Industrial Waste Treatment Process Engineering

Biological Processes, Volume II

Gaetano Celenza

• Publisher: CRC Press
• Year: 2019
• Print ISBN:
  9780367455576
• eBook ISBN:
  9781000725520
• Topic: Technology & Engineering
• Subtopic: Environmental Science
• Index: Technology & Engineering

Industrial Waste Treatment Process Engineering is a step-by-step implementation manual in three volumes, detailing the selection and design of industrial liquid and solid waste treatment systems. It consolidates all the process engineering principles required to evaluate a wide range of industrial facilities, starting with pollution prevention and source control and ending with end-of-pipe treatment technologies. Industrial Waste Treatment Process Engineering guides experienced engineers through the various steps of industrial liquid and solid waste treatment. The structure of the text allows a wider application to various levels of experience. By beginning each chapter with a simplified explanation of applicable theory, expanding to practical design discussions, and finishing with system Flowsheets and Case Study detail calculations, readers can "enter or leave" a section according to their specific needs. As a result, this set serves as a primer for students engaged in environmental engineering studies AND a comprehensive single-source reference for experienced engineers. Industrial Waste Treatment Process Engineering includes design principles applicable to municipal systems with significant industrial influents. The information presented in these volumes is basic to conventional treatment procedures, while allowing evaluation and implementation of specialized and emerging treatment technologies. What makes Industrial Waste Treatment Process Engineering unique is the level of process engineering detail. The facility evaluation section includes a step-by-step review of each major and support manufacturing operation, identifying probable contaminant discharges, practical prevention measures, and point source control procedures. This theoretical plant review is followed by procedures to conduct a site specific pollution control program. The unit operation chapters contain all the details needed to complete a treatment process design.

CHAPTER II-1

Aeration

Aeration devices are employed to supply oxygen to suspended growth biological systems or to induce oxidation conditions for chemical treatment.

BASIC CONCEPTS

AERATION units are common components of chemical or biological oxidation systems, allowing a simple and inexpensive method of supplying oxygen by injecting air into the reactors. Their principal use is in suspended growth biological systems, where they supply oxygen, disperse biological solids, and mix the system reactants. Figure 1.1 illustrates the elements of any oxygen transfer process, with a device’s oxygenation capacity defined by a rate coefficient (K_la), propelled by the driving force (C_s − C), delivering oxygen at the rate of dC/dt.

Chemical process oxygen requirements are stoichiometric quantities, easily established by material balance of simple reactions. Biological oxygen requirements are more difficult to estimate because they represent the stoichiometric sum of simultaneous reactions involving (1) biological carbonaceous oxygen demand, (2) nitrification, and (3) inorganic chemical oxygen demand. Chemical oxidation reactions are discussed in Chapter I-7, and biological systems are covered in Chapters II-2,II-3,II-4.

OXYGEN TRANSFER RATE

Gas transfer rate in a reactor is defined by Equation (1.1) [7].

$dC\text{/}dt=K\cdot A\left(C_s-C\right)$

(1.1)

where dC/dt is the change in oxygen concentration with time, K is the oxygen transfer coefficient, A is the gas transfer area, C_s is the oxygen saturation concentration in the waste at the process temperature and represents the maximum oxygen solubility, and C is the solution oxygen concentration in the waste and will always be lower than the saturation concentration.

Drawing an analogy between heat transfer and mass transfer, heat transfer rates (Q) are calculated on the basis of an overall heat transfer coefficient (U), a transfer area (A), and the temperature difference driving force (T − t):

$\left(\text{BTU/hr}\right)=U\cdot A\cdot \left(T-t\right)$

(1.2)

As indicated by Equation (1.1), the basic oxygen transfer between two points is similar to the heat transfer relation, Equation (1.2).

Another analogy between heat transfer and mass transfer further illustrates the factors affecting oxygen transfer. Any action increasing transfer from the “film” transfer surface into the bulk volume will increase the heat transfer rate. High turbulent fluid conditions, measured by increased Reynold’s number, increases the overall heat transfer coefficient and the heat transfer rate. Aeration devices essentially perform that function by dispersing oxygen to the liquid volume. This is accomplished by turbulent mixing of oxygen and waste in as much of the reactor volume as possible, increasing the contact area between the oxygen (air) and waste by forming small air or waste droplets. The net result is an increase in the mass transfer coefficient K_l and the area A. The contact area is difficult to establish because it is much greater than the basin surface area contributing to diffusion transfer; it is the total liquid droplet area for mechanical aeration or the total air bubble area for dispersed air systems. As a result the transfer capacity is represented by a new factor K_la, a product of the transfer coefficient and the area.

Figure 1.1 Oxygen system.

Using the combined coefficient, Equation 1.1 can be expressed as Equation (1.3) [6,8].

$dC\text{/}dt=K_{la}\left(C_s-C\right)$

(1.3)

The relation defines the concentration change between points. The mass oxygen transfer rate for the entire basin can be calculated on the basis of an overall mass transfer rate (K_la), the aeration volume, and the bulk oxygen concentration difference (C_s − C), as defined by Equation (1.4).

$\text{OTR}=K_{la}\cdot V\cdot \left(C_s-C\right)$

(1.4)

where OTR is the overall mass transfer rate for the system, mass/time, V is the aeration volume, and K_la is the “apparent” overall mass transfer coefficient, 1/time.

The first process consideration is maximizing the driving force (C_s − C). In process calculations, C_s is taken as the maximum possible value, the solubility of oxygen in the waste at the system temperature. C is taken as the minimum practical number to assure aerobic conditions/one to two parts per million (ppm), never below 0.5 ppm. For all practical purposes the driving force (the maximum concentration difference) is fixed in the process design, any change due to an uncontrollable variable, temperature. Therefore, oxygen transfer can only be improved by increasing the device capacity (K_la), the effected transfer area (the dispersion and mixing capability), or both. A device’s energy output is considered a measure of its oxygen transfer and mixing capacity. As will be discussed in the next section there are many available devices capable of accomplishing the dual function required of aeration equipment, oxygen transfer and mixing.

PROCESS OXYGEN CAPACITY

The oxygen requirements of a treatment system are a function of the waste oxidation demand, which will be depleted at a rate defined by the reactor operating conditions and the waste reactivity. The specific process demand must be obtained from laboratory generated data or established from similar operating treatment facilities. This oxygen demand must be delivered by the aeration device at specific operating conditions, primarily defined by the release pressure and reactor temperature. Therefore, a rating system is required to compare equipment oxygen delivery rate on a common basis. Conventionally, the aeration capacity of a device is standardized to accepted reference conditions, reported as the standard oxygen transfer rate (SOTR) at the following conditions:

(1) 20°C

(2) One atmosphere oxygen release pressure and a specified relative humidity.

(3) Zero mg/L initial oxygen concentration

(4) Oxygen saturation concentration of tap water at 20°C

The SOTR capacity of specific devices is frequently reported as follows:

• Diffuser air system: The capacity is expressed as the diffuser standard air flow rate (L/S or SCFM) at SOTR conditions. Significantly, the standard oxygen transfer efficiency (SOTE) is used to estimate the portion of the diffuser oxygen transfer rate that is absorbed in the waste.

• Turbine system: The turbine diffuser capacity is expressed as the SOTR, similar to an air diffuser system. The horsepower ratio of the turbine to the diffuser compressor is often quoted to further define the turbine system effectiveness.

• Mechanical aerators: SOTR is expressed as the oxygen mass transfer rate per horsepower or kilowatt at 20°C, 1 atm, and an initial dissolved oxygen (DO) concentration of 0 mg/L.

Some device capacities may be defined by the supplier at conditions other than those mentioned. It is important that the units be understood, and that all process evaluations are based on consistent units.

The required or actual oxygen rate (AOR) and the standardized oxygen transfer rate (SOTR) must be equated to a common basis for design purposes. The relation used is a modification of Equation (1.4) [8]:

$\text{AOR}=\text{SOTR}\cdot \frac{\beta \cdot C_{swc}-C_l}{C_s}\cdot {1.024}^{T-20}\cdot \text{α}$

(1.5)

where AOR is the actual oxygen requirements, kg (lb) per hour; SOTR is oxygen requirements in tap water at 20°C and 0 mg/1 oxygen, kg (lb) per hour; β is correction of oxygen solubility for wastewater characteristics; C_swc is oxygen solubility at given temperature, altitude, and pressure at point of release; α is correction for oxygen transfer rate in wastewater; C_l is operating dissolved oxygen concentration, mg/L; C_s is oxygen solubility of tap water at 20°C, 9.17 mg/L; and T is wastewater temperature, °C.

Process calculations presented in design texts are...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Introduction
• II-1. AERATION
• II-2. AEROBIC BIOLOGICAL OXIDATION
• II-3. ACTIVATED SLUDGE SYSTEM
• II-4. BIOLOGICAL OXIDATION: LAGOONS
• II-5. BIOLOGICAL OXIDATION: FIXED-FILM PROCESSES
• II-6. AEROBIC DIGESTION
• II-7. ANAEROBIC WASTE TREATMENT—ANAEROBIC SLUDGE DIGESTION
• II-8. SEDIMENTATION
• Index