Acid+ A Novel Treatment Process Configuration for Simultaneous Process Intensification and Struvite Management

Acid/gas anaerobic digestion process is based the premise of environmental phasing, such that the conditions with the digestion process are first optimized for hydrolysis, acidogenesis and acetogenesis followed by a methanogenic phase for conversion of the solubilized organics to methane. The City of Columbus Southery WRF in Columbus, OH has operated an acid/gas system for more than a decade, treating a mixture of primary and secondary sludge. Over this period of time the City has conducted research surrounding improving process stability and reliability. One factor that has played a significant role in the operational reliability of the process, for Columbus, is the operation of the acid phase digestion. Reducing the acid phase operation to 1.8 days or less eliminated chronic foaming in the methane phase with the addition of thickened waste activated sludge. The addition of TWAS to the digestion process, reduced the solids disposal costs but increased the production of struvite, which fouls dewatering equipment and return lines. Acid reactors or fermenters have demonstrated very effective release of phosphorus from sludge. This likely make struvite production from the digestion process at Columbus a magnesium limited system. Based on operating observations, there is sufficient magnesium in the water to create a nuisance levels of struvite in the system. Concurrent with the struvite issues, there is a desire on the City's part to better understand the process limits and increase capacity, as Southerly moves into a facilities expansion project. Based on these operational issues, process interests and an impending capital program Columbus wanted to explore the novel process concept of Acid+, a process flow of which is shown in Figure 1. Acid+, modifies a base acid/gas system through the addition of a harvesting digester and recuperative thickening on the methane phase digesters. The harvesting digesters are designed to create an environment where struvite precipitation can be forced and managed. By recycling a flow of neutral pH sludge to the incoming acidified sludge and adding magnesium hydroxide the conditions are set for struvite formation. Using a high-rate gas mix system creates the micro-environment where pH is further increased and granular struvite can be formed, collected and harvested, as show in Figure 2. A collaborative research project was created between the City of Columbus, NYCDEP, University of Massachusetts- Amherst and City College of New York and Brown and Caldwell with funding provided by the Water Research Foundation under project WRF-5108. The project plan looks to understand the different process elements of Acid+, in sequence, such that a controlled approach process evaluation could be conducted. The phase 1 work focused on optimization of the acid phase both in batch and continuous operation, phase 2 looks to evaluate the continuous operation of acid/gas with the findings of phase 1 and then finally phase 3 would integrate the harvesting reactor on the optimized acid gas system, creating an Acid+ configuration. The phase 1 work is complete and the phase 2 is on-going. Figure 3,4 and 5 summarizes the influence of temperature and sludge type on key factors to struvite control for acid plus and overall acid phase digester performance. The fermentation/acidification process, as shown in Figure 4, demonstrates that temperature primarily impacted the rate of acidification, as measured by total VFA. This observation while not unexpected, did also show that the extent of conversion, maximum VFA produced did not appreciably change over the course of the full study period. This suggests that from an intensification strategy, temperature elevation can successfully deployed to reduce reactor size or increase throughput without a loss in performance. However, the degree of acidification achieved will be very sludge specific, for waste activated sludges. This would suggest that the sludge composition and/or biology has an influence of the acidification process. This is demonstrated by the shape of the acid production curves, with different sludges having different rates and time to onset of VFA production. This also holds true for both ammonia release and phosphorus release. Thermophilic operation appears to provide significant benefit to both. A key consideration for Acid+ would be the impact of operating at thermophilic temperatures and and need to reduce temperature to improve struvite precipitation. Based on these data it could be assumed that the treatment process would impact that performance, however when secondary treatment type was considered there was not a clear correlation. Other actors such as EPS content and metals may be influencing the performance as well as wastewater source. These will be explored in greater detail. In the associated paper.