LÅNGSAMFILTRERING – EN ENKÄTUNDERSÖKNING AV SVENSKA FILTER Slow Sand Filtration – an Investigation of Swedish Slow Sand Filters
At present there is an increasing interest in rhe reduction of natural organic matter, NOM, from drinking water by biological processes instead of chemical processes. In Europe as well as in the US a lot of research is done to increase the knowledge of biological processes in slow sand filters. Removal of organic matter in drinking water treatment may lead to a reduction of the demand of disinfection chemicals (chlorine).
In the Nordic countries the raw waters are characterised by low temperature, oligotrophic quality and they are rich in humic acids, which means that research done in Central Europe may not be applicable to these waters.
The investigation of 35 slow sand filter plants showed that:
• the production of water per filter period expressed in cubic meter per square meter is as high for slow sand filters without pre-treatment as for those preceded by chemical flocculation.
• slow sand filters with lower filtration rate are producing less water expressed in cubic meter per square meter compared to those with higher filtration rate.
• high reduction of turbidity was achieved in the filters but the colour reduction was low.
• the chemical oxygen demand, CODMn, was reduced, in average by 20%.
• some filters could not meet the Swedish water directives (SLV FS 1993: 35) concerning CODMn, and colour.