Identifying the Perfect Fit – Deploying the Right Sidestream Nitrogen Removal Process

Increasing pressures to reduce energy demand, coupled with the ever present need to minimize biosolids production and reuse costs, have resulted in a renewed interest in processes that allow for enhanced anaerobic digestion efficiency. The field has recently seen a larger number of successes, where integration of advances in treatment process energy management, digestion, and cogeneration, have permitted municipal wastewater treatment plants to achieve energy self sufficiency for extended periods of time (Wett, 2005). Key to this achievement has been the optimization of conventional anaerobic digestion processes or the utilization of enhanced solids digestion.Whereas the specific mechanisms utilized by each digestion or solids pre-treatment process vary, a common feature is that with increasing volatile solids destruction there is a concomitant release of nutrients to the reject liquor after dewatering. The increased nitrogen loadings to the wastewater treatment plant, imparted by the higher sludge destruction achieved, can negatively impact the treatment economics and nitrogen removal capacity of the facility. These effects may be manifested as a reduction in the peak nitrogen loading capacity of the plant, as well as an increase in operating costs in the form of aeration energy, supplemental alkalinity and potentially supplemental carbon to nitrify and denitrify the additional nitrogen load. As a result the overall effect of implementation of an enhanced solids destruction/digestion process may not result in the initial cost efficiencies identified during the evaluation of the economics of the enhanced digestion process, and may result in new operational challenges for the WWTP.The economics of enhanced destruction/digestion technologies can be improved significantly while minimizing impacts on plant process capacity and improving overall treatment process reliability by incorporating separate treatment for nitrogen removal from the anaerobically digested sludge dewatering recycle stream.A comparative economic and plant-wide performance analysis of the application of an enhanced solids digestion/destruction process coupled with three alternative sidestream treatment processes will be presented for two representative types of facilities – a 100 MGD high rate step feed BNR facility and a 100 MGD four-stage Bardenpho facility. The model facilities were selected as they cover the two extremes in process configurations for nutrient removal – the high rate step feed BNR, as practiced for example in New York City with detention times of less than 5 hours (only nitrogen removal is provided in this case) with solids retention times of 6 to 8 days represents one extreme. The four stage Bardenpho process with nominal detention times typically on the order of 12 to 16 hours and solids retention times of greater than 15 days, represents the other extreme. Thus this analysis attempts to bracket the variability observed on the ground.The three sidestream treatment alternatives considered for this set of evaluations were:Bioaugmentation and enrichment of sidestream recycle using the MAUREEN-type processes (Katehis, 2002; Constantine, 2005)Nitritation/denitritation of sidestream recycle using a chemostat process such as SHARON (Hellinga, 1998)Intermittent deammonification of sidestream recycle using the Strass or DEMON process (Wett, 2005)The three operating scenarios are compared to the baseline scenario of providing conventional mesophilic digestion with no sidestream treatment (recycles to the head of the plant).Envirosim's BioWin process model, calibrated based on full scale experiences with sidestream treatment (Katehis, 2002 and 2006), is utilized for the process analysis. Based on actual operating experience gained in full scale application of the AT-3 sidestream treatment process (Katehis, 2006), the BioWin process model has been re-configured to consider the reduced nitrifier seeding efficiency and nitrification capacity observed in the full scale implementation of the sidestream nitrogen removal process. This modification significantly affects the results and thus the direction of the sidestream treatment strategy employed.Facility-wide comparative capital and operating costs are developed for each scenario using CH2M HILL's proprietary cost estimation software package, to allow an assessment of the range of impacts on the facility, including both the liquid and solids processing sides of the plant. Key elements include:Aeration energy (for nitrification)Supplemental carbon utilization (for denitrification)Biosolids processing and reuse/disposal costsDigester gas handling/cleanup costsCogeneration facility capital and operating costsElectrical energy cost recoveryGreenhouse gas reductionEnvironmental impacts (Truck traffic for biosolids, chemicals, etc.)The result of the analysis is the development of a better understanding of the true impacts and costs of utilization of enhanced digestion processes and sidestream treatment processes in nutrient removal facilities, while clearly showing the impact of deploying a MAUREEN type process in enhancing overall treatment process performance in the high rate nutrient removal systems. The results indicate that in facilities that have deployed high detention time and high age bioreactors (as would be the case for the model Bardenpho facility) the enhancement in treatment performance and reliability is not material, and thus a sidestream process such as the Strass or DEMON process would be most applicable to allow the facility to benefit from the reduction in energy and alkalinity costs associated with this class of integrated nitritation-deammonification processes.