Study examines using MBR technology for more complete nutrient removal process for secondary effluent end-use from activated sludge plants

By Zeynep Erdal, Regional Technology Leader and Sustainability Coordinator, CH2M HILL

Dr. Erdal presented the paper titled “Achieving Sustainable Nutrient Removal for Tailored Water Treatment: A Novel Application of the MBR Technology” on Monday, Oct.7 during WEFTEC 2013 in Chicago, Illinois. Learn more about CH2M HILL’s WEFTEC 2013 participation and technical sessions.

The secondary effluent from the activated sludge plants that operate at low solids retention times to achieve basis of design (BOD) and technical service (TSS) removal are currently considering the need to either upgrade the main treatment facility or to add tertiary facilities to achieve a more complete nutrient removal for various end use driven reasons.  Especially in the cases where a high quality product water is needed in addition to the nutrient removal, such as industrial reuse, membrane technologies are being considered. However, due to the lack of accumulated knowledge in tertiary application of biological and membrane processes, in most cases the facilities are facing a scenario where large capital investments in upgrading existing facilities to generate feed water suitable for tertiary filtration technologies are being considered. In most cases this means that the main facility will have to be derated, unless the original design conditions are amenable to direct conversion to nitrogen removal configuration and operation at longer solids retention time.

In the case of tertiary membrane bioreactor (T-MBR) implementation, the main facility continues to operate at low solids retention time conditions providing BOD and TSS removal, and the effluent is then treated in a MBR system operated at a sufficient solids retention time to permit year round nitrification. The process can generally be described as two-stage activated sludge with the second stage employing a membrane bioreactor. Considering the limitations on the knowledge base on the design and operation criteria related to separate sludge systems, CH2M HILL has conducted pilot testing at two facilities to obtain proof of concept and establish site specific design and operating parameters for particular feed water quality and the end point drivers.

Following key conclusions were made as a result of the study findings:

  • Nitrification can be successfully sustained through stable operation as well as short duration challenge conditions such as ammonia spikes, power losses, feed quality degradation and feed shut downs, with key process parameters maintained. During the test events the feed was spiked with ammonia as part of the test plan to demonstrate the ability of the system of maintaining performance. The systems consistently produced an effluent that contained less than 0.1 mg/L ammonia-N.
  • Membrane maintenance was similar to secondary treatment MBR operation, with temperature corrected transmembrane pressure (TMP) maintained at or less than 4 psi during the entire test period.
  • Fouling potential of the nitrifying sludge was monitored through soluble and extracellular microbial products characterization, and filterability/ time-to-filter tests, as well as close monitoring of the biological activity of the nitrifying sludge. Adaptation of the microbial population to the specific operating conditions of the system were observed. For example, a significant disappearance of the humic substance-like microbial products was observed over the course of the testing.
  • The T-MBR effluent is of high quality suitable as direct feed for reverse osmosis (RO) treatment for water conditioning for reuse at industrial facilities such as cooling tower and boiler make up water. The silt density index (SDI) of the effluent was less than 2 at all times, iron was non-detectable and turbidity less than 0.2 NTU.
  • Development of the treatment features for tight control of the feed quality and operating conditions was incorporated into the test and design criteria developed reflect the site specific custom design of the T-MBR system for nitrified effluent production. The knowledge is directly transferrable to other facilities where similar customization can allow the necessary level of treatment for the ultimate beneficial use of the treated water.
  • Process modeling before pilot testing allowed proper planning anticipating the test conditions and system failure points, and modeling during the testing allowed further refinement of the model parameters for more accurate simulation of the system. Now the various scenarios can be considered during the design of the full scale system.

 

Dr. Zeynep Erdal is a Senior Technologist in CH2M HILL’s Water Business Group and has more than 16 years of experience in applying systems modeling and process technologies for engineering solutions. She has extensive experience in wastewater treatment, technology implementation, biological systems optimization, climate change regulations and sustainable solutions. She is the Vice Chair of the Research and Innovation Symposia of the WEFTEC Program Committee. She also serves on the Technical Practice Committee of WEF, and has authored research and practice papers on many facets of wastewater management and book chapters on nutrient removal and plant operations.