Water And Wastewater Disinfection Treatment

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One of the most significant public health advances over the past century was that of developing, and then routinely applying, suitable engineering methods for disinfecting potable waters that could retard, and ideally obviate, the transmission of waterborne disease. Rudimental disinfection measures based on water filtration (used by the ancient Egyptians) and heat treatment have long been practiced, but the advent of commercially available chlorine during the late nineteenth and early twentieth centuries effectively revolutionized the wide-scale utility and efficacy of this practice (Baker, 1948).

Indeed, chlorination has subsequently served as the dominant disinfectant "tool" for potable waters since the early twentieth century, but there are now at least six strategies by which waters, wastewaters, or sludges might be duly processed to achieve a desired reduction in their microbial content (Bryant et al., 1992; U.S. EPA, 1999):

1. Physical filtration (e.g., using ultrafiltration or reverse osmosis)

2. Heat treatment (e.g., boiling and pasteurization)

3. Physical sonication

4. Strong oxidant chemical treatment (e.g., using halogens such as chlorine, bromine or iodine, or ozone)

5. Nonoxidizing chemical treatment

6. Radiation treatment

These disinfection strategies can largely be subdivided into the following four options, although in many instances there are likely to be significant overlaps with the causative impacts imposed by any one disinfection procedure:

1. Remove cells physically

2. Alter and disrupt cell membrane permeability

3. Alter and disrupt metabolically essential proteins and enzymes

4. Alter and disrupt genetically essential nucleic material

The first such approach to using physical filtration to remove microbes depends on cell size, which is rather fortunate since chemical-based disinfection tends to become problematic with large cells or cysts, such as might be experienced with protozoan forms of Giardia lamblia and Cryptosporidium. Conventional slow- and rapid-sand filters have been used for more than a century to effectively reduce, if not completely eliminates, these larger microbes, and as a result these types of filters have been widely used for processing surface water sources susceptible to microbial contamination. However, the success of these operations can be compromised by human and technical shortcomings (e.g., inadequate filter backwashing regimes), as has been demonstrated in several U.S. cities over the past few decades. One such well-publicized incident with mismanaged water filter operations in Milwaukee, Wisconsin, during 1994 affected 400,000+ residents. The effectiveness of these operations has been improved with tightened stipulations (i.e., the U.S. EPA Interim Enhanced Surface Water Treatment Rule) on routine filter monitoring (U.S. EPA, 1998). In recent years, the disinfection effectiveness of media-based filtration has been eclipsed by the use of micro- and ultrafiltration systems with pore sizes in the double-digit nanometer range (e.g., typically 30 to ^100 nm), which are small enough to prevent the passage of any pathogens.

As for those disinfection strategies intended to negatively change the permeability or perhaps water content of cells, numerous examples can be seen with foods prepared in percentile-level salt, sugar, or organic-acid-rich conditions (e.g., pickles, candied fruits, cheeses, vinegar, tomato catsup). These preservation environments, many of which yield osmotic pressures intolerable to active cell growth, facilitate a bacteriostatic condition in which microbial growth has been effectively stopped without specifically killing the original cells. Large-scale adjustments to the osmotic pressure within water, wastewater, or sludge treatment processes would, of course, be infeasible given the necessary chemical dosage requirement (i.e., at expensive, high percentage levels).

Chemical disinfection agents that alter the form and function of membrane-bound transport enzymes could disrupt the transmembrane passage of essential substrates or nutrients. Whether or not the latter membrane-specific impact is realized in conjunction with the use of the more widely used antimicrobial chemicals (e.g., chlorine, ozone), enzyme disruption and denaturation are widely considered to be their dominant disinfection mechanism. These agents, and particularly those involving either strong-oxidant (e.g., chlorine) or superoxide (generated by means of irradiation) chemicals, readily disrupt a cell's hydrogen- and covalently bonded three-dimensional enzyme conformation. Having lost the enzyme's catalytic contribution to energy-yielding catabolism, these disinfected cells subsequently lack sufficient energetic resources to reproduce effectively.

Four different strong oxidants are widely used for disinfection: (1) halogens (chlorine, bromine, and iodine), (2) halogen-containing compounds (e.g., chlorine dioxide, chlora-mines, bromochlorodimethylhydantoin), (3) ozone, and (4) hydrogen peroxide (H2O2). In each case, the standard engineering application is that of dosing the applied chemical into a short-term contact chamber (i.e., typically designed for 15 minutes' retention) using a chemical delivery system preset to achieve a desired disinfectant concentration relative to measured flow. As shown in Figure 16.53, these disinfection contact chambers are often designed with a serpentine configuration in an attempt to secure a quasi-plug-flow regime.

