Wastewater Characterization for Evaluation of
Biological Phosphorus Removal

Introduction

Background

Controlling phosphorus discharged from municipal and industrial wastewater treatment plants is a key factor in preventing eutrophication of surface waters. Consistent with International Joint Commission agreements, the Wisconsin Department of Natural Resources (DNR) has, since the mid-1970s, required all municipal treatment facilities that discharge to the Great Lakes basin and have a population equivalent of 2,500 or greater to meet a total limit of 1 mg phosphorus/L (1 mg-P/L). A new regulation (Ch. NR 217, Wis. Admin. Code), which became effective in 1992, expanded the requirement for phosphorus removal to include the entire state. The new rule requires that all existing wastewater treatment plants discharging in excess of 150 pounds of total phosphorus per month to surface waters meet a 1 mg-P/L effluent limit. This effectively lowers the threshold for the size of plant required to remove phosphorus from about 250,000 gallons/day to 100,000 - 150,000 gallons/day. The need to retrofit many small- and medium- sized treatment facilities for phosphorus removal has lead to increased interest in alternatives to chemical addition. At the same time, biological phosphorus removal (BPR) technology has been steadily developing.

To encourage the use of biological removal techniques, the DNR regulation provides an alternative limit if an enhanced BPR process is used. The alternative limit requires the removal of 90% of the phosphorus that would have been removed to achieve a 1 mg-P/L effluent limit. For example, the Madison Metropolitan Sewage District's (MMSD) Nine Springs Wastewater Treatment Plant has an average influent total phosphorus concentration of 6 mg-P/L. Since the MMSD wastewater treatment plant will fall under the new regulation, it will be required to meet either the 1 mg-P/L effluent standard if chemical phosphorus removal is used or the alternative effluent limit of 1.5 mg-P/L [6-(6-1).0.9] if the BPR process is used.

The overall total phosphorus removal obtained in a conventional biological wastewater treatment is generally less than 20% and is even less in wastewater treatment plants where anaerobic digester supernatant is recycled to the head of the plant. Since it is not possible to achieve the 1 mg-P/L effluent limit with conventional biological wastewater treatment processes, additional or alternative treatment methods must be employed.

Many treatment plants have been designed or upgraded to remove phosphorus by the addition of chemicals. Chemical precipitation increases the volume of sludge produced and often results in a sludge with poor settling and dewatering characteristics. Also, precipitation with metals salts can depress the pH. If nitrification is required, additional alkalinity will be consumed and the pH will drop further.

Besides reducing or eliminating the need for chemical addition, BPR systems can offer the following benefits:

• reduced sludge production,
• improved sludge settleability and dewatering characteristics,
• reduced oxygen requirements, and
• reduced process alkalinity requirements.

Pilot-scale tests are generally conducted to evaluate the feasibility of biological phosphorus removal processes. However, pilot tests are expensive and time consuming and generate limited data. Because of this, smaller wastewater treatment plants may not be able to consider BPR as an alternative to chemical phosphorus removal.

The development of activated sludge process design computer programs provides an alternative design method. Computer models can be used to determine the process volume and to evaluate the effect of chemical oxygen demand (COD) loading, biomass concentration, and sludge age on the nutrient removal efficiency. Multiple process design configurations can be evaluated, and the sensitivity of designs to variations in wastewater characteristics can be economically evaluated.

However, several physical, chemical, and biokinetic parameters of the wastewater must be determined in order to use the activated sludge models. The wastewater characterization methods presented in this report will provide the inputs to the computer design programs. These procedures were developed in conjunction with a study in which the ENBIR program¹ was used to evaluate BPR alternatives for the City of Ashland, Wisconsin. Test data from the Ashland study are used to illustrate the characterization methods.

Principle of Biological Phosphorus Removal

The theory of luxury uptake of phosphorus is now well developed (Wentzel et al. 1990; Wentzel et al. 1991). It has been shown that exposing the mixed liquor to an anaerobic/aerobic sequence in the biological reactor selects microorganisms that accumulate higher levels of intracellular phosphorus than other microorganisms. Phosphorus-removing microorganisms are able to rapidly assimilate and store volatile fatty acids (VFAs) and other fermentation products under anaerobic conditions. Phosphorus is released in the anaerobic zone to produce the energy needed to take up the fermentation products, which are stored as poly-ß-hydroxybutyrate. Phosphorus-removing microorganisms produce energy by oxidizing the stored fermentation products in the aerobic zone while simultaneously accumulating intracellular phosphate. The ability of phosphorus-removing microorganisms to rapidly assimilate the fermentation products under anaerobic conditions gives them a competitive advantage over other microorganisms and results in their preferential growth in the wastewater treatment system. Thus, the anaerobic-aerobic sequence allows the selection of a large population of phosphorus-removing microorganisms.

In BPR systems, phosphorus accumulates in the biomass and is removed in the form of waste-activated sludge. A recent study showed that nearly all the enhanced phosphorus removal is due to the storage of polyphosphates. This results in an increase in the inorganic sludge mass but no significant increase in organic sludge production when compared to a conventional activated sludge process without chemical addition (Jardin and Popel 1995). Chemical precipitation of phosphorus has been estimated to increase sludge production by an average of 26% (Sedlak 1991).

Several process configurations (some patented, others not) are currently being applied worldwide for biological phosphorus removal. Some process configurations incorporate nitrogen removal by nitrification and denitrification along with biological phosphorus removal. However, all are based on the sequential exposure of microorganisms to anaerobic and aerobic conditions in the biological reactor.

Problems

Conventional activated sludge treatment was initially developed to remove carbonaceous and nitrogenous biochemical oxygen demand (BOD) from sewage. Activated sludge systems have been modified to enhance biological phosphorus removal by providing aerated and non-aerated reactors in series, along with various internal recycle systems. Not only have the system configurations increased in complexity, but the number of design parameters involved in the processes has also increased. Therefore, additional wastewater characteristics are necessary to evaluate the feasibility of biological phosphorus removal and to design a biological treatment process for phosphorus removal.

Objectives

The main objective of this report is to provide a simple procedure to determine wastewater characteristics necessary for the design of BPR systems, with specific emphases on:

These parameters can be used in biological nutrient removal process design computer programs such as ENBIR, which is based on the model developed by Ekama et al. (1984), or BIOSIM^TM, a menu-driven personal computer-based simulation program that solves the equations of the International Association on Water Pollution Research and Control (IAWPRC) (now the International Association on Water Quality, IAWQ) task group model for activated sludge systems extended for enhanced BPR (EnviroSim Associates 1993). These models can be used to determine the process volume and to evaluate the effects of COD loading, biomass concentration, and sludge age on the phosphorus and nitrogen removal efficiencies. These methods will allow smaller wastewater treatment plants or industries to evaluate the feasibility of BPR of their wastewater with minimum cost.

¹A public-domain computer program. To obtain a copy, contact Professor Jae K. Park. See "About the Authors," for address.

Abstract
Wastewater, Sampling Preservation, and Anaylsis Methods
Wastewater Fractionation || Biological Kinetic Parameter Estimation
Summary || List of Selected Symbols Used in this Report
Literature Cited || Credits

More information on this topic: Gerry Novotny