Membranes Filtration Techniques for Wastewater from Tanneries

1. Water Pollution

It is well known that water pollution is caused due to the release of waste products and contaminants into surface runoff into river drainage systems like the dumping of industrial wastes, improper disposal of human and animal wastes and residue of agricultural practices, including fertilizers and pesticides.

2. Membranes for Wastewater Treatment

Membrane technologies have developed as one of the main contributors to the resolution of water-related problems over the past two decades. Increasing water scarcity, followed by severe regulations in industrialized countries, have promoted the use of membranes for water and wastewater treatment

. 3. Membrane Theory

Membrane separation processes are characterized by instantaneous retention of species and product flow through the semi permeable membrane. Membrane performance depends on High permeate flux and selectivity, good mechanical, chemical and thermal stability of membrane materials, minimal fouling during operation, good compatibility with the operating environment. A membrane is defined as a perm selective barrier between two homogeneous phases. For many processes in wastewater treatment, the membrane acts to reject the pollutants, which may be suspended or dissolved and allows the “purified” water through it. Membranes can be characterized as porous and nonporous, based on the mechanism by which separation is actually achieved. Membranes are divided into two main categories of symmetric or asymmetric. The active layer on the top of the membrane determines its separation behaviour and the porous layer below assists as the top layer. The supporting layer ensures the mechanical stability of the membrane with a low resistance to the permeate flow such as proteins. Membrane processes are continuous steady-state operations consisting of three streams: Feed, Retentate, Permeate (product) streams.

. Membrane processes are continuous steady-state operations consisting of three streams: feed, retentate and permeate (product) streams. The membrane, a semipermeable barrier, selectively allows the passage of some components, but not others, and allows some components to pass through more rapidly than others. In principle, two operating modes of filtration exist, cross-flow and dead-end. In the cross-flow mode the feed is pumped parallel to the membrane surface, while in dead-end operation the membrane is fed orthogonally.

. 4. Water Contaminants

. 5. Filtration Range

. 6. Membrane Separation Technologies Features

Continuous process resulting in automatic and uninterrupted operation

Low energy utilization involving neither phase nor temperature changes

Modular design – no significant size limitations

Minimal moving parts with low maintenance requirements

No effect on form or chemistry of the contaminant

Discrete membrane barrier to ensure physical separation

No chemical addition requirements

7. Membrane Separation Techniques in Industry

. 7.1. Reverse Osmosis

RO (Reverse Osmosis) membranes are effectively nonporous. They retain particles and even some low molar mass species such as salt ions. NF (Nanofiltration) membranes are relatively porous and their performance is between that of RO and UF (Ultrafiltration) membranes, which can be useful in some applications. For instance, using NF membranes for production of drinking water can reduce the post-treatment cost such as demineralization, because of limiting salt rejection. Membrane technology offers the advantage of selectively removing contaminants based on their sizes. Membranes with different pore-size distributions and physical properties remove a wide range of pollutants. MF (Membrane Filtration) membranes have the largest pore size among other membrane technologies, they reject large particles and various microorganisms as well as bacteria. UF membranes have smaller pore size than the MF membranes, thus they can reject viruses and slightly soluble macromolecules.

. The rejection is based on size exclusion, charge exclusion and physiochemical interactions between solute, solvent and membrane. The fraction that appears in the product is usually measured in terms of the rejection coefficient of the membrane. An ideal RO membrane has a salt rejection of more than 99%.

. Semi-permeable membrane uses two mechanisms for removal of impurities: 1) Rejection-Repels mineral salts involving dielectric and molecular forces. 2) Sieving – Does not allow particulate matter to pass on a small scale > 0.0005 microns. Only tiny organics and gas molecules can pass.

. 7.1.1. RO System Controls

Product Water Check Valve: Protects membrane from back pressure.

Automatic Shut-off Valve: Maintains storage tank pressure between ½ to 2/3 feed line pressure.

Brine Flow Restrictor: Maintain reject rinse flow at 3x to 5x product flow; Membrane life and water quality; Prevent water wasting.

. 7.2. Nanofiltration

NF is closely associated with RO and is sometimes known as loose RO, the basic principles of the RO process are valid for the NF process. The rejection of solutes is different and depends both on molecular size Donnan exclusion effects, which are due to the functional groups on the polymer backbone. The Donnan-exclusion effect is based on the exclusion (inability to deeply penetrate inside the polymer) of cations when the sign of their charge coincides with that of the polymer.

. 7.2.1. Applications

Decontamination and recycling of all kinds of water, including ground, surface and wastewater generated in many industries . Ability to reduce the wastewater organic loading in terms of chemical oxygen demand (COD) and to promote its partial desalination for possible water reuse NF as a standalone process has been shown in many cases to reduce total organic carbon (TOC) to less than 0.5 mg/L . NF and RO can be applied for treatment of a wide range of wastewaters involving petrochemicals, electroplating, coal and gasification, pulp and paper, electronic wastewaters , toxic metals and cyanides , municipal leachates , landfill leachate.

