Fellow: Lynne Seth
Program: Civil Engineering, North Dakota State University
Advisor: Wei Lin

Description of the Critical Water Problem

High concentrations of ammonia in wastewater discharges have been shown to be toxic to fish and cause dissolved oxygen (DO) depletion in receiving streams, especially during periods of low river flow. However, when the City of Moorhead Wastewater Treatment Facility (WWTF) was designed in 1983, no ammonia discharge limit was implemented and no treatment process for the removal of ammonia was employed. Since operation began at the Moorhead WWTF, the typical discharge of ammonia to the Red River of the North had been near 19 parts per million (ppm).

In the mid-1990's, the reach of the Red River of the North from the Cities of Moorhead and Fargo to the confluence with the Buffalo River in Minnesota was identified as impaired for both ammonia and dissolved oxygen (i.e. violating water quality standards for these parameters at low river flow). In 1994, a workgroup was formed to address this impairment and establish a Total Maximum Daily Load (TMDL) study to set allowable loadings of CBOD5 and ammonia discharged from each plant and corresponding permit limits for the treated wastewater discharges. To aid that effort, a water quality model (QUAL2E), developed by the EPA, was used to simulate the wastewater discharges. The model was found to be sensitive to the assumed reaeration rate in the river, and therefore, the workgroup concluded that calibration at critical low river flow conditions was necessary to simulate the impacts of CBOD5 and ammonia on dissolved oxygen in the river.

In 1999, the USEPA revised their water quality criteria for ammonia. This criteria is established for ammonia toxicity and unrelated to dissolved oxygen impacts associated with ammonia discharges. The Minnesota Pollution Control Agency subsequently adopted a site specific standard for ammonia for the impaired reach of the Red River of the North based on the new criteria and developed a new discharge limit for the City of Moorhead (equivalent to 8 ppm for the months of June through September when the river flow is less than fifty cubic feet per second). A compliance date of September 30th, 2003 was also established. The City of Moorhead developed a facility plan and, based on that plan, constructed an innovative process, the attached growth moving bed biofilm reactor (MBBR), to meet the new limits at a cost of $3.3 million in 2002.

This process is the only full scale separate stage nitrifying MBBR in the country. The MBBR process utilizes floating media placed in an aeration basin. In the basin, an aeration system supplies oxygen and provides mixing for the process while the media supply the necessary surface area for attached growth of nitrifying bacteria

In 2003, low river flows were experienced in the Red River of the North. In response, a sampling plan was carried out by the USGS and state agencies from North Dakota and Minnesota to determine the reaeration coefficient of the Red River at low flow conditions. The results of this study will be used to reassess the Red River’s capacity for receiving CBOD5 and ammonia at low flow conditions while maintaining proper water quality. If an appropriate water quality can not be maintained, the permit limits will be reduced even further.

An evaluation of this new, innovative process is necessary in order to determine the most critical operational parameters. A better understanding of the process gained by studying the key parameters via a kinetic model will result in improved operational efficiency and reduced effluent concentrations of ammonia, thus improving the overall water quality of the Red River of the North.

Key Literature

Nitrification is the conversion of ammonia to nitrate mainly by autotrophic bacteria. This process consumes alkalinity and oxygen. As a result, pH and dissolved oxygen are two important parameters in nitrification process selection and system operation (Tchobanoglous and Schroeder, 1987). The two major nitrifying bacteria are known as Nitrosomonas and Nitrobacter. Carbon dioxide is utilized as the only source of carbon for cell synthesis while energy is obtained by oxidizing ammonia to nitrate.

Nitrification can be achieved in single or separate sludge processes. In a single sludge system, nitrification occurs concurrently with the removal of organic matter (i.e. BOD) in combined carbon oxidation-nitrification reactors (Grady and Lim, 1980). In a separate stage system, nitrification is accomplished in a separate reactor after the majority of BOD is removed in the first stage. Both single and separate sludge systems can be designed as suspended growth or attached growth processes. Because of the low specific growth rate of nitrifying bacteria, certain design complications are presented. One aspect of importance is the influence of sludge retention time on the rate of nitrification (Hamoda, 1996). Additionally, the efficiency of nitrification in an attached growth system is affected by biofilm thickness, biomass density, DO level, and water temperature (Bonomo, 2000).

The issue of ammonia removal has been studied extensively by the City of Moorhead WWTF and at NDSU. Jayme Klecker (1998) studied the feasibility of using the existing polishing ponds at the Moorhead WWTF for nitrification. The polishing ponds provided an appropriate detention time, but a low biomass concentration in the ponds limited the removal efficiency of the nitrification process. Klecker recommended that modified operation be evaluated in order to achieve nitrification on a full-scale.

Based on Klecker’s study and research by Dr. Robert A. Zimmerman, one of the existing polishing ponds at the Moorhead WWTF was later converted to the MBBR process. Feasibility of the full-scale nitrification process was evaluated by a pilot-scale study using the separate stage attached growth MBBR process. Addition of the media allowed for the development of a suitable biomass population (Zimmerman, 2003).

