Being one of the oldest water treatment systems in the world, slow sand filters (SSFs) have played a big role in the water treatment sector due to their reliability and their effective physico-chemical and biological purification capabilities. They are still widely used as a disinfection step in many countries, especially those that do not use chlorine for disinfection. SSFs are gaining popularity in recent times, especially due to their ability to deactivate pathogens like Giardia lamblia and Cryptosordidium parvum, which have been posing significant challenges to the public drinking water systems for years.
Over time, researchers have mainly focused on understanding the importance of the schmutzdecke layer of the SSFs, as it is understood that the schmutzdecke is essential for their functioning. This has limited the research done in the deeper layers of the sand bed, resulting in a lack of understanding of the activities here.
The aim of this study is to investigate the physico-chemical and biological functions happening in the deeper layers of the SSF, and to determine whether parameters like nitrate, turbidity, DOC, FEEM, ATP and particle cell counts can evaluate the ripening of a scraped bed. Furthermore, the ability of these parameters to sufficiently indicate biological stability of the sand bed is to be assessed.
In order to determine this, two full scale filters were sampled and analysed. One was a scraped bed and the other was a mature bed. They were also compared to judge how differently they behaved throughout the ripening phase.
The results of the tests showed that nitrate, turbidity and ATP are good indicators of ripening. ATP and particle cell counts, when used in parallel, are good assessors of biological stability. FEEM also serves as a useful method to determine why the DOC concentrations in the water samples were similar for both the filter beds. ATP tests conducted on the schmutzdecke sand samples provide evidence of ripening over time for the scraped bed. Further research using parameters like iron, manganese, ammonia and oxygen is required to strengthen the understanding of the workings of the deeper layers of the sand bed and the various physico-chemical and biological mechanisms happening within.