Safe Disposal of Wastewater – Disinfection

2.1.4 Disinfection

Disinfection is the primary mechanism for the inactivation or devastation of infective beings to forestall the spread of waterborne disease to downstream users and the environment. ( Municipal Technology Branch U.S. EPA, 1999 )

Chlorination is the most common method of disinfection. Chlorine is effectual at killing most bacteriums, viruses and Protozoa that rise hazards to human wellness. The reactions of Cl in effluent are shown as below:

Chlorine2+ H2O > HOCl + H-Cl

Chlorine Hypochlorous

Gas Gas

HOCl + OCl + H



In chlorination intervention, Cl must be removed before treated H2O is released due to chlorine is itself toxic to marine life. Sodium bisulfite is added to the treated H2O to take any Cl residue. ( Patrick, n.d. )

Besides, ultraviolet light disinfection is a physical procedure that transportations electromagnetic radiation with a wavelength of about 4 to 400 nanometers from a quicksilver discharge lamp to an being ‘s familial stuff ( DNA and RNA ) . When UV radiation, generated by an electrical discharge through quicksilver vapour, penetrates the familial stuff of micro-organisms and idiots their ability to reproduce. ( Anon. , 1999 ) UV disinfect at wavelength of about 254nm is most efficient for disinfection intent. This procedure adds nil to the H2O but UV light, therefore, there is no residuary consequence that can be harmful to worlds or aquatic life. In add-on, UV is an effectual germicide for chlorine-resistant Protozoa like Cryptosporidium and Giardia. However, UV disinfection is non every bit cost-efficient as chlorination.

Recently, ozone has begun to acquire attending as a effluent disinfection method. Ozone is an unstable gas comprising of three O atoms, the gas will readily degrade back to O, and during this passage a free O atom or free group is formed. Most wastewater intervention workss generate ozone by enforcing a high electromotive force jumping current ( 6 to 20 kVs ) across a dielectric discharge spread that contains an oxygen-bearing gas. Ozone is a really strong oxidizer and viricide. It has greater disinfection effectivity in taking virus and bacteriums present in H2O due to direct oxidization or devastation of the cell wall with escape of cellular components outside of the cell, reactions with extremist byproducts of ozone decomposition and harm to the components of the nucleic acids. ( Anon. , 1999 ) Ozone is besides known to cut down the concentration of Fe, S and manganese and extinguish olfactory property and gustatory sensation jobs. Ozone oxides the manganese, sulfur and Fe into indissoluble metal oxides. These indissoluble atoms are subsequently removed by filtration. Furthermore, ozone increases the O content of H2O, a good side-effect of disinfection.

2.1.5 Third Treatment

After biochemical debasement of the sewerage in the secondary intervention, the clarified wastewater is farther treated to take non-biodegradable toxic organic pollutants, disable disease doing beings and viruses, and other man-made pollutants. ( Anon. , n.d. ) After this intervention, the waste H2O becomes relatively safer and can be discharged into the environment. There are three chief third intervention procedures, which are filtration, lagooning, and alimentary remotion.

In the filtration procedure, sand or activated C are used to filtrate the effluent. The H2O is made to go through through a bed of sand activated C, so that the particulate affair in the H2O adheres to the filter medium and gets removed from the H2O. Sand filtration removes much of the residuary suspended affair whereas filtration over activated C removes residuary toxins. ( Anon. , n.d. )

Lagooning provides colony and farther biological betterment through storage in big semisynthetic pools or lagunas. These lagunas are extremely aerophilic and colonisation by native macrophytes, particularly reeds, is frequently encouraged. Small filter feeding invertebrates such as Daphnia and species of Rotifera greatly assist in intervention by consuming the biodegradable atoms in the sewerage H2O ( taking all right particulates ) .

Removal of foods nitrogen and P is necessary as their inordinate release to the environment can take to a buildup of foods, called eutrophication that in a manner lead to the giantism of weeds, algae, and blue-green algaes ( bluish green algae ) . This lead to a phenomenon called Algae Bloom, where there is a rapid growing in the population of unsustainable algae which finally dies. As the bacterium works on the decomposition of these algae, they use up excessively much of O which may take to the decease of most of the aquatic biology which creates more organic affair for the bacteriums to break up. In add-on to doing deoxygenation, some algal species produce toxins that contaminate imbibing H2O supplies. Therefore, remotion of foods is indispensable measure in third effluent intervention. Phosphate remotion is achieved by precipitation as Ca phosphate. Besides, N is removed through the biological oxidization of N from ammonium hydroxide to nitrate ( nitrification ) , followed by the decrease of nitrate to nitrogen gas ( denitrification ) . Nitrogen gas is released in the ambiance and therefore removed from the H2O. ( Anon. , n.d. )

2.1.6 Sludge Treatment

Sludge is produced from the intervention of effluent in on-site ( infected armored combat vehicle ) and off-site ( activated sludge ) systems. The sludge are likely to incorporate microorganisms which may lend to the transmittal of diseases, every bit good as organic and inorganic contaminations which may be risky or toxic to worlds or have damaging effects on the environment in general. Therefore, all sludge must be treated before recycling or disposal and the grade of intervention depends on the intended concluding usage. The intervention procedure may affect thickener, dewatering, digestion, composting, and concluding disposal. Sludge Thickening

