Conventional water traveling screens, in-line debris filters, strainers, and ultrification can be effective for blocking adult mussels and shells. Typical screen mesh openings (3- to 10-mm) and filter or strainer opening size (3- to 5-mm) are too large to block mussel veligers, however, (EPRI 1992) these screens and filters can be obtained in sizes suitable for the largest once-through circulating water systems down to small raw water makeup systems. Although screens and filters can control the carryover and carry-through of translocators and shells from the intake to the downstream piping system. They cannot control the growth of mussels within the piping system. Ultrafiltration (40) is a feasible zebra mussel veliger control technology for small flow cooling water systems Claudi and Mackie 1994, (EPRI 1992, Smythe and Short 1995). Rigden (1993) reported on filters with 25- and 40 mesh screens. After testing in adverse conditions such as high turbidity, the filters successfully stopped all zebra mussel veligers utilizing either if the screens. Other filtration tests (e.g., Dardeau and Bivens 1995, Smythe and Short 1995) are currently underway.
Traveling screens at raw water intakes are effective blocking the passage of adult zebra mussels into raw water intakes. Traveling screens in either the through-flow or dual-flow configurations are typically located downstream from trash racks. Baskets with screen mesh are mounted on a vertically rotating chain and continuously screen the flowing intake water. Mesh openings are usually small enough to collect adult zebra mussels, though veligers are not blocked. As baskets travel along the rotation cycle they are exposed to air and subjected to water jet cleaning. Debris and shells washed from the screen are carried off in discharge flow. This system allows for uninterrupted, unobstructed screening.
Because traveling screen mesh openings are too large to restrict the flow of mussel veligers, they do not prevent subsequent infestations within the piping systems. Traveling screens should, therefore, be used in conjunction with other control strategies for the protection of water intakes. In addition to the cyclic areal exposure and spray cleaning, screen can be kept unfouled by using nonmetallic, smooth screen baskets, or antifoulant-coated baskets. Water spray pressure should be in the 550 kPa range for effective cleaning (EPRI 1992).
Preventive Control Methods