Entre las comunidades de aprendizaje y las comunidades de prácticas

The LT2ESWTR allows the option of changing the timing of withdrawals to obtain a lower source water concentration of Cryptosporidium for bin assignment (40 CFR

141.716(b)(1)). If the system calculates its bin assignment based on this alternative timing, then after the compliance deadline, it must continue to draw its source water in the same manner (40 CFR 141.716(b)(4)). The operating conditions under which the samples were collected for the LT2ESWTR must be reported and submitted to the state with the monitoring results (40 CFR 141.716(b)(3)).

3.4.1 Toolbox Selection Considerations

As stated above, the change in timing must be consistent during Cryptosporidium monitoring and during routine operation after monitoring. Additionally, the LT2ESWTR does not allow source water monitoring to deviate from a predetermined schedule by more than 2

days, unless extreme conditions or situations arise that prevent sampling (40 CFR 141.702(b) and (c)). Given these limitations, the following provides examples of approaches that are recommended and others that are not recommended.

Recommended Approaches

Changing the timing of withdrawal on a daily basis (e.g., from the afternoon to morning to avoid suspended material stirred up by recreational water use).

Use a water quality indicator to avoid short-term increases in Cryptosporidium due to short-term weather or source water contamination events. For example, if a system routinely experiences a spike in turbidity and subsequently, Cryptosporidium, for a 12-24 hour period following a storm event, then the system may choose to set up a monitoring plan that delays withdrawal for a 24 hour period when detecting a spike in turbidity.

Approaches Not Recommended

Limiting withdrawal in response to seasonal effects or weather effects lasting on the order of days. This would be a difficult monitoring strategy to follow and stay in compliance with the 2 day sampling window.

3.4.1.1 Advantages and Disadvantages

The advantage of changing the timing of withdrawals is it requires no treatment changes, only a change in operations. For systems with multiple sources it also allows the greatest

flexibility in meeting water quality goals.

A disadvantage of relying on changing withdrawals to lower Cryptosporidium

concentrations is that it may result in decreased flexibility, since systems must follow the same withdrawal practices they did during Cryptosporidium source water monitoring. If electing to practice a withdrawal approach that defers withdrawal during likely Cryptosporidium events, then a system may need some raw water storage capacity.

3.5 References

Gregory, J. 1994. Cryptosporidium in water: Treatment and monitoring methods. Filtr. Sep. 31(3): 283-289.

Kortmann, R.W. 2000. Reservoir management approaches exemplified. Proceedings of American Water Works Association Water Quality Technology Conference.

Kortmann, R.W. 1989. Raw water quality control: an overview of reservoir management

LT2ESWTR Toolbox Guidance Manual 3-7 April 2010 Swabby-Cahill, K.D., G.W. Clark, and A.R. Cahill. Buoyant qualities of Cryptosporidium parvum oocysts. AWWA Water Quality Technology Conference. Boston: AWWA, 1996.

Walker M.J., C.D. Montemagno, and M.B. Jenkins. 1998. Source water assessment and nonpoint sources of acutely toxic contaminants: A review of research related to survival and transport of

4.1 Introduction

Bank filtration is a surface water pretreatment process that uses the bed or bank of a river (or lake) and the adjacent aquifer as a natural filter. The natural filter performs most efficiently when the surface water passes slowly through unconsolidated granular material. In such locations and under normal ground water flow conditions, bank filtration is suitable for accomplishing sufficient Cryptosporidium removal to partially meet the requirements of the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR). To accomplish this, a pumping well located in the adjacent aquifer induces surface water infiltration through the bed and bank.

Bank filtration differs significantly from artificial recharge and from aquifer storage and recovery, both of which rely on engineering works to move water into specially constructed and maintained recharge basins or wells for infiltration into or replenishment of the aquifer. Although microorganism removal can occur in such engineered systems, they are not bank filtration. This is because bank filtration relies solely on the natural properties of the surface water bed and aquifer, unmodified by engineered works or activity, except for the recovery of ground water via a pumping well. Sites with artificial recharge and aquifer storage and recovery operations may also receive bank filtration Cryptosporidium removal credit after a suitable site-specific study but are not eligible for automatic credit. Slow sand filtration also relies on engineered materials as the filter medium and so is not bank filtration.

A significant proportion of microorganisms and other contaminants are removed by contact with the aquifer material as the water travels to the well through the subsurface. Flow to the well may be horizontal or vertical, but more typically will take a variable path with both horizontal and vertical components. The water which has been induced to infiltrate through the river’s bed and bank is known as “bank filtrate.” It will be mixed with ambient ground water that has taken a different and typically longer path to the well. The ambient ground water may have originated as bed or bank infiltration from an upstream portion of the river or from a lake. It may have originated from infiltrating precipitation. Regardless, ambient ground water is likely to contain different contaminants and contaminant concentrations than bank filtrate because its origin and flow pathways differ significantly. Ambient ground water should not be assumed to be uncontaminated.

Aquifers suitable for bank filtration are composed of unconsolidated, granular material (i.e., grains) and have open, interconnected pores that allow ground water to flow. Pathogen removal is enhanced when fine-grained sediment is present along the flow path. Geologic units consisting primarily of fine-grained (e.g., clay-sized) materials will have higher removal but will be incapable of yielding economically significant water flow rates. In aquifers containing both sand-sized and finer grains, the presence of fine grains increases the possibility that pathogens will encounter a grain surface. This is because flow is slower and flow paths are longer than they would be in aquifers without such fine grains. Microorganisms will be removed from flow as they contact and attach to grain surfaces. Although microorganism (e.g., Cryptosporidium) detachment can occur, it usually does so at slow rates (Harter et al. 2000). When little or no

LT2ESWTR Toolbox Guidance Manual 4-2 April 2010 detachment occurs or when detachment is slow, microorganisms can become non-viable while attached to grain surfaces. Thus, bank filtration provides physical removal, and in some cases, inactivation, to remove pathogens from water supplies.

The purposes of this chapter are: 1) to clarify the requirements of the LT2ESWTR related to receiving Cryptosporidium removal credit for the use of bank filtration systems; 2) to present the current state-of-the-science, advantages and disadvantages of Cryptosporidum removal by bank filtration; 3) to explain how local geologic and hydrologic conditions affect the functioning and effectiveness of bank filtration systems; 4) to provide suggestions for optimal operation of bank filtration systems; and 5) to discuss necessary and sufficient elements of a field and

laboratory investigation as part of a demonstration of performance (DOP) at a bank filtration site (with or without engineered systems) to qualify for additional Cryptosporidium removal credits.

This chapter is organized as follows: