Biological Pre-filtration and Surface Water Treatment - Microbial barrier function and removal of natural inorganic and organic compounds

Doctoral thesis, 2005

Waterworks in Sweden that apply conventional chemical surface water treatment are facing a number of challenges, including changes in raw water quality and demands for improved particle removal. The chief objective of this work was to evaluate biological pre-filtration for typical Swedish surface water, regarding the removal of natural organic matter, particles, iron and manganese, and taste and odour compounds. Pilot-scale experimental work included the investigation of biofilters fed directly with surface water, using non-adsorptive expanded clay and partly exhausted granular activated carbon. Process combinations with conventional chemical treatment and nanofiltration were investigated. Biological pre-filtration decreased the load of particles and biodegradable organic matter to subsequent treatment processes. Peak loads of added particles were equalised by high initial retention followed by a slow release of attached particles. In chemical treatment with pre-filtration, the removal of µm-size particles became less dependent on the post-sedimentation rapid filters. The experimental study contributed with data on the microbial barrier function of chemical surface water treatment under Swedish conditions with regard to particle-, bacteria- and virus removal. Simple rapid media filtration pre-treatment of surface water caused fast pressure drop development in nanofilter membranes. Biofiltration moderated the increase in pressure drop in comparison to rapid filtration. Destructive analysis of the nanofiltration elements was performed to study the fouling layer on the membrane. Biological pre-filtration alleviated occasional episodes of dissolved manganese and odour compounds in humic surface waters, which are difficult to control by conventional chemical treatment alone. The mechanism of biological removal of two biogenic odour compounds (geosmin and MIB) was found to completely depend on metabolic activity on a non-adsorptive filter medium (expanded clay). Within the Sustainable Urban Water Management program, a systems analysis study was conducted. Hypothetical decentralised systems with drinking water treatment closer to the consumer were compared to conventional centralised treatment regarding energy consumption and microbial risk.