Membrane filtration water disinfection

Tertiary treatment is required often to meet the industry or irrigation standards, especially when disinfection is needed. This step is known as water regeneration. Typical tertiary treatment proposed in the literature [23] is composed of the following stages low pressure membrane filtration (e.g., MF) followed by disinfection stage and finally high pressure membrane filtration (e.g., RO). Industrial... 

Tests should also be done in the presenee of organic matter (e.g. albumin) and in hard water. It is important to remember when performing viable counts that care must be taken to ensure that, at the moment of sampling, the disinfection process is immediately arrested by the use of a suitable neutralizer or ensuring inactivation by dilution (Table 11.4). Membrane filtration is an alternative procedure, the principle of whieh is that treated cells are retained on the filter whilst the disinfectant forms the filtrate. After washing in situ, the membrane is transferred to the surface of a solid (agar) reeoveiy medium and the eolonies that develop on the membrane are counted. 

Pitfalls of the different water treatment processes are the formation of extensive amounts of sludge, which has to be deposited off, as is the case with flocculation, the formation of fouling layers during membrane filtration, or DBP formation after disinfection of NOM-containing waters. 

The disadvantage of a NF treatment is the high energy cost, and the generation of waste streams that need further treatment prior to disposal (Wale and Johnson (1993)). Howev er, the generated waste stream is only a concentration of the natural components of surface water, and not a sludge of added chemicals as in coagulation or PAC. Moreover all water treatment processes inevitably produce a waste (residue stream). Product water stabilisation may also be of concern in NF, and more so, in RO. Kasper (1993) discussed different possibilities of membrane filtrate stabilisation and water disinfection. 

The single most important use of chlorine-containing compounds is water disinfection. About 98% of the drinking water in the US and 96% of the waste water is treated with chlorine. There are four technologies that could replace chlorination membrane filtration, ultraviolet irradiation, filtration on activated carbon bed and treatment with ozone (ozonolysis). All of them are more expensive than chlorination, and none of them were studied in as much detail as chlorination was. If ozone is used, the by-products formed in the reactions of ozone with organic compounds have to be removed in a separate step using activated carbon. Overall, there is no viable alternative to chlorination today. 

Another consideration of growing importance in water treatment is the development of new technologies. These include special membrane processes for water filtration, alternatives to chlorine for water disinfection, advanced oxidation of impurities, and the use of ultraviolet radiation for water disinfection and as an aid to destruction of organic contaminants by oxidants. It is important to consider the sustainability of developing techniques including costs and by-product generation. 

The agents that can be used to disinfect water include (1) chlorine, (2) chloramines, (3) ozone, (4) chlorine dioxide, (5) ultraviolet radiation, (6) membrane filtration to remove pathogens, and (7) miscellaneous agents including the evolving use of ferrate (iron(VI)). Of these, chlorine and chloramines have been the most popular, but are becoming less so because of the by-products... 

The physical process of filtration is rather effective in removing pathogens from water. During the 1800s, before chlorine came into widespread use, filtration with simple sand filters employed in just a few cities cut down significantly on the incidence of waterborne cholera in those cities. With modern membrane technology (see Section 5.10), ultrafiltration can remove even viruses from water. Small amounts of chlorine or chloramines (see Section 5.11.3) can be added to maintain sterile water in distribution systems, but much less of these agents are required for membrane-filtered water than are required for total disinfection. 

Ultrafiltration can adequately produce disinfected water directly from strrface water for different applications. MF can also be used for disinfection, although not all viruses are removed. However, direct membrane filtration is limited by fouling, which, during constant-flux filtration, leads to a continuous increase in transmembrane pressure. In addition, UF and MF membrane treatment alone cannot effectively and consistently remove organic material, measured as total organic carbon (TOC), and THM (tri-halo-methane) precursors, measured as chloroform formation potential (Berube et al., 2002). 

Membrane bioreactors are an option for municipal wastewater treatment when high effluent water quality is required, for example, bathing water quality, or when the receiving water body is very sensitive or when the water is to be treated for reuse. As mentioned before (see Section 9.2.5.1), the effluent quality is superior to that of secondary sedimentation. To attain a similar effluent quality by conventional treatment, effluent filtration and disinfection would be required in addition. This needs to be taken into account when comparing the cost of MBR and conventional activated sludge treatment. 

Table 12 shows the typical LRV values obtained using a polymeric and ceramic microfilter. Sterile filtration requires 100% bacteria retention by the membrane, whereas in many industrial bacteria removal applications the presence of a small quantity of bacteria in the filtrate may be acceptable. For example, drinking water obtained by microfiltration may contain nominal counts of bacteria in the filtrate which is then treated with a disinfectant such as chlorine or ozone. The use of ceramic filters may allow the user to combine the sterile filtration with steam sterilization in a single operation. This process can be repeated many times without changing filters due to their long service life (5 years or longer). 

Patel et al (1994) employed a combined process of coagulation and MF to avoid a disinfection posttreatment. The coagulation step was used to eliminate phosphorus, arsenic, and viruses, to avoid fouling, decrease particle accumulation on the membrane surface, and improve backflush characteristics. MF pilot plant studies in constant permeate flux mode showed that turbidity, particles, and faecal coliforms could be removed, but TOC removal was unreliable. Crossflow MF showed no difference to dead-end filtration, and both methods were similar to or better than sand filtration. Results with coagulation and MF improved phosphorous and turbidity removal, but the process was not optimised. The treatment lead to a reduction of chlorine demand in the product water. 

The flow scheme of these facilities is very similar to the one of a conventional water supply facility which treats surface water (in spite of the very different use). It usually consists of a physicochemical treatment (to reduce pollution associated with the colloids that escape from the secondary clarifiers of the WWTF) and a disinfection unit (to remove pathogens and prevent health issues related to the wastewater reuse). The first treatment follows a four-stage scheme coagulation, flocculation, clarification, and filtration. Membrane technology (i.e., reverse osmosis or electrodialysis) is sometimes proposed... 

Chlorine (or other disinfectant) is required to minimize the potential for fouling the membranes with microbes (see Chapters 8.2.1, 8.2.2, and 8.5.2.1). Once membranes are fouled with microbes, it is very difficult to remove them. A free chlorine residual of about 0.5 to 1.0 ppm in the pretreatment system is desirable. Feed water to the RO must be dechlorinated prior to the membranes because the membranes are sensitive to oxidizers, which will degrade the membrane. Sodium bisulfite is the preferred method to dechlorinate unless the RO feed water has a high organic concentration, in which case, carbon filtration at a flow rate of 2 gpm/ft is recommended. (see Chapters 8.1.4 and 8.2.3) Sodium metabisulfite is typically about 33% active, and the stoichiometic dosage of sodium metabisulfite is about 1.8 ppm per ppm free chlorine. So, the stoichiometric dosage of 33% active sodium metabisulfite is 5.4 ppm. For safety, a factor of 1.5 is used to increase the dosage of sodium metabisulfite to ensure complete elimination of free chlorine. 

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