Zimmerman and his collaborators suggested that the nitrification rate of the MBBR process can be modeled as a 1st order decay model. The equation implies that nitrification is primarily dependent on the influent loading. However, a number of other parameters were recognized as affecting the process. Further research of the new MBBR process was recommended in order to resolve unanswered questions regarding the nitrification rate and its relationship to ammonia concentration, dissolved oxygen concentration, and detention time, as well as influent loading. Developing definitive relationships require evaluation where these parameters are maintained as independent variables.

Scope and Objectives

The objectives of the proposed research include (1) monitoring the system operation under various flow and ammonia loading conditions and (2) developing and calibrating a kinetic model to further evaluate and optimize critical design and operational parameters for the separate stage nitrifying MBBR. Improved understanding of this process will enhance operational efficiency of the facility and further reduce effluent ammonia concentration discharged into the Red River. This objective becomes particularly important if, in fact, the permit limits are reduced based on the results of the USGS calibration of the water quality model. The research will also be widely applicable to the MBBR process in general, and thus, expand the body of current knowledge associated with this new process.

Methods, Procedures, and Facilities

The proposed research will be conducted at the Moorhead WWTF laboratory and the NDSU Environmental Engineering laboratory. A literature review will be conducted to gather available information specific to the kinetic modeling of a separate stage nitrification process (i.e. model equations, key parameters to consider, etc.). A vendor search will also be conducted for commercially available software with the capabilities required for the research objectives. Software packages will be evaluated with regard to the specific objectives of this research and the most suitable package will be purchased. The model will be studied in detail to gain a full understanding of the software requirements such as influent wastewater characteristics, kinetic model equations employed, key kinetic parameters involved, and effluent wastewater characteristics. This information will be used to establish data collection requirements.

A significant amount of full-scale operational data has been collected since the process began operation in April, 2003. However, based on the model requirements, it is anticipated that additional specific data collection (i.e. actual biomass characteristics which may include thickness and mass) will be required in conjunction with the on-going operational data collection. A sampling and analysis protocol will be established to collect the additional required information. Utilizing this information, in conjunction with the historical operational data collected, the model will be calibrated. Following calibration, the model will be verified by additional data collection to determine the usefulness of the calibrated model.

An understanding of the factors that affect the kinetics of nitrification in the MBBR process will be gained through experimentation and analysis of key parameters in the model. These parameters include but are not limited to temperature, dissolved oxygen, pH, residence time, loading, and other environmental considerations. Substrate growth on the media will be predicted as well as measured in-situ.

Once the model is calibrated and verified, key parameters relating to operation and optimal design of the process will be determined through sensitivity analysis. This analysis will involve examining changes in key operational parameters and the corresponding changes in process performance predicted from the verified model. If appropriate, a simplified model may be presented for design and operational consideration.

Anticipated Results and Deliverables

This study will result in a calibrated and verified kinetic model for the MBBR process. In addition, an analysis of important operational and design parameters for the system will be reported. If possible and/or appropriate, a simplified model will be proposed for design and operational considerations with regard to the process. Improved understanding of the MBBR will optimize operation which will, in return, reduce ammonia concentrations in the Red River of the North.

Other benefits included in the study are related to the uniqueness of the process. As mentioned earlier, this is the only separate stage nitrifying moving bed biofilm reactor in the country. Attention has been growing with regard to the MBBR process. As a separate stage system, an opportunity is presented to study and evaluate this full-scale system, and thus, provide valuable operational and design information which may be utilized in similar systems; not only for separate stage systems, but also in combination with activated sludge. In conclusion, this endeavor will provide valuable information to improve the water quality of the Red River of the North as well as establish an important kinetic model for the MBBR process that has a potentially wide application.

Literature Cited

     Bonomo, L., et. al., Tertiary Nitrification in Pure Oxygen Moving Bed Biofilm Reactors. Water Science and Technology, 2000, Volume 41 Issue 4, pp. 361-368, Milano, Italy.

     Grady, Leslie C. P. Jr., and Henry C. Lim, Biological Wastewater Treatment. New York: Marcel Dekker, Inc., 1980.

      Hamoda, M. F., et. al., Biological Nitrification Kinetics in a Fixed-Film Rector. Bioresource Technology, October 1996, Volume 58 Issue 1, pp. 41-48, UAE University, Al-Ain.

      Klecker, Jayme J., Feasibility Study of Achieving Ammonia Removal by Nitrification in Polishing Ponds. Master of Science Thesis, May 1998, North Dakota State University, Fargo, ND.

     Tchobanoglous, George, and Edward D. Schroeder, Water Quality. University of California at Davis, CA: Addison-Wesley, Inc., 1987.

     Zimmerman, Robert A., et. al., Pilot-Scale Evaluation of Separate Stage Nitrification Utilizing an Attached Growth Moving Bed Media Process. Water Environment Research, September/October 2003, Volume 75 Number 5, pp. 422-433, Alexandria, VA.