Thickening of sludge increases its solids content and reduces the volume of free H2O thereby minimising the unit burden on downstream procedures such as digestion and dewatering. The most normally used inspissating procedures include gravitation thickener, dissolved air floatation and extractor thickener. Gravity thickener is normally carried out in a round armored combat vehicle where the sludge is fed to the armored combat vehicle through a centre provender good. The provender sludge is allowed to settle and the thickened sludge is withdrawn from the underside of the armored combat vehicle. Conventional sludge roll uping mechanisms with deep trusses or perpendicular lookouts are used to stir the sludge gently thereby easing the release of H2O from the sludge. The supernatant flow is returned either to the primary subsiding armored combat vehicles or to the influent of the intervention works, whereas the thickened sludge is pumped to digesters or dewatering installations. Typically, sludge can be thickened to approximately 2 to 3 % for solids derived from activated sludge or RBC systems, whilst for primary sludge and dribbling filter sludge, they can be thickened to approximately 4 to 6 per centum solids concentration. ( Alturkmani, 2012 )

Flotation thickener is a solid-liquid separation procedure. Separation is unnaturally induced by presenting all right gas bubbles into the floatation procedure system. The gas bubbles become affiliated to the solid particulates, organizing a gas-solid sum with an overall majority denseness less than the denseness of the liquid. Therefore, these sums float on the surface of the fluid. Once the solid atoms have been floated to the surface, they can be collected by a skimming operation. ( Lawrence K Wang, n.d. )

Centrifuges are a compact, simple, flexible, self-contained unit. They have the disadvantages of high capitals, care and power costs and frequently a hapless, solids-capture efficiency if chemicals are non used for bio sludge. However, they have extra advantage of less infinite demand, less odour potency and housework demand. ( Anon. , n.d. ) Centrifugal thickener is acceleration of deposit through the usage of centrifugal force. Centrifuges are normally used for inspissating waste activated sludge. Primary sludge is usually non fed to centrifugate as it may incorporate scratchy stuff. Stabilzsation

Stabilization is the term used to denote the procedure of biochemical O demand ( BOD ) decrease. The stabilisation procedure can be carried out under aerobic or anaerobiotic conditions. In anaerobiotic system, the sludge is passed through a well-mixed digester at 35°C. The procedure is uninterrupted and takes 15-20 yearss. The organic stuff interruptions down in the digester to bring forth methane gas and C dioxide. The gas is burned to heat the digester or in really big workss to bring forth electricity through a combined heat and power works. Digested sludge has a low smell and H2O is easy removed to cut down volume. The procedure kills potentially harmful bacteriums and can be enhanced by usage of high force per unit area or sonic systems which destroy bacteriums cells. ( Anon. , n.d. )

Aerobic stabilisation is carry out in an aeration armored combat vehicle with the nowadays of O which is same manual as in an activated sludge procedure. Due to the high O demand, this procedure is energy intensive and high costs. In aerophilic digestion, sludge stabilisation is achieved when aerophilic and facultative micro-organisms convert biodegradable organic affair in an environment where aeration is provided. End merchandises of the digestion are chiefly C dioxide, H2O and non-biodegradable stuffs. Sludge Conditioning

Sludge conditioning is a procedure that sludge solids are treated with chemicals to fix the sludge for dewatering procedures. Chemical conditioning prepares the sludge for better economical intervention with vacuity filters or extractors. Sulfuric acid, alum, chlorinated copperas, ferric sulphate, and ferrous chloride with or without calcium hydroxide are common chemicals that used in this procedure. The intent of adding chemicals to the sludge is to take down or increase its pH value to a point where little atoms coagulate into larger 1s and the H2O in the sludge solids is given up most readily.

Thermal conditioning has two rudimentss which are wet air oxidization and heat intervention. Wet air oxidization play a function in reduces the sludge to an ash whereas heat intervention improves the dewaterability of the sludge. This procedure produces a more readily dewaterable sludge and besides provides effectual disinfection of the sludge.

Blending is a procedure where two or more types of sludge are “blended” together to ease a higher sludge solids concentration and a more homogeneous mixture of sludge prior to dewatering. Blending operations tends to diminish the chemical demand for conditioning and dewatering sludge. The blending operation normally takes topographic point in sludge keeping armored combat vehicles usually where primary sludge is assorted with waste activated sludge. ( Anon. , n.d. ) Dewatering

Sludge dewatering is a procedure that taking H2O from sludge. The most common manner to dewater sludge is to physically squash the H2O out of the sludge. The usual dewatering method are pressure filtration dewatering, belt imperativeness dewatering filtration, air sludge drying procedures, sludge dewatering centrifugation and vacuity filtration. Furthermore, a sludge desiccant can be utilized at the terminal of the procedure. Desiccants are oven like equipment that really bakes out the H2O. Volume Decrease

Sludge incineration is the thermic devastation of sludge by oxidization at high temperature, this procedure besides known as burning. The gases from burning must be kept at temperature of 677?C to 760?C until they are wholly burned.

Wet air oxidization is one of the methods for the intervention of aqueous effluents. In wet air oxidization aqueous waste is oxidized in the liquid stage at high temperatures of 400K to 573 K and force per unit areas of 0.5MPa to 20 MPa in the presence of an oxygen-containing gas. ( Anon. , n.d. ) Sludge Disposal

2.1.7 Safe Disposal of Wastewater

Wastewater intervention workss must dispose of the remainders ensuing from effluent intervention.

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