A number of chemicals have been tested with varying degrees of success and acceptability. Claudi and Mackie (1994) pointed out that the major advantage offered by most chemical treatments is that they can be engineered to protect almost the entire facility. The disadvantage rests in limiting the discharge of toxic materials to the environment and meeting environmental regulations.
Chemical treatment technologies are subjected to continued scrutiny, and environmental concerns may further limit their use. However, until suitable substitutes are found, facility managers will have no choice but to rely on chemicals as a component of their overall control strategy for problem infestations of zebra mussels. Many chemical treatments have been tested, but chlorination, specifically, sodium hypochlorite (NaOCl) seems to have the widest use and acceptance. Section IV (Mitigation) of the volume edited by Nalepa and Schloesser (1993) contains case studies dealing with successes and failures of various chemical applications to control zebra mussels.
Oxidizing Chemicals
Of all the chemical methods tested thus far, chlorination seems to have almost universal acceptability in that it generally satisfies environmental concerns and is also affordable and reasonably easy to apply at most facilities. Change is on the horizon, however, and new control strategies are being proposed and tested in anticipation of more stringent environmental limitations. Chlorination forms trihalogenated methanes and other hydrocarbons that are carcinogenic. In 1993, Ontario Hydro gave its researchers 5 years to develop alternative control strategies that might replace chlorination. Even though this goal is being pursued, the Canadians believe that chlorination will always have to be a viable control option especially with problem infestations that they have experienced on the Great Lakes and St. Lawrence River. There is also a trend in the United States, toward tighter environmental restrictions regarding to the use of chlorination. Like their counterparts in Canada, United States facility managers will need to have chlorination available to them for zebra mussel control, especially when operation is in jeopardy or efficiency is greatly reduced. Thus, guidelines are needed for the reasonable and prudent use of chlorination as a means of zebra mussel control at hydropower facilities.
Chlorination systems have gained wider acceptance than other treatment technologies, mainly because they effectively control zebra mussel infestations. McMahon, Ussery, and Clarke (1994) reviewed zebra mussel control methods, and noted that "there is a large and varied body of literature from Europe and, more recently, from North America, describing the relative of chemical and nonchemical control technologies for zebra mussels." Their tables are useful for comparing the merits of individual control methods. The treatment methods listed with the oxidizing molluscicides compiled by McMahon, Ussery, and Clarke (1994) include chlorination, chlorine dioxide (ClO2), and chloramines, with the latter being broadly classified as a reaction product of "chlorine with any compound containing the nitrogen atom with one or more hydrogen atoms attached (mostly inorganic nitrogen)" (Claudi and Mackie 1994). The literature clearly shows a variation in not only the methods of chlorine treatment but also the concentrations and duration of application and the lethality of these treatment. McMahon, Ussery, and Clarke (1994) report the following application and effect ranges for chlorine treatments (Table 1):
Hypochlorite Reaction
Chlorine treatments have relied on the use of pressurized gas; liquid sodium hypochlorite is the chlorine source of choice because of safety concerns. Sodium hypochlorite, NaOCl, is considered a safe and versatile chlorinating liquid. Claudi and Mackie (1994) described the reaction that takes place when sodium hypochlorite is added to water, with hypochlorous acid (HOCl) formed as the oxidizing agent in this reaction. As a "weak" acid, hypochlorous acid tends to undergo partial dissociation, to produce a hydrogen ion (H+) and a hypochlorite ion (OCl-). Hypochlorous acid has more biocidal effect than the hypochlorite ion because of its ability to penetrate cell walls (White 1986). The FAC is the combined amount of HOCl and OCl- in the water.
