Treatment of Industrial Wastewater
Treatment of Industrial Wastewater
Water is used in the industry for process needs, for cooling, for steam generation, for dust suppression and many other uses. Industrial wastewater is the aqueous discard that results after the water is used for the above purposes. Industrial wastewater is the result of substances other than water having being dissolved or suspended in water.
Treatment of industrial wastewater covers the mechanisms and processes used to treat waters that have been contaminated in some way or other due to use of the water in the above activities. The objective of treatment is to remove the dissolved and suspended substances of the wastewater so that treated water can be safely discharged into the environment or can be recycled back in the same process or can be used in a different process.
Constituents of wastewater
Industrial wastewater contains the following constituents that need proper treatment.
- Suspended solids – These are the contaminants that are visible to the naked eye and can generally be filtered out of the water using common filter paper. Although there is no hard and fast definition, suspended solids tend to be larger than 1 – 2 microns in size. If the water is left to stand without disturbing it, the suspended solids will settle to the bottom of the container over time.
- Dissolved solids – These are contaminants that are not visible to the naked eye and cannot be removed from the water by filtration. The dissolved solids are defined as the materials that are generally smaller than 0.45 microns in size. The dissolved solids are either normally soluble substances or inversely soluble substances. The normally soluble substances are those materials that become more soluble with increasing temperature. The inversely soluble substances normally referred to as ‘hardness’ ions and are generally limited to salts of calcium, magnesium, strontium and barium in water treatment. These materials become less soluble as temperatures increase. These materials form scale on the hot surfaces of boiler or heat exchanger tubes.
- Colloidal solids – These are solids that are not quite small enough to be considered dissolved but not quite large enough to be considered suspended solids. Generally colloidal materials will appear as a haze in the water, and it will not be possible to see distinct particles with the naked eye. Colloidal materials are typically within the size range of ~0.45 to ~2.0 microns. Colloidal solids do not settle out from the water because they are so small that they are greatly affected by their ionic surface charges. A colloidal suspension in water is said to be a stable suspension.
- Colour – It is a type of colloidal suspension. Organic molecules that contribute colour to raw surface water are simply macromolecules that fall into the smaller colloidal size range. In water these macromolecules take on an ionic surface charge that stabilizes them and they cannot settle out.
- Biological oxygen demand (BOD) and chemical oxygen demand (COD) – BOD is a measure of the amount of oxygen that is consumed by bacteria during the decomposition of organic matter. Having a safe BOD level in wastewater is essential to producing quality wastewater. If the BOD level is too high then the water could be at risk for further contamination, interfering with the treatment process and affecting the end product. There are several factors that can contribute to high BOD levels such as nitrates and phosphates present in the wastewater, water temperature, and others. Each factor affects the plant life in the water, such as algae, and in turn also has an effect on the organisms that help decompose water contaminants in the wastewater treatment process. The best quality wastewater treatment will occur in an environment that supports the life of these bacteria while maintaining a controlled population of them so as to not encourage rapid bacterial decomposition, which would create higher BOD levels. Similar to BOD is COD. COD measures the amount of oxygen that is consumed by the water in the decomposition and oxidation processes, specifically the decomposition of organic matter and oxidation of inorganic matter, or chemicals. COD is an application that is usually used for industrial wastewaters.
Wastewater treatment technologies
Technologies for treating industrial wastewater are usually divided into the following four categories namely (i) chemical technologies, (ii) physical technologies, (iii) biological technologies, and (iv) membrane technologies.
Chemical technologies – Major chemical technologies for industrial wastewater are as follows.
- Neutralization – It is the adjustment of alkalinity and acidity of wastewater to a neutral value with pH 7.
- Precipitation – Precipitation is addition of chemicals to wastewater to change the chemical composition of pollutants so that the newly formed compound settles down during sedimentation. Precipitation is usually employed for removal of heavy metals from the wastewater which are normally precipitated as hydroxides. However it is necessary to pre-treat the wastewater to remove the substances which interferes the precipitation of the heavy metals.
- Coagulation – It is the use of chemicals to cause pollutants to agglomerate and subsequently settle out during sedimentation. Coagulation is used for the clarification of wastewater containing colloidal and suspended solids. Silica or polyelectrolyte aids in the formation a rapid settling material. Waste containing emulsified oils can be clarified by coagulation process. The process is very efficient for colour reduction of the waste water but less effective for COD (chemical oxidation demand) reduction.
- Adsorption – Adsorption is use of a chemical which causes certain pollutants to adhere to the surface of that chemical for subsequent removal. Activated carbon or synthetic active surfaces are used for adsorption.
- Ion exchange – Ion exchange process is normally used for the removal of undesirable anions and cations from the wastewater. Cations are exchanged for hydrogen or sodium and anions for hydroxyl ions. Ion exchange resins consist of an organic or inorganic network structure with attached functional groups. Most ion exchange resins used in wastewater treatment are synthetic resins made by the polymerization of organic compounds.
Physical technologies – Major physical technologies for the industrial wastewater are as follows.
- Screening – It is the removal of coarse solids out of wastewater by use of a straining device.
- Clarification and sedimentation – Wastewater clarification and sedimentation is a common and essential process in industrial wastewater treatment plants. Clarifiers consist of tanks or basins which hold wastewater for a period of time, allowing solids or other materials suspended in the water to settle to the bottom.
- Floatation – Floatation is carried out with the help of small air/gas bubbles injected into the wastewater. The air/gas bubbles cause pollutant particles in the wastewater to rise to the surface for subsequent removal. Floatation process is usually employed for oil separation. Free oil is floated to the surface of the tank and then skimmed off.
- Air stripping – It is removal of volatile and semi volatile organic compounds from wastewater by the use of air flow.
Biological technologies – Biological treatment is a more natural wastewater treatment process than other wastewater treatment methods. Microorganisms feed on the complex materials present in the wastewater and turn them into simpler substances, preparing the water for further treatment. The main objective of the biological technologies is to reduce the BOD level. Major biological technologies for the industrial wastewater are as follows.
- Air activated sludge process – It is an aerobic process in which bacteria consume organic matter, nitrogen and oxygen from the wastewater and grow new bacteria. The bacteria are suspended in the aeration tank by the mixing action of the air blown into the wastewater.
- High purity activated sludge process – It is an aerobic process which is similar to air activated sludge process except that in place of air pure oxygen is injected into the wastewater.
- Aerated pond/lagoon process – It is an aerobic process which is similar to air activated sludge process. In this process mechanical aerators are used either to inject air into the wastewater or to cause violent agitation of the waste water and air in order to transfer oxygen to the wastewater.
- Trickling filter process – It is a fixed film aerobic process in which a tank containing media with a high surface to volume ratio is used. Wastewater is discharged at the top of the tank and trickles (percolates) down the media. Bacteria grow on the media utilizing the organic matter and nitrogen from the wastewater. A typical trickling filter process is shown in Fig 1.
- Rotating biological process – It is a fixed film aerobic process similar to the trickling filter process except that the media is supported horizontally across the tank of the wastewater. The media upon which the bacteria grow is continuously rotated so that it is alternately in the waste water and the air.
- Oxidation ditch process – This process is similar to the activated sludge process. Physically an oxygen ditch is ring shaped and is equipped with mechanical aeration devices.
Fig 1 Typical trickling filter process
Membrane technologies – In treatment of industrial wastewater, membrane technologies are becoming increasingly important. With the help of these technologies it is possible to remove particles, colloids and macromolecules, so that wastewater can be disinfected. Membrane technologies are usually classified according to the size range of the separated species.
Major membrane technologies for the industrial wastewater are as follows.
- Reverse osmosis – Reverse osmosis process is used to separate dissolved salts and small organics.
- Nanofiltration – This process is used for selective demineralization of water or concentration of organic solutions. The process is used for separation of antibiotics.
- Ultra filtration – The process is used to separate emulsions, colloids, macromolecules or proteins.
- Micro filtration – The process is used to separate small particles, large colloids and microbial cells.