In the mid-to-late 1800s, chlorine had been used sporadically to help control waterborne illnesses and pollutants in drinking water. This also prompted utilities across the world to begin large-scale filtration of water supplies and wastewater treatment. However, back then filtration alone was inadequate to reduce widespread illnesses and disinfection with chlorination virtually eliminating waterborne epidemics, increasing life expectancy by 50 per cent.
Conventional water and wastewater treatment facilities use various disinfection processes at the head of the treatment work with the aim of not creating chlorine byproducts, thereafter chlorine is applied to the effluent or finished water product to maintain a residual throughout the water distribution system. With increasing water pollution and stringent environmental regulations, it is becoming more difficult to achieve all of the requirements for safe Water and Wastewater disinfection with one treatment alone. Today, a more layered approach for disinfection with multiple technologies used together is often necessary. Some of the most reliable disinfectants and disinfection processes used in industry today include:
(I) Chlorine gas
Used in water treatment plants since the early 1900s, it is essentially pure chlorine, typically delivered in pressurised cylinders. It is generally the most cost-effective, efficient, and easiest to maintain the method of disinfecting with chlorine. Chlorine dioxide is generated by mixing acid or chlorine gas and sodium chlorite then mixed with ejector water as an entrained gas to form a solution that is applied to the process. Because it cannot be compressed and liquefied for transportation, it is often generated on-site close to its intended use. It is a strong oxidant and a very popular disinfectant across a wide pH range for both water and wastewater. It does not react with ammonia to become a weaker disinfectant, important for water and wastewater treatment plants where the water has a high ammonia content, often resulting in lower operating costs. Chlorine dioxide is frequently used in water treatment plants as a primary disinfectant early in the treatment process to prevent the formation of Trihalomethanes (THMs) which environmental pollutants, and many are considered carcinogenic.
(II) Sodium Hypochlorite
Manufactured at approximately 12 to 15 per cent chlorine by weight with a pH greater than 11. Delivery systems of this liquid disinfection include storage tanks, chemical dosing pumping systems with associated valves and piping, and a control method such as flow control, residual control or compound loop (flow + residual). Although more expensive commercially than chlorine gas it is considered simpler to maintain and operate with on-site generation. Bulk sodium hypochlorite concentration decays over time and higher volumes are required to achieve the same result.
(III) On-site hypochlorite generation (OSHG)
An On-site hypochlorite generation (OSHG) system uses electrolysis to generate a nominal solution of hypochlorite on-site as needed. A brine solution passes through an electrolytic cell, converting the chloride ions from salt to hypochlorite. The process typically uses three pounds of salt, two kW hours of electricity and 15 gallons of water to produce a pound of chlorine in 15 gallons of solution, the equivalent of the active chlorine present in one gallon of 12.5 per cent bulk hypochlorite or one pound of chlorine gas. OSHG systems have moderate maintenance requirements and are considered safer than transporting chlorine gas under pressure.
(IV) On-site chlorine generation (OSCG)
Similar to OSHG, an On-site chlorine generation (OSCG) system uses brine solutions and membrane electrolysis to produce higher concentrations of hypochlorite up to 12 per cent of chlorine gas. On-site chlorine generation reduces transportation and delivery hazards associated with shipping commercial hypochlorite and eliminates transportation safety concerns of chlorine gas delivered in liquid (bulk) containers.
(V) Calcium hypochlorite
A solid tablet, calcium hypochlorite is typically 60 per cent available chlorine delivered via a dilution tank or tablet feed system whereby the calcium hypochlorite is dissolved into solution then dosed with a metering pump. It is commonly used in swimming pools. Because calcium hypochlorite is expensive per pound of chlorine and is difficult to accurately dose when it is occasionally used for water and wastewater treatment, it is typical for smaller remote plants or treatment works where other methods of chlorine feed are not feasible.
In the presence of ammonia, chlorine combines with the ammonia to form either mono-chloramine, di-chloramine or tri-chloramine depending on the ammonia-to-chlorine ratio. Mono-chloramine is a relatively weak disinfectant but maintains a very stable long-lasting residual in water and is therefore often used as a secondary disinfection method in systems with a long water age. Chloramine systems can be challenging to control and water and wastewater operators and engineers must carefully monitor and maintain their dosing systems for maximum accuracy.
Comprissing three oxygen atoms (O3), ozone is a very strong oxidant. It deteriorates rapidly to oxygen and is usually generated on-site using either air or pure oxygen. Ozone does not produce disinfection byproducts (DBPs) and can be used as a primary disinfectant for water treatment to reduce THMs and DBPs. Ozone also is used for taste, odour and colour control in potable water treatment, as well as Iron-Manganese (Fe/Mn) removal when THMs are a concern. Ozone can also be used to remove micro-pollutants including pesticides at disinfection dosages. Ozonation is typically not used for primary disinfection of wastewater effluent with high levels of suspended solids (SS), biochemical oxygen demand (BOD), chemical oxygen demand (COD), or total organic carbon (TOC) since the cost of treatment can be relatively high in capital and in power intensiveness.
(VIII) Ultraviolet Light (UV radiation)
Ultraviolet light energy at 254 nm wavelength is absorbed by the DNA of a microorganism, stopping the reproductive process and rendering it non-infective and microbiologically dead. UV systems operate at varying pressure and output, depending on application, target pathogens and water quality involved. UV has most recently been used to treat wastewater effluent since environmental regulations globally require more stringent chlorine discharge limits for various receiving streams. UV is effective in removing chlorine-resistant pathogens from drinking water including Cryptosporidium, Giardia and various viruses that have proven to be resistant to traditional disinfection methods such as chlorine and filtration.
(IX) Peracetic Acid (PAA)
Peracetic acid (CH3CO3—also known as peroxyacetic acid, or PAA—is a liquid that functions as a strong oxidising agent, has an acrid odour and can also be used as a disinfectant. PAA is generally commercially available as an equilibrium mixture of 12-15 per cent peracetic acid and 18-23 per cent hydrogen peroxide. PAA is available in gallon containers and in bulk, requires stainless steel piping, and is administered using a metering pump. Since PAA is a highly effective bactericide, does not form disinfection by-products (DBPs), has minimal dependency on pH and does not leave a residual, it has received significant consideration for the disinfection of wastewater effluent.
With a well-defined water and wastewater treatment process goals and experience, knowledgeable partners working together, developing the most effective, simple and budget-conscious water and wastewater disinfection plan should be a simple, sustainable and economical process.