Chloramine
As indicated earlier, chloramines are produced in the reaction of free available chlorine with various forms of nitrogen containing compounds occurring in the water such as ammonia, nitrites, nitrates, and amino acids. Chloramines are formed naturally when chlorine or sodium hypochlorite is added to raw water. The chloramines include monochloramine (NH2Cl), dichloramine (NHCl2), and trichloramine (NCl3), together designated as TCC (Claudi and Mackie 1994). The more ammonium that is present, the higher the level of chloramines that are formed. Claudi and Mackie (1994) stated that chloramines are considered less powerful as oxidants than hypochlorous acid. At sites where the formation of trihalomethanes is a concern, the use of chloramines offers some advantages. Chloramine treatments are applied by co-injection of ammonium as either ammonium gas or ammonium hydroxide and sodium hypochlorite. Exact dosing requirements for effective zebra mussel control is unknown.
Chlorine Dioxide
Chlorine dioxide (ClO2) has been an effective disinfectant in the water industry for over 50 years (Claudi and Mackie 1994). Unlike the hypochlorite reaction, its by-products are primarily sodium chloride and sodium chlorite and does not lead directly to the formation of trihalomethanes. Opinions differ as to its effectiveness in zebra mussel control. The use of chlorine dioxide may not offer significant advantages over sodium hypochlorite when cost and ease of use are considered. Chlorine dioxide must be manufactured on-site with the use of specialized equipment. Chlorine dioxide control methods may be beneficial if a chlorine dioxide system is already in place or the formation of trihalomethanes is a serious problem.
Factors Influencing Chlorine Effectiveness
A number of raw water parameters influence the effectiveness of chlorine treatments. These factors include organic and inorganic compound concentrations, temperature, and pH (Claudi and Mackie 1994, EPRI 1992). The physical state of the zebra mussel and the extent of infestation will also influence the effectiveness of the chlorine treatment (Claudi and Mackie 1994).
Water chemistry has a very important impact on the toxicity of chlorination to zebra mussels. Claudi and Mackie (1994) stated that waters rich in organic and inorganic compounds have high chlorine demand, consuming larger amounts of chlorine residuals through oxidation-reduction reactions. The presence of reducing agents, such as S2-, Fe2+, Mn2+, and NO2-, accelerate the chlorine decomposition rate and should be taken into account to ensure expected zebra mussel mortality.
Water temperature effects both the dissociation of hypochlorous acid into the hydrogen and hypochlorite ions and the metabolic rate of zebra mussels. As water temperatures rise, the concentration of the more effective hypochlorous acid decreases as the concentration of the dissociated ions increase. Higher temperatures also seem to escalate the intake of chlorine compounds as the zebra mussel's metabolic rates increases. As a result, even though higher temperatures lower the toxicity of the chlorine, the increased uptake of chlorine compounds increases the overall chlorine effectiveness.
Water pH strongly influences the dissociation of hypochlorous acid into the hydrogen and hypochlorite ions. Claudi and Mackie (1994) presented a graph showing dissociation of hypochlorous acid versus pH, showing that when the pH of the chlorinated water is approximately 7.5, 50 percent of the chlorine concentration present will be undissociated hypochlorous acid and the remainder, the hypochlorite ion. A 100 percent hypochlorite ion concentration is attained at a water pH of 10. Conversely, at pH 5, 100 percent of the chlorine concentration will be the more effective undissociated hypochlorous acid.
Chloramine formation is pH-dependent; a lower pH will yield a higher concentration of dichloramines, whereas a higher pH will yield a higher concentration of monochloramines. Dichloramines are more potent disinfectants than monochloramines (Claudi and Mackie 1994). Maximum (100 percent) dichloramine concentrations occur at pH 4.5. At pH 8.5, 100 percent monochloramine concentrations exist.
Toxicity studies have shown that mature zebra mussels are slightly more resistant to chlorine than are various veliger stages (Claudi and Mackie 1994). Chlorine treatments are more effective at the end of a growing season due to the physiologically exhausted state of the mussel following the reproductive effort. There is an inverse relation between the population biomass and the treatment effectiveness. Larger populations, particularly individuals farther away from the surface layer, are less vulnerable than are single layer colonies (Claudi and Mackie 1994). Thus, multiple applications or multiple treatment methods may be necessary in problem infestations.
