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Establishing Nitrification Reliability Guidelines for Water Reuse

Project: 10-15
Year Release: 2015
Product: Scientific Investigation

Program: Principle
Funding Partner: Bureau of Reclamation
Total Investment: $351,766.83 (WRRF: $150,000, In-Kind cash and service: $201,766.83)

Principal Investigator: Gordon Williams, Ph.D., Trussell Technologies

Background

In an effort to satisfy growing water demand in the face of dwindling water resources, many communities throughout the U.S. and the world are turning to water reclamation and reuse. Recycled water supplements conventional water supplies by capturing and treating wastewater that would otherwise be disposed of to the environment. Various disinfection processes may be used to meet pathogen removal requirements, but chlorine disinfection is by far the most common. Chloramines are formed when chlorine is added in the presence of ammonia. When adding chlorine to a water containing ammonia, the chlorine will stay as free-chlorine only if enough chlorine is added to consume all the ammonia present.

Nitrification, the biological oxidation of ammonia to nitrate, is a widespread treatment process in the production of high-quality recycled water. In a non-nitrified wastewater, ammonia is typically present at concentrations in the range of 20 to 35 mg-N/L, making disinfection with freechlorine prohibitively expensive. However, in a nitrified wastewater, ammonia concentrations are typically are much lower, and chlorine remains as free-chlorine when added for disinfection.

Even though the disinfection potentials are greatly different between chlorine and chloramines, most water reuse regulations make no disinfection between the two. This is because at the time the disinfection regulations were made nitrification was not considered reliable. Fast forward to the present day, nitrification is now a well-understood process that can be designed and reliably operated using a number of different technologies.

Goals and Objectives

This project is intended to build a foundation for the water reuse industry to begin differentiating between disinfection by free-chlorine and chloramines. The process of nitrification is now well understood, but the reliability must be documented in the context of water reuse. The specific objectives of this study are two-fold:

  1. Document the reliability of nitrification processes used in water reuse using laboratory and continuous online instrumentation
  2. Develop industry guidelines for ensuring nitrification reliability at recycled water treatment facilities

Research Approach

Task 1 – Site Visits and Facility Review

Participating facilities were surveyed to gather background information about the plant in preparation for onsite visits and meetings with plant operators and engineers. These surveys will detail the facility design and operating parameters.

Task 2 – Field Assessment of Nitrification Reliability

Participating facilities collected samples over a three-month period to perform laboratory analyses to monitor concentrations of pertinent water quality parameters related to nitrification in the effluent. In addition, other process parameters were recorded daily to enable a complete picture of the facilities nitrification process over the study period.

Task 3 – Development of Nitrification Reliability Guidelines for Water Reuse

Findings and Conclusions

On the basis of the results of this study, the nitrification process can be operated, monitored, and verified with sufficient reliability to pursue a free chlorine disinfection strategy. Ensuring consistent free chlorine residuals, however, will require a significant shift from the current operational paradigm. In this new context, reliability is no longer framed in terms of nutrient loading but in achieving consistent, low ammonia levels at all times. Ammonia peaks—both transient and long-term—offer challenges to the free chlorine strategy; therefore, both preventive and responsive strategies need to be incorporated into design and operation.

Through this study, a number of knowledge and technical gaps were identified that should be addressed in future efforts. One of the most important next steps is to engage the relevant regulatory agencies on the issue of free chlorine disinfection credit for recycled water. These discussions should include defining the operating conditions needed to obtain free chlorine disinfection credit. One likely outcome of these discussions is the need to demonstrate the operation of a free chlorine system using a pilot- or full-scale demonstration system.

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