Engineered Systems for Water Purification.pdf

ENGINEERED SYSTEMS FOR WATER PURIFICATION ▪ ▪ ▪ ▪ Many aquifers and isolated waters are of high-water quality and may be pumped from the supply and transmission network directly to any number of end uses, including human consumption, irrigation, industrial processes, or fire control. However, there...

ENGINEERED SYSTEMS FOR WATER PURIFICATION ▪ ▪ ▪ ▪ Many aquifers and isolated waters are of high-water quality and may be pumped from the supply and transmission network directly to any number of end uses, including human consumption, irrigation, industrial processes, or fire control. However, there are regions which cannot be directly supplied, particularly in regions with dense populations or regions that are heavily agricultural. Here, the water supply must receive varying degrees of treatment prior to distribution. A typical water treatment plant is made up of a series of reactors or unit operations, with water flowing from one to the next. When stacked in series, achieve a desired end product. Each operation is designed to perform a specific function and the order of these operations is important. 1) Coagulation and Flocculation • Raw surface water entering a water treatment facility usually has significant turbidity caused by tiny colloidal clay and silt particles. These particles have a natural electrostatic charge that keeps them continually in motion and prevents them from colliding and sticking together. Engineered Systems for Water Purification│ 1 Prepared by: Engr. MAAbellera • Chemicals such as alum (aluminum sulfate) are added to water (stage 1), first to neutralize the charge on the particles and then to aid in making the tiny particles “sticky” so they can coalesce and form large particles (stage 2). • The purpose is to clear the water of suspended colloidal solids by building large particles from the stable colloidal solids so that these large and heavier particles could be readily settled out of water. • Coagulation – the chemical alteration of the colloidal particles to make them stick together forming large particles called flocs. It is the destabilization of the colloids. Two mechanisms are thought to be involved in coagulation. Two mechanisms are thought to be involved in coagulation. a) Charge neutralization ‒ the aluminum ions are used to counter the charges on the colloidal particles. The colloidal particles in natural waters are commonly negatively charged and when suspended in water, repel each other due to their like charges. ‒ Aluminum ions in aluminum sulfate are positively charged and when these are drawn to the negatively charged particles, they compress the negative charge on the particles, making them less stable in terms of their charges. They form then larger particles. b) Interparticle bridging ‒ this involves the sticking together of the colloidal particles by virtue of the macromolecules formed by the aluminum hydroxides ‒ The polymers bridge the gap between adjacent particles, thereby creating larger particles. ‒ In addition, because aluminum hydroxide is soluble in low pH, there is a need to raise pH and that is done by addition of calcium hydroxide. ‒ Some of the calcium precipitates as calcium carbonate which assist also in settling. • Flocculation – the physical combination of the coagulated particles forming flocs 2) Sedimentation or Settling • Flocs formed must be separated from the water. This is done in gravity settling tank that simply allow the heavier – than – water to settle at the bottom. • The sludge in water treatment plants is aluminum hydroxides, calcium carbonates, and clays, is not highly biodegradable and will not decompose at the bottom of the tank. • Typically, sludge is removed every few weeks through a valve at the bottom and is wasted either into sewer or into a sludge holding/ drying pond. Engineered Systems for Water Purification│ 2 Prepared by: Engr. MAAbellera • Settling tanks work because the density of the solids exceeds that of the liquid. The movement of a solid particle through a fluid under the pull of gravity is governed by a number of variables such as particle size, particle shape, particle density, fluid density and fluid viscosity. Important calculations: a. Overflow rate (surface loading) 𝑚3 𝑄( 𝑠 ) 𝐻 𝑣𝑜 = = (𝑐𝑟𝑖𝑡𝑖𝑐𝑎𝑙 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒 𝑠𝑒𝑡𝑡𝑙𝑖𝑛𝑔 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦) 𝐴 (𝑚2 ) 𝑡 Velocity of the particles (vp) must overcome vo for it to settle. b. Residence time (retention time) 𝑡= 𝑉 (𝑚3 ) =𝑠 𝑚3 𝑄( 𝑠 ) 3) Filtration • Environmental engineers have learned to apply natural purification of water through its movement to soil and sand. They have developed what is now known as the rapid sand filter. The operation of this process involves two phases, filtration and backwashing. • The suspended solids that escape the flocculation and settling processes are caught on the filter sand particles and eventually the rapid sand filter becomes clogged and must be cleaned through a process called backwashing BACKWASHING ‒ The suspended solids trapped within the filter are released and escape with the wash water. ‒ Other filters make use of activated carbon (powder or granules in form) as filtering media in place of sand and gravel. 4) Disinfection • Water is disinfected in order to destroy whatever pathogenic organisms present. The disinfectant commonly used is chlorine. • Chlorine is purchased as liquid but is released to the water as gas using a chlorine feeder system. The presence of a residual of active chlorine in the water is an indication that no further organisms remain to be oxidized and that the water can be assumed to be free of disease – causing organisms. ‒ Usually, 5 mg/L of chlorine is added but residual chlorine must be 2 mg/L to ensure disinfection. Engineered Systems for Water Purification│ 3 Prepared by: Engr. MAAbellera • • ‒ Possible formation of the following compounds makes chlorination quite dangerous: Formation of trihalogen methanes (THM), chlorinated phenols, halogenated methanes, ethanes and ethenes, halogentaed polynuclear aromatic hydrocarbons and chlorinated aldehydes and ketones. Chlorine dioxide can also be used as a disinfectant. It possesses theoretically 25 times greater oxidizing power than chlorine. ‒ Advantages of using this as disinfectant are no deterioration of taste and odor, formation of THM can be neglected, and it does not react with ammonia. Drawbacks include formation of toxic chlorite (ClO2-) and chlorate (ClO3-) which can cause methemoglobinemia in babies just like nitrates if concentration in water exceeds 0.1 mg/L. Ozone is also used as disinfectant. ‒ This is produced from oxygen in pure form or from ionization of clean dry air. ‒ Drawbacks include it being expensive (2-3 times higher than chlorine). Ozone is very reactive and corrosive, thus requiring corrosion-resistant material such as stainless steel. Sample Problems on Sedimentation and Filtration 1. A water treatment plant settling tank has an overflow rate of 600 gal/d.ft2 & a depth of 6 ft. What is the residence time in hrs? 2. A small water plant has a raw water inflow rate of 0.6 m3/s. It was found through experimentation that the particles after flocculation all settle at a rate of va= 0.004 m/s. A proposed rectangular settling tank has an effective settling zone of L=20m, H=3m &W=6m. (a) Can 100% removal be expected? (b) If not, what percent of the particles are removed? 3. A settling tank in water treatment plant has an inflow of 2m3/ min and a solids concentration of 2100mg/L. The effluent from this settling tank goes to sand filters. The concentration of sludge coming out of the bottom of the settling tank (underflow) is 18000mg/L & the flow to the filters is 1.8 m3/ min. (a) What is the underflow flowrate? (b) What is the solids concentration in the effluent (flow to the filters) (c) How large must the sand filters be (m2) if the filter loading is 4 gal/ ft2. min? Engineered Systems for Water Purification│ 4 Prepared by: Engr. MAAbellera