Fundamentals of Biological Processes for Wastewater Treatment
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1 FUNDAMENTALS OF BIOLOGICAL PROCESSES FOR WASTEWATER TREATMENT JIANLONG WANG Laboratory of Environmental Technology, INET, Tsinghua University, Beijing, China 1.1 INTRODUCTION In this chapter we offer an overview of the fundamentals and applications of biological processes developed for wastewater treatment, including aerobic and anaerobic processes. Beginning here, readers may learn how biosolids are even- tually produced during the biological treatment of wastewater. The fundamentals of biological treatment introduced in the first six sections of this chapter include (1) an overview of biological wastewater treatment, (2) the classification of microorgan- isms, (3) some important microorganisms in wastewater treatment, (4) measurement of microbial biomass, (5) microbial nutrition, and (6) microbial metabolism. Following the presentation of fundamentals, the remaining four sections introduce applications of biological wastewater treatment, including the functions of waste- water treatment, the activated sludge process, the suspended and attached-growth processes, and sludge production, treatment, and disposal. The topics covered include (1) aerobicCOPYRIGHTED biological oxidation, (2) biological MATERIAL nitrification and denitrifi- cation, (3) anaerobic biological oxidation, (4) biological phosphorus removal, (5) biological removal of toxic organic compounds and heavy metals, and (6) bio- logical removal of pathogens and parasites. Biological Sludge Minimization and Biomaterials/Bioenergy Recovery Technologies, First Edition. Edited by Etienne Paul and Yu Liu. Ó 2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc. 1 2 FUNDAMENTALS OF BIOLOGICAL PROCESSES FOR WASTEWATER TREATMENT 1.2 OVERVIEW OF BIOLOGICAL WASTEWATER TREATMENT Biological treatment processes are the most important unit operations in wastewater treatment. Methods of purification in biological treatment units are similar to the self- purification process that occurs naturally in rivers and streams, and involve many of the same microorganisms. Removal of organic matter is carried out by heterotrophic microorganisms, which are predominately bacteria but also, occasionally, fungi. The microorganisms break down the organic matter by two distinct processes, biological oxidation and biosynthesis, both of which result in the removal of organic matter from wastewater. Oxidation or respiration results in the formation of mineralized end products that remain in wastewater and are discharged in the final effluent, while biosynthesis converts the colloidal and soluble organic matter into particulate biomass (new microbial cells) which can subsequently be separated from the treated liquid by gravity sedimentation because it has a specific gravity slightly greater than that of water. The fundamental mechanisms involved in biological treatment are the same for all processes. Microorganisms, principally bacteria, utilize the organic and inorganic matter present in wastewater to support growth. A portion of materials is oxidized, and the energy released is used to convert the remaining materials into new cell tissue. The aim in this chapter will help students and technicians in environmental science and engineering to recognize the role that environmental microbiology plays in solving environmental problems. The principal purposes of this chapter are (1) to provide fundamental information on the microorganisms used to treat wastewater and (2) to introduce the application of biological process fundamentals for the biological treatment of wastewater. 1.2.1 The Objective of Biological Wastewater Treatment From a chemical point of view, municipal wastewater or sewage contains (1) organic compounds, such as carbohydrates, proteins, and fats; (2) nitrogen, principally in the form of ammonia; and (3) phosphorus, which is principally in the form of phosphate from human waste and detergents. In addition, municipal wastewater contains many other types of particulate and dissolved matter, such as pathogens, plastics, sand, grit, live organisms, metals, anions, and cations. All these constituents have to be dealt with at wastewater treatment plants. However, not all of these are important for the modeling and design of a wastewater treatment plant. Usually, the carbonaceous, nitrogenous, and phosphorus constituents are mainly objects to be considered because they influence biological activity and eutrophication in the receiving water. When municipal wastewater is discharged to a water body, the organic compounds will stimulate the growth of the heterotrophic organisms, causing a reduction in the dissolved oxygen. When oxygen is present, the ammonia, which is toxic to many higher life forms, such as fish and insects, will