Membranes Based Processes Industrial & Operation Challenge and Brine Disposal Management

Membrane processes cover a group of separation processes in which the characteristics of a membrane (porosity, selectivity, Hydrophobicity, electric charge,....) are used to separate the components of a solution or a suspension. In these processes the feed stream is separated into two streams: the fraction that permeates through the membrane, called Permeate, and the fraction containing the components that have not been transported through the membrane, usually called the Retenate or Concentrate.

Specific energy consumption, water cost, desired water quality, OPEX and environmental impacts are the set of parameters that are used to determine the most desirable process to be applied in separation process.

In spite of the potential advantages of the membrane based processes, they have some limitations and industrial challenges that render the process uneconomic.

Most of these limitations are not only contributed by membrane characteristics, but also the hydrodynamics within the membrane module, such as recovery and permeation flux.  Therefore, numerous attempts have been made to obtain optimized geometry or shear induced accessories in membrane modules. A properly designed module will improve the hydrodynamics within the module and enhance the overall system performance. However, module design and fabrication itself also faces industrial challenges, such as compatibility of adhesive, seal, spacer, and feed distributors.

Developments in the membrane material and module have been conducted to improve membrane process efficiency, that includes the increase in membrane performance, such as selectivity and flux, membrane durability, chemical resistance, pressure and temperature resistance, high packing density, and lower membrane cost. The development of membrane material will then reduce investment cost related to membrane cost and overcome other limitations.

Among those limitations, fouling is considered as the major obstacle in membrane application, which leads to the rapid flux decline and high energy consumption. There are different types of fouling that might occur at the surface of membranes

Qualitative classification of fouling phenomena

1-Particulate fouling

Particulate fouling is a deposition of suspended solids, colloidal maters, corrosion products, silt and clay, precipitated crystals, colloidal silica and sulfur, precipitated iron and aluminum compounds from incomplete treatment. Commonly monitored by silt density index (SDI) test, modified fouling index (MFI-UF) etc.

2-Organic fouling

Adsorption of natural organic matters (NOMs): proteins, carbohydrates, fats, oil, tannins, aromatic acids such as Humic acid and Fulvic acid to the surface of RO membranes also disinfection byproducts (DBPs) generated during disinfection processes of water and wastewater treatment, soluble microbial products formed during the biological treatment.

3-Inorganic fouling

Deposition and precipitation of inorganic compounds such as calcium carbonate (CaCO3), barium sulfate (BaSO4), silica (SiO2), calcium phosphate (CaSO4), coagulant/flocculant residuals may also be present as inorganic foulants (aluminum silicate clays and colloids of iron–iron oxide, aluminum oxide, silica)    

4-Biological fouling

Formation of biofilm due to accumulation of bacteria, algae, fungi, viruses, protozoa, bacterial cell wall fragments on the surface of membrane due to improper early stages pre-treatment.

Additionally, fouling influences capital investment, which is attributed to the need of additional pretreatment units and energy, materials, and chemicals required to overcome the fouling. This fouling phenomenon also contributes to the increase of mass transfer resistance that leads to the increase of trans-membrane pressure to maintain constant flux which finally results in higher operation expenditure.

Consequently, higher energy is consumed during the desalination process. Moreover, the operational cost is also increased to remove foulants by chemical cleaning. Due to its major impact on process efficiency and economics, strategies in fouling mitigation have been proposed by many studies researches.

Fouling Control & Minimization

·        Fouling control

·        Pretreatment technologies

·        Anti-fouling membranes and modules

In the fouling control strategies, membrane processes could be operated under a particular condition where in the fouling phenomena remained negligible, such as by operating below critical flux and critical conversion or injecting antiscalant/antifouling agent.

Appropriate pre-treatment strategies have a significant effect on reducing fouling in membrane systems. Microfiltration (MF)/UF techniques have been progressively used to reduce fouling of RO membranes instead of conventional pretreatment. The integration of MF/UF as RO pretreatment could significantly reduce the chemical consumption in the pretreatment step. Nevertheless, an optimized design of the membrane module and development of the fouling-resistance membrane also has significant contributions to fouling mitigation as well.

There have been many studies that contribute to the reduction of operating cost for membrane processes especially in energy consumption. For the pressure driven membrane such as RO, the main energy consumption is energy required by the pump to deliver feed water with high-pressure conditions into RO units. Reversible pumps, pelton turbine, turbo exchanger, pressure exchanger, and hydraulic pressure booster are typical energy recovery devices, which have been used to recover energy from the brine water.

 By using those devices, dramatic reduction in desalination cost can be achieved. In spite of its success on energy recovery, further development is still investigated to gain more reduction in energy consumption.

Also contaminants such as Boron are another challenge for membrane technology in the desalination process to meet the regulation of fresh water quality.

Boron is present in seawater at average concentrations of 4.6-4.9 ppm. WHO regulated that the maximum concentration of boron in drinking water is 0.5 mg/l then  updated to 2.4 mg/l at 2011. However, Boron rejection over 90% is typically hard to be achieved by the RO membrane.

System design is one of the alternative solutions to improve the boron rejection. Typically, the RO membrane system is operated under neutral pH, which is considerably ineffective for removing Boron due to boron exists in the form of Boric acid which is weakly ionized at neutral pH.

