Mikrobiologiska barriärer i vattenrening

Report, 2005

The objective was to investigate the microbial barrier function of the most common process combination applied to Swedish surface waters – coagulation / flocculation followed by settling and rapid filtration. The effect of operational conditions was studied, as well as alternative and add-on processes, such as biological pre-filtration of raw water without ozonation, ultrafiltration (UF) and nanofiltration (NF). Analytical parameters included the removal of natural organic matter (NOM) and regrowth potential. The work was carried out in close cooperation between engineers and microbiologists. The barrier function of chemical treatment was investigated in pilot-scale by challenge tests and a novel method for following raw water particles, based on the detection of algae by flow cytometry. Results indicate that chemical treatment represents a robust yet mediocre barrier. The measured barrier function for added viruses was around 99.99 %, but considerably lower for particles in the size range of protozoan parasites (~99 %) and in bacteria size (~90 %). The measured reduction in numbers of indicator bacteria added in high concentrations was however higher. Considering the sparse information on the microbial quality and variability of Swedish surface water, the barrier function at many waterworks appears insufficient over chemical treatment alone, and therefore requires additional processes and/or disinfection to reliably eliminate the risk of waterborne disease. Processes were investigated that increase microbial barrier function. Chemical treatment was complemented with UF with a nominal pore diameter close to virus size. Alternatively, chemical treatment may be replaced by NF with high removal of NOM. Since the feasibility of membranes processes is limited by fouling, different pre-treatment options were compared. The UF and NF pilot plants achieved very high removal efficiencies for particles in the size ranges of protozoan parasites and bacteria, as well as two surrogate viruses (bacteriophages) with different surface properties. Both NF and postchemical treatment UF were subject to irreversible fouling that could not be solved by chemical cleaning. Compared to conventional pre-treatment with rapid filters, biological pre-filtration significantly amended fouling problems in the NF pilot plant. Bulk NOM was reduced to a minor degree in the biofilters (~10 %), while higher removals of the biodegradable fractions (~20–30 %) and biofilm formation potential (80–90 %) were experienced. Biological pre-filtration was furthermore evaluated as a way to stabilize the feed water quality to chemical treatment. Particle removal became less dependent on filter function, which increased the robustness of the process. Batch tests revealed potential savings of coagulant with pre-filtration, due to lower organic and particle load. Natural odour substances (geosmin and MIB), almost unaffected by conventional treatment, were reliably removed in the pre-filters. For further details in English, see the scientific publications on the project.