Treatment Strategies
The applied chemical treatment strategy is as important as the type of chemical used. There are five different chemical treatment strategies proposed by Claudi and Mackie (1994) for zebra mussel infestations: end-of-season, periodic, intermittent, continuous, and semicontinuous. A chemical zebra mussel control strategy may consist of a single treatment scenario or a combination of treatments used in concert. The treatments most applicable to a particular facility depends on the extent of zebra mussel infestation, the degree of permissible infestation, water quality, existing facility systems, economics, permit requirements and environmental regulations. An effective chemical treatment design allows for flexibility in treatment applications in accordance with the facilities entire zebra mussel control program for each facility.
End-of-Season Treatment
End-of-season treatment is generally a reactive strategy, acceptable in systems that can tolerate limited macrofouling. Limited macrofouling can be anticipated if chemical treatments are applied once during the year, usually after the spawning season or at the end of the growing season. Treatments after the spawning season increase chemical effectiveness and reduce required concentrations as individuals are fatigued and weakened. Also, shells and soft tissue debris of young-of-the-year mussels more easily pass through facility systems.
Mitigation of established mussels by end-of-season treatments requires higher dosages of chemicals over an extended period of time (2 to 3 weeks) (Claudi and Mackie 1994). Chemical concentrations and exposure times are dependent on the chemical used, water quality, and health of the mussels. Defining absolute levels applicable to all locations at all times is very difficult. Byssal threads remaining after end-of-season treatment can promote the settlement of veligers, and cause corrosion, and add surface friction.
Periodic Treatment
Periodic chemical treatment, like end-of-season treatment, is usually a reactive treatment (usually conducted on a regular basis, such as every 2 months) designed to eliminate adults that have accumulated since the previous application. Again, limited infestations must be tolerable, but because treatments are more frequent, infestations will be proportionally smaller. The chemical concentration and exposure time should be comparable to end-of-season values, though the total removed biomass will be smaller.
Intermittent Treatment
Intermittent chemical use is designed to prevent initial zebra mussel infestation at facilities that cannot tolerate macrofouling. Dosing at frequent intervals (e.g., 6, 12, 24 hr) destroys post-veligers that have settled since the previous treatment. Post-veligers are more susceptible to oxidizing chemicals than are adults; thus, the concentration of the chemical and exposure times will be considerably less than if adults were the target. Because post-veligers with shells about 250 mm long can easily pass through the system, disposal and under-deposit corrosion is eliminated.
Semicontinuous Treatment
Semicontinuous treatment is a preventive control method developed by Ontario Hydro. Because zebra mussels will stop filtering and close their shell when exposed to a toxic substance, the utility postulated that frequent on-off cycling of chlorine was more effective than continuous chemical treatments. Treatment schedules can be adjusted to 15 min on and 15 to 45 min off. Chlorination treatments consisting of 15 min on and 15 or 30 min off at the 0.5-mg/ l level have been as effective as continuous treatment (Claudi and Mackie 1994). Semicontinuous treatment is ideal for facilities where several discrete systems need to be treated and results in less chemical usage, than continuous chlorination.
Continuous Treatment
Continuous chemical treatment is designed for facilities that cannot tolerate any level of macrofouling. Low chemical concentrations, applied continuously, prevent any post-veliger settlement and is stressful enough to either kill adult mussels or cause them to detach and move out of the system. Continuous treatment should be carried out for the entire zebra mussel breeding season.
Nontarget Effects of Chlorine
Chlorine, chloramines, and chlorine dioxide are nonselective and highly toxic to nontarget fish and invertebrates. Claudi and Mackie (1994) have provided a detailed information on the impacts of chlorination on fishes, invertebrates, and phytoplankton, which can be consulted for guidance. Fish seem to be more negatively affected than are other aquatic organisms (Claudi and Mackie 1994), though literature related to the effects on other aquatic organisms (i.e., invertebrates and phytoplankton) is less abundant. Following chlorine treatment, phytoplankton populations may drastically decrease; however, their recovery is generally rapid.
