Introduction

Biological wastewater treatment is often associated with secondary waste-water treatment and intends to treat the dissolved and colloidal organics after primary treatment. The goal of all biological wastewater treatment systems is to coagulate and remove or reduce the nonsettling organic solids and the dissolved organic load from the effluents by using micro-bial communities to degrade the organic load through biochemical reactions. Biological wastewater treatment is generally a major part of secondary treatment design of wastewater and characterized by reduction of the oxygen demand of an influent wastewater to a given level of purification. The microorganisms responsible for reducing the organic matters and consequently the oxygen demand of incoming wastewater can be classified based on the way in which they utilize oxygen: aerobic (need oxygen for their metabolism), anaerobic (thrive in the absence of oxygen), or facultative (can live on oxygen and live without it through different metabolisms). Aerobic biological treatment dominates secondary wastewater treatment scenes and is performed in the presence of oxygen by aerobic microorganisms (principally bacteria) that metabolize the organic matter in the wastewater, thereby producing more microorganisms and inorganic end products (principally CO2, NH3, and H2O). Several aerobic biological processes are used for secondary treatment, differing primarily in the manner in which oxygen is supplied to the microorganisms and in the rate at which organisms metabolize the organic matter. From a nutritional point of view, the majority of microorganisms in biological wastewater treatment systems uses the organic matters in the wastewater as the energy source for growth and maintenance of microorganisms.

Anaerobic processes sometimes are also used in the secondary biological treatment of wastewater. Anaerobic processes, in addition to sludge digestion, are employed to treat high-strength wastewater, such as high-strength food-processing wastewater streams when the prospect of difficulty associated with oxygen supply to the reactor and large biomass produced in an aerobic process is deemed uneconomical.

In secondary wastewater treatment, sedimentation is also employed to remove settleable solids after microorganisms have done their work. The microorganisms must be separated from the treated wastewater by sedimentation to produce clarified secondary effluent. The sedimentation tanks used in secondary treatment, often referred to as secondary clari-fiers, operate in the same basic manner as the primary clarifiers described in Chapter 3. The biological solids removed during secondary sedimentation, called secondary or biological sludge, are normally combined with primary sludge for sludge processing. The main difference between sedimentation in a secondary treatment reactor (tank/basin) and sedimentation in a primary treatment reactor (tank/basin) is that sludge in the secondary treatment is composed of biological cells. There are two main types of bioreactors used in the secondary treatment: those where microorganisms are attached to a fixed surface (e.g., trickling filter), and those where microorganisms run freely in the wastewater stream (e.g., activated sludge). The sludge settled in the sedimentation tank in the latter type of reactors is usually recycled back to the system for continuing operations.

Widespread high-rate processes in relatively small reactors include the activated sludge processes, trickling filters, and RBSs (rotating biological contactors). A combination of two of these processes in series (e.g., trickling filters followed by activated sludge) is sometimes used with certain wastewaters containing a high concentration of organic materials from industrial sources, such as food and agricultural processing.

Because a secondary treatment process uses microorganisms to break down the organic matters in order to clarify the wastewater, it is important to know the biology of the secondary wastewater treatment process. The most widely present microorganisms in wastewater treatment are bacteria, and this group of microorganisms is responsible for degrading organic matters present in the wastewater. The following section is centered on bacteria's role in biological wastewater treatment; the essence of the description, however, is applicable to all microorganisms in biological conversions in biochemical systems.

Chapter 4: Biological Wastewater Treatment Processes 115 Kinetics of Biochemical Systems in Wastewater Microbiology

The fundamentals of wastewater microbiology include the roles of micro-bial groups in specific biological transformation of wastewaters, nutritional requirements, the effects of environmental conditions on microbial activities, and enzymatic reactions that underpin biological conversions of waste materials.

Chapter 2 presents the classifications of various microbial groups and their roles in biological wastewater treatment based on cell structures and function as eukaryotes and prokaryotes. However, microorganisms in wastewater treatment can also be described based on nutritional requirements. Like all living things, nutrients play a critical role in development of microbial communities; they supply the energy source for cell growth and biosynthesis and provide the materials necessary for synthesis of cy-toplasmic materials, as well as serve as electron acceptors from biochemical reactions. The nutritional requirements provide a basis of microorganism classifications based on carbon source and energy source. Fig. 4.1 illustrates a general classification of microorganisms based on nutritional requirements.