. 7.3. Ultrafiltration & Microfiltration

Ultrafiltration (UF) membranes have pore sizes up to around 0.1 μm in diameter, whereas membranes with pore diameters in the range of 0.1 to 10 μm are considered to be microfiltration (MF) membranes. The separation mechanism is normally the molecular sieve.

. 7.3.1. Applications

1. Food industry to recover milk proteins and to remove lactose and salts.

2. Metal finishing industry to concentrate oil emulsions.

3. UF membranes have been applied for removing organic compounds with a high molecular weight such as proteins, colloids and oils.

4. Some MF/UF plants are applied to treat industrial wastewater streams

. 7.4. Membrane Bioreactor (MBR)

Membrane bioreactor (MBR), combining membrane filtration with biological treatment, is recognized as one of the most successful hybrid membrane systems in wastewater treatment. Typically, low-pressure membranes such as UF and MF can be used in MBR systems. The MBR system offers various advantages over the conventional biological treatment.

. 7.5. Integrated Membrane Systems

Since any single treatment process cannot achieve all the treatment objectives, an IMS is frequently used for wastewater recycling to attain multiple treatment purposes. Nowadays, a number of IMSs have been developed to alleviate membrane fouling, particularly when the feed water contains high concentrations of organic matter. Typically, in IMS, a pre-treatment will be applied prior to the membrane filtration unit. This pre-treatment process may involve conventional units such as coagulation, flocculation, sedimentation or a membrane pre-treatment method.

. 8. Heavy Metals

The term heavy metal refers to any metallic chemical element that has a relatively high density and is toxic or poisonous at low concentrations. Some of the heavy metals are chromium, arsenic, mercury and lead. They are natural components of the Earth's crust. They cannot be degraded or destroyed. To a small extent they enter our bodies via food, drinking water and air as trace elements, some heavy metals (e.g. copper, selenium, zinc) are essential to maintain the metabolism of the human body. However, at higher concentrations they can lead to poisoning. Heavy metal poisoning could result, for instance, from drinking-water contamination (e.g. lead pipes), high ambient air concentrations near emission sources, or intake via the food chain.

8.1. Hexavalent Chromium (Cr (VI)) in Industrial Wastewaters

Cr (VI) is present in the wastewaters of a number of industries producing steel, chrome plated products, tannery products, dye stuff, paints, etc.

. 8.2. Tanning Process

The process of converting hides and skin into leather is called tanning. Though any animal skin can be converted into leather, cattle hides, sheep and goat skins are the major type of hides and skin used to manufacture leather. Traditional vegetable tanning is highly labour intensive. It is an extremely slow process rendering it economically less viable compared to chrome tanning. Chrome tanning is a more modem process of tanning using powdered chromium as the tanning substance. Depending on the nature of chemicals used, the characteristics of wastewater discharge from different stages of leather processing differ across stages .

. Chromium can exist in several chemical forms displaying oxidation numbers from 0 to VI. Trivalent and hexavalent forms of chromium are the most stable in the environment. Chromium in tannery wastewater is found to be 390 mg/L. Toxic Impacts of Cr (VI) are allergies, irritations, eczema, ulceration, nasal and skin irritations, perforation of eardrum, respiratory track disorders, lung carcinoma, genotoxic and mutagenic effects.

. 8.3. Microbial Fuel Cell Technology

A microbial fuel cell (MFC) is a bioreactor that converts chemical energy in the chemical bonds in organic compounds to electrical energy through catalytic reactions of microorganisms under anaerobic conditions.

. Since acidic Cr(VI) can be used as cathodic electron acceptor in microbial fuel cells, it makes it possible to reduce Cr(VI) to much less toxic Cr(III) with simultaneous electricity generation in a microbial fuel cell. The applications of MFCs are electricity generation, wastewater treatment & recovery of pure materials, removal of organic matters, treatment of specific pollutants, bioremediation, heavy metal recovery, nitrification & denitrification, dye decolourization, water softening, biogas production, biomass production, robotics, biosensor application, insitu power supply, implantable power supply.

. 8.3.1. Membrane Processes for Tannery Wastewater Treatment

Several studies showed as crossflow microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO) and supported liquid membranes (SLMs) can be applied in leather industry for the recovery of chromium from spent liquors. Some of the proton exchange membranes (PEMs) in MFC are Nafion, Ultra Filtration membranes, Ultrex, Salt bridge, Clay. Of all Nafion is preferred for its high proton conductivity and electricity generation.

. 9. Conclusion

MFC is a promising technology for wastewater treatment and bioelectricity generation. Recent research and development and analysis of literature show that higher power densities can be obtained from improved MFC designs with the use of cost effective materials. Some companies have emerged to use MFC technology for fuel and other potential applications including remote power, bioremediation and biosensors proving that this technology could have greater impact in development of clean energy within a few years.