Sammendrag
In micro scale jar tests an approximately 75 % removal of turbidity from wastewater taken from Holen WWTP was achieved by precipitation with iron chloride sulphate (as JKL) in combination with different anionic, cationic and non-ionic polymers and subsequent separation through a mesh with 300 μm characteristic pore size. The good results with chemical coagulation in lab scale were not obtained in full scale at the Sobyefilter with a 0.35 mm mesh size, even if apparently the same chemical doses were used. During the main part of the test period the inlet wastewater was severly influenced by a lime/lye discharge to the sewage network, resulting in an unfavourable high pH during the chemical precipitation. The lack of pH control during the dosage of JKL was unfortunate and may in part have been the cause for the weak results. However, the results from a final week with normal pH gave no indications that the high pH was the only unfavourable parameter during the test period. Even if the floc production was somewhat improved at close to optimum pH conditions, no significant effect was observed on the removal of suspended matter or biochemical oxygen demand (BOD5). Inadequate intermixing of the polymer, and possibly of JKL, in the water volume and unfavourable conditions ahead of the filter cloth, with i.e. turbulence and feed of wash water from the flushing of the filter, were considered to be important causes for the lack of improvement in the suspended matter and BOD5 removal compared to mere mechanical treatment.
As an improvement of the process conditions compared to the conditions during the test period it is suggested that the intermixing of polymer is made more efficient, e.g. by in-pumping in the bottom of the grit chamber together with aeration, or by adding dilution water directly intoto the dosage tube for polymer, to obtain improved spreading in the whole water column. The dosage of JKL/PAX XL60 can be improved by installing pH control and by including the organic load (measured as e.g. turbidity) as an operating parameter in addition to the water flow. Furthermore, the turbulens during flocculation should be reduced by, among other measures, relocating the inlet of the filter back wash water from in front of the filter to in front of the flocculant dosing point. Other measures to improve the flow conditions in front of the filter may require a somewhat larger intervention. Flotation should also be attempted established in front of the filter to give a thicker sludge cake before the flocs reach the filter cloth. Such flotation should be provided by combining compressed water and compressed air. Hot water back wash, or an air-cutter with sufficient capacity, should also be installed on the Sobye filter to remove possibly precipitated fat on the filter cloth and thereby increasing the capacity of the filter. Futhermore, it may be advisable to reduce the steepness of the filter and the rotor speed of the filter cloth.
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