Session 06C: Wastewater Process
3:00pm - 3:45pm
Meeting Stringent Ammonia and Disinfection Byproduct Limits with Preformed Monochloramines
1Jacobs, United States of America; 2Clean Water Services, Oregon; ,
The Rock Creek Advanced Wastewater Treatment Facility (RCAWWTF) must balance low effluent ammonia limits, disinfection requirements, and potential low disinfection byproduct (DBP) limits.
The ammonia and disinfection limits are currently met through stable nitrification and use of sodium hypochlorite (SHC) disinfection, respectively. Effluent characterization identified the presence of two trihalomethane DBPs of potential regulatory interest when effluent ammonia concentrations were low: bromodichloromethane (BDCM) and chlorodibromomethane (CDBM). Initial estimates suggest that future discharge limits for CDBM and BDCM may be as low as 1.1 µg/L and 1.5 µg/L, respectively.
Preformed monochloramine (PFM) disinfection was identified as an operational strategy for reducing DBP production. This method utilizes monochloramines that are formed by mixing ammonia and sodium hypochlorite in carrier water before mixing with process water, greatly reducing the opportunity for DBP formation. Bench-scale testing followed by pilot testing was conducted to determine if utilizing a PFM disinfection approach could be an effective solution.
Preliminary bench-scale testing of insitu monochloramines (ISM) versus PFM indicated that PFM was a promising option to meet the disinfection and DBP formation goals; therefore, pilot-scale testing was pursued.
A flow-through pilot system was constructed at the RCAWWTF to receive tertiary effluent where a PFM solution could be added. Testing evaluated:
Testing demonstrated the benefits of using PFM over free chlorine and ISM by meeting disinfection permit limits and significantly reducing DBP formation potential while still maintaining effluent ammonia concentrations that met permit limits. These results show the viability of implementing PFM disinfection as a solution to more stringent DBP limits. Other clean water utilities may benefit from modifying existing chlorine disinfection to PFM disinfection as a much more cost-effective alternative relative to converting to another disinfection technology.
3:45pm - 4:30pm
BNR Conversion of the Oro Loma/Castro Valley Water Pollution Control Plant
Kennedy Jenks, United States of America;
Maintaining reliability in aging infrastructure has become a significant investment for wastewater agencies as facilities approach the end of their useful life. One such agency, Oro Loma Sanitary District (OLSD), was faced with a significant investment to rehabilitate a 7-mile long 189 MGD deep-water outfall in San Francisco Bay shared by six agencies. At the same time, nutrient regulation was being considered through a region-wide watershed permit that would require higher levels of treatment at OLSD’s 20 MGD Water Pollution Control Plant (WPCP). OLSD identified a project that would address these two challenges.
Implementation of $26M biological nutrient removal (BNR) upgrade of the existing secondary treatment process allowed OLSD to cost-effectively comply with anticipated regulation in the future watershed permit for nitrogen removal. In addition, an improved effluent quality allowed OLSD to renegotiate its NPDES permit to allow for the use of a near-shore outfall during wet-weather as an alternative discharge location to the deep-water outfall. Permitted use of the alternative outfall allowed OLSD’s partner agencies additional capacity in the shared deep-water outfall, as well as reducing OLSD’s liabilities for future outfall maintenance. The BNR upgrades were designed and constructed over a 3-year period and went into operation in September 2020.
This presentation will highlight how BNR was incorporated into the WPCP, the anticipated benefits of the project, and how the improvements are performing based on the first months of operation.
4:30pm - 5:15pm
Primary Sludge Fermentation: A Wretched Hive of Scum and Villainy
Clean Water Services, United States of America;
The Unified Fermentation and Thickening (UFAT) process is used at two Clean Water Services (CWS) Resource Recovery Facilities (Rock Creek and Durham) for primary sludge fermentation, generating volatile fatty acids (VFA) to support biological phosphorus removal. The Durham plant has a problem with fermenter scum buildup, forming dense mats during warm periods; a problem not shared by the Rock Creek fermentation system. The goal of this project was to inform design decisions on an upcoming fermenter expansion at the Durham plant by clarifying the role played in scum mat formation by three significant design differences between the facilities: 1) sludge heating, 2) sludge screening, and 3) scum removal.
Laboratory experiments using heated primary sludge reactors showed a correlation between scum mat formation and temperature, with mats forming more quickly with increasing heat. Observations of gas bubbles caught in and under scum mats in the laboratory and at full-scale suggest that solids from the sludge blanket may be floated to the surface of the fermenter by biologically-generated gas during fermentation. Laboratory experiments also showed that unscreened primary sludge produced scum mats more quickly than screened sludge, and that these mats persisted longer. Unscreened Durham primary sludge contains irregular, thread-like solids like hair and fibers. We observed these in dense formations in the full-scale mat, suggesting that they provide a structural matrix for scum to congeal to, trapping gas bubbles.
Our observations suggest that gas production from fermentation activity causes solids from the sludge blanket to rise to the surface, creating or exacerbating scum mats. The fermentation process is accelerated at warm temperatures, which is likely why mat formation worsens during warm periods. Screenable solids also likely play a substantial role in providing a stabilizing network for scum solids to adhere to, worsening scum problems at plants that do not utilize primary sludge screens. Installation of capital improvements such as sludge screens and scum handling systems are multi-million dollar investments; this work offers valuable insight into the roles played by fermenter design differences in mitigating scum accumulation and can inform design decisions regarding installation or improvement of UFAT fermentation systems.