In this article we will discuss about the technologies used for recycling waste-water.
Total Water Management Approach to Recycle of Waste-Water:
Waste-water recycle should take shape at the drawing board stage in contrast to the traditional approach of designing the raw water and waste-water treatment plants (end of pipe solutions) separately (Fig. 11.1). This will enable planning for water recycle at the design stage itself. The benefits are many as shown in Fig. 11.2.
Firstly, because water is recycled to the process, raw water consumption is reduced. The designer can, therefore, plan for a raw water treatment plant of lower capacity and lower cost. Secondly, effluent treatment is essentially not required and the quantity of waste disposed is less—which again, leads to cost reduction.
Investment is certainly required for the product recovery and water recycle plants; but it gives a good payback and provides value for money spent. Pollution is not just abated but prevented; pollutants are separated not destroyed; energy is saved and the total cost of water and waste-water treatment is reduced. How do we use the experience gained in successful on/off site recycle and integrated solutions for water and waste-water treatment? To achieve the ultimate goal, total water management should be applied right at the design stage. We need to apply these approaches in a complex industry in multiple ways.
Guidelines for Selection of Recycle Schemes:
1. Study the manufacturing process thoroughly and identify areas where reduction of water consumption is possible.
2. Identify the process where reduction of pollution load is possible by changing raw material or adopting cleaner manufacturing processes.
3. Identify streams that can be segregated and treated economically. In case of electroplating, the rinse water can be segregated and treated for recovery of plating metal. This not only reduces the overall cost of recycle but also facilitates the recovery of valuable products from the waste-water stream.
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4. Identify effluents which are relatively clean and can be treated with simple processes so that they can be recycled internally without letting the water out into an effluent treatment plant.
5. Identify the quality of water required at various manufacturing stages. For steam generation, high quality water may be required. For washing or for cooling water make up, high quality water may not be required. It is always economical to design a recycle system to produce water suitable for lower end uses.
6. Select a technology that is easy to implement, operate and service.
7. Look for the availability of spare parts that may be needed in future.
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8. Reliability of performance is important.
9. Low in operating cost.
10. Good service network of the plant supplier
Recycle Technologies:
Any waste-water recycling plant requires three stages of treatment:
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1. Effluent treatment.
2. Tertiary treatment.
3. Advanced treatment.
1. Effluent Treatment:
A good effluent treatment is a pre-requisite for a good effluent recycle system. Unless we remove the easily removable pollutants with cost-effective methods, it would be difficult to recycle the effluents economically.
Usually effluent treatment plants (ETPs) are designed to meet statutory requirements for disposal. When recycling is considered, the ETP should also be designed considering overall requirements of treatment. For example, in India, disposal standards do not require complete removal of nutrients. But, when we are installing a downstream reverse osmosis system, it is better to remove nutrients in the biological system of the ETP. This will help in reducing fouling of the reverse osmosis system.
There are different technologies available for effluent treatment to remove different pollutants. Table 11.1 lists some generic technologies applied in effluent treatment.
2. Tertiary Treatment:
Treatment beyond disposal norms for re-using effluents for low end usages is called tertiary treatment. It acts as pre-treatment to advanced treatment for complete recycle of effluents. Table 11.2 enlists some generic technologies applied in tertiary treatment.
3. Advanced Treatment:
Further treatment is required after tertiary treated effluents for conforming to the requirements of various end usages of treated water.
Technologies for Recycling of Effluents:
I. Biological Treatment:
Biological treatment is mainly carried out for removal of bio-degradable organic matter in the effluents. Biological treatment can be classified into aerobic and anaerobic treatment. Anaerobic treatment is particularly applied for medium to high concentration effluents (BOD values above 2000 mg/l). At these concentrations, aerobic treatment systems require high power for aeration whereas in anaerobic treatment, it produces power in the form of bio-gas which is used as fuel. Up-flow anaerobic sludge blanket (UASB) reactor is one such technology for anaerobic treatment of waste-water.
Activated sludge process (ASP) system is an age old technology for aerobic biological treatment. Sequential batch reactor (SBR) is an improvement over ASP wherein nutrient removal is also carried out in the treatment unit. There are many variants of SBR.
A modified SBR is a cyclic operating activated sludge system with a compact and modular design. This design combines the advantages of the conventional and the sequencing batch reactor technology. Like in the conventional system, the reactor volume and the level in the tanks are always constant. It is a continuous system for both influent feed and effluent discharge. Like the fill-and-draw system, the reactor operates according to a time controlled process cycle that allows for the alternation of all essential processes (aeration, mixing, sedimentation) in a single compartment.
The advanced treatment leads to sustainability (effluent recycling). A biological reactor consists of three units. Each unit is a similar in design, equipment and functional cycle. All units are hydraulically inter-connected and completely redundant.