Chlorine has been, and remains, the dominant disinfectant chemical with waters and wastewaters in the United States, applied either in gas (Cl2), liquid (NaOCl), or solid [Ca(OCl)2] form at what is likely to be the least possible cost (i.e., in the range of pennies per pound) for any disinfection option (White, 1992). Aside from cost, chlorine's advantages include its range of delivery options and expected efficiency. However, there are also shortcomings with its use, including the fact that there are significant safety issues to be addressed when storing and metering chlorine gas.

One key aspect of chlorine use is that of its sensitivity to pH. Above pH 7.5 the desired hypochlorous acid (HOCl) species found in aqueous environments (e.g., produced by the

Figure 16.53 Serpentine chlorination reactor for wastewater effluent disinfection.

hydration of chlorine: Cl2 + H2O ! HOCl + H+) will disassociate into a hypochlorite (OCl~) anion (HOCl ! OCl~ + H+) whose antimicrobial efficiency is far lower than that of the hypochlorous acid (i.e., HOCl) form.

Another important fact is that hypochlorous acid reacts readily with reduced ammonia (NH3), leading to a series of amination reactions and products [i.e., monochloramine, NH2OCl; dichloramine, NH(OCl)2; and nitrogen trichloride, NCl3] whose bacteriocidal efficacy is again less that of the original HOCl.

Bromine use as a disinfectant also has its own set of unit features in terms of chemistry, benefits, and shortcomings (Water Environment Federation, 1996). First, the operative hypobromous acid (HOBr) species does not disassociate until it reaches a pH of about 8.5, so offering a wider range of serviceability them chlorine. Second, bromine tends to have a higher level of efficacy at equivalent concentrations, so lower dosage levels might be used. Third, the bromamine forms are all far better disinfectants than are the chlora-mines. Indeed, a solid-phase, bromine-bearing compound called bromochlorodimethylhy-dantoin (BCDMH) is widely marketed for hot tub and spa applications, given the particular prevalence of urinary ammonia release. One clear disadvantage, though, is that of cost, which tends to be several times higher than that of chlorine if the latter's efficacy attributes are ignored.

Hydrogen peroxide has little, if any, credible role as a disinfectant in environmental engineering systems, but ozone has found widespread acceptance, particularly in Europe, as a potable water disinfectant. As compared to any of the halogen-based options, ozone has the unique ability to dissipate shortly after its addition, without any semblance of a lingering residual. This lack of a residual is considered a disadvantage in the United States, where residual chlorine levels are maintained routinely in potable water delivery systems as a safeguard against subsequent contamination that can occur within the distribution system. However, the conventional wisdom in Europe is that disinfecting chemical residuals are both unwarranted and undesired. Yet another important aspect of ozone use is that it must be produced on-site, using electrical hardware that is not all that simple and at a cost that is considerably higher than that of chlorine.

A wide range of nonoxidizing organic and inorganic chemicals are used for, or are able to provide, disinfecting effects, including aldehydes (formaldehyde and glutaraldehyde), phenolics, alchohols (ethanol and isoproponal), cationic detergents, nitrites, and heavy metals (e.g., mercury, silver nitrate, tin, arsenic, copper). Although most of these chemicals have little relevance for the disinfection of waters, wastewaters, or sludges, there are two noteworthy exceptions. Specifically, silver-impregnated filters are sometimes marketed for point-of-use water conditioning devices, such as those that are sometimes screwed onto the outlets of sink faucets. In this instance, the silver is intended to be slowly leached from the filter medium (typically, activated carbon) at a rate that, hopefully, will retard the opportunistic formation of microbial biofilms intent on using sorbed organics as their energy source. A second nonoxidizing chemical disinfectant option is that of using cationic detergents in the form of quaternary ammonium compounds (formed as organic salts of ammonium chloride and commonly referred to as quats). One such common application is that of controlling biofilm growth on cooling tower surfaces, in which aggressive (i.e., oxidative) disinfection agents such as chlorine, bromine, and ozone would unacceptably attack exposed wood or metal heat-transfer surfaces.

One of the unique disinfection features of the bactericidal quaternary compounds is that they have a distinctly higher level of effectiveness with many gram-positive bacteria, probably due to the added depth and complexity of their membrane structure. Conversely, gram-negative cells as a whole are often similarly considered to be somewhat more resistant, perhaps due to the added depth and complexity of their membrane structure. Indeed, Pseudomonas probably tops the list in terms of durability, under conditions that would foil the vast majority of other cells (e.g., growth in distilled water). Similarly, gram-positive Mycobacteria species, as well as spore formers, also tend to exhibit this resistant nature when challenged with quaternary disinfectants, apparently based on the protective capacity of their respective outer cell coatings. Finally, given their chemical nature, these quat compounds also bear a unique sensitivity to inactivation when exposed to complexing soaps, detergents, and organic materials.

