Recovery of Water Using Membrane Technology

9.3.1.1 Concepts of Membrane Technology

Membranes are considered as thin semi-permeable sheets, usually made of biological, synthetic, or polymeric materials, which act as a selective barrier (Salas, 2017). In general, a membrane is a thin barrier that permits selective mass transport (Mortazavi, 2008). This is a vital property that the membrane techniques exploit, as it allows only specific components of the solution to permeate through the membrane freely while impeding the permeation of other components (Salas, 2017). Transport of permeating species through the membrane matrix is achieved by the application of a driving force across the membrane which provides a basis for the classification of membrane- separation processes (Mortazavi, 2008). Indeed, the classification is based on the type of driving force that drives mass transport across the membrane such as mechanical (pressure), concentration (chemical potential), temperature, or electrical potential (Porter, 1989; Mortazavi, 2008). Table 9.4 shows the

TABLE 9.4

Driving Forces and the Related Membrane Separation Processes

Driving Force

Membrane

Pressure difference

Microfiltration, ultrafiltration, nanofiltration, reverse osmosis or hyper filtration

Chemical potential difference

Pervaporation, per-traction, dialysis, gas separation vapour permeation, liquid membranes

Electrical potential difference

Electrodialysis, membrane electrophoresis, membrane electrolysis

Temperature difference

Membrane distillation

driving forces and the related membrane-separation processes. The characteristics of the feed solution and the desired permeate quality also dictate the choice of the membrane process, the membrane type, and module design and configuration (ITRC, 2010b).

The most broadly applied membrane technology processes are pressure driven which include reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF). A brief summary of the comparison amongst the four classes of pressure-driven membrane-separation processes is given in Table 9.5.

Membrane-separation processes have become a viable alternative to other physical methods of separation (Mortazavi, 2008; ITRC, 2010b). Some of their

TABLE 9.5

Comparison amongst the Four Classes of Pressure-Driven Membrane Techniques

Reverse

Osmosis

Nanofiltration

Ultrafiltration

Microfiltration

Membrane

Asymmetric

Asymmetric

Asymmetric

Asymmetric

Symmetric

Thin film thickness

1 pm 150 pm

1 pm 150 pm

  • 1 pm
  • 150-250 pm

1-150 pm

Rejection

High and low molecular weight compounds, NaCl, glucose, amino acids

High molecular weight compounds, mono-, di-, and oligosaccharides, polyvalent ions

Macromolecules,

proteins,

polysaccharides,

vira

Particles, clay, bacteria

Applications

Ultrapure

water;

desalination

Removal of (multivalent) ions and relatively small organics

Removal of macromolecules, bacteria, viruses

Classification; pre- treatment; removal of bacteria

Membrane

materials

Cellulose acetate, thin film

Cellulose acetate, thin film

Ceramic, polysulphonic, poly vinylidene flouride, cellulose acetate, thin film

Ceramic, polysulphonic, poly vinylidene flouride, cellulose acetate

Pore size

<0.002 pm

<0.002 pm

0.02-0.2 pm

0.02-4 pm

Module

configuration

Tubular, spiral wound, plate- and-frame

Tubular spiral wound, plate- and-frame

Tubular hollow fibre spiral wound, plate-and-frame

Tubular, hollow fibre

Operating

pressure

15-150 bar

5-35 bar

1-10 bar

<2 bar

Separation

mechanism

Diffusion + exclusion

Diffusion + exclusion

Sieving

Sieving

Permeate flux

Low

Medium

High

High

Sources: van der Bruggen, 2003; Mortazavi, 2008.

benefits include relatively low energy consumption, possibility of separation of both organic and inorganic substances, process selectivity, low chemical consumption, and operation at room temperature (Garcia et al., 2013; Agboola, 2019). In addition, membrane processes are capable of continuous operation, require low capital, and their footprint is low (Garcia et al., 2013).

The membrane technology can be used to treat AMD due to their capability in removing suspended solids, dissolved organic compounds, metallic ions, microbes, and multivalent ions from wastewater (Agboola, 2019). However, AMD treatment by membrane technology was very rare, in the past, owing to the moderately high cost of the membrane and high membrane fouling because of the susceptibility of membrane systems to high-salt concentrations found in AMD (Mortazavi, 2008; Agboola, 2019). Nevertheless, membrane technology processes are currently playing a crucial role as new state of the art methods used in treating AMD (Agboola, 2019), and recent studies on the treatment of AMD using semi-permeable membranes can be found in several literature (Mortazavi, 2008; ITRC, 2010b; Ambiado et al., 2017; Salas,

  • 2017). In fact, various studies have shown successful application of various pressure-driven membranes for the treatment of AMD (Mortazavi, 2008).
  • 9.3.1.2 Selected Typical Studies of Pressure-Driven Membrane Technology

The increasing demand for clean and portable water in countries that have or had mining industries has stimulated research and/or utilisation of membrane-separation processes for water and wastewater purification aimed at the use/reuse of water resources such as AMD. This section discusses selected examples of the use of membrane technology from published literature for the treatment of AMD with the view of recovering water using various classes of pressure-driven membranes. In fact, individual and/or combinations of MF, UF, NF, and RO membrane processes have been extensively studied for the recovery of both water and metals (Ahn et al., 1999; Garba et al., 1999; Zhong et al., 2007).