Besides killing the nontarget organisms, sublethal life parameters of nontarget species that chlorine may affect include behavior, reproduction, growth and mutagenesis. Claudi and Mackie (1994) stated the most important aspect of behavior affected by chlorination is avoidance, and fishes have received more attention in the literature with regard to their avoidance of chlorine. Reproduction is a sensitive indicator of sublethal toxicity. Chlorination adversely affects the reproduction of certain nontarget aquatic organisms and its presence inhibits the growth of both plant and animal species. Chlorine can also react with dissolved organic material to form chlorinated organics, some of which are suspected mutagens.
Dechlorination
Federal and state statutes regulate the concentrations of chlorine that can be released into the environment and require that water samples be analyzed accurately for the presence of free and residual chlorine. A major concern when using chlorine in fresh waters is that it will combine with various organic compounds to form trihalomethanes, which are considered carcinogenic. Stringent requirements are also placed on the level of total residual chlorine allowed in the discharge. Facilities unable to meet TRC water quality limits must dilute the discharge with raw water or neutralize the chlorine prior to release. Sodium sulfite (Na2SO3), sodium bisulfite (NaHSO3), sodium metabisulfite (Na2S2O5), or sulfur dioxide (SO2) may be used. The most convenient chemical used to neutralize residual chlorine is sodium bisulfite (sometimes called "liquid sulfite"), with the dosage requirement being a concentration of 1.8 to 2.0 units of sodium bisulfite for each unit of TRC (Claudi and Mackie 1994). Sodium bisulfite can be fed directly into the discharge prior to reintroduction into the waterbody because the reaction of chlorine with sulfite is almost instantaneous.
Bromine
Several forms of bromine can be used as antifoulants, including activated bromine, sodium bromine, bromine chloride, and proprietary mixtures of bromine and chlorine or other chemicals (e.g., Acti-Brom by Nalco Chemical Company and BromiCide by Great Lakes Chemical Corporation). Generally, the same precautions for chlorine apply to bromine (Claudi and Mackie 1994). Bromine in all forms has been shown as a more effective oxidizing agent than chlorine when water pH levels are greater then 8.0. Bromine ultimately forms Br- in aquatic systems; however, the pathways are largely specific to environmental conditions. Furthermore, a single bromine atom may undergo a series of cyclic transformations. Hence, exact mechanisms and temporal relations are not well understood (EPRI 1993). The type of information available for treatment using chlorine is not readily available for bromine or bromine-based products. However, as a rough guide, the amount of total oxidant required would be the same with bromine and chlorine. Bromine has the reputation of being less toxic to nontarget species than chlorine. However, recent data suggest that the toxicity to nontarget species is in fact higher than that of chlorine (Howe et al. 1994).
Ozone
Ozone is a well-known bacterial agent, used in Europe to disinfect drinking water and industrial and municipal wastewater (EPRI 1993). Ozone also improves taste, odor, and color of drinking water and can be used to prevent biofouling. Ozone out performs chlorine in terms of contact time at comparable residual levels. Lewis, VanBenschoten, and Jensen (1993) indicated that at 15 to 20°C, a minimum of 5 hr contact time was required at 0.5 mg/ l for a 100-percent mortality of veligers and post- veligers in the water column 1. Ozone residuals of 0.5 mg/ l or greater for 7 to 12 days will cause 100-percent mortality of adult zebra mussels. Time to death is inversely related to both on concentration and ambient temperature.
1 (Lewis, D., Van Benschoten, J.E., and Jensen, J. N. (1993). "A study to determine effective ozone dose at various temperatures for inactivation of zebra mussels," unpublished report, Ontario Hydro, Toronto, Canada.)
