Water is life, water is unique and water is scarce.
Three quarters of our earth is water but only a tiny percent of this quantity is available and fit for our use. The oceans and seas account for 97.2% of the total water. Further 2.15% is locked up as polar ice and glaciers. The different sources of water are outlined in Table 1.
World’s Water Supply Based on V.S. Geological Survey Estimates
|Location||Water volume||% of Total|
|Fresh water lakes||1,20,000.00||00.009|
|Saline lakes & inland seas||1,00,000.00||00.008|
|Average in rivers & streams||1,200.00||00.0001|
|Soil moisture & near surface ground water||64,000.00||00.005|
|Deeper ground water||80,00,000.00||00.61|
|Ice caps & glaciers||2,80,00,000.00||02.15|
Just as living organisms, industry is dependent upon water. It is used as a source of power, a raw material, a heat transfer fluid, a solvent and universal transport medium. The accepted maximum TDS levels for different applications are enumerated in Table 2.
The water salinity spectrum (Annexure I) indicates the salinity in different sources of water.
Water: Quality – Required
|Industrial||10 – 200 ppm|
As can be seen from the spectrum the salinity of seawater is very high and is not suitable for use in applications mentioned in Table 2.
We are therefore left with only a miniscule percent of the total water to meet all our requirements.
Water is becoming an increasingly costly resource and therefore the safe and economical use, reuse and disposal of water is the focus of much environmentally motivated concern.
Water treatment is normally required in the form of pretreatment plant, chemical inhibitors and effluent control to ensure that the appropriate quality of water is available whatever the required end use.
Water – A Unique Molecule
The simple formula of water, H2O gives no indication of its true molecular complexities. The unique properties of water are due to the weak electrostatic forces of
attraction between neighboring water molecules, a phenomenon known as hydrogen bonding. Many compounds such as ammonia, hydrogen fluoride and alcohols also exhibit hydrogen bonding but to a much weaker extent than water.
Table 3 contrasts the properties of water with a number of chemically similar compounds.
|Element/Hydride||MP oC||BP oC||LV Kcal/g|
|Nitrogen/NH3||- 77||- 33.4||327|
|Phosphorus/PH3||- 133||- 88||60|
|Sulphur/ H2S||- 85||- 61||132|
|Fluorine/HF||- 83||+ 19.5||361|
|Chlorine/HCl||- 115||- 85||99|
The relatively high melting point, boiling point and latent heat of vaporization are an indication of the disinclination of the hydrogen bonded molecules to separate. Surface tension, viscosity and heat of fusion values are all high fort he same reason.
The relatively easy availability of water in all the three physical states together with the high latent heat of transformation, particularly vaporization, makes it a convenient and valuable heat transfer medium.
The strong dipole moment, which results from the charge separation and the shape of molecules, gives rise to a high dielectric constant making water a good ionic solvent.
In fact water is often called “A Universal Solvent”.
Water pollution is not a new phenomenon. Water has always been cleaning the environment. In the earlier years it was mainly vegetative matter, organic in nature and in the course of its flow the pollutants were degraded. Today the nature of pollutants has changed and waste water from industries and other varied sources carry with it chemicals that are not easily degraded by nature. There has therefore been a qualitative change along with the quantitative problem. Water, like for living organism, is also the life of industry. There is no economical, non-toxic, ready available alternative to water. The use of water is not as simple as merely piping up to the nearest supply. Impurities in the water cause problems within the plant which result in reduced efficiency, increased maintenance and lost production.
The need for water will always remain, but control and adjustment of water quality to the requirements of the end use, is ever more necessary.
“Waste Water”, always implies water with a load of pollutants. When one looks at water as a process raw material the entire outlook is different. All necessary pretreatments are meticulously taken note of to ensure the right quality and purity of water.
Why then can we not look at all available water in the same way? If wastewater is minimized, if pollutants in the water are reduced at source and then logically treated we can obtain water of a quality that can be reused rather than disposed.
This then is the objective of this paper rather than describing specific methods to handle specific pollutants, a subject on which volumes are available.
Generally industry uses water in the following areas:
- Steam raising
- Cooling purposes
Every area of its use dictates the quality of water required as also the quality of waste generated.
The critical step in reducing water pollution and wastewater is to conduct an audit. This is not a compliance audit but a fact finding procedure. This implies finding the source and amount of pollutant and periods of discharge. Once a closer look is taken, every area of the plant that produces waste is an area for improvement. Many industries discharge wastewater to publicly owned treatment works while some treat it themselves. The reduced pollutant load and quantity of wastewater therefore directly means reduced cost and often in such cases treated water obtained is of a quality that can be reused.
