By Holly Shorney-Darby, PhD, PE, and Jumeng Zheng, PhD
Filtration and disinfection are used in surface water treatment to remove and inactivate harmful pathogens, such as cryptosporidium, giardia, and viruses. While conventional sand and mixed media filters remove larger diameter pathogens and reduce turbidity to meet the regulatory limits, these filtration methods are susceptible to upsets and have ripening times, which increase the risk of pathogen breakthroughs.
Membranes have been promoted as a barrier to these pathogens and particles, or turbidity. Polymeric membranes have been implemented since the 1990s, but a problem polymeric membrane systems face is the durability of the polymer material over time. Some polymerics are susceptible to exposure to oxidants and some experience material expansion due to physical stresses during operations. For example, having high solids concentration in feed water and backwashing for membrane cleaning can lead to changes in membrane properties, which result in the weakening of fibres or passage of particles and viruses though the membrane. These changes help explain failures in water quality observed in polymeric membrane systems, which either have breaches or weakened fibres over time.
The CeraMac microfiltration process by PWNT from Netherlands has a record for exceeding filtration water quality requirements while providing a system that can be cleaned and maintained for over 15 years. The system uses Metawater’s ceramic microfilter membranes, with multiple monolith membranes in one steel vessel. This membrane has a nominal pore size of 0.1 micron and has been in service in Japan since 1998, with more than 24 years in operation at one site. With five full-scale CeraMac plants worldwide of 424MLD in total treatment capacity, and over 140 installations by Metawater in Japan of 900MLD in total treatment capacity, there have been no reported breaks of ceramic membranes in service.
The CeraMac installations typically achieve turbidities less than 0.05 nephelometric turbidity unit (NTU). They have little variability in turbidity levels after backwashing or cleaning. These help utilities that have difficulty meeting turbidity limits with sand or have failing polymeric membrane systems, and ensure quality water is supplied to customers in the service area.
Its ceramic microfiltration serves as a barrier to particulate contaminants, but in terms of other dissolved water quality parameters, pre-treatment process is required to convert them to a removable form by a different process such as oxidation or coagulation. Coagulation can be conducted in different pre-treatment steps, including in-line coagulation, conventional clarification, or dissolved air flotation. The in-line coagulation process, sometimes with oxidation, can coagulate dissolved organic carbon (DOC) and colour, with DOC removal at 30-70% depending on raw water properties. The in-line coagulation results in the highest solids loading onto the membranes, which is viable due to the monolith structure, though it does not impact the filtered water quality. It may improve the filtered water quality by providing a layer of flocculated materials, sometimes referred to as a ‘cake’ layer on the membrane for contaminants to pass through. While water passes through this ‘cake’ layer, contaminants are potentially captured within the layer and removed by backwashing to waste.
It is not a barrier for dissolved metals. Dissolved metals, such as manganese and aluminium can pass through the membrane. It is, however, possible to oxidise manganese and iron upstream of the membrane, or provide a dedicated metals removal process, such as a manganese reactor bed, upstream or downstream of the CeraMac to remove dissolved metals. For aluminium, un-optimised coagulation chemistry is often why too much dissolved aluminium passes through the membrane when aluminium-based coagulants such as aluminium sulfate or polyaluminium chloride are used. Typically, a pH adjustment or a change in the coagulant dose will bring the aluminium concentration to low levels, for instance, less than 50ug/l.
Ferric coagulant can also be used for coagulation pre-treatment, and the iron concentration in the filtrate is usually below detection limits.
For disinfection credits for surface water treatment, CeraMac will achieve 4.0 log cryptosporidium, 4.0 log giardia, and 1.0 log virus inactivation, based on log removal credits from the California Department of Public Health. The actual level of removal is likely to be higher, due to the ceramic barrier and tight nominal pore size distribution of the membrane. Natural virus tests show what log removal of viruses are achieved in an installation, and those tests show higher than 1.0 log virus removal in practice.
Another mode of disinfection can be with an oxidant applied to the feed water. The CeraMac system is designed with materials that are compatible with different oxidants, including ozone. These chemicals can be for cleaning or dosed for oxidation in the main process flow. For example, some plants operate with ozone in the feed water to the membrane such that an ozone residual persists at typically 0.5-1.0mg/l on the membrane surface. The filtration of ozonated water by CeraMac differs, with higher flux and lower operating transmembrane pressure than without ozone. With ozone application, utilities with taste and odour compounds can expect removal of geosmin and 2-methylisoborneol (MIB).
Ozone contact within the CeraMac vessels may also provide additional disinfection due to the improved ozone transfer efficiency through membrane pores. Another reason is the additional contact of pathogens that are captured on the membrane surface and are exposed to continuous ozone until these pathogens are backwashed from the surface.
No specific disinfection credits have been quantified by a third party or regulator for this combined ozone and CeraMac process, but full-scale installations and pilot-scale studies indicate that these are added water quality and performance benefits of the CeraMac system when ozone is applied. It is also possible to continuously dose chlorine or peroxide upstream of the membrane. It would be beneficial to perform a cleaning function which could assist with disinfection.
The first CeraMac microfiltration surface water treatment plant has been in operation since 2014. This and other ceramic membrane plants have different pre-treatments to meet water quality goals for a specific site. It is a system that allows efficient operation with the ability to clean if necessary. The quality of the water is maintained over time due to the system’s resilience. This is supported by the monolith membrane, which is made of inorganic material that is not subjected to physical pore size changes or breakages which can affect water quality in the filtrate over time.