The aim of a study by Andrade et al. (2017) was to investigate the use of NF and RO for gold mining effluent treatment in order to obtain water for industrial reuse. Two effluents from a gold mining company were studied, i.e., an effluent from a sulphuric acid production plant and the water from the calcined dam. The effluent from gold mining was first pre-treated for each test performed. The pre-treatment used a commercial submerged poly- vinylidene difluoride (PVDF) based UF membrane with an average pore diameter of 0.04 pm and a filtration area of 0.047 m2 operating at a pressure of 0.7 bar. For the NF and RO filtration tests a bench scale unit was used. The RO membranes were TFCHR and BW30, while the NF membranes were MPF34, NF90, and NF270. The NF and RO filtration took place at a fixed pressure of 10 bar, feed flow rate of 2.4 L/min (corresponding to a cross-flow velocity on the surface of the membrane of 1.9 m/s and Reynolds number of 840), while the permeate was continuously removed and the concentrate returned to the supply tank. Before every test, all membranes were washed in two consecutive ultrasound baths for 20 min each; the first containing citric acid solution at pH 2.5 and the second containing 0.1% NaOH solution. After chemical cleaning, the membranes were flushed with distilled water.

A study by Andrade et al. (2017) concluded that in an effluent in which the major contaminants are bivalent ions, such as effluent from gold mining with high concentrations of sulphate, calcium and magnesium, effluent treatment with NF membranes was more effective than with RO membranes because they allow permeate fluxes of 7 to 12 times higher and compatible retention efficiencies. In the study, NF90 showed the best performance, and subsequently NF was declared to be a suitable treatment for gold mining effluent at an estimated cost of US$0.83/m3. However, it was observed that retention efficiencies of NF90 were similar to those of RO membranes except that the permeate fluxes obtained were 7 to 12 times higher for NF90. The study found that a feed pH of 5.0 provided both greater permeate flux and higher retention efficiencies. It was noted that slightly above the membrane isoelectric point (IEP) (pH 4.3), at pH 5.0, the membrane has a small negative charge and, therefore, it repels anions, which prevents fouling formation. It was observed that permeate flux decreased linearly as the permeate recovery rate increased. Additionally, there was a significant increase in conductivity for recovery rate above 40%. In other words, at a recovery rate above 40%, there was a significant decrease in permeate quality. Consequently, a permeate recovery rate of 40% was selected as the ideal for this process.

Three commercial NF (NF99, DK, and GE) membranes were employed in a laboratory scale study by Al-Zoubi et al. (2010) to investigate their performance in handling AMD collected from a copper mine in Chile. Table 9.6 shows the properties of the AMD solution used in the study. The pressure of the membrane in the laboratory scale test cell was set at 20 and 30 bar

TABLE 9.6

Properties of the Original Acid Mine Drainage and the Standard Concentration for Potable Water According to the World Health Organisation

Metal

Concentration (ppm) in AMD Solution

Standard Concentration for Potable Water (ppm)

Aluminium(Al)

1139.0

0.20

Sulphate (SO.,)

14 337

250

Calcium (Ca)

325.9

40

Copper (Cu)

2298.0

2.0

Iron (Fe)

627.5

0.200

Manganese (Mn)

224.5

0.050

Magnesium (Mg)

630.60

20

Sodium (Na)

6.89

200

Potassium (K)

4.31

12

for both the original and concentrated AMD. The concentrated AMD was obtained from RO of the original AMD until 50% of permeate (pure) water were removed and the total ion content was accordingly raised by a factor of 2.

The results of the study showed that DK and NF99 NF membranes successfully treated AMD with a very high rejection (>98%) of all divalent cations and anions for both 20 and 30 bar, which confirms that the maximum applied pressure on the NF membrane cell should not exceed 20 bar. The results indicate the suitability of NF membranes in treating AMD in a more environmentally friendly process. Moreover, the results showed that NF membranes are capable of reducing the heavy metals concentration found in AMD to low levels that are accepted by many international organisations especially for industrial and agricultural use. Detailed analysis of the results showed that the DK membrane is preferable for high concentration of AMD, while NF99 is used, when high permeate flux is required. The GE had the lowest rejection and permeate flux at all investigated conditions indicating its inappropriateness in treating AMD solution.

