Achieving New Phosphorous Compliance at Noblesville and Rockville WWTPs Pilot Testing and Modeling Results
Viraj deSilva, PhD, PE, BCEE, American Structurepoint, Inc., Derek C. Urban, PE, American Structurepoint, Inc., Ray Thompson, City of Noblesville Utility Director
INTRODUCTION
The need for preserving the environment is tightening regulations limiting the discharge of contaminants into water bodies. Nowadays most of the effort is done on the removal of more specific contaminants such as Phosphorous since they are becoming of great concern due to its impact on the quality of water bodies. The presentation will focus on recent pilot studies at City of Noblesville, Indiana and Rockville Indiana. American Structurepoint completed the pilot study at Noblesville and completed the Model runs at Rockville to achieve the effluent Phosphorous limit. These two case studies along with the theory behind the removal technologies will be presented.
NOBLESVILLE WWTP
The City of Noblesville currently operates a conventional activated sludge wastewater treatment plant (WWTP) in a contact stabilization configuration. While the WWTP has no current phosphorus discharge limit, the City has an interest in converting to Enhanced Biological Phosphorus Removal (EBPR) in advance of an upcoming discharge limit of 1.0 mg/L to optimize operations ahead of the limit and realize energy savings. The capacity of the WWTP is 10.0 MGD average flow and 20.0 MGD peak hourly flow.
The WWTP was modeled in GPS-X to develop the schematic design of the improvements to achieve EBPR. Based on the model results, it was determined that EBPR was feasible, and that the amount of food was close to being a limited factor. Based on these results, the City decided to implement a quarter scale pilot of the process using existing tanks, temporary pumps, rental mixers and rental pumps. Because the WWTP did not already measure several key parameters, monitoring of several parameters was completed during the pilot, including phosphorus, volatile fatty acids (VFAs), soluble BOD, nitrate (NO3-N), and ammonia (NH4-N) profiles in the aeration basins. This data along with approximately three years of monthly report of operation (MRO) data was used to calibrate the model.
Several modeling runs will be performed, including average and peak design conditions, seasonal conditions such as warm and cold weather, and maintenance requirements, i.e., with one process unit such as a biological reactor or final clarifier removed from service. It was evident that EBPR could be most easily achieved by converting from contact stabilization to plug flow configuration. An anaerobic selector basin and/or an anoxic zone will be placed in the front end of each aeration basin to allow for orthophosphate (Ortho-P) release and the subsequent luxury uptake in the aeration portion of the reactor. The Pilot showed that there would be release and luxury uptake, as shown in Figure 2 below. Because of the additional phosphorus that will be directed to the solids handling (Biosolids) part of the WWTP, changes to this portion of the WWTP are required as well. The final results, pilot data, and proposed plan to achieve EBPR will be presented.
Figure 1: Orthophophate – Primary Effluent, Pilot Effluent and Regular Plant Effluent
Figure 2: Orthophosphate Profile – Pilot Results
ROCKVILLE WWTP
At Rockville WWTP, Indiana 3 alternatives for Phosphorous removal are evaluated:
GPS-X process model is used to evaluate the feasibility of biological phosphorous removal. The model is calibrated using the 2014 to 2015 MRO data and any other process monitoring data. Different scenarios are then evaluated to find best option to remove highest phosphorus content without affecting ammonia and BOD removal efficiency under average design and peak flow and load conditions.
Chemical phosphorus removal option is evaluated to meet the phosphorous discharge requirements, determining the requirements of the chemical feed, chemical storage system, chemical injection, and mixing, and estimating the amount of chemical sludge production and impact on biosolids handling system and disposal. Since the WWTP utilizes UV for disinfection, it is likely that alum will be the chemical of choice to prevent fouling of the UV system.
The combination of biological and chemical phosphorus removal scenarios were evaluated to estimate degree of phosphorus removal that can be achieved utilizing the existing tankage with a selector compartment or with a new selector tank under average and peak flow and load conditions including hydraulic evaluation. Also evaluated are the chemical requirements to remove the remaining phosphorus to meet the discharge requirements and the impact to land application with the combined biological and chemical sludge.