Ozone is highly explosive, especially when solutions are warmed. Commercial ozone is not available due to shipping problems, and ozone used in water treatment is always generated on site (EPRI 1993). Ozone is a powerful natural oxidant in the atmosphere not occurring naturally in surface waters. When released in natural waters, residual ozone concentrations quickly dissipate. Dissipation in raw water is so rapid that, if injected in pipe intakes or forebays, no ozone residual can be found in facility discharge.
Properties of ozone offer both advantages and disadvantages. Ozone treatments do not exhibit downstream environmental impacts, making it attractive for use in once through systems. This characteristic, however, is undesirable when considering control of downstream zebra mussel settlement and growth. Maintaining sufficient residual ozone levels required to kill adult zebra mussels in an extensive piping system is very difficult and expensive, requiring multiple injection points would be required.
Potassium Permanganate
Potassium permanganate (KMnO4) is another oxidizing chemical commonly used in municipal facilities for water purification. It is widely used for oxidation of iron and manganese and for control of taste and odor problems. Cost and effectiveness have limited municipal use of potassium permanganate to control zebra mussels. Unlike chlorine, potassium permanganate does not eliminate the mussels except at high, continuous dosage. The greatest advantage of potassium permanganate use is that it does not produce THMs. Some studies suggest that it can be used against adult zebra mussels although it is less effective than chlorine (Klerks, Fraleigh, and Stevenson 1993; Claudi and Mackie 1994). In flow through experiments using 1.0 and 2.5 mg/ l potassium permanganate, veliger densities in the outflows of treatment bioboxes were reduced 90 percent from inflow densities. At these concentration, potassium permanganate also prevented settlement of zebra mussels in the test tanks. In static experiments, Klerks, Fraleigh, and Stevenson (1993) found 27-percent mortality in veligers exposed to 2.5 mg/ l of potassium permanganate for 3 hr. These results suggest that potassium permanganate may prevent settlement of zebra mussels, but it is not acutely toxic to the veligers.
Sodium Chlorite
Sodium chlorite (NaClO2) solutions appear to have numerous advantages over other chemicals. They have more environment-friendly characteristics, such as the non-generation of undesired byproducts common to the use of chlorine. Their use does not induce increased water oxidation/reduction potential and they are non-corrosive. These solutions are stable and easy to apply with the existing equipment in most industries that commonly use hypochlorite as a treatment. If the concentration and exposure time required for an efficient kill of the mussels could be lowered, there is a great potential for future use of sodium chlorite treatment (Dion, Richer, and Messer 1995)
Sodium chlorite is an oxidant. When dissolved in water, sodium chlorite produces the chlorite ion, ClO-2. The other ingredients in solution set up an oscillation reaction that quickly converts ClO-2 to chlorine dioxide (ClO2) and dichlorine dioxide (Cl2O2), which in turn, produces superoxide, O-2 and up to 62 other intermediates. In the oscillation reactions, the intermediates have very short lifetimes that do not allow the formation of undesired by-products, without stopping their known biocidal activity.
For all mussel sizes, increases in treatment concentration above 80-ppm dilution, offer no benefit. Even at the highest concentration tested, the time required for 50 percent sample mortality (LT50) is over 10 days. A shock treatment is therefore not a good option for these chemicals.
Non-oxidizing Chemicals
Non-oxidizing chemicals are effective in controlling zebra mussels because the organisms appear insensitive to such compounds. In the presence of oxidizing chemicals such as chlorine, zebra mussels will close their valves to avoid the chemical. When non-oxidizing compounds are applied, the valves remain open, while water is actively filtered through their gills, exposing the tissues to the toxic actions of compounds even when the chemicals are present in surrounding waters in relatively high concentrations. The toxic actions of the non-oxidizing chemicals prevent the zebra mussel from maintaining its chemical balance, resulting in death.
A number of efficacious non-oxidizing chemicals have been developed to control zebra mussels in raw water systems. Very few of these chemicals have EPA registration for use in once-through cooling systems. Primarily, there is a concern with the persistence of these chemicals in the environment after discharge. Those that have been registered are used primarily for end-of-season or periodic treatments (Claudi and Mackie 1994).
