Biological Wastewater Treatment Processes

Mass and Heat Balances

Davide Dionisi

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351670890
• Edition: 1
• Topic: Scienze fisiche
• Subtopic: Chimica industriale e tecnica

The focus of the book is on how to use mass and heat balances to simulate and design biological wastewater treatment processes. All the main processes for biological wastewater treatment are covered viz. activated sludge processes for carbon and nitrogen removal, anaerobic digestion, sequencing batch reactors, and attached growth processes.

CHAPTER 1

Biological Wastewater Treatment Processes

1.1 POLLUTING PARAMETERS IN WASTEWATERS

Raw wastewaters from urban or industrial discharges may contain many substances which can cause pollution to the environment and can cause in the end very negative effects on human life and health. Therefore, wastewaters cannot in general be discharged to the receiving water body, which may be a river, lake or sea, unless they have been treated to remove the polluting substances or to reduce the concentration of these substances below some safe levels (Figure 1.1).

In general, the main parameters or substances which need to be removed from wastewaters are suspended solids, soluble organic matter, heavy metals, toxic organic chemicals, nitrogen and phosphorus.

Suspended solids are usually measured as total suspended solids (TSS) and volatile suspended solids (VSS). The TSS are the total solids which are deposited on a filter of specified pore size (typically around 1 μm) and are composed of both organic and inorganic solids. The VSS are the fraction of the TSS which volatilise at a specified temperature, typically 550°C. The VSS are in general considered to represent the fraction of the TSS which is organic in nature. The difference between TSS and VSS is called fixed suspended solids (FSS), which is considered the inorganic fraction of the TSS. As an example, starch, cellulose or microorganisms contribute to the VSS, whereas calcium carbonate (CaCO₃) is part of the FSS.

FIGURE 1.1 Wastewaters and treatment processes.

Soluble organic matter has to be removed from wastewaters because if large amounts of organic matter are discharged into water bodies they will cause growth of microorganisms. Microorganisms’ growth will have a negative effect on aquatic life since it will cause a depletion in dissolved oxygen in water causing death of many aquatic species. Also, uncontrolled growth of microorganisms in water bodies may originate pathogen bacteria which may spread diseases among fish, animals and humans. Soluble organic matter is composed of many different organic species which cannot be measured individually. Therefore, concentration of soluble organic matter in wastewaters is generally expressed via two lump parameters, the biochemical oxygen demand (BOD) and the chemical oxygen demand (COD). The BOD measures the amount of oxygen that an inoculum of microorganisms consumes when is in contact with the considered wastewater for a prescribed period of time. This time is often taken as five days, and the BOD measured with five-day contact time between microorganisms and wastewater is referred to as BOD₅. So the BOD of a wastewater is considered an indicator of the biodegradable organic matter in that wastewater. The COD measures the amount of oxygen that is necessary for the complete oxidation of all the organic matter contained in the sample. The COD test is carried out heating a sample of the considered wastewater in the presence of a strong acid and of a strong chemical oxidant, typically potassium dichromate (K₂Cr₂O₇). The ratio BOD:COD is taken as an indicator of the biodegradability of the organic matter contained in the wastewater. The higher the BOD:COD ratio, the higher is the biodegradability of the organic matter. This is correct; however, it is only an approximate indication. Indeed, even for a completely biodegradable substance, for example, glucose, the BOD will be lower than the COD because in the BOD test part of the substance is assimilated into new microorganisms and therefore is not oxidised by oxygen, even though it is biodegraded.

Heavy metals such as chromium, copper, palladium and nickel are usually not present at harmful concentrations in urban wastewaters, although they may be present in industrial wastewaters, depending on the nature of the industrial discharge. Heavy metals are toxic, above certain concentrations, to living organisms and, if present in the wastewater above certain limits, they must be removed before discharge into the receiving water body.

Toxic organic chemicals are substances which can be toxic or harmful to living organisms even when present in wastewaters at very low concentrations. Many different categories of chemicals can fit into this definition, for example, pharmaceuticals, detergents, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and others. These substance are also called xenobiotics, micropollutants or priority pollutants. They can be present in municipal wastewaters (e.g. pharmaceuticals and detergents) and in industrial wastewaters (e.g. PAHs and PCBs).

Nitrogen and phosphorus compounds can be present in municipal and industrial wastewaters. Nitrogen is often present as ammonia (NH₃ or NH₄⁺) or as organic nitrogen, for example, in amino acids. Phosphorus is mainly present as phosphates (PO₄^3–). In municipal wastewaters, nitrogen is mainly present as a product of human metabolism, whereas phosphorus may be present due to its presence in laundry liquids, even though more stringent regulations and improved formulations by the detergents industry have greatly decreased the presence of phosphorus in municipal wastewaters.

Table 1.1 summarises the typical concentration of the main polluting parameters in municipal wastewaters.

1.2 COD AND BOD

COD and BOD are the two key parameters used to characterise the organic content of wastewaters and the efficiency of biological wastewater treatment plants.

TABLE 1.1 Typical Range of Polluting Parameters in Raw (before Treatment) Municipal Wastewaters

Parameter	Value
TSS	100–350 mg/l
BOD5	100–400 mg/l
COD	250–1000 mg/l
Total nitrogen	20–100 mgN/l
Total phosphorus	4–15 mgP/l

Note: TSS: total suspended solids; BOD: biochemical oxygen demand; COD: chemical oxygen demand.

1.2.1 COD

The COD of an organic compound represents the amount of oxygen that is required to oxidise the substance to carbon dioxide and water. For a wastewater, where many organic substances may be present, the COD is the amount of oxygen that is required to oxidise all the organic substances present in the wastewater. Examples of oxidation reactions for some organic species are shown below.

${\text{C}}_{\text{6}}{\text{H}}_{\text{12}}{\text{O}}_{\text{6}}(\text{glucose})+6{\text{O}}_{\text{2}}\to 6{\text{CO}}_2+6{\text{H}}_2\text{O}$

(1.1)

From this stoichiometry it can be calculated that 1 g of glucose corresponds to 1.067 g of COD.

${\text{CH}}_{\text{4}}(\text{methane})+2{\text{O}}_{\text{2}}\to {\text{CO}}_2+2{\text{H}}_2\text{O}$

(1.2)

1 g of methane corresponds to 4 g of COD.

${\text{C}}_{\text{5}}{\text{H}}_9{\text{O}}_{\text{4}}\text{N(glutamic}\text{acid)}\text{+}\text{4}{\text{.5O}}_{\text{2}}\to 5{\text{CO}}_2+3{\text{H}}_2\text{O}\text{+}{\text{NH}}_{\text{3}}$

(1.3)

1 g of glutamic acid corresponds to 0.98 g of COD. Note that in the COD reaction nitrogen is not oxidised and remains at the oxidation state at which is present in the organic compound, in this case NH₃.

The COD can be calculated, and ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Preface
• About the Author
• CHAPTER 1 ■ Biological Wastewater Treatment Processes
• CHAPTER 2 ■ Modelling Processes in Biological Wastewater Treatment
• CHAPTER 3 ■ Mass Balances, Energy Balances and Parameter Estimation
• CHAPTER 4 ■ The Activated Sludge Process
• CHAPTER 5 ■ The Anaerobic Digestion Process
• CHAPTER 6 ■ The Sequencing Batch Reactor
• CHAPTER 7 ■ Attached Growth Processes
• APPENDIX A
• APPENDIX B
• APPENDIX C
• APPENDIX D
• APPENDIX E
• BIBLIOGRAPHY
• INDEX