be converted to nitrate by the nitrifying microorganisms, resulting in a further demand for oxygen. Depending on the volume of wastewater discharged and the amount of oxygen available, the water body can become anoxic. If the water body does become anoxic, nitrification of ammonia to OVERVIEW OF BIOLOGICAL WASTEWATER TREATMENT 3 nitrate by the autotrophic bacteria will cease. However, some of the heterotrophic bacteria will use nitrate instead of oxygen as a terminal electron acceptor and continue their metabolic reactions. Depending on the relative amount of organics and nitrate, the nitrate may become depleted. In this case, the water will become anaerobic and transfer to fermentation. When the organic compounds of the wastewater have been depleted, the water body will begin to recover, clarify, and again becomes aerobic. But most of the nutrients, nitrogen (N) and phosphorus (P), remain and stimulate aquatic plants such as algae to grow. Only when the nutrients N and P are depleted and the organic compounds sufficiently reduced can the water body became eutrophically stable again. From these considerations, the overall objectives of the biological treatment of domestic wastewater are to (1) transform (i.e., oxidize) dissolved and particulate biodegradable constituents into acceptable end products that will no longer sustain heterotrophic growth; (2) transform or remove nutrients, such as ammonia, nitrate, and particularly phosphates; and (3) in some cases, remove specific trace organic constituents and compounds. For industrial wastewater treatment, the objective is to remove or reduce the concentration of organic and inorganic compounds. Because some of the constituents and compounds found in industrial wastewater are toxic to microorganisms, pretreatment may be required before industrial wastewater can be discharged to a municipal collection system. 1.2.2 Roles of Microorganisms in Wastewater Treatment The biological wastewater treatment is carried out by a diversified group of microorganisms. It is the bacteria that are primarily responsible for the oxidation of organic compounds. However, fungi, algae, protozoans, and higher organisms all have important roles in the transformation of soluble and colloidal organic pollutants into carbon dioxide and water as well as biomass. The latter can be removed from the liquid by settlement prior to discharge to a natural watercourse. Many water pollution problems and solutions deal with microorganisms. To solve water pollution problems, environmental scientists require a background in micro- biology. By understanding how microbes live and grow in an environment, envi- ronmental engineers can develop the best possible solution to biological waste problems. After construction of the desired treatment facilities, environmental engineers are responsible to operate them properly to produce the desired results at the least cost. Learning to use mixtures of microorganisms to control the major environmental systems has become a major challenge for environmental engineers. Environmental microbiology should help operators of municipal wastewater and industrial waste treatment plants gain a better understanding of how their biological treatment unit work and what should be done to obtain maximum treatment efficiency. Design engineers should gain a better understanding of how microbes provide the desired treatment and the limitations for good design. Even regulatory personnel can obtain a better understanding of the limits of their regulations and what concentration of contaminants can be allowed in the environment. 4 FUNDAMENTALS OF BIOLOGICAL PROCESSES FOR WASTEWATER TREATMENT The stabilization of organic matter is accomplished biologically using a variety of microorganisms, which convert the colloidal and dissolved carbonaceous organic matter into various gases and into protoplasm. It is important to note that unless the protoplasm produced from the organic matter is removed from the solution, complete treatment will not be accomplished because the protoplasm, which itself is organic, will be measured as biological oxygen demand (BOD) in the effluent. 1.2.3 Types of Biological Wastewater Treatment Processes Biological treatment processes are typically divided into two categories according to the existing state of the microorganisms: suspended-growth systems and attached-growth systems. Suspended systems are more commonly referred to as activated sludge processes, of which several variations and modifications exist. Attached-growth systems differ from suspended-growth systems in that microor- ganisms attached themselves to a medium, which provides an inert support. Trickling filters and rotating biological contactors are most common forms of attached-growth systems. The major biological processes used for wastewater treatment are