Non-oxidizing Molluscicides
Non-oxidizing molluscicides are one of the primary chemical treatment methods available to control zebra mussels. Non-oxidizing biocides are a group of proprietary chemicals that have been found effective in producing mortality in zebra mussels. Quaternary ammonium compounds (QACs) are the most frequently used non-oxidizers. A number of chemical companies market these chemicals under various trade names such as Clam-Trol CT-1 (Betz Chemicals), H-130 (Calgon Corporation), Macro-Trol 7326 (Nalco) and poly-quaternary ammonium compound Bulab 6002 (Buckman Laboratories). More recently, a propriety variation of endothall has been tested against zebra mussels and found effective. This product is marketed by Elf Atochem and is being tested as TD 2335 (Claudi and Mackie 1994, Green 1995).
In addition, Mexel 432, a product marketed by RTK Technologies, Inc., of Baton Rouge, LA, has received EPA approval for use as a molluscicide. It is an aqueous dispersion of straight-chain aliphatic hydrocarbons with alcohol and amine functionality. It controls zebra mussels in three ways: (a) on clean surfaces, the film prevents settlement; (b) on infested surfaces, it attacks the byssal thread and inhibits formation of more byssal threads, causing many viable zebra mussels to detach; and , (c) it forms a film on zebra mussels that remain in the system, causing lesions on the gill surfaces and, ultimately, death on the organism.
Mexel 432 biodegrades rapidly to harmless substances, necessitating daily dosage to sustain the film. Its half-life in river water is 22 hours at 19 º C. Biodegradation is accelerated by agitation and aeration. Mexel 432 decomposes immediately in the presence of oxidizing agents such as chlorine or ozone (Giamberini, Caembor, and Pihan 1995; Khalanski 1994; Van Donk 1995).
The success of a non-oxidizing zebra mussel control treatment is directly related to the time spent planning the treatment. Procedures should be in place prior to the treatment to overcome any obstacles that may prevent completion. Contingency planning, facility preparation, and accurate chemical application will ensure an effective treatment.
Treatment
Application of non-oxidizers in once-through systems typically consists of short periodic applications during the warm water season. Depending on the chemical used, permit restrictions, and ambient water temperatures, significant mortality can occur in 4 to 24 hr (Claudi and Mackie 1994, Green 1995). Non oxidizing treatments are used to cleanse the system of recently settled mussels. Mortality is reported at or near 100 percent (EPRI 1992). Such treatments are effective provided a regular program of periodic treatments is employed.
Because treatments are designed to cleanse the system of any settled zebra mussels, application of non-oxidizing mulloscicides is heavily dependent on the history of the local mussel population. Frequency and timing of treatments are established based on the settlement history and veliger population dynamics. Therefore, thorough monitoring programs are essential. A complete veliger monitoring program along with accurate data relating to the lake or river temperature regime, will result in and effective and economic treatment schedule.
Normally two or three applications a year have proved sufficient to kill all the mussels in a system. A treatment should, therefore, occur immediately after peak veliger activity and the beginning of settlement evidence. Normally this is approximately 4 to 6 weeks after the veliger peak (Green 1995). Timing a treatment in this manner allows the remaining shells to be flushed through the piping system without causing flow blockage in small pipes, valves, and screens. Scheduling a treatment at this time also assures that the settled mussels are quite small. An early season treatment is also recommended to cleanse the system of any late season settlement and to prevent any translocaters from becoming established in the piping systems.
Because plant configurations and water volumes vary, several differing strategies can be employed for the application of non-oxidizing biocides. These methods include targeted treatments, entire system treatments and recirculation treatments (Green 1995). Treatments are coordinated with veliger and mussel settlement data to minimize the frequency of applications.
