Disinfection of wastewater is a preventive method of minimizing the numbers of actual or potential microorganisms reintroduced to the environment. It is designed as one of the multiple barriers against the transmission of infectious disease. The number of pathogenic organisms that could be present in treated but undisinfected sewage effluent are sufficient to result in a reasonable probability of infection upon contact with waters to which the sewage is discharged. Thus, the policy of the VDH is that adequate protection of public health requires elimination of possible contact (with an infective dose of pathogens) through use of disinfection and dilution. Sewage effluents should be disinfected and adequately diluted when discharged:
1.) Within 10 miles of a water supply source.
2.) Within 5 miles or a tidal cycle of estuary flow to Shellfish growing or harvesting waters.
3.) Within 5 miles or a tidal cycle of swimming areas.
Tests for the indicator organism fecal coliform are used to assure the efficiency of the disinfection process. Undisinfected sewage effluents treated to a secondary level (30 mg/l BOD5) can contain up to one million (6 logs) of the bacteria per 100 milliliters. The disinfection step must be capable of achieving a four log reduction (in the level of organisms showing a positive reaction upon incubation in lactose media) in order to achieve the effluent standard of 200 per 100 milliters. It is assumed that such a reduction is paralleled by a significant reduction in pathogens as virus studies have previously shown.
Water and wastewater may be disinfected by a variety of physical, chemical and biological mechanisms that are used in conventional treatment processes. However, a separate disinfection unit operation must be provided in addition to other upstream treatment operations in order to achieve health protection standards. The level of treatment or contaminant removal achieved prior to the disinfection step greatly affects the performance of disinfectants that prevent microorganisms from surviving and reproducing (inactivation). A high degree of microorganism inactivation can be achieved through the application of chlorine (Cl2) in concentrated solution to water and wastewater.
However, the efficiency of any chemical disinfectant is directly affected by the amount and type of solids content present. The amount of chlorine (dose) applied to secondary effluent (30 mg/l or less TSS) to achieve an effluent standard of 200 fecal coliform per 100 milliliters would be several times the dosage necessary to disinfect treated drinking water to completely inactivate the fecal coliform organism as required for potable water.
Chlorination is the disinfection method of choice in water and wastewater as its performance reliability is well established. The toxicity impacts of chlorination by-products have been studied and have raised concerns over the long term chronic effects of these compounds (4). Drinking water standards have been established for the trihalomethane series of compounds, but additional studies of chlorinated organics are being planned. Possible chronic toxicity impacts on aquatic life has been a controversial issued relative to wastewater disinfection for more than 20 years. These concerns have resulted in-stream standards for residual chlorine compounds (in the tenths of a part per million) that mandate dechlorination or the use of alternative disinfectants.Chlorinated wastewater effluents can be effectively dechlorinated to remove the stable compounds formed by the reaction between chlorine and the wastewater ammonia content. These combined chlorine compounds (principally monochloramine) can be reduced by sulfur compounds to non-toxic forms. The addition of a dechlorination step has made the use of alternative wastewater disinfectants more cost competitive. More than 20 installations of ultraviolet light irradiation systems are now operating in Virginia providing disinfection of secondary and better treated effluents. Only one ozonation system has been installed in Virginia (Henrico County) and it has been beset by operational problems leading to non-compliance with effluent standards.
A conventional gas chlorination system usually consists of a supply system, a dosage metering system, a solution discharge system and control equipment. The supply system includes weighing scales to monitor chlorine usage and a gas withdrawal system of valves and gages for compressed liquid-chlorine containers. The chlorinator features a pressure-vacuum regulating valve to reduce the supply pressure of the chlorine gas to a negative (vacuum) level. The gas flow through the chlorinator can be fine-tuned by adjustment of a metering orifice, which is in-line with a vacuum differential regulating valve. Gas flow from the chlorinator passes into an injector, where it is mixed with an outside supply of water or treated wastewater. The chlorine mixture is then pumped through a diffuser mechanism into the influent to the chlorine contact chamber.
As a result of safety concerns over possible toxic gas leaks, many owners of metropolitan treatment works are switching to dry chemical hypochlorite systems. Hypochlorite systems usually feature gravity or pumped feeding of the chlorine solution formed from the hypochlorite. The feed rate can be controlled by valves or metering pumps. Dry tablet hypochlorinators can be used to disinfect small flows (50,000 gallons per day (gpd) or less). The tablet chlorinators contain stacked columns of tablets which dissolve at a controlled rate as the contact tank influent passes around the tablet, releasing hypochlorous acid. As the bottom tablet dissolves, the tablet immediately above it drops into place within the flow depth established by a flow control weir. On-site generation of hypochlorite is infrequently used to treat larger flows, as it is a more expensive process than the use of liquid chlorine containers. However, many larger urban located treatment works are being retrofitted with hypochlorite generation facilities as a result of safety and liability concerns associated with the transportation and storage of large quantities of compressed chlorine gas within highly populated areas.
Most gas chlorination systems feature an open-loop, flow proportional control, in which the gas flow through the chlorinator metering orifice may be increased or decreased in direct proportion to the contact tank influent flow rate. By adding a chlorine residual analyzer to signal and feedback the residual changes in the contact tank effluent, a closed loop system can be developed. The residual analyzer most often utilized is an amperometric device, which requires constant attention to ensure proper operation. Recently, oxidation-reduction (redox or ORP) devices have been shown to provide an accurate means of detecting residual chlorine levels in water and wastewater. The residual feed-back signal can be used to increase or decrease the chlorine feed rate through direct adjustment of the differential regulating valve on the chlorinator. The chlorinator compounds the residual signal with the flow rate signal to achieve a more flexible and closely controlled operation. More exact chlorination control is obtained by placing a second analyzer in the control system to monitor upstream chlorine residuals in the chlorine contact tank and by using that signal to trim the chlorine feed rate up or down. This cascade control system develops a quick response to residual fluctuations caused by changes in wastewater quality.
