Membrane and Desalination Technologies - TCE Moodle Website

Treatment of Food Industry Foods and Wastes by Membrane Filtration 247
Table 6.3
Typical recoveries, trans-membrane pressures, power requirements, consumption, and
resulting energy costs based on applied pressures with an overall motor/pump efficiency
of 40%. Assumed energy cost 0.05 US$/kWh
Recovery (%) Pressure (kPa) Consumption (kWh/day) Energy cost (US$/m 3 )
MF 90–98 100 77.5–70.8 0.004–0.0035
UF 90–98 300 231.5–212.5 0.012–0.011
NF 75–95 500 463.5–365.5 0.023–0.018
RO 50–80 1000 1389–868.3 0.070–0.043
3. OPERATIONAL PROBLEMS AND ENGINEERING SOLUTIONS
All aspects of fouling involve the trapping of some type of material within the membrane
process device itself or on the surface of the membrane. Since the causes, symptoms, and
cures are different, the following five types of fouling are discussed below (23, 24):
1. Membrane scaling
2. Metal oxides fouling
3. Colloidal fouling
4. Biological fouling
5. Cleaning agent fouling
The problems of membrane scaling and metal oxides fouling are common to all membrane
systems, regardless of the type or source of feed water stream (i.e., sea water or food
processing water). Colloidal and biological types of fouling are specifically important in
the design of a membrane system for treating food processing water and wastewater. To
establish the optimal membrane cleaning regime, it behooves the operator to understand
clearly the nature and the extent of the interaction of cleaning agents with the membrane.
3.1. Membrane Scaling
Precipitation of dissolved salts results from exceeding the solubility limits in a concentrated
stream. This may occur because the salts in feed water are normally concentrated from
two to four times. This is a function of system recovery; 50% results in a concentration of
2 and 75% results in a concentration of 4. The most common precipitates encountered are
those of calcium carbonate, calcium sulfate, and silica. Calcium carbonate precipitation can
be prevented by lowering the feed pH by the addition of hydrochloric acid. Carbonates and
bicarbonates are then converted to carbon dioxide gas that passes through the membrane
without difficulty. The precipitation of calcium sulfate can be retarded by the addition of a
sequestering agent, such as sodium hexametaphosphate. The silica precipitation can be
prevented by reducing system recovery to keep silica concentrations below saturation.