Texas Waste Water Treatment Ch#12 Chlorination and Disinfection

In general, methods of disinfection can be sub-divided into two categories, chemical and physical. Chemical methods include chlorine and other strong oxidizing agents, such as ozone, chlorine dioxide, and bromine. Physical methods include ultraviolet radiation and irradiation with x-rays (also known as E-Beam). Heating water to boiling for small quantity treatment, such as the “boil water” notices for household treatment of drinking water, is also a physical method of disinfection.

The effectiveness of any disinfection program is measured by the use of indicator bacteria, usually fecal coliform. These organisms are excreted by all warm-blooded animals, are present in sanitary wastewater in high numbers, tend to survive in the natural environment as long or longer than many pathogenic bacteria, and are easy to detect and quantify. Effective disinfection will destroy fecal coliform as well as disease-causing organisms. If fecal coliform remains, then effective disinfection has not taken place. Chlorine (a pure element used frequently in waste water treatment for disinfection) refers to a halogen element of the periodic table. Chloride is formed from chlorine. It can either mean any compound with chlorine in it, or specifically a salt of hydrochloric acid consisting of two elements, one of which is chlorine. An example of a chloride is sodium chloride (sodium chloride = NaCl = table salt). An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. In everyday life, iodide is most commonly encountered as a component of iodized salt, which many governments mandate. While iodine can be used for disinfection the answer choice given is specifically iodide. Ammonia in wastewater refers to nitrogen in the form of free ammonia and ionic ammonium, mainly from the decomposition of nitrogen-containing organic matter in domestic sewage, coking, ammonia synthesis and other industrial wastewater, as well as farmland drainage. This is a normal constituent of waste water and is not used for disinfection.

Chlorine is available in three forms: • Pure chlorine (liquid or gas) • Granular and tablet chlorine (65% chlorine) • Chlorine solutions such as bleach (5–15% chlorine)

Pure chlorine in liquid and/or gas form IS kept in cylinders, Calcium oxide or hydroxide (Lime) is the solidifying agent used to make granular or tablet form chlorine which typically consists of about 65% chlorine and 35% lime, sodium hypochlorite is the liquid form diluted bleach solution you generally get from the store NaOCl and is typically only about 5-15% chlorine.

When chlorine is added to water, it forms a mixture of hydrochloric acid (HCl) and hypochlorous acid (HOCl). Hydrochloric acid is not a disinfectant. The hypochlorous acid compound ionizes into H+ and OCl. The degree of ionization depends on the pH. Cl2 + H2O  HOCl (Hypochlorous) + HCl (Hydrochloric) Hypochlorous acid is extremely effective as a disinfectant, but the hypochlorite ion (OCl) is not. At a pH of 4 or 5 the HOCl concentration is almost 100%. As the pH increases, the percent HOCl decreases, thus decreasing its disinfecting capabilities. At a pH of 7.0 the HOCl concentration is about 70%–80% (Figure 12.2). Most wastewater effluents fall in this range. NOTE: the book mentions nothing about CaCl2 (calcium chloride) being formed when adding gaseous chlorine to water and per the snippet i started with the answer should be HCL AND HOCL but that is not the answer given and is not apparently going to be the answer you see on the test for your license....

At a pH of 4 or 5 the HOCl concentration is almost 100%. As the pH increases, the percent HOCl decreases, thus decreasing its disinfecting capabilities. At a pH of 7.0 the HOCl concentration is about 70%–80% (Figure 12.2). Most wastewater effluents fall in this range.

The reaction between chlorine and ammonia can be written as NH 3 +HOCl -> NH 2Cl + H 2O. In this chemical equation NH 3 is ammonia nd HOCl is hypochlorous acid which is formed when the chlorine first dissolved in the water. The primary result of this chemical reaction is NH 2Cl, a chemical know as chloramine.

***** this is another question TEEX seems to have gotten wrong. the answer per the key is "Sufficient AND a monochloramine" however the chapter states "Laboratory experiments and actual plant experience show that disinfection is accomplished when enough chlorine is applied to give a 1.0 mg/L residual after a 20 minute contact time. In addition, these specific concentration and time (CT) limits are required by the TCEQ" and the terms dosage and demand refer to the total amount of chlorine applied and used. Residual = Dosage - Demand = (12-11.5)mg/L = 0.5 mg/L so it should NOT be sufficient unless the question states that the retention time is >20min. on the subject of monochloramines it says the following "Chlorine reactions occur in stages, depending on the material in the water (Figure 12.3). Stage 1 No residual is formed because inorganic demand destroys the chlorine. Stage 2 Monochloramines are formed as chlorine combines with organics containing ammonia (rising curve). The chlorine to ammonia weight ratio at this point is 5:1. As combined and total residual increases, chloro-organics are formed. Stage 3 Increasing chlorine dosage (declining curve) to a chlorine/ammonia weight ratio of 7.6:1 destroys chloramines and chloro-organics. Combined chlorine decreases and free chlorine increases. Stage 4 Enough chlorine is added to reduce all demand. The combined residual remains the same, but free residual increases with dosage. This stage is called breakpoint chlorination. The chlorine to ammonia weight ratio of 10:1 satisfies all demand." this means that while you might find residual monochloramines in your residual they would make up the smallest portion of the combined residual. in short it seems the answer SHOULD be 0.5mg/L however as stated before the answer on the test is going to be what you see here.

