Surface water membrane filtration

For turbidity, color and microbiological control in surface water treatment filtration. Common variations of filtration are conventional, direct, slow sand, diatomaceous earth, and membranes. 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

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. 

The SBP membrane filtration system concentrates contaminants and reduces the volume of contaminated groundwater, surface water, storm water, landfill leachates, and industrial process water. This hyperfiltration system consists of stainless steel tubes coated with a multilayered membrane, which is formed in-place using proprietary chemicals. The membrane filtration system can be used with an SBP bioremediation system or another technology as part of a treatment train. 

Reddersen, K., Altmann, H.J. and Zimmermann, T. (2002) Production of drinking water from highly contaminated surface waters removal of organic, inorganic and microbial contaminants applying mobile membrane filtration units. Acta Hydrochim. Hydrobiol., 30 (1), 24-33. 

Bioburden loading levels were determined by a membrane filtration procedure prior to washing and also after the spiking to confirm that the desired challenge level was achieved. Following the cleaning cycle, the same procedure was used to evaluate residual bioburden. To recover the residual contaminants, sterile peptone water USP is used to rinse the entire inner surface of each vial. Results are reported as CFU per vial. 

The main cost factors are represented by equipment depreciation, membrane replacement, and electric power consumption. The other costs (man power and water consumption) are of minor influence. Membrane filtration plants equipped with a low level of automation require only a few hours of attention and direct surveillance by the operator per day. Direct surveillance of membrane filtration equipment is necessary at times when batches need to be changed or during membrane cleaning. The electric power consumption is associated with the use of pumps to move the viscous yeast slurry over the membrane surface, and its estimation is relatively straight forward. 

Membrane filtration is the technique recommended by most pharmacopoeias and, consequently, the method by which the great majority of products are examined. It involves filtration of fluids through a sterile membrane filter (pore size "0.45pm) any microorganism present being retained on the surface of the filter. After washing in situ, the filter is divided aseptically and portions are transferred to suitable culture media which are then incubated at the appropriate temperature for the required period of time. Water-soluble solids can be dissolved in a suitable diluent and processed in this way and oil-soluble products may be dissolved in a suitable solvent, e.g. isopropyl myristate. 

Membrane filtration method. This method is widely used for water but has some applications for food examination. A certain volume of the water or diluted food sample is filtered through a membrane filter. The filter is placed on the surface of a solid medium (usually selective). After incubation for the selected time and temperature the bacteria retained on the surface of the membrane develop into visible colonies, from which a value for the number of cfp in the sample can be calculated. 

All surface waters and municipal effluents contain suspended solids as well as dissolved solids and the presence of suspended solids dictates the need for a pretreatment section. Experience has shown that effective removal of the suspended solids in pretreatment is a prerequisite to efficient reverse osmosis membrane performance. Suspended solids in secondary effluent are primarily organic in nature and, due to their small size, it is difficult to remove them by settling. Therefore, it is necessary to aggregate the smaller particles into larger particles which can more easily be removed by settling and filtration. 

It is evident that many of the sampling devices described in the previous section in fact collect thin layers of water adjacent to and including the air/ sea interface itself. These type of surface film samples have become known as surface microlayers (Liss, 1975). Although such layers are in fact operationally defined by the devices used for their collection, there is the understanding that something happens to the properties of ordinary, bulk seawater at and near the air/sea interface which is contained in such microlayer samples. In this way, the microlayer becomes a real phenomenon in much the same way that the particulate state, understandable in a common-sense sort of way, is also usually defined by operational methods such as membrane filtration. 

RO membranes have a very small molecular weight cut-off (MWCO) and are expected to retain a large fraction of LMW compounds, such as amino acids or sugars, and are therefore useful for extracting a representative mixture of NOM from surface waters. For nanofiltration smdies it is important to retain the LMW fraction, as these molecules could be major contributors of pore pluming in membrane filtration. Pure HS solution cannot be obtained from such a mixture with a the salt content of the sample depending on its origin. 

In summary, although the MF of coUoids is generally well understood, the literature is somewhat limited in the areas of filtration of colloids much smaller than the membrane pore size, and in systems where aggregation occurs. Systems are, in this regard, often poorly characterised, especially in the presence of humic substances. As shown in Chapter 2 organics stabilise inorganic colloids at sizes much smaller than pores, and their behaviour in MF or surface waters is largely unknown. 

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 results confirm the presence of an organic cake, observed in the filtration of surface water. Flux decline is very high considering the small amount of organics deposited, as it would take a considerable amount of time to fill the membrane pores with the small organics. In a surface water system, particles are also important contributors to membrane fouling (as was shown in Figure 5.1). The colloidal systems of interest were described and characterised in Chapter 4. 

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