Environmental factors such as temperature, pH, and oxygen requirements for aerobic or facultative microorganisms are of great importance to microbial growth or even survival. The temperature effect on microbial

Figure 4.1. Classification of microorganisms based on nutritional requirements.

growth tends to be positive; however, overheating may inhibit or kill microorganisms. The effects of pH on microbial communities are more varied; some microorganisms do well in slightly alkaline conditions—such as most bacteria with pH ranging from 6.5 to 7.4—but fungi prefer slightly acidic conditions. Oxygen requirements are critically important for aerobes and are optional for facultative bacteria. Anaerobes are not affected by absence of oxygen; they thrive without it.

Bioconversion of organic matters by microorganisms takes place in a series of biochemical reactions with participation of a class of biological catalysts called enzymes. Enzymes are specific proteins that catalyze reactions but do not undergo permanent changes themselves. They work by forming complexes with organic substrate and inorganic molecules, facilitating reaction of these substances resulting in end products, and releasing the enzymes in the original state so the biochemical reaction cycle can continue. Enzymes are substrate-specific; thus, bacteria usually have many different enzymes performing different catalytic roles in converting a broth of organic substances into end products. In general, these enzymes belong to one of two groups: extracellular and intracellular. As the names suggest, extracellular enzymes convert organic substances outside the cell into a form of intermediate products; intracellular enzymes can take over from there to complete the biochemical reactions within the cells. These enzymatic reactions often occur sequentially among different enzymes in a cell. A portion of the organic substrate that attaches to the enzyme is utilized as an energy source while the remainder is scavenged to reproduce more cells.

The microbial population of biological wastewater treatment systems contains a large number of species of microorganisms with diverse physiological and genetic variations; as a result, the properties of the colony in wastewater systems may be described only as averaged behaviors of the microbial population. Additionally, the properties of the microbial population are described in terms of easily quantifiable parameters. For example, the size of the population is often measured in dry weight or nitrogen content because microbial colonies are established along the line of functionally discrete units or cell mass. Thus, the microbial population in wastewater treatment systems is considered a mixture of microorganisms as biomass distributed continuously in treatment systems (or reactors). With that view, we can treat biochemical reactions involving microbial populations in reactors as averaged reactants (biomass and organic substrate) undergoing enzymatic reactions. The models that describe characteristics of these abstract "reactants" are therefore deterministic despite the fact that the behaviors of individual cells within the microbial community can only be described only as stochastic.

The kinetic models that describe biological conversion of organic matters in wastewater are those of the reactions that result in changes in concentration of an organic substrate or microorganism responsible for the conversion and may be modeled using simple reaction rate theory, which describes the rate of change in concentration of an organic substrate or microorganism (Equations 4.1 and 4.2):

Reactant Product rate = k (concentration of A)n

The rate of reaction can further be expressed as the following (Equation 4.3):

-r = kCAn where k is the reaction rate constant, CA is concentration of species A (substrate or biomass), and n is the order of the reaction. The negative sign in the equation signifies the disappearing of reactant A as the reaction progresses. The order of reaction is a parameter that reflects the kinetics of a biochemical reaction and can theoretically be any number, but often it is one of the three basic reaction kinetics: zero-order, first-order, and second-order, as described in Chapter 1.

Taking the log on both sides of Equation 4.2 yields the following (Equation 4.4):

log r = nlog( concentration of reactant A) + log k

A plot of log r versus log CA will produce a linear line with a slope of n. Effects of temperature on reaction rates

Like any other reactions, the effects of temperature on reaction rates originate from the temperature effects on rate constants. Rate constant, k, is a lumped parameter that encompasses many environmental factors, such as pH; oxygen concentration; concentrations of trace elements; and, in the case of photosynthesis, light intensity. One particularly important parameter is temperature, which affects the rates of both chemical and biochemical reactions. It is observed by Van't Hoff that a reaction rate roughly doubles for every 10°C increase in temperature. The effects of temperature on reaction rates often follow the Arrhenius model (Equation 4.5):

where T is the absolute temperature or thermodynamic temperature in Kelvin, Ea is activated energy that reactants must overcome in order to proceed with reactions, R is the gas constant (R = 8.314 J K^1 moP1), and A is the preexponential factor or frequency factor that is related to the collision frequency of reactants.

