose of this precipitation is to separate phosphorus and particulate matter from the effluent. Previous investigations have shown that most of the phosphorus leaving ponds, in which chemical precipitation by slaked lime is used, is in the particulate form. It is therefore important to identify processes that regulate the particulate content of the wastewater. An investigation was conducted to determine the particle size distribution within wastewater taken from a slaked lime settlement pond. Experiments were conducted in the second pond at the Tänndalen wastewater treatment plant in central west Sweden. Wastewater flowed into the first, volume buffering pond, then to an automatic lime feeding unit immediately prior to the second settlement pond. Water quality measurements and samples were taken at various depths and locations within the settlement pond. Changes in the particle size distribution, within a range of 1.39 to 43.30 µm diameter, using a Coulter Multisizer particle analyser, were determined. Statistics summarising these changes were related to the pH, temperature and total phosphorus concentration of the samples. The majority of particles at the effluent to the pond were so small that they were unable to settle. Improvements to the pond detention time were therefore considered unnecessary, but the initial precipitation environment at the pond influent required attention.

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