Texas Basic Waste Water Operations Ch#12 Disinfection

Disinfection of wastewater effluent prevents waterborne (water-carried) disease by protecting public water supplies, recreation water, and shellfish growing areas.

There are numerous waterborne diseases. These include the following: • Typhoid • Cholera • Paratyphoid • Dysentery • Poliomyelitis • Gastroenteritis • Hepatitis A • Giardiasis* • Cryptosporidiosis* (*Giardia and Cryptosporidium are chlorine-resistant microorganisms and are not inactivated at normal dosage and contact times.)

Disinfection destroys pathogenic (disease-causing) microorganisms. Pathogens include bacteria, viruses, and protozoan. Wastewater treatment alone may remove 98% of the pathogens, but the TCEQ requires, as part of the facilities discharge permit, continuous disinfection as a precaution to ensure the safety of downstream users.

There are two methods often used to disinfect wastewater effluent: >ultraviolet< radiation and chlorination.

Chlorine gas is greenish-yellow in color, 2½ times heavier than air, and has a strong, pungent odor. It is irritating to the respiratory tract and very toxic. In the presence of moisture, it is highly corrosive.

Chlorine is nonflammable, but is a strong oxidizer and reacts violently with oil and other hydrocarbons.

Calcium hypochlorite (CaOCl2), commonly known as HTH, is available in granular or tablet form and contains 65% chlorine by weight. A 100 lb barrel of calcium hypochlorite contains 65 lbs of chlorine. The remaining 35 lbs is lime. The chlorine separates from the lime when the product mixes with water. Calcium hypochlorite is a strong oxidizer. It reacts violently with oil or other hydrocarbons.

Chlorine dosage is the amount of chlorine applied. Chlorine demand is the amount of chlorine used up by organic and inorganic reducing agents. Residual is the amount of chlorine left after demand is met. Each is measured in milligrams per liter (mg/L) or parts per million (ppm). Dosage = Demand + Residual

When chlorine is mixed with water, hypochlorous and hydrochloric acids form: Cl2 + H2O = HOCl(Hypochlorous) + HCl(Hydrochloic)

Hypochlorous acid (HOCl) is the free, >chemically uncombined residual<, and a powerful disinfectant.

NOTE: since this is a false statement there is no direct reference to put here. "lime" can refer to dry/quick lime (calcium oxide CaO) or hydrated/slake lime (calcium hydroxide CaO2H2). this question gets more complicated when you realize that the form of the chlorine introduction (HTH, Cl2, bleach) will change what reactions are happening and therefore the products of those reactions. chloramines however are defined as "an organic compound containing a chlorine atom bonded to nitrogen, especially any of a group of sulfonamide derivatives used as antiseptics and disinfectants." and none of the proposed reactants contain nitrogen.

Chloramines exist in three forms according to the water pH: monochloramine, dichloramine, and trichloramine. All have disinfecting power. Chloramines are referred to as combined residual.

Chlorine irritates the skin, eyes, and respiratory tract. Strong concentrations cause coughing, vomiting, and difficulty in breathing. Suffocation can occur. There is no antidote for chlorine gas inhalation. Relief of symptoms is the goal.

In case of exposure, the following are highly recommended: • Wash under a safety shower or with a garden hose. • Have the victim sit or lie down with the head and trunk elevated. • If chlorine gets into the eyes, flush with cool water for 15 minutes. • Milk, coffee, or peppermint candy relieves throat irritation; do not give alcohol. • Call for medical assistance, dial 911.

In case of exposure, the following are highly recommended: • Wash under a safety shower or with a garden hose. • Have the victim sit or lie down with the head and trunk elevated. • If chlorine gets into the eyes, flush with cool water for 15 minutes. • Milk, coffee, or peppermint candy relieves throat irritation; do not give alcohol. • Call for medical assistance, dial 911.

Use DPD (N-N-diethyl-p-phenylenediamine) as a chlorine indicator in the titrimetric and spectrophotometric methods. Chlorine reacts with DPD to form a pink to red color; the stronger the color, the stronger the residual.

There are several factors that affect disinfection processes. Some of these include the following: • pH • Temperature • Contact time • Chlorine dose • Residual type

Because HOCl (hypochlorous acid) is an acid, it is more effective at low pH. High pH neutralizes acids. Hypochlorous acid (HOCl) is the free, chemically uncombined residual, and a powerful disinfectant. It works well at low pH, but weakens as the pH increases.

The higher the water temperature, the faster the disinfection rate. The lower the water temperature, the slower the disinfection rate.

The lower the dosage, the longer the required contact time (due to slower disinfection). The higher the dosage, the shorter the required contact time (due to faster disinfection).

There are three types of residuals: free, combined, and total. Free chlorine residual is chemically uncombined and is composed of chlorine in the form of hypochlorous acid (HOCl) and hypochlorite ion (OCl). A combined residual, which TCEQ calls a chloramine residual, is composed of chloramines and other organic compounds. A chloramine is chlorine that has reacted with ammonia to form monochloramines, dichloramines, and trichloramines. Together, free chlorine residual and combined residual equal total residual. The free residual is more powerful and faster acting than the combined residual, but the combined residual lasts longer. Domestic wastewater contains ammonia, which forms a combined residual and is reported to the TCEQ as a total residual.

In addition to disinfection, chlorine has other uses at wastewater treatment plants, including the following: • Reduces BOD • Reduces odor • Inhibits algae growth • Aids in grease removal • Helps control sludge bulking • Helps control digester foaming • Aids in settling sludge • Controls trickling filter flies/snails

Gas units connect to cylinders or manifolds and use vacuum produced by the injector or ejector to suction chlorine into a water stream.

The rotameter (rate indicator) indicates feed rates in pounds/day or grams/hour.

The TCEQ requires chlorination equipment to be capable of continuously applying chlorine at the highest expected dosage with the largest unit out of service (30 TAC 217.271). Effluent dosage in the contact chamber must produce a total combined residual of 1.0 mg/L after 20 minutes of contact time at peak flow.

Wrenches must be no longer than 8 inches. Do not force a valve.

Detect leaks with 10% ammonium hydroxide vapor. Household ammonia is not approved. The chlorine and ammonia reaction produces white smoke.

Do not spray water on a leak; water makes the leak worse. Large leaks may require evacuation.

State waters may provide habitat for aquatic organisms, such as fish. To protect these organisms from toxins, such as chlorine, the TCEQ requires dechlorination in some discharge permits.

Dechlorination may be achieved by the following: • Long detention time • Aeration • Sunlight • Activated carbon • Chemicals (EX: SO2)

The TCEQ allows the reuse of treated wastewater (30 TAC 210) for specific purposes, which include the following: • Lawn irrigation • Park irrigation • Golf course irrigation • Aquifer recharge • Pasture irrigation • Cooling towers • Agricultural irrigation • Fire protection

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