Chlorine may be used for disinfection, Biochemical Oxygen Demand(BOD) reduction, grease removal, filter fly and ponding control in trickling filters, sludge bulking treatment, sludge thickening, and odor control.

Laboratory experiments and actual plant experience show that disinfection is accomplished when enough chlorine is applied to give a 1.0 mg/L residual after a 20 minute contact time. In addition, these specific concentration and time (CT) limits are required by the TCEQ. NOTE: the 20 minute contact time is based on peak flow through the treatment process so a longer contact time with a lower residual can still be acceptable.

Chlorine may be useful in trickling filter operations in controlling both psychoda (filter flies) and ponding. No control method for flies has been entirely satisfactory, but chlorine is used with some success, especially on standard rate filters. The trickling filter is an ideal place for flies. Eggs are laid on the media surface and in the zoogleal mass. They hatch into larvae, then in a few days develop into pupae. After a day or two the pupae shell bursts and the fly emerges. The life cycle of the filter fly is 10–12 days during the summer. The best time to control psychoda by chlorination is in the larvae and pupae stages. These are most abundant 3–12 in. below the filter surface. In fly control, chlorine should be added during night flows when a minimum amount is required to obtain a combined chlorine residual. Chlorine is applied to the filter influent one night in every 10, one night a week, or at a frequency determined by plant experience. Ponding results from an excess growth of zoogleal mass on the filter. Chlorine will kill the microorganisms and cause the mass to slough off and wash through the filter. Generally, chlorine is applied to the filter influent to produce a chlorine residual of from 2 to 10 mg/L at the spray nozzles. The lower residual value allows continuous application for several hours. The higher value is used as a shock treatment for only a short time, usually at night when the chlorine demand is low.

The following is taken from 30 TAC §217.281(b) Disinfection Contact Basins: (1) A chlorine contact basin must provide a minimum chlorine contact time of 20 minutes at the peak flow.

All potential chlorine leaks should be located by using a 10% ammonium hydroxide vapor, not by smelling the chlorine. (Ammonia vapors react with chlorine to form a white ammonium chloride smoke.)

Chlorine Installation Features: • Chlorinators should be located near point of application. >>Application should be continuous.<< • If possible, keep chlorinators in a separate room above the ground. • Maintain ample work space around equipment and storage space for spare parts. • It is necessary to have an ample supply of water under pressure of at least 15 psi and three times back pressure at point of treatment. • Booster pumps may be necessary. In general, a minimum of 40–50 gal./day is required per pound of chlorinator capacity. >>Capacity should be sufficient to add the highest expected dosage at any time.<< • Water supply failure means chlorinator failure. The supply must be clear enough not to clog strainers. Minimum water pressure should be 10–15 lb. • Safe vacuum system gas withdrawal rate for 150 lb. cylinders is 1 lb./°F/day. • Safe gas withdrawal rate for ton containers is 8 lb./°F/day. • For pressure systems, the safe withdrawal rate is 42 lb./day (150 lb. cylinders) and 360 lb./day (ton containers). • Adequate lighting is required. • Continuous ventilation is desirable to remove leaking chlorine gas. Forced ventilation must be provided to remove gas if a large leak develops. The forced ventilation system inlet must be near the floor. • Convenient handling and storage of chlorine containers is important. It is dangerous and costly to move cylinders up and down stairs and through narrow doors or passageways. • Adequate measuring and controlling of chlorine dosage must be practiced. Scales and loss-in-weight recorders are desirable as a continuous check on the continuity of chlorination and as a record. >>Feed control should be automatic.<

Ultraviolet (UV) radiation is sandwiched between visible light and x-rays. UV disinfection incorporates the spectrum of light between 200 nanometers (nm) and 300 nm, with 254 nm being the most lethal. This area of light waves is also known as low-wave ultraviolet light, Because low-wave UV is screened out by the earth's atmosphere, it is rarely found at the earth's surface. UV radiation has been found to be very effective against a range of microorganisms, including bacteria, viruses, algae, and most protozoa spores and cysts, such as Entamoeba histolytica, Giardia lambia, and Cryptosporidium parvum. UV disinfection works by penetrating the cell walls of organisms and structurally altering their DNA. This prevents the cell from being functional, rendering it incapable of reproducing or causing disease. >>Because it is not a chemical method of disinfection, UV radiation does not alter the physical or chemical properties of water.<

The TCEQ allows the reuse of treated wastewater (30 TAC 210, included on resource thumb drive) for specific purposes, including the following: • Lawn irrigation • Park irrigation • >>Golf course irrigation<< • >>Aquifer recharge<< • >>Pasture irrigation<< • Cooling towers • >>Agricultural irrigation<< • Fire protection

Exposed piping must be purple pipe or painted purple with the warning “NON-POTABLE WATER” stenciled in white. Buried pipe must be purple pipe, painted purple, taped with purple metallic tape, or bagged in purple.

Reclaimed water providers must report monthly to TCEQ the volume and quality of reclaimed water delivered to a user. The report is due by the 20th of the following month.