JÄRNKLORID SOM KOAGULANT VID DRICKSVATTENFRAMSTÄLLNING - ETT PILOTFÖRSÖK VID LOVÖ VATTENVERK / Ferric chloride coagulation in drinking water treatment - a pilot plant study at Lovö waterworks
Stockholm Water Company operates two waterworks, Norsborg och Lovö. Both treat raw water taken from Lake Mälaren with the following process: coagulation with aluminium sulphate, sedimentation, rapid sand filtration, slow sand filtration and desinfection with chloroamine. A pilot plant scale test was performed at Lovo waterworks during spring 1996. The main objective with the study was to evalute the effects by treating raw water with ferric chloride. The pilot plant consisted of two identically process lines with coagulation, sedimentation and rapid sand filtration (Q=0,5 m3/h, line). One of the lines was used as a reference with aluminium sulphate as coagulant.
The results from the study showed that the removal of organic matter was significantly improved with ferric chloride. CODMn, TOC och UV-absorbance at 254 nm was 31-38% lower compared to water precipitated with aluminium sulphate. The optimal dose was in the range 120-125 mmol Fe/m3 and pH 4,7-5,3 in the coagulation step. At higher pH-values the treatment results become unsatisfied. pH should be adjusted before
the filtration step to pH >7 to avoid iron residuals in the filtrate. Coagulation at a low pH value led to a reduction in alkalinity. Precipitation with ferric chloride is more expensive for Lovö waterworks than using aluminium sulphate because of the need of sulfuric acid and extra alkali.