-1- The Effects and Processes for Removal of Chromium in Activated Sludge Treatment Jennifer Merical ABSTRACT The presence of chromium in wastewater can impact the efficiency of activated sludge treatment plants that do not usually treat chromium contaminated wastewater. Pretreatment regulations have been passed by the United States Environmental Protection Agency (EPA) for industries who produce chromium contaminated wastewater. Two forms of chromium exist: trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)), which is more toxic than Cr(III). Activated sludge that has acclimated to chromium can efficiently remove chromium from wastewater. An increased concentration of suspended solids and a decreased activated sludge age can aid in the removal efficiency; however, a higher concentration of chromium present in wastewater can lead to decreased filamentous bulking and nitrification. Chromium’s toxicity decreases the diversity of microorganisms, but low doses of chromium do not affect microorganisms such as γ-Protebacteria and free swimming ciliates that are present in activated sludge. These microorganisms aid in the chromium removal process provided by activated sludge. KEYWORDS Activated Sludge, chromium, removal, biosolids, toxicity INTRODUCTION Activated sludge treatment is a biological solution to wastewater treatment. Municipal wastewater treatment plants not only receive wastewater from residential areas, but also from industrial and commercial establishments. Influent can contain compounds or metals that the treatment plant does not usually treat and can have potentially negative effects on the efficiency and quality of treatment achieved by the plant. Chromium is a naturally occurring metal and is often found in rocks, plants, soil, volcanic dust and gases, animals and humans. Chromium can have negative impacts on activated sludge systems if high enough concentrations of the metal are present in wastewater. Leather tanning, electroplating, wood preservation, textile manufacturing, and pulp processing facilities are most frequently the source of chromium contaminated wastewater. The United State Environmental Protection Agency (EPA) has established the National Pretreatment Program, which requires industries who discharge chromium containing wastewater to remove most of the toxic metal with pretreatment. The municipal wastewater treatment plants can then proceed in treating the water to ensure that chromium does not affect the aquatic life or drinking water sources (U.S. EPA, 1999). The main concern for municipal treatment plants is receiving chromium contaminated wastewater that can interfere with the activated sludge process. In order to fully understand the effects and processes for removal of chromium in activated sludge, a brief background on the behavior of chromium and the regulations regarding chromium removal will be discussed. The method of chromium removal and the toxic effect of chromium on activated sludge will also be discussed. Finally, chromium’s effect on the activated sludge microbiology will be addressed. CHROMIUM STATES Chromium is commonly found in the most stable states of hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)). Cr(VI) is more toxic than Cr(III) and less common in the natural environment. One aspect of the removal process is to ensure that aquatic life, animals, and humans are not exposed to chromium. According the EPA Technology Transfer Network (2007), humans consume about 2.0 µg/L of chromium in water per day and about 60 µg/L of chromium in food a day. The human body has the ability to reduce Cr(VI) to Cr(III) via metabolic processes. Upon exposure to large concentrations of chromium, humans can develop health problems, which include acute respiratory effects, cancer, reproductive and developmental effects (US EPA Technology Transfer Network, 2007).-2- WASTEWATER TREATMENT REGULATIONS Municipal wastewater treatment plants are generally not able to treat large concentrations of heavy metals, such as chromium, that come from industrial effluents (US EPA, 1999). Hence, pretreatment is required of industrial wastewater sources by Title 40 of the Code of Federal Regulations (CFR) in an effort to limit or eliminate chromium loading in municipal treatment plants (US EPA, 1999). Chromium can negatively affect the efficiency of activated sludge treatment processes and increase the chance of emitting chromium into the environment. A lack of pretreatment would also reduce the ability of the wastewater to be recycled or reused. CHROMIUM REMOVAL WITH ACTIVATED SLUDGE Traditional methods of chromium removal from wastewater usually require chemical processes; however, biological processes like activated sludge have also been proven to be efficient in chromium removal. In order to fully understand the importance and impact that chromium can have on an activated sludge treatment plant, the following topics are discussed: • Removal mechanisms, • Reduction of Cr(VI) to Cr(III), • Absorption capacity of activated sludge, • Suspended solids affect on absorption, • Activated sludge age affect on removal, and • Chromium effect on biomass acclimation. Removal mechanisms For the reasons stated above, removal of chromium during pretreatment is important. Traditional processes such as chemical precipitation, electrochemical treatment, and ion exchange have been used for pretreatment of industrial wastewater. Another approach to chromium removal is the use of activated sludge. According to Koçberber and Dönmez (2006), the microorganisms in activated sludge can be an effective tool in the removal of heavy metals from wastewater. The efficiency of biological removal resides in the microorganisms’ tolerance and ability to absorb chromium. Three methods of biological Cr(VI) removal with activated sludge exist (Koçberber and Dönmez, 2006; Imai and Gloyna, 1988): 1. Positively charged Cr(VI) is attracted to the negatively charged cell wall of the microorganisms; 2. Adsorption of Cr(VI) into microorganisms; and 3. Reduction of Cr(VI) to Cr(III), which is then settled. Reduction of Cr(VI) to Cr(III) The most common removal mechanism of chromium is the reduction of Cr(VI) to Cr(III). The Cr(III) is then adsorbed by the activated sludge and precipitated as Cr(OH)3 (Imai and Gloyna, 1988; Stasinakis, Thomaidis, Mamais, and Karivali et al., 2003). Figure 1 represents the