The fourth and final disinfection mechanism is that of altering and disrupting a cell's genetic makeup so that the cell is prevented from reproducing even though it may still have the energy to do so. This disinfection effect is largely associated with the use of ultraviolet irradiation, and its consequent high-energy cross-linking of adjacent nitrogen base groups poised side by side at various points within stranded DNA (see the thymine dimer reaction in Figure 16.54). The resulting impact of polymerizing DNA and formation of thymine dimers follow much the same path, such that cells are effectively sterilized by this UV exposure.

The range of wavelengths associated with ultraviolet irradiation actually has considerable breadth, from 4 to 300 nm, but the highest level of absorbance by DNA appears to fall nearly coincident with the maximal output value (i.e., at approximately 254 nm) for the emission spectrum of light emitted by mercury-vapor light bulbs. The standard engineering application of UV irradiation involves an array of mercury bulbs placed inside an irradiation chamber, through which the water or wastewater is passed, with each such tube being jacketed inside a UV-transparent quartz jacket. These tubes are aligned vertically or horizontally in a fashion where the hydromechanics of the operation provides maximal opportunity for exposure of cells flowing through the chamber while obviating

Thymine dimer

Figure 16.54 Thymine dimer formation along DNA strand produced by polymerizing ionizing or UV irradiation.

Thymine dimer

Figure 16.54 Thymine dimer formation along DNA strand produced by polymerizing ionizing or UV irradiation.

short-circuiting pathways that would degrade process efficiency. Figure 16.55 depicts one such array used for disinfection of a wastewater effluent immediately prior to discharge. UV irradiation with mercury bulbs offers an effective means of sterilization for those engineering applications involving fairly clear or optically transparent waters and wastewaters. Conversely, sludge is not amenable to UV disinfection given the unacceptably shallow degree of light penetration into this material.

Figure 16.55 Ultraviolet irradiation reactor for wastewater effluent disinfection.

Ionizing radiation using x-ray and gamma-ray beams with even higher energy levels than that of UV irradiation, can also be used in water, wastewater, and sludge disinfection. These ionizing mechanisms displace electrons during beam bombardment (i.e., at which point they are said to ionize), and in the presence of oxygen these displaced electrons elec-trochemically form a type of free radical (called hydroxyl radicals) that is highly toxic to microbial cells. Free radicals are highly reactive, having essentially no activation energy for their reaction. Given the acutely reactive nature of these radicals, they readily attack and destroy hydrogen bonds, double bonds, and ring structures essential to the metabolic utility of various cellular molecules. Yet another operative mechanism, and perhaps the key factor behind disinfection with ionizing irradiation, is that of a polymerizing impact (e.g., DNA thymine dimerization) whereby the biochemical effectiveness of complex molecules is degraded or terminated. This technology bears a degree of complexity and technical hazard that is not typically appropriate for most municipal applications, however, so that only a limited number of sites, the majority of which involve sludge disinfection presently rely on its use. On the other hand, ionizing radiation is used widely for the disinfection of pharmaceuticals and of disposable dental and medical supplies (e.g., syringes, gloves).

In reviewing the various options for disinfection, the mere fact that there is such a diverse range of disinfecting mechanisms and associated effects demonstrates that there is no perfect solution for all engineering applications. Excluding heat treatment and osmotic pressure, which have little if any pragmatic utility for large-scale engineered disinfection, the remaining options exhibit unique differences in their associated costs, technical complexity, residual impacts, environmental sensitivity, and relative performance, such that an appropriate assessment of many such site-specific issues have to be completed prior to making a final decision.

As for defining the precise goal of disinfection, the normal benchmark for potable water is that of zero residual fecal coliform presence. In the case of wastewater disinfection, though, effluent standards tend to be targeted for a somewhat higher level of residual bacterial presence in a fashion that implicitly reflects the latter phenomenon of microbial resistance. Compared against the typical levels of bacterial presence within raw, undisin-fected wastewaters (such as those shown in le 16.15), therefore, effluent criteria for fecal coliform usually tend to be somewhat more tolerant (i.e., typically at 200 fecal coliform colony forming units per 100 mL). However, compliance with this sort of effluent fecal coliform standard would still require a sizable four- to five-log reduction (i.e., based on influent fecal coliform densities in the range 106 to 107 cells per 100 mL).

Extending beyond the theoretical aspects of these disinfection mechanisms there are pragmatic uncertainties and inherent regulatory concerns tied to the nature and degree

TABLE 16.15 Bacterial Levels Present in Representative Wastewaters

Observed Bacterial Densities (106 viable cells/liter)

Wastewater Total Coliform Fecal Coliform Fecal Streptococci FC/FS Ratio

Wastewater Total Coliform Fecal Coliform Fecal Streptococci FC/FS Ratio

TABLE 16.15 Bacterial Levels Present in Representative Wastewaters

Observed Bacterial Densities (106 viable cells/liter)

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