Mikrobiologiska barriärer i vattenrening
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
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
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.