The Arrhenius equation is widely used in wastewater treatment systems to model the effects of temperature on reactions. It first starts with taking the derivative of Equation 4.5 and is then integrated between the limits T0 and Tf this gives the following (Equation 4.6):

KRTfToJ

where k0 and kf are the rate constants at temperatures of T0 and Tf, respectively. The reason for taking the derivative and following with integration is that the rate constant is a function of time. Equation 4.6 has been used in the temperature range of 5-25°C; outside this range, the equation is not valid due to significant change in microbial composition in the microbial population.

Effects of pH and dissolved oxygen concentration on reaction rates

Effects of pH and dissolved oxygen concentration on reaction rates are more substrate-specific. The optimum pH range for carbonaceous oxidation lies in the range of 6.5-8.5. At pH above 9.0, microbial activity is inhibited. At pH below 6.5, fungi dominate over the bacteria in the competition for the substrate. Some types of reactors are less affected by fluctuations of influent pH; obviously, completely mixed reactors, such as CSTRs, will minimize the effect of pH fluctuation. If the pH fluctuation is significant, some adjustment to pH may be needed. Some bacteria have less tolerance toward pH fluctuation than others; for example, anaerobic bacteria have a viable pH range of 6.7-7.4, with optimum growth occurring in pH 7.0-7.1.

Dissolved oxygen (DO) concentration obviously affects rates of reactions in aerobic biochemical reactions. For instance, DO concentration of 1-2 mg/l may be sufficient for active aerobic heterotrophic microbial activity provided that sufficient nutrients and trace elements are available to microbial activities.

Kinetic equations of bacterial growth

The kinetics of biochemical reactions in bioreactors described in the previous sections and in Chapter 1 are simplified mathematical descriptions that are not all-inclusive in terms of all aspects of the mechanisms under consideration. Successful environmental control in biological wastewater treatment, however, is rooted in an understanding of the basic principles governing the growth of microorganisms where substrates are assimilated and biomass in the system accumulates. The studies of microorganism growth in pure media under controlled pH, temperature, oxygen concentration, and other substances have produced reliable kinetic models of microorganism growth.

The general growth pattern of bacteria in a batch culture is illustrated in Fig. 4.2. The diagram is a record of a number of viable microorganisms in the culture medium of fixed volume over time. The pattern of the curve shown in Fig. 4.2 shows four distinct phases: the lag phase, the log-growth phase, the stationary phase, and the log-death phase:

• The lag phase. Once the inoculum is introduced in the culture medium, the microorganisms take time to acclimate themselves in the environment.

• The log-growth phase. This is the normal growth pattern under sufficient food (organic substrate) and nutrients for microbial growth.

• The stationary phase. The microbial population is stabilized as a result of a stand-off between growth of microorganisms and death of old cells. Usually, there is some insufficiency of substrate and/or nutrients available for microbial growth.

• The log-death phase. During this phase, the death rate of microbial cells exceeds the growth rate of new cells. This is an indication of deteriorating environmental conditions in addition to lack of substrate.

Figure 4.2. A diagram of a number of viable microorganisms in the culture medium of fixed volume over time.

The common autocatalytic equation, a first-order reaction, is used to describe the log-growth phase (Equation 4.7):

where rX is the rate of production of viable bacteria, X is the concentration of viable microorganisms, and ^ is the specific growth rate constant (t-1). If the cell concentration of microbial cells at to is Xo, then after a time interval, t, the viable cell concentration Xt is the following (Equation 4.8):

Xt = XoeMt

Taking logs of Equation 4.8 shows a linear relationship between ln X and ln X0.

When cell concentration doubles between td and t0, it shows the following relationship (Equation 4.9):

There are several models that relate cell growth to substrate utilization; Monod model is the most widely used model. Monod model describes the relationship between the residual concentration of the cell-growth limiting substrate or nutrient and the specific growth rate of biomass of cells, ^ in the following mathematical expression (Equation 4.10):

S + ks where is the maximum specific growth rate at saturation concentration of the growth limiting substrate; S is the substrate concentration; and ks is the saturation constant (mg/l), which is the concentration of growth-limiting substrate at which the specific growth rate ^ = ^/2.

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