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Monochloramine Disinfection for Alternative Water Supplies Sean P. Menard and Thomas W. Friedrich
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quifer recharge, including aquifer storage and recovery (ASR), aquifer recharge (AR), and managed aquifer recharge (MAR), has become an important component of alternative water supply projects. The water sources available as options for AR projects include potable water, reclaimed water, stormwater, and raw surface water. The level of pretreatment (prescreening, physical/ chemical, biological treatment, filtration) and final disinfection is determined by the aquifer type (greater than 10,000 total dissolved solids [TDS] or less than 10,000 TDS), the end use, and the Florida Department of Environmental Protection (FDEP) regulatory requirements. Monochloramine is a combined chlorine disinfectant that is being increasingly used in the municipal wastewater industry; it’s also being evaluated to treat stormwater and/or surface water before discharge to AR wells or aquifer storage and recovery wells. Wastewater facilities that have final effluent and/or reclaimed water quality requirements limiting the level of total trihalomethanes (TTHMs) and haloacetic acids (HAA5) are evaluating the feasibility of converting to the monochloramine process as a stable total chlorine disinfectant to mitigate the formation of disinfection byproducts (DBPs), while still meeting disinfection requirements for total and fecal coliforms. An added benefit is a reduction in free chlorine demand and a corresponding decrease in the total chemical use and cost for disinfection. Jones Edmunds has had the opportunity over the past several years to work with multiple Florida-based clients to evaluated adding a monochloramine disinfection system to a facility to begin disinfecting raw water or to replace and optimize an existing disinfection system for more-efficient wastewater treatment. In recent years, clients have requested that monochloramine disinfection be implemented in a facility before the effluent enters recharge wells as an optimized method of disinfection that will reduce DBPs and fecal or total coliforms, and will be stable to preserve the water in the aquifer without harm to a future water resource. The key purposes of implementing these monochloramine projects have been to address the following challenges before sending water
to the AR wells or sending excess reclaimed water to a surface water outfall: S Reducing TTHMs, HAA5, dibromochloromethanes (DBCMs), and dichlorobromomethanes (DCBMs). S Meeting the required reduction of fecal coliforms and total coliforms. S Reducing chlorine demand and subsequent sodium hypochlorite chemical use and reducing overall operational costs due to reduced dosing requirements and morestable total chlorine residual. This article highlights the consulting firm’s approaches (completed and pending) to address a client’s individual challenges on a project and corresponding coordination used to implement the monochloramine into new and existing disinfection systems, including: S Monochloramine Formation Approaches S Key Components of Retrofitting a Monochloramine Design to an Existing System S Client Coordination and Communication S Specific Equipment Analysis S Discussion of Chemical Maintenance Requirements S Findings, Results, Conclusions, and Recommendations of Improvements Regulations—current and pending—will also be discussed, and program costs (capital and operation and maintenance [O&M]) that are related to these projects are summarized.
Client Backgrounds City of St. Cloud The City of St. Cloud owns and operates the Southside Water Reclamation Facility (WRF), which is a Modified Ludzack-Ettinger (MLE) process with effluent disk filtration and high-level disinfection that currently treats approximately 3.75 mil gal per day (mgd) average annual daily flow (AADF) of wastewater from the city, which has a population of approximately 50,000. The WRF is permitted for a design AADF of 7.6 mgd. The WRF is designed to produce water for 100 percent unrestricted public access reuse, with no alternative disposal methods. A 95-mil-
24 August 2022 • Florida Water Resources Journal
Sean P. Menard, P.E., CDT, ENV SP, is an engineer and department manager, and Tom Friedrich, P.E., BCEE, is a vice president with Jones Edmunds & Associates Inc. in Tampa.
gal (MG) reclaimed storage pond and 29-MG reject storage ponds, along with a 20-mgd high-service pump station, are located at the wastewater treatment facility (WWTF).
City of St. Cloud Southside Water Reclamation Facility
A Class V, Group 3 injection well, and associated monitoring wells, were permitted and constructed onsite from 2014 through 2017. The goal of the Southside injection well—through the injection of excess reclaimed water—is to offset wet season storage volume concerns (overflow of storage ponds), protect the impacted aquifer, and eliminate unpermitted discharges of reclaimed water from the ponds. The injection well permit requires that only reclaimed water be injected into the well. Water not meeting Chapter 62610, Florida Administrative Code (FAC), Part III, unrestricted public access reuse water standards cannot be injected. The city’s injection well was permitted for construction and testing and placed into operation July 2017. The injection well is constructed into the lower portion of the Avon Park Formation within the Lower Floridan aquifer (LFA) with a 14 in. steel casing set to approximately 300 ft below land surface (bls). The well transitions to a 12-in.-diameter steel casing set to approximately 1,652 ft bls,