Main advantages of modified SBR compared to conventional systems:
1. No need for external clarifiers, sludge rakers, recycle pumps/screws/piping.
2. No extra disturbances in clarifier caused by sludge recirculation.
3. Optimal dynamics in substrate gradient and integrated selector effect, favours the formation of well settleable sludge flocks.
4. Control in time enables flexibility by adapting times for nitrification, denitrification, biological phosphorus removal, sedimentation, depending on influent characteristics.
5. Easy and very compact construction, small footprint.
6. Maximum redundancy.
7. Easy to cover.
8. Easy to extend, modular construction.
9. Common walls.
Main advantages of modified SBR compared to conventional sequencing batch reactors:
1. No head loss (volume is always used for 100 per cent).
2. Raw waste-water does not enter the compartment in the phase preceding the sedimentation sequence. This guarantees optimal BOD, COD, N, P removal efficiencies, no short-circuiting.
3. Continuous influent and effluent flow means lower maximal flow rate and therefore smaller piping diameter and less installed capacity of pumps, aerators, weirs.
4. No moving mechanical parts.
5. No varying pressure on walls.
6. Enables use of surface aerators.
7. Enables use of fine bubble aeration.
II. Clarification:
Clarification is a simple process that is used as a polisher after biological treatment for the removal of suspended solids. Chemicals are often dosed to obtain good quality of treated water.
High rate solid contact clarifiers (HRSCC) or ultra-high rate clarifiers (UHRC) are very useful as they work on the principle of solid contact which has inherent advantage in terms of reliable performance, low chemical consumption, etc. In many cases, the effluent characteristics may vary with the design values due to some process changes. In such scenarios, HRSCC or UHRC provides flexibility in terms of dosing additional chemicals to keep parameters such as hardness, silica, etc. within the operating limits of subsequent membrane systems.
Treatment units such as clariflocculators, tube settlers, parallel plate settlers are not suitable for hardness and silica removal. Parallel plate settlers and tube settlers are ideal for systems where space is a constraint. Ultra high rate systems which use a combination of technologies of solid contact and parallel plate separation are ideal in all cases.
III. Filtration:
Filtration is a basic treatment process in any recycle system. Some systems require only suspended solid removal for recirculation of effluents. These applications include side stream filtration to increase the cycle of concentration in cooling towers, mill scale filtration in steel industry, etc. In many cases, filtration works as a pre-treatment to many advanced treatment systems such as membrane filtration or ion exchange process.
IV. Adsorption:
Adsorption is a process used for removal of trace organics, TOC, Cl2 and activated carbon is mainly used for the purpose. Some ion exchange resins are also available for adsorption of colour, organic matter, etc.
V. Membrane Bio-Reactor:
Membrane bio-reactor (MBR) technology is one of the latest technologies in biological treatment. Submerged membranes are used in place of clarifiers to separate sludge from the waste-water so as to produce high quality permeate. These MBRs can handle very high sludge concentrations in the aeration tank because of which the size of the aeration tank reduces four to five fold. As the membrane acts as a fine filter, it does not require any further treatment using sand filters, activated carbon filters, etc.
Advantages of MBR over conventional treatment processes:
1. The quality of treated water in case of MBR is much superior to conventional biological systems. As the membrane acts as a physical barrier, it does not allow any sludge particles and to a great extent bacteria and viruses to pass through it. Micro-organisms like coliform or Cryptosporidium can be easily removed in MBR. This increases the reliability of the system.
2. MBR does not require clarifier tank whereas conventional activated sludge process requires clarifier which further adds to area requirement and cost.
3. Conventional biological systems require further costlier tertiary treatment to match the performance of the MBR system. This may include coagulation, filtration, chlorination, adsorption, UV treatment etc.
4. MBR system has minimum number of treatment units and is very simple to operate. It does not require any regular handling of hazardous chemicals. As the treatment units are less, it is less prone to system breakdowns.
5. MBR requires much less space compared to conventional activated sludge process. Biological reaction in MBR can be carried out under the condition of 4 to 5 times of MLSS compared to conventional activated sludge process. It means that the aeration tank volume is 1/4th to 1/5th that of a conventional design. Combining this with other features into a very compact design requiring less space than conventional designs.
6. Conventional treatment systems require disinfection with chlorine, which has to be removed completely before applying onto gardens or for green belt development. Otherwise, high amounts of residual chlorine may damage the plants. Also, disinfection with any disinfectant does not remove organisms, it only inactivates them.
The effect depends on the amount of disinfectant used, the quality of filtration applied, the retention time available for oxidation and the existence or nonexistence of other competing reaction partners (scavenging). As MBR acts like a physical barrier, it completely removes bacteria and viruses up to a degree of 4-7 log removal (104 to 107 times reduction), independent of type or life form of organisms. Hence, MBR does not require any other chemical disinfection.
7. As chlorination is not required, MBR does not produce disinfection by-products which are toxic to many living beings.
8. The sludge produced is only 1/5th of conventional systems. It is also highly stable and hence easy to dispose of.
9. MBR treatment does not produce any objectionable odours and hence it is very safe to house near or inside basements of housing or commercial complexes.
10. As the MBR system does not require sludge recirculation system, clarifier, filter feed pumps, filter back wash pumps, chemical dosing pumps, UV sterilisation, etc. it results into low energy consumption when compared to other conventional systems.
11. The treated sewage from MBR has low silt density index (SDI) and can be fed directly into RO plant without any further treatment.
VI. Ultra Filtration:
Ultra filtration (UF) is a very important technology in waste-water treatment. It has many applications in waste-water recycle.
Some of the widely used applications are:
1. Pre-treatment to RO for producing good quality water.
2. Colloidal silica removal.
3. Caustic recovery.
4. Colour removal.
5. Bacteria and virus removal.
6. Oil emulsion waste treatment.
7. Treatment of whey in dairy industries.
8. Electrocoat paint recovery.
9. Concentration of textile sizing.
10. Concentration of gelatine.
UF is mainly used as a pre-treatment to nano filtration (NF) and reverse osmosis (RO) so as to reduce the silt density index (SDI), a parameter important to avoid NF/RO fouling.
Advantages of using UF as pre-treatment to RO are:
1. Increased flux in RO.
2. Reduced cleaning frequencies of RO, thereby reducing the RO downtime and chemical cleaning costs.
3. Minimising cartridge filter consumption. Cartridge filter is used only as safety barrier after UF to take care of any possible contamination resulting from chemical dosing.
4. Increases the life of RO membranes.
UF also selectively rejects some sparingly soluble dyes such as indigo. But many soluble dyes may pass through the UF system. In some cases, charged UF membranes have been successfully employed to remove soluble dyes.
VII. Nanofiltration (NF):
NF has a pore size much smaller than UF and hence can reject many colour-causing elements. NF can very effectively separate dyes and concentrate them too. This way of concentration and purification reduces the loss for dyes. Also, when dyes are removed from the concentrated salt solution, it can be reused in the process thereby reducing the pollution load and also saving water and salt.
In general, it may be stated that this separation technology which depends both on size separation as well as the charges on the membranes can be economically applied to separate organics and also to separate higher valency cations or anions and associated salts from monovalent salt. Thus, softening of any aqueous stream is possible by separating out Ca++ or Mg++ or SO4– –, CO3– – from Na+, CI–, etc.
VIII. Reverse Osmosis (RO):
Reverse osmosis is increasingly used in recycle of waste-water for producing good quality water for reuse in the process. There is good improvement in membrane element design in terms of reducing fouling potential and increasing the life of membrane. Low cost of membranes is making its application a viable option.
RO is a membrane technology used for separation of salts from waste-water so as to make it reusable in the process.
RO is more useful in separating salts and organic compounds from textile effluent which are pre-treated for removal of suspended/colloidal matter and certain pollutants which are likely to foul the RO membrane systems. As textile effluents contain high amount of dissolved salts, RO is a suitable technology for separation of these salts and for producing permeate which can be used in the process.
IX. Eletrodialysis (ED):
Eletrodialysis is an electro-membrane process in which the ions are transported through a membrane from one solution to another under the influence of an electrical potential. ED can be utilised to perform several general types of separations such as separation and concentration of salts, acids and bases from aqueous solutions or the separation and concentration of monovalent ions from multiple charged components or the separation of ionic compounds from uncharged molecules. ED membranes are usually made of cross-linked sulphonated polystyrene. Anion membranes can be of cross-linked polystyrene containing quaternary ammonium groups. Usually, ED membranes are fabricated as flat sheets containing about 30-50 per cent water. Membranes are fabricated by applying cation and anion-selective polymers to a fabric material.
The system consists of two kinds of membranes – cation and anion, which are placed in an electric field. The cation-selective membrane permits only the cations, and anion-selective membrane only the anions. The transport of ions across the membranes results in ion depletion in some cells, and ion concentration in alternate ones.
Electrodialysis is used widely for production of potable water from sea or brackish water, production of ultra-pure water, recovery of organic acids from salts, deacidification of fruit juices, heavy metal recovery, acid recovery from etching baths, pickling liquors, etc. treatment of plating waste-waters, demineralisation of whey, soya sauce, sugar, fruit juice and organic acid.