For two passes of the RO system, pH of permeate in the first pass could be elevated up to 11 to improve boron rejection but higher pH will lead to precipitation of calcium carbonate and metal hydroxide on the surface of RO membranes which impact their performance due to scale formation.

Suitable design of post treatment can also enhance the boron removal by incorporating a conventional ion exchange bed with boron selective ion exchange resin or ED. Boron-selective resin is able to remove boron from the first pass RO permeate to below 0.1 mg/L. It should be noted that ion exchange requires chemical regeneration when the resins become exhausted. Consequently, it results in another problem that is associated to chemical regeneration and the potential of hazard chemical treatment.  

EDI is the deionization process that combines the advantages of conventional ion exchange and ED. Benefiting from the combined process, EDI is able to achieve high removal of ionic components to a relatively low concentration including weakly ionized substances such as boron and silica.

It is proven that EDI has a capability to remove boron for more than 99 % of boron. For example, the study investigated the performance of EDI using feed water with a boron concentration of about 3 mg/L. The results of the study showed that EDI was able to produce dilute water with 25 μg/L (ppb) of boron.

Therefore, EDI has the potential to be applied as SWRO post treatment in order to achieve high quality water that meets the requirement of boron maximum level.

Challenges and limitations in membrane based processes. Nowadays, the way to satisfy the increasing demand of water by increasing desalination plants through scaling-up into larger capacity is challenged by its complexity in the larger system which is associated with the spaced required, number of components (membrane elements, pressure vessels, piping and instrumentation) uses, and the large quantity of waste disposal as brine. In some places, land is very expensive which is considered as the major limitation for obtaining larger capacity due to the investment cost required.

To overcome this problem developing the membrane system that requires a smaller footprint, particularly when the membrane system is constructed at a very large capacity. One of the alternatives is developing a membrane element with larger size. This larger element is expected to reduce the footprint and the number components. Typically, the desalination process extracts a large volume of water from saline water and discharges concentrated brine back into the environment. The brine concentration is higher than the original feed water (brackish or seawater). Furthermore, for preventing membrane scaling, scale inhibitor or antiscalant is injected prior to RO and remains in the concentrate streams, thus becoming concentrated in the rejected brine. In addition, chemicals used for pretreatment and cleaning are also present.

Moreover, pollutant components in the brine can be classified as: corrosion products, antiscaling additives, antifouling additives, halogenated organic compounds (formed by reaction of residual chlorine and bromine with natural organic matters), antifoaming additives, acid, and concentrate. The brine could potentially impact the environment due to its salt concentration and chemical content. The impact involves physicochemical and ecological attributes of receiving the environment where in the brine is discharged.

Brine disposal has the potential to degrade characteristics of receiving water and its severity depends on volume, characteristics, dilution rate prior to disposal, and characteristics of receiving water. Due to the aforementioned problems, many technologies have been investigated to manage brine reject from desalination plants.

Brine management technologies.

Brine management are categorized into four different groups:

 (1) Methods for reducing and eliminating brine disposal.

 (2) Methods for commercial salt recovery.

 (3) Brine adaption for industrial uses

 (4) Metal recovery

The comparative study revealed that zero discharge of desalination has very high costs. Meanwhile, the emerging technologies are promising in reducing brine volume although still in a laboratory scale. MD is one of potential alternatives for reducing brine disposal. It requires lower energy than conventional evaporation and could be coupled with solar ponds or residual heat and thus reduces energy consumption.

Scaling might be formed on the MD membrane surface due to precipitation of salts that contributes to performance deterioration. However, simply membrane washing using water could be applied to remove salt crystal built on the membrane surface. Technologies, which have been purposed to obtain commercial salts from brine show greater potential than those for reducing waste brine volume. For example, the recovery of gypsum, sodium chloride, magnesium hydroxide, calcium chloride, calcium carbonate, and sodium sulfate could improve cost-effectiveness of the desalination process. However, such an integration process is complex and results in higher cost. In addition to recovering commercial salt, the treatment of brine for industrial usage, such as brine adaption to feed chlor alkali industry, is also a complex process. This option involves appropriate treatment for contaminant removal, and thus needs high capital and operational cost. Metal recovery from brine is another promising issue. However, more research is needed to develop a selective separation process for recovering specific and valuable metals from seawater. Overall, it could be concluded from the comparative studies that technologies for recovering water and reducing brine as performed by the MD process are the most promising process, which exhibited high recovery, a simple process, and lower capital cost.

Another option to solve brine disposal is the application of membrane crystallization (MCr), as the NF-RO-MCr integration process. MCr is not limited by osmotic pressure and can be therefore operated at high water recovery. The MCr unit further concentrates the brine from NF and RO brine up to salt crystal formation and thus no more brine disposal.

The possible results that could be obtained by using MCr are: reduction of brine disposal; improvement of total water recovery; and production of valuable crystalline products. In the so-called integration process that incorporates MCr drives, a new alternative process for achieving zero liquid is discharged.

The FO membrane is also considered as a sustainable solution to treat brine disposal from the desalination plant, before it is discharged into the environment. More than 90% of water recovery could be achieved from brine. However, the application of FO for brine treatment is limited by fouling.