Membrane and membrane bioreactor process

Membranes are special devices, generally made up of synthetic polymers or inorganic material, that operate like barriers to divide a liquid from a solid stream. They are characterised by their different compositions and pore sizes. In food industry and wastewater treatment the size generally ranges between microfiltration (MF, 0.1-10 |im) and ultrafiltration (UF, 0.001-0.1 |im). However, reverse osmosis (RO, 0.0001-0.001 |im) can also be of interest if salinity is excessive. All these systems are pressure driven (Cheryan, 1998). Membranes can be used for treating high-strength waste-waters like those coming from food processing or in downstream processing to recover products, or to polish and recycle wastewater (Moresi and Lo Presti, 2003).

Raw or pretreated food processing wastewater can be treated by MF. MF can remove suspended solids, bacteria and large molecules without the need for chemicals. In addition, membranes are backwashable which allows them to be operated under low or no cross-flow and with high yield and flux. If RO is done after MF or UF, a number of chemicals can be removed. However, carbonates can precipitate and therefore some chemicals may be needed to reduce water hardness.

Although this technology is used frequently in the food industry it should be emphasised that, in this role, membranes merely serve to separate or fractionate wastewater components, hopefully into more useful and/or less polluting streams, and cannot break down or chemically alter the pollutants. At the very least, a membrane could render a permeate stream ready to discharge into the sewer or to be reused within the production process. In any case, a membrane process requires a proper pretreatment technology for removing coarse particles and suspended solids.

In order to improve the capability of removing pollutants, membranes can be coupled to activated sludge processes to form a membrane bioreac-tor (MBR; Stephenson et al., 2000). The membrane bioreactors are systems in which a membrane is used as a liquid-solid separator and permits a high solids concentration in the bioreactor in order to intensify the treatment process. As the settling properties of the biomass are no longer driving the capacity to divide the liquid from the solid stream, the hydraulic and solid retention time are not linked, therefore the system can be operated with very high solid retention times (typically greater than 50-60 days or more) which reduces the production of excess sludge (Rosenberger et al., 2000; Bae et al., 2003).

Moreover, these systems are well known for their ability to completely remove suspended solids and produce virtually solids-free permeate without the presence of micropollutants like organic compounds and heavy metals (Innocenti et al., 2002; Clara et al., 2005; Fatone et al., 2005). A typical characteristic of these processes is the production of treated waters with very high quality standards. Table 21.8 shows some typical data reported in the literature for this process: these show very high yields for COD

Table 21.8 Efficiency of the MBR for different food processing wastewaters

Wastewater

removal (%)

removal (%)

removal (%)

Reference

Food (generic)

94

74

91

Wang et al., 2005

Food (generic)

99

Katayon et al., 2004

Meat industry

98.1

99.6

98.2

95

87.3

Sroka et al.,

2004

Winery

97

Artiga et al., 2005

Vegetable oil factory

90

85

Mohammadi and Esmaeelifar, 2004

Pet food

94-98

Kurian et al., 2005

Vegetable

96-99

95-96.5

Nakhla et al., 2006

OP w

CO NO

Table 21.9 Efficiency of the jet loop reactor for different food processing wastewaters

Wastewater

COD removal (%)

Reference

Winery

9G.7-93.1

Petruccioli et al, 2GG2

Brewery

9G-97

Bloor et al., 1995

Cheese whey

97

Farizoglu et al., 2GG4

removal, always >90%, while solids removal is virtually 100%. If a biological nutrients removal process is applied, nitrogen and phosphorous can also be removed.

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