The paper will present results of a trial initiated in February 2004 by the San Antonio Water System (SAWS) to quantify the impacts of hydrogen peroxide injection prior to dissolved air flotation thickening of wastewater sludges.
Since February 2000, the San Antonio Water System has successfully used iron salts (FeSO4) for odor and corrosion control in the Dos Rios Water Recycling Center (WRC) collection system. In May of 2003, the Dos Rios WRC began receiving about 300,000 gallons per day of a mixture of primary and waste activated sludges from SAWS’ Leon Creek WRC. When the Dos Rios WRC facility started receiving sludge from Leon Creek, several negative impacts were observed. Operators immediately noticed a significant increase in sulfide odors from the DAF units treating the Leon Creek sludge. In addition, a 2-3% decrease in percent solids of combined Leon Creek/Dos Rios belt filter press dewatered sludge was observed. In addition, volatile solids reduction through the anaerobic digestion process decreased dramatically. Finally, an expected increase in methane production due to increased sludge volume was not realized.
In order to investigate the efficacy of hydrogen peroxide (H2O2) towards resolving these negative impacts, SAWS initiated a trial involving injection of H2O2 into the Leon Creek sludge line several minutes ahead of the DAF units. Impacts to be quantified were H2S odors, polymer usage, volatile solids destruction, methane production, and dewatering efficiency. Past experience had suggested that hydrogen peroxide may enhance the performance of flotation thickening and sludge dewatering, and may result in a reduction of polymer use. It had also been suggested that hydrogen peroxide (in the presence of iron salts) could potentially increase methane production through partial oxidation of organic compounds (e.g. lignins) in the sludge, which may be recalcitrant to anaerobic digestion. Finally, it was thought that the hydrogen peroxide residuals in the Leon Creek sludge would also react to regenerate the iron salts from the Dos Rios sludges and achieve additional odor control in a blend tank where Leon Creek and Dos Rios sludges are mixed.
History of the San Antonio Water System
SAWS was created through the consolidation of three predecessor agencies on May 19, 1992: the City Water Board (the previous city-owned water supply utility); the City Wastewater Department (a department of the city government responsible for sewage collection and treatment); and the Alamo Water Conservation and Reuse District (an independent city agency created to develop a system for reuse of the city's treated wastewater).
SAWS currently operates four major Water Recycling Centers (WRCs) with a combined capacity of 225.5 MGD. The largest and most modern is the Dos Rios WRC with a rated capacity of 125 MGD; the Salado Creek and Leon Creek WRCs are SAWS' oldest plants and are rated at 46 MGD each. These three facilities operate as conventional activated sludge plants. The smallest of SAWS' facilities is the Medio Creek WRC, a carrousel plant, with a capacity of
Sludge Consolidation Process
Through various studies, SAWS had shown that consolidation and centralization of all sludge dewatering activities at the Dos Rios WRC would be the most efficient and economical method for the future treatment and utilization of its biosolids. Construction of a series of force mains from the Medio Creek and Leon Creek WRCs to transfer sludges produced at these facilities was begun. Waste sludge from the Medio Creek WRC is transferred via a force main to a major trunk line on the Leon Creek collection system where it is transported and mixed with raw sewage. This mixture arrives at the headworks of the Leon Creek facility for additional treatment. Leon Creek WRC produces both primary and waste activated sludges. These are then blended together in an 800,000-gallon blend/holding tank. The blended sludge at a concentration of about 1.0 to 1.5 % solids is then pumped through a seven-mile force main to the Dos Rios facility for thickening, digestion and dewatering. This force main was completed and in full operation by April 2003.
Existing Odor Control and Issues
Since February 2000, the San Antonio Water System has successfully used iron salts (FeSO4) for odor and corrosion control in the Dos Rios Water Recycling Center (WRC) collection system. In April of 2003, the Dos Rios WRC began receiving about 300,000 - 400,000 gallons per day of a mixture of primary and waste activated sludges from SAWS’ Leon Creek WRC. While the Dos Rios WRC collection system has the benefit of iron salt addition for odor control, the Leon Creek WRC collection system does not. SAWS has not considered the addition of iron salts on the Leon leg as odors in this collection system are not considered to be a problem of the same magnitude as on the Dos Rios system.
When the Dos Rios WRC facility started receiving sludge from Leon Creek, several negative impacts were observed. Operators at Dos Rios immediately noticed a significant increase in sulfide odors from the DAFT units treating the Leon Creek sludge. The units treating Leon Creek WRC sludges are fed directly from a seven-mile force main directly into the DAFT units on the north side of the sludge thickening processes area at the Dos Rios facility. This configuration was found to be a perfect source for odor generation.
An additional unexpected problem occurred when an expected increase in methane production due to increased sludge volume was not realized. This loss of gas production would have a very negative affect on SAWS intention to utilize digester gas in an up coming project for electrical generation. These issues, especially the decrease in gas production sent SAWS' staff into a high level of activity searching to identify the reasons for the loss of gas productivity and odor