Andalaft et al. (2018) analysed, evaluated, and modelled the behaviour of two commercial spiral-wound NF membranes (NF270 and NF90) in the treatment of real AMD. A 150-L sample of AMD was made available by a large copper mining company in central Chile. The sample was micro-filtered with a 0.10-pm cut-off ceramic membrane to remove any existing colloidal solids. The results revealed that both membranes showed operational differences regarding the effect of pFl, temperature, continuous operation, concentration polarisation, and fouling. Flowever, the suitability of NF in treating AMD was confirmed, as the two tested membranes succeeded at removing almost all of the dissolved ions. Specifically, both NF90 and NF270 indicated a high rejection of divalent ions (-100%). However, the permeate flux modelling detected the occurrence of surface fouling on the NF90 membrane by colloid particles due to its elevated roughness, a theory supported by the elevated physically reversible resistance in the resistance model. The modelling of the permeate flux at various recoveries enabled the researchers to distinguish between the decline in permeate flux resulting from the increase in osmotic pressure and that caused by membrane fouling. As a result, the study was able to identify the maximum design recovery at which fouling begins to occur (-75%). Following this value, gypsum (CaS04-2H20) was considered to be the only foulant, and its potential fouling on the tail elements of an industrial system could be controlled with antisealants. Furthermore, it was also noted that adjustments of pH in the continuous tests reduced the precipitation of aluminium, and gypsum was identified as the main compound generating a significant decrease in the permeate flow in the membrane.

Sierra et al. (2013) studied the treatment of AMD from an abandoned mercury mine in Spain using NF270-2540 NF membrane (Filmtec). The sediment geochemistry and the origin of acid waters were analysed in order to understand the geochemical factors involved in NF. The study showed that 99% of the dissolved solids such as As, Fe, and Al could be removed. In particular, the researchers found that NF has the capability of rejecting up to 99% of aluminium, arsenic and iron content, and 97% of sulphate content. The studied NF membrane was found to be effective in the treatment of metallic acid drainage contaminants, even at low pFI and low pressures, which, without doubt, can decrease the costs associated with the process at industrial scale.

A comparative study of the performance of the commercially available polyamide ultra-low-pressure RO (RE-4040-BL) and NF (DK4040F) processes for AMD treatment and reuse was investigated by Zhong et al. (2007). Samples of AMD for the study were collected from the Dong Gua Shan copper mine in Anhui province, China. The evident property of the AMD was the high conductivity that resulted from various ions such as sodium, chloride, sulphate, nitrate, heavy metal, and many others. Effects of operation pressure, pH, temperature, water recovery efficiency, and operation time on ultra-low- pressure RO and NF performance were studied. The experimental results showed that the removal efficiency of total conductivity of AMD by NF was low, although the removal efficiency of the heavy metal ions was very high while ultra-low-pressure RO effectively removed both total conductivity and heavy metal ions from AMD. In addition, the results of the study showed that the rejection increased with an increase in feed pressure and decreased with an increase in feed temperature, and it was dependent on the pH of the feed. The pH was found to influence the rejection of heavy metals because the charge property of the surface of polyamide ultra-low-pressure RO and NF membrane changes and heavy metal ions are capable of interacting with OH- ions and precipitate onto the membrane surface. The effect of the feed temperature on the permeate flux was very sensitive, and the removal efficiency kept almost constant below 25°C while it began to decrease above 25°C. The results suggested that with the rejections of heavy metals and total conductivity being greater than 97% and 96%, respectively, for the ultra-low- pressure RO, it showed that the membrane was suitable for the recovery of heavy metals and reclaiming of water from AMD. Compared to the ultra- low-pressure RO, the NF process had the capability of removing close to 90% of the ions in the AMD with 48% reduction of the total conductivity.

Haan et al. (2018) investigated the potential of different membrane modules (UF membrane with the nominal molecular weight cut-off of 10 kDa, UF membrane with molecular weight cut-off of 5 kDa, and RO membrane) in treating mine water collected from three different places in Selangor, Malaysia. The UF membranes were made from polyethersulphone whilst RO was made from polyamide thin film composite with 95% MgS04 and 99.4% NaCl rejection. The results indicated that different membrane modules demonstrated different extents of removal efficiency, depending on the rejection capability of the membranes, which in turn was predominantly governed by the membrane morphology. Given its tight membrane structure, the RO membrane module had the best performance in producing permeate water of an excellent quality. However, RO produced lower permeate flux due to the smaller pore size of the membrane module. Unfortunately, none of the water produced from the three studied single-membrane systems was able to meet the standard for residential use. Therefore, an integrated membrane filtration system with three stages of membrane filtration is required so as to further improve the water quality which could eventually lead to the attainment of the standard of water required for residential use.

In one study, Vaclav and Eva (2005) used RO technique to recover water of good quality from three different sources of AMD. Gunther and Mey

(2008) evaluated different water treatment technologies including the membrane technology with the view of treating AMD from a coal mine. The results showed that only the biological sulphate removal process and RO membranes could be used to produce a high water recovery, whilst being cost effective. Bhagwan (2012) studied the use of a high recovery precipitating reverse osmosis (HiPRO) process for the recovery of low salinity water from mine waters. The main advantage of the developed process is that it makes use of RO to concentrate the water and produce supersaturated brine from which the salts can be released in a simple precipitation process. Furthermore, Bhagwan (2012) states that the technology offers other key advantages including (1) very high recovery, (2) simple system configuration, (3) low operating and capital costs, (4) easy operation, and (5) minimum waste.

 
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