Introduction
It is important to note that water supply and wastewater management have been in the prime interest of human communities for several millennia. One of the key aspects of such a drinking water supply revolves around ensuring that it is treated to make it potable. The latter can be achieved through a wide range of means, among which direct potable reuse or DPR and indirect potable reuse or IPR are the most prominent and critical to the water supply system. In the case of DPR, the untreated water is directly supplied upstream of the distribution system, whereas, under IPR, an environmental buffer is utilized.
Definitions and Differences
One should be aware that IPR and DPR can have practical uses depending on the conditions, needs, and water specificities. In order to understand these differences, it is important to define these systems of potable reuse frameworks beforehand. It is stated that “the introduction of purified water into an environmental buffer such as a groundwater aquifer or a surface water such as a water reservoir, lake or river before the water is blended with a treated water supply system is called indirect potable reuse” (Davis, 2019, p. 1586). In other words, the wastewater is not directly introduced to the water distribution system, because it is relocated into a buffer before the treatment. For DPR, “the introduction of treated wastewater directly into a potable distribution system or into the raw water supply immediately upstream of the water treatment plant is called direct potable reuse” (Davis, 2019, p. 1586). Thus, unlike in IPR, the wastewater is directly relocated to the supply system, which can either go to the treatment plant or be distributed with the raw water.
Underlying Factors Behind IPR and DPR
With an ever-increasing population size as well as water demand in many regions of the United States, IPR and DPR are becoming plausible and feasible solutions. Additional factors include “water scarcity because of climate effects such as drought and seawater rise, as well as saltwater intrusion because of overdrawn potable water aquifers” (Davis, 2019, p. 1587). Although the drivers for IPR implementation vary across many regions, the high demand and low supply due to many factors make IPR attractive. However, when it comes to DPR, “the amount of water supplied is relatively small compared to the communities” (Davis, 2019, p. 1589). Another explanation can include a high level of drought occurrence rates and an ever-improving technological sophistication in identifying and removing major contaminants.
Water Quality Standards and Log Removal Value
One of the key aspects of both IPR and DPR is centered around the strict adherence to national and local standards. The National Safe Drinking Water Act SDWA sets the core standard for the minimum requirements of drinking water. Although the act has no specific IPR-related or DPR-related standardization needs, the wastewater reused in both cases must adhere to the SDWA. The log removal value, or LRV, is an important standard of measurement for the finished water contaminant and pathogen concentration numbers, which tend to be in smaller ranges (Davis, 2019). Thus, various processes, such as membrane filtration or reverse osmosis, are evaluated and measured in accordance with their corresponding log removal values.
Core Design Principles and FAT
When it comes to potable reuse, the treatment process performance must be effective at all times over a long period. The contaminants that cause chronic health issues are more dangerous because the exposure is longitudinal (Davis, 2019). Therefore, the core design principles include reliability, redundancy, robustness, and resilience. In other words, a potable reuse system must be reliable in terms of consistent delivery of high-quality water. Redundancy refers to “the use of measures beyond minimum requirements to ensure that the treatment goals are reliably met, or that performance can be more reliably demonstrated” (Davis, 2019, p. 1592). Robustness determines the resistance to failures and capability to address a wide range of contaminants, whereas resilience is ensured through the treatment system’s adaptability to failure.
In the case of design practices, both DPR and IRP must undergo treatment processes. It is stated that “full advanced treatment (FAT) trains typically consist of (1) microfiltration (MF), (2) reverse osmosis (RO), and (3) an ultraviolet light (UV)–based advanced oxidation process (AOP) with hydrogen peroxide” (Davis, 2019, p. 1594). In other words, FAT is a highly effective method of achieving the desirable log removal value and elimination of many contaminants.
Conclusion
In conclusion, indirect potable reuse of wastewater uses an environmental buffer, but DPR uses the untreated water directly supplied upstream of the distribution system. Direct potable reuse ensures that the wastewater from the reclamation plant is treated with FAT through screening, membrane biological reactor, chloramine disinfection, reverse osmosis, and UV oxidation. After these processes, the wastewater is blended with raw water for further treatment. However, for IPR, wastewater is cleaned, after which it is added to the water reservoir.
Reference
Davis, M. (2019). Water and wastewater engineering: Design principles and practice (2nd ed.). McGraw Hill.