Let us now look at some areas where we could implement this approach:
i. Raw Material
Raw material often is a major pollutant in/of wastewater. This means not only extra cost in its removal from waste water but also a loss of revenue due to wastage and inventory loss of a costly final product input. Raw material can be lost from leaking pumps and glands, overflowing tanks; discharge of left overs from batch processes etc. Reviewing this area could also result in process optimization leading to lowered operating costs. An exercise that started as pollution prevention could lead to better raw material inventory control.
ii. Steam Raising
To protect the boiler from the detrimental effects of water, apart from demineralising/softening, chemical inhibitors are added. These inhibitors find their way in the effluent through blow down. The regenerant from demineralisers and softeners also all to the effluent. The areas that need to be looked into are:
a) Select a boiler treatment plan that permits operation at the highest cycle of concentration. This will help in reducing boiler blow down, which not only reduces the effluent load but also helps in energy conservation.
b) Use chemical inhibitors that are easily biodegradable and non-toxic.
c) Proper selection of demineralising or softening plant. Improperly designed plants require frequent regeneration increasing the effluent load. Saline water leads to loss of fertility of the land surrounding the discharge area. Therefore plants using large demineralisers/softeners also need to pay attention to this pollutant. This need today has prompted recovery of the salt for reuse with the help of R.O. plants or evaporation or a combination of the two. The necessity has not only prevented pollution but has also resulted into recovery and reuse of a material, which was earlier being merrily dumped. The end result – cost savings.
iii. Cooling Systems
Cooling water system blow down often constitutes a large part of the wastewater stream. But this is also an area where substantial improvements can be implemented. This can be achieved in the ways suggested below:
a) First and foremost, select a treatment regime, which permits operation at the highest possible cycles of concentration. This will straightway result in lower blow down and savings in energy. Consider a system with an evaporation loss of 100 M3/hr. The blow down at different COC is graphically represented in Annexure II.
As can be seen from the graph proper selection of the treatment programme to operate the system at optimum COC will considerably reduce the load on the ETP.
b) Select chemicals that are non toxic. A classic example in this case is the switching of chromate based programmes to non-chromate programmes.
Hexavalent chromium is highly toxic to aquatic life and to meet the stringent discharge limit of 0.1 ppm of Cr (VI) capital intensive chromate removal plant is needed. Today dependence on such chemical and other difficult to degrade products can be done away with as non-toxic inhibitors are available which can be directly discharged in to the effluent without any treatment.
Washings may look an innocuous area but here is one of the most important areas where improvements can be brought about. Under washings are included product washing and floor and equipment washing. Usually it has been observed that product washings are directly discharged to the sewers and not reused. Usable quantities of raw material and finished products can be recovered if the washings are reused. Sometimes minor treatment may be necessary before reuse. Every application needs to be independently studied and then the washing cycles determined. However, in every case where washings are recycled substantial savings can be affected.
Floor and equipment washings also generate pollutants or add chemicals that create problems in the ETP. Many varieties of soaps used contain chelating agents and this may interfere with the removal of metal ions in the waste treatment process. Similarly, rector and equipment cleanings may add to the wastewater, material that needs to be treated before discharge. It may be a good idea to use treated wastewater for floor and equipment washings.
Sewage water is usually easily degraded. Various areas needs to be looked at to reduce the waste of sanitation water to minimize the quantity of sewage water generated. The important thing to consider here is the ways to use or recycle this water. Sewage water is a good nutrient for bacteria and can therefore be mixed with the total effluent to ensure better biological degradation.
In cases of acute water shortage, sewage water in a large plant was treated to finally yield a quality of water which was used as make up to the cooling tower. This involved various steps like filtration, clarification, biological degradation, further clarification and finally through reverse osmosis to the area of reuse.
Water the Final Treatment:-
As mentioned earlier in the paper, in the years gone by the pollutants in the water were easily degradable and often biological degradation was sufficient. The nature of
a pollutant over the years has changed and with increasing amounts of non biodegradable contaminants being added the conventional treatment regime of wastewater may not be enough. The treatment philosophy will need to undergo a change depending on the nature of contaminants biological degradation process will need to be supplemented or replaced with physico chemical treatment systems : an area which involves most of the unit operations of chemical engineering, knowledge base for which is available in house and one need not look outwards.
The Water Salinity Spectrum
|Water Source||Salinity (mg/l)|
|Typical brackish water||3,000|
|Typical potable water||300|
|Industrial process water||30|
|Typical De-ionized water||3|
|High purity, high pressure boiler feed water||0.3|
|Ultra high purity electronics industry water||0.03|