Treatment of the entire system is recommended for facilities with relatively small water usage. To treat the entire system, the non-oxidizing chemical is added to either the forebay or injected into either the suction or discharge of system pump piping. Forebay addition is preferred. Addition into the forebays should be as far in advance of the pumps as possible to allow for proper mixing to occur and the forebays to be treated.
Targeted treatment should be used by facilities when only select components are fouled by zebra mussels. Facilities may have several raw water systems being fed by a single forebay or intake. Treating the entire system by injection at the intake may be cost prohibitive and environmentally damaging. Targeted treatments are utilized in the individual fouled systems. Thus, a smaller water volume and a lesser amount of non-oxidizing biocide will be required, multiple systems being supplied by a common forebay. Injection of the non-oxidizing biocide is made either directly in front of the smaller pumps or to the piping pump area.
Large-volume systems that can be isolated are ideal for recirculation type treatments. Recirculation also makes possible the eradication of mussels in the forebays. Once a particular system is isolated from external water contact, the non-oxidizing mulluscicide can be added to the system (usually to the forebay) and the water recirculated for a predetermined length of treatment. After the desired time has elapsed the forebay and discharge bay can be reconfigured, and the water systems flow paths returned to normal.
Detoxification
Effects of non-oxidizing mulluscicides on nontarget species are a major concern. Non-oxidizing chemicals are normally added at dosages that are toxic to zebra mussels as well as other aquatic organisms. Consequently, detoxification is required to meet state and Federal discharge requirements. Bentonite clay, added to the plant discharge upstream of its entry into an aquatic ecosystem, is the standard detoxification agent (Claudi and Mackie 1994, Green 1995). Additional downstream sampling for chemical residuals is a normal permit requirement.
Potassium Ions
A number of metallic salts have been tested for toxicity to zebra mussels. Of these, potassium (K+) may have the greatest potential for use in on-line control of zebra mussel macrofouling. Normal concentrations are between 88 and 228 mg/ l, depending on the potassium compound used, permit restrictions, water quality, and ambient water temperature (Fisher 1991). In flow-through experiments, Fisher, and Polizotto (1993) found 50 mg/ l of potassium chloride prevented settlement of zebra mussels in test chambers.
Potassium compounds are nontoxic to higher organisms, such as fish (Claudi and Mackie 1994). Unfortunately, many native freshwater mussels are even more sensitive to potassium salts than are zebra mussels (tolerance level of 4 to 7 mg/ l), making their use and approval for control of mussel fouling in once-through raw water systems unlikely. In closed-loop systems however, these compounds can be an attractive, economic alternative.
Flocculation
Flocculation is used to remove unwanted suspended particles from drinking water supplies. This process causes small particles to agglomerate into larger particles or floc which is of sufficient size and density to settle. The agglomeration of fine suspended particles is a result of interparticle polymer bridging. Aluminum sulfate (alum) is the flocculant most frequently used in the drinking water industry. Alum will remove zebra mussel veligers by causing chemical toxicity in the mixing zone and by flocculation of both living and dead veligers in other areas. Studies by Mackie and Kilgour (1993) investigated the effect of alum on zebra mussel veligers. Mackie and Kilgour (1993) found that the alum concentrations used in most water treatment plants (i.e., 20 to 50 ppm) is not sufficient to kill zebra mussel veligers. Studies indicated that the lethal alum concentration for 50 percent mortality is near 126 ppm. Most veligers remain alive for at least 24 hr in the floc at concentrations below 100 ppm. The studies also indicated a pH below 5 caused by the addition of alum, especially in the area of alum addition (mixing zone), caused instantaneous kill of veligers. Mackie and Kilgour (1993) also found that the role of alum in removal of veligers appeared to be mainly a physical one, with the floc physically removing even living veligers. Prechlorination improves the efficacy of alum in removing veligers from raw water supplies. Flocculation may be an appropriate zebra mussel mitigation treatment for some drinking water plant intakes, provided that flocculation does not cause sediment formation problems in the intake (Claudi and Mackie 1994).