Many studies indicate that uniform injection and proper mixing of a disinfectant dose with an adequate exposure period is more important to disinfection efficiency than is an arbitrary increase in dosage. Results of in-plant modifications demonstrate that uniform dispersion of chlorine through mixing upstream of the contact basin, will increase disinfection efficiency without an increase in chlorine dosage. Uniform dosage application must be attained to optimally disperse the disinfectant within the wastewater flow. Apparently, uniform injection and rapid mixing of the chlorine dose exposes a larger number of microorganisms to the disinfectant. Upstream dose mixing prior to dilution and retention is important to obtaining efficient inactivation of pathogens. Adequate contact time with residual chloramine is necessary to accomplish the inactivation of microorganisms and this extremely important when utilizing a chlorination-dechlorination process in which the residual chlorine is removed prior to either discharge, or withdrawal for reuse, or consumption. Similar problems related to adequate disinfectant exposure or contact times may exist with other alternative disinfection processes. Many contact tank designs are subject to severe hydraulic short-circuiting effects that allow some of the flow to leave the basin after short residence periods. Contact basins must be designed to achieve maximum flow retention. Use of longitudinal end-around baffles provide extended retention times and prevent short-circuiting caused by hydraulic currents. A flow path length to width ratio exceeding twenty to one provides protection against short-circuiting.
Ultraviolet light irradiation (UV) is a physical process which provides disinfection through the transference of electromagnetic energy, emitted by UV lamps, primarily at a germicidal wavelength that is absorbed by the cell's genetic material. This absorbed energy damages the genetic makeup of the cell by structurally altering the DNA molecule, producing either lethal effects, or inhibiting cell replication. UV light also alters virus RNA. The germicidal effectiveness of UV is optimum at the 250 to 270 nm wavelength (maximum absorption by nucleic acids). The most efficient commercial source of UV energy is the low pressure mercury arc lamp with 35 to 40 percent of the electrical energy input converted to light energy with 85 percent of that input monochromatic at 253.7 nm (UVC). Light output of the low pressure lamp typically ranges from 40 to 60 watts and must remain germicidal in excess of 7500 hours of operation. However, high intensity UV lamps, both low and medium pressure are coming into more widespread use. The high intensity lamps may deliver anywhere from 90 watts for low pressure, up to 400 watts for medium pressure lamps. Low pressure lamps are commercially available and inexpensive compared to medium pressure lamps that are usually proprietary and sole source. In addition, the medium pressure UV lamps may deliver less than 10 percent of the input energy as UVC.
A UV disinfection system consists of a grouping of lamps called an assembly designed to expose all microorganisms, within the flow passing through the assembly, to a lethal dose of UVC. Lamp assemblies may be installed within enclosed reactors or within open channels. The current configurations acceptable for UV disinfection equipment include contact systems with submerged UV lamps enclosed in quartz tubes called sleeves, that are placed parallel to channel flow (horizontal), or placed perpendicular to flow (vertical). Noncontact systems feature compartmentalized UV lamps situated adjacent to the flow surface or adjacent to Teflon-lined tubular channels carrying treated effluent. Conventional UV disinfection system design include, as a minimum, two (2) separate lamp assemblies with each assembly capable of providing the level of disinfection necessary to meet the disinfection standard at average daily flow. If no more than two (2) lamp assemblies are provided for treatment works discharging to critical waters, then each assembly must be capable of disinfecting the maximum daily flow. The design dosage to be provided by the lamp assembly is a function of the UV absorbance characteristics of the water or treated wastewater. Typical design dosages vary from more than 60,000 to 35,000 microwatts per square centimeter of flow area per second of exposure for secondary to filtered effluents. This dosage could require nearly 50 low intensity lamps (40 watts UVC) to less than 10 high intensity lamps to treat a 1.0 mgd flow of secondary effluent.
Ultraviolet irradiation can be an effective method of effluent disinfection when properly designed. However, the process requires dose control and must be properly operated. Efficient design of a UV systems must include adequate light intensity (I) and contact time (T) to provide the proper dose of ultraviolet light to the microorganisms being inactivated. As flow passes through UV lamp assemblies, mixing should occur between the lamp sleeve surfaces along the light intensity gradients (lateral mixing in the direction of flow). Such mixing action or turbulent dispersion, will enhance the level of inactivation achieved for a given dose of UVC.
Lamp spacing in channels or reactors should be sufficient to use the light in the solution rather than absorb it on adjacent lamps and walls. The lamp spacing should provide for the absorbency of the fluid disinfected. For secondary effluent the spacing between lamps should be no more than eight (8) cm with good mixing provided along intensity gradients. The arrangement and numbers of lamps included in each assembly shall be designed to obtain the design dosage while allowing proper maintenance. Electronic ballasts matched to no more than two (2) lamps are preferred. All electrical connections to submerged lamps must be watertight and designed so as to remain dry during maintenance operations. As so-called self-cleaning systems have not proven reliable, submerged quartz tubes must be routinely removed and cleaned of surface deposits of metal salts and absorbed organics that block UV transmission. Periodic testing for the biological indicator must be performed to verify the level of disinfection achieved at the monitored lamp intensity readings.