Disinfectants and Biocides

A disinfectant is a chemical (commonly chlorine, chloramines, or ozone) or physical process (e.g., ultraviolet light) that kills microorganisms such as bacteria, viruses and protozoa. For example, chlorine is often used to disinfect sewage treatment effluent, water supplies, wells, and swimming pools. Most forms of disinfection, including chlorination and ozonation kill organisms by oxidation. The exception is ultra violet (UV) treatment which kills with UV radiation. In the U.S., chlorination is by far the most common disinfectant in use.

Most disinfectants are strong oxidants and/or generate oxidants as byproducts (such as hydroxyl free radicals) that react with organic and inorganic compounds in water. Most disinfectants are also used for other purposes in drinking water treatment, such as taste and odor control, improved flocculation, and nuisance control.

Disinfectants are typically identified by the EPA as a primary or secondary disinfectant, as follows:

• Primary disinfection refers to the inactivation of microorganisms to meet the regulatory bacteriological requirements.  This requirement typically is met by achieving certain CT requirements to assure a target log inactivation of target organisms as set forth in the Surface Water Treatment Rule (SWTR).
• Secondary disinfection refers to application of a disinfectant to meet regulatory requirements for distribution system bacteriological quality as set forth in the Total Coliform Rule (TCR).  The SWTR requires that a residual disinfectant be measured in the distribution system, or that the bacteriological quality meet certain standards (heterotrophic plate count (HPC) less than 500/100mL).

A biocide is a substance capable of destroying (killing) living organisms. There are two major types of biocides; oxidizers and non-oxidizers:

• Oxidizers destroy important cellular components in microorganisms, resulting in death of the organisms. Chlorine gas and liquid sodium hypochlorite chlorine are the most widely used oxidizer products. Another oxidizer that is fed in many applications is bromine, which is available as a solid or liquid. Bromine offers several performance and safety advantages over chlorine and sodium hypochlorite.
• Non-oxidizing biocides have been typically used to control severe upset conditions or to enhance oxidizing biocide programs. Due to increasing environmental concerns, use of non-oxidizers is becoming more commonplace. Non-oxidizing biocides are organic compounds which react with specific cell components within a microbe to ultimately destroy that cell.

An oxidizing agent is defined as any substance, such as oxygen (O₂) or chlorine (Cl₂), that will readily add (take on) electrons.

Chemical Engineering says oxidation processes can be biological, chemical or a combination of both. The efficacy of such processes is differentiated by their removal efficiencies for biological oxygen demand (BOD) and chemical oxygen demand (COD).

For removal of biodegradable contaminants, oxygen can assist microorganisms in the conversion of such contaminants into water and carbon dioxide. Likewise, oxygen can be used as an oxidizing agent to remove nonbiodegradable contaminants, as in physico-chemical oxidation processes such as wet oxidation. Ozone, one of the most powerful oxidizing agents, can also convert pollutants into unproblematic reaction products.

Biological wastewater treatment imitates nature's ways of degrading organic contaminants. When oxygen is used to assist and accelerate conversion, the process is aerobic. Such methods are effectively used to treat municipal sewage, which tends to be higher in volume, but smaller in concentration and range of organic contaminants than industrial wastewater.

A common feature of all aerobic microorganisms is that they form water and CO₂ as metabolic products from the buildup of new cell material present in contaminants. However, this conversion process is only possible when all the substances are present in sufficient quantities.

For purification of wastewaters that are heavy in organic matter, nutrients and energy are available in abundance. Lacking, typically, is a sufficient supply of oxygen -- usually due to overload on the wastewater treatment facility. As a result, purification capacity is compromised and obnoxious odors are released.

Modifying the activated sludge process to increase the supply of oxygen to the aeration tank can alleviate these problems and increase purification capacity. Typically, mechanical or compressed air systems are used to introduce dissolved oxygen to the activated sludge process. Mechanical systems entrain atmospheric air through impellers, propellers, or turbines. Compressed air is introduced to the tank bottom, through porous diffuser plates, spirally wound tubes, spargers, nozzles or injection jets.

Increased microbial activity and purification capacity can be achieved by enriching the activated sludge system with pure oxygen. This can be done without additional energy costs or structural changes by a process known as partial oxygenation. Fine bubbles of oxygen are injected into the mixture of activated sludge and water via gasifying mats, consisting of thick-walled perforated hoses, over a wide area of the aeration tank. As a result, biological activity is intensified in a process that is competitive in price, but more efficient than conventional aeration systems.

To meet regulatory requirements, wastewater treatment facilities are using biological processes to degrade ammonium compounds. Nitrification occurs in an aerobic system, mainly by the slow-growing Nitrosomonas and Nitrobacter. In a subsequent step, depleted of free oxygen, microbes, such as Alcaligenes and Achromobacter attack nitrates and nitrites. The microbes strip away the oxygen atoms, freeing the nitrogen in a denitrification step.

