Off-stream raw water storage and pre-sedimentation are similar in many respects because both act as additional barriers to passage of Cryptosporidium and other contaminants, and both provide general benefits to subsequent treatment processes by equalizing the raw water source, including dampening of both hydraulic and water quality spikes. However, off-stream storage generally refers to structures such as lakes or reservoirs with storage times on the order of weeks, versus pre-sedimentation that refers to tanks and retention basins with storage times on the order of hours or days. Another difference is that metal salts, organic polymers, or recycled softening sludge are commonly added in order to enhance physical separation within the pre-sedimentation basin, though some facilities do not add coagulant on a regular basis.
The LT2ESWTR allows Cryptosporidium removal credit for pre-sedimentation under certain conditions. With pre-sedimentation, utilities may either sample their source water after the pre-sedimentation basin and use that for determining bin assignment, or else use the pre-sedimentation system for Cryptosporidium removal credit. If off-stream storage is used, then sampling for bin assignment is done downstream of that storage, but the storage can not be used for removal credit under LT2ESWTR.
Pre-sedimentation is also similar to conventional clarification, except that pre-sedimentation basins are only intended to meet pretreatment objectives and, consequently, are often operated at higher overflow rates than conventional clarification. When coagulant addition is occurring and the treatment process is being actively managed to achieve clarification, solids removal achieved in pre-sedimentation reduces the solids load on the primary clarification process in the water treatment plant. The water treatment plant can operate the two processes in a complementary fashion so that the overall treatment process is less dependent on the removal of solids during filtration.
Pre-sedimentation is a useful tool in this process because it can help manage the initial particulate load reaching the primary treatment process. Pre-sedimentation not only initiates the solids removal process but it also helps to equalize water quality reaching the primary treatment process. The equalization, for example, can help dampen impacts of short periods of high contaminant concentrations (i.e., a chemical spill event). Such an approach is used by the City of Philadelphia Water Department, and others (AWWA, 2004). The removal of total aerobic spores (including Bacillus subtilis) during operation of full-scale pre-sedimentation basins spore has been shown to range from about 0.5 to 1.0 log for pre-sedimentation basins operated between 0.3 and 2.8 gpm/ft2 (AWWA, 2004) All of these basins either included addition of an organic polymer, a metal salt coagulant (e.g., ferric sulfate), or recycled lime softening solids.
One of the benefits of off-stream storage is that an off-stream storage reservoir can be isolated from the raw water source when needed, for example during the first flush after a storm or during a chemical or other spill. The treatment plant can operate using the reserve storage in the reservoir until the contamination in the raw water source passes the intake. LeChevallier et al. (1998) in a study on the Delaware River noted that peak levels of microbial contaminants following storm events were associated with peak turbidity spikes. By discontinuing pumping from source water during this period, Cryptosporidium spikes of 12 to 16 times baseline levels could be avoided. This illustrates that off-stream storage coupled with proper intake management practices can allow operations personnel to discontinue raw water pumping to avoid exposure to high risk events, such as first flush following a storm.
Several studies have investigated the reduction of Giardia, Cryptosporidium, and other microbial indicators during off-stream reservoir storage in the Netherlands (van Breemen et al., 1998; Ketelaars et al. 1995; Oskam, 1995; Medema, et al. 1998). These data indicate average of 1.7-log Cryptosporidium removal during 5 or more months of off-stream storage for one system, and about 1.3 log after 70 days storage in another reservoir. Bertolucci et al. (1998) report data from Turin, Italy demonstrating mean Cryptosporidium removals of about 1 log after 18 days of storage.
LeChevallier and Au (2001) have summarized many of the literature sources currently available that address the removal or reduction in viability of Cryptosporidium and other microbial indicators during off-stream storage. Appendix 2 of the AWWA formal comments to USEPA on the LT2ESWTR cited earlier (AWWA, 2004) includes literature references for studies investigating microbial removal and inactivation in off-stream raw water storage, especially focusing on Cryptosporidium.