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Establishing Additional Log Reduction Credits for Wastewater Treatment Plants

Project: 14-02
Estimated Release: 2017
Type: Report

Program: Principal
Funding Partner:
Total Investment: $590,546 (Cash: $400,000, In-Kind cash and service: $ 190,546)

Principal Investigator: Zia Bukhari, Ph.D. American Water


California has established 12, 10, 10-log removal requirements for enteric virus, Cryptosporidium, and Giardia for potable reuse projects. Recent research from the U.S., Australia, and other international locations has characterized raw, primary and secondary effluent pathogen concentrations (Cryptosporidium, Giardia, total coliform, E. coli, and viruses) to some degree. More data collection from various types and sizes of WWTPs is expected to be necessary to further characterize influent pathogen concentrations and log reductions achieved at WWTPs.

Goals and Objectives

The project will:

  • Obtain a more accurate picture of the microbial treatment requirements by addressing a major source of uncertainty—the concentration of pathogens in raw wastewater and secondary effluent.
  • Provide data relevant to the protection of public health in a Direct Potable Reuse scenario. Quantifying pathogen levels in raw wastewater and quantifying the removal that occurs through the wastewater treatment plant (WWTP) will ensure that treatment processes can meet removal requirements in a potential DPR system.
  • Establish removal credit for biological treatment provided in the treatment train for protozoa, bacteria, and viruses. Treatment train proposed should represent a DPR scenario and include activated sludge. Other treatment trains (e.g. activated sludge with trickling filter) may be included as appropriate.
  • Determine validity of pathogen log-removal requirements identified for WWTPs by the Division of Drinking Water (formerly CDPH) for potable reuse projects. NWRI’s report “Examining the Criteria for Direct Potable Reuse” (published by WRRF project 11-02 and available here: http://watereuse.org/product/11-02-1) is a suggested source to inform this objective.

Research Approach

Task 1: Literature Review and Data Gathering. The project team will conduct a literature review of existing peer-reviewed and grey literature data of pathogens in raw wastewater and their removal through WWTPs. In addition to the general literature review above, perform a focused review to obtain a comprehensive dataset of the occurrence of pathogens identified in the project WRRF-11-02 report (including bacteria, virus, Cryptosporidium, and Giardia.)

Task 2: Develop Study Plan and Collect Additional Data to Fill Gaps. In this phase, the project team will:

  • Identify participating utilities, sampling location, number of sampling events, etc.
  • Collect data over a period of one year from participating utilities that represent a wide range of activated sludge processes (including bacterial ratio of atypical colonies to total coliform colonies AC/TC), seasonal changes, wet weather flow conditions, geographic locations, system sizes, etc. Assume at least 20 samples from raw wastewater and 20 samples from secondary effluent per WWTP with analysis of bacteria (e.g. E. coli), protozoa (e.g. Cryptosporidium and Giardia), and viruses, and budget permitting, surrogates and/or indicators of for pathogen removal through WWTPs as determined to be most appropriate by the project team
  • Collect WWTP operational data at time of sample collection

Task 3: Analyze Data and Determine Log Reduction Credits Attributable to WWTPs. In this phase, the project team will:

  • Consider various statistical techniques (e.g., Monte Carlo) in analysis of data
  • In final analysis, extrapolate how extreme conditions (both weather and water quality) might affect pathogen concentrations in influent and effluent. Predict safety measures that could be utilized by treatment plant operators to manage extreme events.
  • If indicators and surrogates are included in Task 2, provide recommendations on the suitability of the indicators and/or surrogates used in this study and the confidence with which they can be used for future prediction and measurement of pathogen reduction by activated sludge plants.
  • Consider whether the data supports using raw sewage versus secondary treated sewage as the starting point for calculating pathogen removal requirements and make a recommendation.

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