Dallas Water Utilities,
Solids Handling Improvements at the Central and Southside Wastewater Treatment Plants
- Solids handling optimization for increased digester gas production
- Grease co-digestion for increased digester gas production
- Evaluation of sludge pretreatment technologies
- Evaluation of sidestream treatment technologies
- Phosphate scaling mitigation
- Digester foaming mitigation
Carollo Engineers was selected for the Dallas Water Utilities (DWU) Solids Handling Improvements Project. This project included the evaluation of improvements at the Southside Wastewater Treatment Plant (SWWTP) and the Central Wastewater Treatment Plant (CWWTP), which have a combined capacity to treat an average daily flow of 260 mgd. The SWWTP stabilizes the solids generated at both plants.
Boosting Digester Gas Production
The SWWTP gas production per volatile solids treated is unusually low because a large fraction of the organic material present in the raw sludge does not reach the anaerobic digesters, decreasing the gas available for the cogeneration facility. Carollo evaluated alternatives to increase the digester gas production and improve operability of the solids handling facilities and provided the following recommendations:
- Separate the CWWTP primary sludge and waste activated sludge streams to increase the degradability. Co-settling and infrequent primary sludge pumping results in solids degradation in the CWWTP primary clarifiers.
- Replace the thickened sludge well with three small concrete hoppers to decrease the thickened sludge fermentation upstream of the digesters. Degradation occurs in the existing thickened sludge well because of a very long solids retention time and poor mixing.
- Other alternatives to increase gas production, including chemically enhanced primary settling at the CWWTP and sludge pretreatment (thermal hydrolysis, electroporation, and cavitation). These alternatives were not economically feasible.
In addition, Carollo completed the preliminary design for a pilot grease receiving facility at the SWWTP. The facility includes a truck unloading station, grease storage tank with odor control, mixing system, and transfer pumps. The pilot facility will have the capacity to receive 20,000 gallons per day and will feed one digester.
Phosphate Scaling Mitigation
Vivianite scaling in the SWWTP anaerobic digester sludge heating and recirculation system results in considerable operations and maintenance costs and is anticipated to aggravate in the future after the conversion to enhanced biological phosphorus removal (EBPR). Carollo characterized the solids streams, performed chemical equilibria model simulations, evaluated scale mitigation alternatives, and provided the following recommendations:
- Reroute the raw sludge feed piping in all digesters to prevent raw sludge from blending with recirculated sludge. Chemical modeling and field trial results showed that scaling occurs when the acidic raw sludge is neutralized by the neutral pH found in the digesters.
- Implement a pipe replacement/cleaning program and, when phosphorus discharge limits are imposed, construct a sidestream phosphorus removal facility. Carollo evaluated different chemical precipitation and struvite harvesting processes. Pipe replacement provided a lower lifecycle cost for vivianite mitigation than the evaluated sidestream phosphorus removal processes.
Digester Foaming Mitigation
The SWWTP digesters have experienced recurring foaming events during the past few years. Some of these events resulted in operations and maintenance problems, such as foam intrusion into the gas collection system and inability to mix the digesters. Carollo investigated the causes of digester foaming, evaluated different foaming mitigation alternatives, and provided the following recommendations:
- Install anaerobic selectors, foam abatement spray bars, and surface wasting at the CWWTP for Nocardia control. Nocardia was detected in the CWWTP aeration basins and was determined to be the primary cause for digester foaming.
- Modify the digester mixing systems to decrease the potential for foaming while maintaining the mixing performance based on proven design criteria and CFD modeling results. The existing digester mixing systems were found to aggravate foaming.
- Modify the gas piping configuration and install foam separators, defoamer addition systems, and foam level monitoring instrumentation to protect the gas handling system, which was found to be prone to foam intrusion.