Typically, microbes suitable for degrading nitrogen have a higher oxygen demand and grow more slowly than the species responsible for the degradation of carbon. By directly injecting oxygen, some systems increase the microbial activity and can increase the treatment capacity of existing activated- sludge basins by as much as 100%.

Use of non-oxidizing biocides has been effective for bio-control, without the help of chlorine or other oxidants. Most often, they are used as a supplementary biocide to produce a broader organism-control spectrum than provided by chlorine or bromine alone. Occasionally, a non-oxidizing biocide has found use as the sole control agent, particularly when oxidant demand is high. Quaternary ammonium salts and certain aldehydes are among the forms most commonly used.

These toxic non-oxidizing biocides are very effective, but not as economical as chlorine and most other oxidants except to deal with certain specific problems. Examples are Asiatic clams and other marine macrofoulants. They can be cost-effective when a high oxidant demand occurs (over 20 ppm) in cooling water, or when sulfate-reducing bacteria (SRB) and other corrosive micro-organisms present a major problem.

Generally, non-oxidizing biocides constitute suitable chlorine alternatives only for specific sites or situations. Some are quite persistent, and will be found in the effluent unless removed or deactivated. Research into new approaches to control technology is under way on several fronts. Specific non-oxidizing inhibitors, for example, are being developed that are adsorbed on component surfaces to render them bio-resistant. A major advantage is that these inhibitors are not discharged with the system effluent.

The most important use of disinfectants in water treatment is to limit waterborne disease and inactive pathogenic organisms in water supplies. The first use of chlorine as a continuous process in water treatment was in a small town in Belgium in the early 1900s. Since the introduction of filtration and disinfection at water treatment plants in the US, waterborne diseases such as typhoid and cholera have been virtually eliminated.

While various types of odor problems occur in industry, sulfur compounds are the most common cause of these problems. These compounds are the result of the use sulfur chemicals, chemical and food processing, or biological action. These are not merely aesthetic problems, but serious health concerns as well. While some of these compounds merely cause nausea, others, such as hydrogen sulfide, are more poisonous than hydrogen cyanide that is used in gas chambers. Even in low concentrations, hydrogen sulfide's "rotten egg" odor is very noxious.

Algae are natural odor generators and generally disrupt water treatment. They prefer warm temperatures (35-40 degrees C) and feed off of organic compounds in the water. Odor-causing organics are byproducts of the algae's metabolism.

Industrial discharges are not the sole source of these odors. These problems strike closer to home, in the sanitary sewers that carry wastes from our homes. Anaerobic bacterial slime layers form hydrogen sulfide from materials in the wastewater. Once inside the wastewater treatment plant, additional bacterial action forms even more hydrogen sulfide. Even the municipal garbage dumps are significant sources of odor.

There are many methods of odor control: masking methods using perfumes, removal methods that act on the odor-causing chemicals, destruction methods that oxidize the odor-causing chemicals, and finally biological control methods that prevent microorganisms from forming these chemicals.

Most water treatment plants disinfect water prior to distribution. The 1995 Community Water Systems Survey reported that 81 percent of all community water systems provide some form of treatment on all or a portion of their water sources (Figure II-4). The survey found that 99% of the surface water systems provide some treatment of their water. Of those systems reporting no treatment, 80% rely on ground water as their only water source.

Chlorine is, by far, the most commonly used disinfectant in the drinking water treatment industry. Today, chlorine is used as a primary disinfectant in the vast majority of all surface water treatment plants, being used as a pre-disinfectant in more than 63 percent and as a post-disinfectant in more than 67 percent of all surface water treatment.

Figure II-4 shows the disinfectant usage at water plants by flow categories for all water sources, surface water sources, and ground water sources, respectively. The figure shows the percentage of utilities that are using the particular disinfectant. Because some facilities use more than one disinfectant, the total usage exceeds 100%.

Figure II-4: Disinfectant Usage as a Function of Flow for all Water Sources

*Source: USEPA, April, 1999. Percentage calculated as a fraction of 527 – the total number of systems. 740 disinfectants are used by the 527 systems. Numbers show the percentage of systems using a particular disinfectant.

Disinfectants are used for more than just disinfection in drinking water treatment. While inactivation of pathogenic organisms is a primary function, disinfectants are also used as oxidants in drinking water treatment for several other functions. An oxidant is a substance containing oxygen that reacts chemically in air to produce a new substance. Oxidants are used for:

• Minimization of DBP formation;
• Control of nuisance Asiatic clams and zebra mussels;
• Oxidation of iron and manganese
• Prevention of regrowth in the distribution system and maintenance of biological stability;
• Removal of taste and odors through chemical oxidation;
• Improvement of coagulation and filtration efficiency;
• Prevention of algal growth in sedimentation basins and filters;
• Removal of color.

Iron and manganese occur frequently in ground waters but are less problematic in surface waters. Although not harmful to human health at the low concentrations typically found in water, these compounds can cause staining and taste problems. These compounds are readily treatment by oxidation to produce a precipitant that is removed in subsequent sedimentation and filtration processes.