Igensättning av långsamfilter i Östby vattenverk i Kramfors– studie av påverkande faktorer
Östby waterworks in the community of Kramfors has since 2003 had problems of fastclogging of the slow sand filters. As the clogging appears more often, they must be cleanedmore frequently which has made the drinking water preparation difficult. Wintertime it hassometimes been impossible to clean the filters which has led to their closing and this hasinfluenced the water quality negatively.The waterworks is a surface waterworks which takes its raw water from the lake Sjöbysjön.The water is flocculated and filtered in a contact filter with the flocculent EKOFLOCK 91.Thereafter it is alkalinized before it reaches the outdoors placed slow sand filters. After thefilters the water is alkalinized and disinfected before it reaches the customers.This thesis work has looked into which factors influence the clogging and trials have beenmade in order to optimize the waterworks and thereby reduce the clogging. The raw water hasbeen studied with aspect to biology and chemistry, the filtered water has been studied withaspect to chemistry and also the pressures in the slow sand filters have been studied.The colour of the raw water from Sjöbysjön and its catchment area has increased since thebeginning of the 90-ies and also the bio volume has increased in the lake.The contents of aluminium before the slow sand filters are far higher than the contents after,which leads to the conclusion that aluminium is accumulated in the filters. The differentialpressure over the sand bed increases with time after a cleaning. This indicates anaccumulation of particles which increases with the load. Observations of the filter surfacebefore cleaning showed that it was covered by a brown, jelly-like film. The internal resistancein the filter beds increases successively after a cleaning and one month after cleaning it ishighest in the upper part of the sand bed. All this points to that flocculated aluminium isgathered in the slow sand filters, on the surface, causing clogging.While the thesis work has been going on a process of alkalinizing before the contact filtershas been reengaged and this has influenced the flocking of organic materials. When raisingthe pH the dose of flocculent was increased and this combined increased the amount offlocculated material. The contact filters could not bear this increased amount of flock load butshowed instead a breakthrough.In a few lab scale trials the flocking pH was varied as well as the dose flocculent to the rawwater and after this the water was filtrated. A tendency was seen that the separation ofaluminium, colour and turbidity increased with increasing pH and dose flocculent. At the pH6.2 and the chemical dose of 60 g/m3 the content of aluminium, the colour and the turbidityshowed the lowest values in the filtrate.
Source Type:Master's Thesis
Keywords:surface water work contact filter slow sand clogging colour flocking ph ekoflock 91
Date of Publication:09/02/2008