ED is an economical process only when used on brackish water, and tends to be most economical at TDS levels of up to 5000 mg/l. But, improved designs in ED makes it suitable for sea water desalination also. There are claims that the power consumption for ED in sea water desalination can be as low as 5 kWh/m3 which is comparable to RO.
X. Plate and Frame RO System:
Plate and frame RO is another technology in reverse osmosis. Although the membrane is same, it offers some advantages because of its construction.
The plate and frame RO technology is designed to treat waste that is higher in dissolved solids content, turbidity, and contaminant levels than waste treated by conventional membrane separation processes. The membrane modules in this design feature larger feed flow channels and a higher feed flow velocity than other membrane separation systems. These characteristics allow the plate and frame RO greater tolerance for dissolved solids and turbidity and a greater resistance to fouling and scaling of the membranes. Suspended particulates are readily flushed away from the membrane during operation.
The high flow velocity, short feed water path across each membrane, and the circuitous flow path create turbulent mixing to reduce boundary layer effects and minimise membrane fouling and scaling. This design also allows easy cleaning and maintenance of the membranes. This technology can use reverse osmosis, ultra filtration, or micro filtration membrane materials. These membranes are more permeable to water than to contaminants or impurities.
Water in the feed is forced through these membranes by pressure. The permeate thus consists of a larger fraction of water with a lower concentration of contaminants. The impurities are selectively rejected by the membranes and are thus concentrated in the smaller fraction of the concentrate left behind. The percentage of water that passes through the membranes is a function of the operating pressure, membrane type, and concentration of the contaminants.
The Plate and Frame RO design has following advantages over conventional spiral RO:
1. It can tolerate higher feed pressures (up to 140 bar) and hence can handle higher dissolved salt concentrations in the feed water.
2. It has higher fouling resistance because of wide feed channels and higher flow velocities.
3. It can be easily maintained. If required, the module can be opened for cleaning.
4. It requires less pre-treatment units compared to spiral RO design.
XI. Photo Chemical Oxidation (PCO):
There are several compounds in the effluent streams that are difficult to treat with conventional treatment processes. Furthermore, many of these compounds find their way to ground water sources thereby polluting them. These compounds need to be reduced to levels so as to meet the environmental standards. There are many upcoming technologies that would be effective in the treatment of these complex compounds.
‘Advanced oxidation processes’ is one such technology that is capable of treating complex organic and refractory compounds by bringing about the oxidation of these compounds. ‘Photochemical oxidation’ and ‘photocatalytic oxidation’ are some of the advanced oxidation processes.
In the photochemical oxidation process, photons of appropriate energy levels from a source of light (UV or solar radiation) interact with oxidants such as hydrogen peroxide, ozone, etc. to generate free hydroxyl radicals that are used to oxidise the wastes directly into carbon dioxide and water without the generation of sludge.
In the photocatalytic oxidation process a catalyst, usually a semiconductor (like titanium dioxide) in aqueous solution is used to generate free hydroxyl radicals by interaction of photons of appropriate energy levels from a source of light (UV or solar radiation) with the catalyst.
Photochemical oxidation has been found to degrade a wide number of refractory compounds such as chlorinated hydrocarbons, aromatic compounds, etc. Similarly, photocatalytic oxidation has been able to degrade compounds such as poly-chlorobiphenyls, which cannot be degraded by conventional treatment methods. Major advantages of photochemical and photocatalytic oxidation processes are that they can be carried out at room temperature and atmospheric pressure.
XII. Ion Exchange Process:
Ion exchange is a proven technology in terms of removal of dissolved salts. Ion exchange is also used as pre-treatment to RO for removal of hardness, alkalinity, heavy metals, etc. Some special applications for treatment of waste-water using ion exchange process include removal of fluorides, nitrates, iron, arsenic, etc.
Ion exchange process is also used to recover metals from waste-water streams. Many applications are in use for recovery of valuable products from waste-water.
Some of such applications include:
1. Nickel or chromium recovery from electroplating rinse waters.
2. Zinc recovery from waste-water of rayon industry.
3. Copper recovery.
4. Mercury recovery from chlor alkali waste-water.
Thus, waste-water recycle is mandatory for many industries because of water scarcity, legislation, rising water costs, unreliable water supplies, environmental requirements from buyers in case of exporters, etc. Apart from these reasons, industries now identify recycling as their social responsibility for environmental friendly manufacturing of goods.
Many technologies are now available for managing industrial waste-water. It is of utmost importance to involve water management specialist’s right from the planning stage of the project so that the best optimum solutions can be developed. Priority should always be given to source reduction and product recovery rather than end-of-pipe waste-water treatment. Best technologies should be adopted for recovery and recycle of water from waste-water. Final effluents which cannot be recycled should be treated and disposed of in an environment friendly way.