Thermal treatment is generally an accepted nonchemical mitigation technology to alleviate macrofouling of raw water system by invertebrates. For the zebra mussel and other macrofouling species, the upper lethal thermal limits on which thermal mitigation strategies are based have generally been determined as either the acute upper lethal temperatures or the chronic upper lethal temperatures (McMahon et al. 1995). The reduced thermal tolerance of zebra mussels relative to other North American biofouling species makes the mollusc more susceptible to thermal mitigation.
Thermal treatment is a cost-effective and efficient method for zebra mussel control. Most regulatory authorities regard heat treatment as a more environmentally safe and benign method than chemical treatment; however, restrictions on the discharge of heated water have to be taken into account. The zebra mussel is capable of extensive temperature acclimation, affecting both its acute and chronic lethal temperature limits. Thus, regardless of the thermal treatment strategy employed, a raw water system will need to be heated to higher temperatures to achieve 100 percent eradication of mussel infestations during summer months when source water temperatures are elevated than in winter months when source water temperatures decline. Initiating either chronic or acute thermal treatments during periods when water temperatures are below maximum summer levels may significantly reduce both the exposure time and treatment temperature required to achieve 100 percent kills of zebra mussels. Also important is the fact that smaller zebra mussels have greater thermal tolerance than larger mussels. Because of their higher thermal tolerance, infestations consisting primarily of smaller individuals (the usual case if a raw water system is subjected to annual or biannual mitigation treatments) will require higher treatment temperatures and/or longer exposure times to induce the desired mussel kill (McMahon et al. 1995).
Acute Thermal Treatment
Acute upper lethal temperatures are defined as the temperature at which death occurs when water temperature is raised at a specific rate. Heating of raw water systems to the acute lethal temperature of zebra mussels followed by rapid return to normal operating temperatures is a promising thermal mitigation technology for zebra mussel macrofouling (McMahon et al. 1995). Use of acute upper lethal temperature treatment to mitigate zebra mussel fouling is most applicable in raw water systems where lethal temperatures are difficult or inefficient to maintain for extended periods. In these systems, increasing water temperature to a level that induces an instantaneous 100-percent mussel mortality followed by return to normal operating temperatures is more practical. Acute thermal treatment, which does not require precise, long-term regulation of elevated temperatures, has been proposed for raw water systems where operation above normal water temperatures for prolonged periods reduces efficiency and increases component wear, making chronic thermal treatment of zebra mussels economically infeasible. Acute thermal mitigation may also be particularly applicable for use in off-line components such as intake embayments heated by steam injection of other means.
The acute upper lethal temperature of zebra mussels is affected by both the acclimation or ambient water temperature and the rate at which the temperature rises and induces instantaneous death. The temperature at which instantaneous death ensues increases with increased acclimation temperature and increased heating rate. McMahon et al. (1995) studied the relation of acclimation temperature and rate of temperature increase versus zebra mussel mortality. The time to achieve 100 percent sample mortality (SM100) was recorded and the time required for induction of 50-percent (LT50), and near 100-percent mortality (LT100), respectively, at a given test temperature, were estimated. McMahon et al. (1995) found that the relation of intake water temperature and system heating rate to a temperature of 100-percent instantaneous mortality (i.e., SM100). Thus, the maximum temperature required for 100-percent mussel kill would be 41ēC if mussels were maximally acclimated to 25ēC and subjected to rapid heating rate of 1ēC/5ēmin. (Figure 4) plots the time required for 100-percent sample mortality versus acclimation temperature for heating temperature of 35, 36, and 37ēC.
Chronic Thermal Treatment
Zebra mussel thermal mitigation strategies based on the chronic upper thermal limits of the organism involve continuous exposure to constant lethal temperatures for durations sufficient to achieve significant mortality. Chronic thermal treatment for mitigation of zebra mussel infestations is most applicable to industrial and steam-electric power station raw water systems that generate heated discharge water and are designed to recirculate or backwash heated effluent into their intakes to maintain operating temperatures at relatively constant, elevated, lethal levels for prolonged periods.
The exposure time of chronic thermal treatments is affected by both the acclimation water temperature and the treatment water temperature. The required exposure time increases as the acclimation temperature increases and treatment temperature decreases (McMahon and Ussery 1995). Mitigation treatment with temperatures greater than or equal to 34 ē C could induce near 100-percent kills of zebra mussel infestations within 6 to 26 hr depending on the prior acclimation/operating temperature (McMahon and Ussery 1995). At treatment temperatures ranging from 34 to 37ē C, exposure times required for 100-percent kill of zebra mussel are short enough to be cost-effective, application temperatures are low enough to prevent major loss of production or excessive equipment wear and/or malfunction, and discharge temperatures are likely to be low enough to meet the discharge temperature restrictions of state and/or federal regulatory agencies (EPRI 1992, Claudi and Mackie 1994).