Municipal Waste Water Treatment
1. Elimination or reduction of colors and odors: Color and odor-producing substances
present in wastewaters are oxidized by chlorine. The oxidizing ability of chlorine is
employed for odor control and color removal in treatment of municipal waste water and many industrial effluents (beet sugar, cannery, dairy, pulp and paper, textiles).
2. Disinfection: Owing to its strong oxidizing capacity, chlorine destroys or inhibits growth of bacteria and alage.
3. BOD reduction: Chlorine accomplishes BOD reduction by oxidation of organic compounds present in wastewaters.
4. Oxidation of metal ions: Metal ions which are in reduced state are oxidized by chlorine (e.g., ferrous to ferric ion and manganous to manganic ions).
5. Destruction of cyanides, phenols and detergents.
6. Control of foams and flies.
7. Aiding scum and grease removal.
Chlorination of wastewater for BOD reduction was practised as long as 1859 in England . A number of studies of BOD reduction by effluent chlorination where it was recognised as such were conducted the 1920s and 1930s. In the USA in the 1920s and 1930s and thereafter the chlorination of Sewage was in use for purposes of protecting water supplies, bathing beaches and shellfish layings situated at the sewer outfalls. Chlorination was reported to reduce Biochemical Oxygen Demand of sewage as well as to control odors . Applications of chlorine in wastewater collection, treatment and disposal are reported in several publications . In spite of the use of chlorine as disinfacting chemical today, renewed interests have been shown in using chlorine for treating textile dyeing-finishing wastewaters through in-situ generation of chlorine by electrolysis of a chlorine bearing salt in the wastewater. Major achivements reported are: color destruction and significant reduction of COD and BOD. Creative Environmental Consultants have offered technology to treat dyebath effluents with chlorine gas
Chlorine is a well-known oxidant and has long been used to purify water, destroy organisms in wastewater and swimming pools and oxidize chemicals in wastewater. The destruction of cyanide and phenols by chlorine oxidation is well-known in waste-treatment technology . US Patent No. 3,910,999 describes the stepwise chlorine oxidation of mixture of compounds present in industrial wastes, such as glycols, chloroalcohols, organic acids and ketenes by controlling the pH in the alkaline range initially to oxidize those most favorably destroyed by hypochlorite ion, then allowing the pH to drop to the acid range to destroy those oxidized more rapidly at low pH .
Chlorine generally reacts in a prescribed order, first with inorganic reducing compounds. If enough chlorine is added to react with these substances, then the addition of more chlorine will result in reactions of chlorine with the organic matter that is present. These chlororgano compounds have little or no disinfecting action. Again, if enough chlorine is added to react with all the reducing compounds and all the organic matter, then the addition of little more chlorine will react with ammonia or other nitrogenous compounds to produce chloramines or other combined forms of chlorine. The continued addition will result in the destruction of the chloramines and organic compounds and the formation of free chlorine
Though all three species, Cl2, HOCl and OCl-, will readily oxidize organic compounds, HOCl is the most reactive for its higher redox potential (3). The reactivity of HOCl is based on the electrophollic nature of the molecule at either the oxygen or chlorine atom. Reactions occurring at the oxygen atom produce a chloride ion by displacement in reactions with organic compounds, the chlorine atom may become electrophillic and may combine with an electron pair in the substrate. This behaviour accounts for the reactions of HOCl with ammonia and amines, with phenols and other aromatic substances and in the formation of chloroform from organic substrate Chemical Oxidation of BOD and COD (2):
The overall reaction for oxidation of organic molecules comprising BOD, for example, with chlorine, ozone and hydrogen peroxide, can be represented as follows:
Organic molecule (e.g. BOD),Cl2,Intermediate,oxygenated,molecules,Cl2 Simple end products (e.g.CO2, H2O, etc) Multiple arrows in the direction of the reaction are used to signify that a number of steps are involved in the overall reaction sequences. The overall reaction rates are reported to be slow. Typical dosages of chlorine for the oxidation of organic in wastewater are reported in the range 1.75 to 2.0 kg/kg BOD destroyed. Chlorine oxidation can be used to oxidize nonbiodegrable organic compounds also. Chlorination of wastewater for BOD reduction was practiced as long as 1859 in England with wastewater putrefaction reported to be delayed by the addition of chlorinated lime (4). This in effect was chlorination for BOD reduction (10). A number of studies of BOD reduction by effluent chlorination where it was recognized as such were conducted in the 1920’s and 1930’s. These studies indicated that chlorination effects a reduction in the 5-day BOD value that increases with increasing chlorine dose, with the optimum dose being that produces a slight residual chlorine after ten minute contact time .
The reactions with chlorine mentioned here above are well known and can be found in books and journal publications . Decolorization (Bleaching) step involves the following reaction with hypochlorous acid:
dye (colored) + HOCl_ HCl + (dye clolorless + O) ......... (1)
Reaction of chlorine with hydrogen sulfide:
H2S (g) + Cl2 (g) _ 2 HCl (g) + S (s) ........... (2)
Many metals salts and alkalies react with chlorine; with alkali solution, hypochlorite and chloride of the metal are formed for example:
Cl2 (g) + 2NaOH (aq) _ NaOCl (aq) + NaCl (aq) + H2O (l) ......... (3)
Zn(s) + Cl2(g) _ ZnCl2(s) ......... (4)
2FeCl2(aq) + Cl2(g) _ 2FeCl3(aq) ......... (5)
Reaction with cyanides (1,2,3):
2CN– + 8OH– + 5Cl2 2CO2 (g) + N2 (g) + 10Cl– + 4H2O ......... (6)
Reaction with phenols (1, 4, 5) C6H5 + Cl2 _ C6H5Cl+ HCl ......... (7)
Chemical Oxidation of BOD and COD (6): The overall reaction for oxidation of organic molecules comprising BOD, for example, with chlorine, ozone and hydrogen peroxide, can be represented as follows: Organic molecule (e.g. BOD) Cl2,Intermediate,oxygenated molecules, Simple end products (e.g.CO2, H2O, etc) .......
Multiple arrows in the direction of the reaction are used to signify that a number of steps are involved in the overall reaction sequences. The overall reaction rates are reported to be slow. The reaction for COD reduction is same as reaction From reactions TDS of the effluent are likely to decrease.