Wastewater characterisation is fundamental to the design and operation of treatment facilities. Understanding the chemical makeup of sewage allows engineers to select appropriate treatment processes, predict environmental impact, and comply with regulatory discharge standards. Sewage characteristics are typically divided into three broad categories: physical, chemical, and biological. The chemical category encompasses oxygen demand parameters, nutrient concentrations, pH, alkalinity, and the presence of organic and inorganic compounds. This article examines the chemical characteristics of sewage in detail, with particular focus on biochemical oxygen demand (BOD), chemical oxygen demand (COD), nutrient content, and the various methods used to quantify organic pollution loads. Understanding these parameters is essential for anyone working with road user characteristics and infrastructure planning where wastewater systems intersect with transportation networks.
Organic Composition and Chemical Makeup of Sewage
In sanitary sewage, approximately 75 percent of suspended solids and 40 percent of filterable solids are organic in nature. These solids originate from animal, plant, and human sources. The organic compounds found in wastewater consist primarily of carbon, hydrogen, oxygen, and nitrogen, along with sulphur, phosphorus, and iron. The major organic fractions present in sewage include:
- Proteins, which constitute between 40 and 60 percent of the organic content
- Carbohydrates, which make up 25 to 50 percent of the organic fraction
- Fats and oils, accounting for approximately 10 percent of organic matter
- Synthetic organic compounds including VOCs, pesticides, and insecticides
- Organic priority pollutants from industrial and domestic sources
Sewage also contains significant inorganic substances such as chlorides, sulphates, phosphates, and heavy metals. The distribution and concentration of these constituents determine the treatment strategy required. For example, the presence of high levels of fats and oils can interfere with biological treatment processes, while elevated nutrient concentrations may trigger eutrophication in receiving water bodies. The first stage of many treatment plants focuses on removing settleable solids and grit, which is why understanding grit chambers sewage treatment operations is important for managing the inorganic fraction of wastewater effectively. Tests such as BOD, COD, nitrogen, phosphorus, and alkalinity measurements collectively define the full chemical profile of a sewage sample.
Biochemical Oxygen Demand and the Dynamics of Aerobic Decomposition
When biodegradable organic matter enters a water body, microorganisms begin to feed on the waste material, breaking complex organic compounds into simpler organic and inorganic substances. Under aerobic conditions, this decomposition produces stable, non-objectionable end products including carbon dioxide, sulphate, phosphate, and nitrate. The simplified chemical equation for aerobic decomposition can be expressed as:
Organic Matter + O₂ + Microorganisms → CO₂ + H₂O + C₅H₇NO₂ (new cells) + stable products (NO₃, PO₄, SO₄)
When dissolved oxygen is insufficient, anaerobic decomposition occurs through a different microbial population. This process produces highly objectionable end products including hydrogen sulphide, ammonia, and methane. The anaerobic pathway is responsible for the unpleasant odours associated with septic sewage and poorly maintained collection systems. The amount of oxygen required by bacteria to oxidise the organic matter present in sewage to stable end products is known as the biochemical oxygen demand.
Two key variants of BOD are used in practice. BODu, or ultimate BOD, represents the maximum amount of oxygen consumed by microorganisms over an extended period and provides a good measure of maximum bioavailability. BOD5 measures the oxygen consumed in milligrams per litre over a standard five-day period at 20 degrees Celsius in the dark. BOD5 is the most widely used regulatory parameter worldwide and serves as the primary indicator of organic pollution strength in domestic and industrial wastewater. Advances in energy recovery from wastewater streams have opened new possibilities, such as sewage source heat pumps that turn wastewater warmth into usable thermal energy, demonstrating how BOD-rich effluent can be viewed as a resource rather than merely a waste stream.
Chemical Oxygen Demand and Laboratory Measurement Methods
While biodegradable organic matter is fully degraded by microorganisms through carbonaceous or nitrogenous oxygen demand pathways, certain organic substances resist biological breakdown. Cellulose, phenols, benzene, and tannic acid are examples of compounds that are resistant to biodegradation. Similarly, pesticides, insecticides, and various industrial chemicals are non-biodegradable and can be toxic to microbial populations. These refractory compounds require a different analytical approach.
Chemical oxygen demand is a measured quantity that does not depend on microorganisms. Instead, a strong oxidising agent, typically potassium dichromate, is used under acidic conditions to oxidise both biodegradable and non-biodegradable organic matter. The reaction proceeds as follows:
Organic Matter (CₐH₂O꜀) + Cr₂O₇²⁻ + H₂O → Cr³⁺ + CO₂ + H₂O
The COD test offers a significant advantage in speed, producing results within hours rather than the five days required for BOD testing. However, it does not distinguish between biodegradable and non-biodegradable material. When all organic matter present is biodegradable, COD values closely match BOD readings. In the presence of non-biodegradable impurities, measured COD exceeds BOD. The ratio between these two parameters provides valuable insight into the treatability of wastewater. Selecting the right materials for plumbing and conveyance systems is equally critical, as aggressive chemical conditions in wastewater can affect infrastructure longevity. Information on pex piping and soil pesticides understanding chemical compatibility in below slab plumbing is highly relevant when designing collection systems that carry chemically aggressive sewage.
| Parameter | Test Duration | What It Measures | Biological Component | Typical BOD/COD Ratio |
|---|---|---|---|---|
| BOD5 | 5 days | Biodegradable organic matter | Microorganisms required | 0.4 to 0.8 |
| BODu (Ultimate BOD) | 20 to 30 days | Maximum biodegradable oxygen demand | Microorganisms required | 0.4 to 0.8 |
| CBOD | 5 days | Carbonaceous demand only (nitrogen suppressed) | Microorganisms required | 0.3 to 0.6 |
| COD | 2 to 3 hours | Total organic matter (biodegradable + non-biodegradable) | Chemical oxidation only | N/A |
| TheoD | Calculated | Stoichiometric oxygen demand from chemical formula | Neither required | N/A |
Carbonaceous and Nitrogenous Oxygen Demand in Wastewater
Carbonaceous biochemical oxygen demand, commonly abbreviated as CBOD, is a method-defined parameter that measures the depletion of dissolved oxygen by biological organisms while suppressing the contribution from nitrogenous bacteria. CBOD provides a focused assessment of the carbonaceous fraction of organic pollution and is widely used as an indicator of pollutant removal efficiency in wastewater treatment plants. It is listed as a conventional pollutant under the United States Clean Water Act and is a standard compliance parameter for discharge permits.
Nitrogenous oxygen demand, or NBOD, represents the oxygen consumed during the biological oxidation of nitrogenous compounds, primarily ammonia, to nitrate. This process, known as nitrification, is carried out by specialised autotrophic bacteria such as Nitrosomonas and Nitrobacter. In standard BOD5 tests, the nitrogenous demand is typically suppressed to isolate the carbonaceous component, as nitrifying bacteria have a longer lag phase. However, in receiving waters, the combined oxygen demand from both carbonaceous and nitrogenous sources determines the overall impact on dissolved oxygen levels. Effective chemical treatment methods extend beyond wastewater alone. For surface finishing applications, understanding refinishing ebonized oak flooring chemical methods for removing pet urine and ammonia stains illustrates how ammonia chemistry plays a role in both wastewater treatment and building material restoration contexts.
Theoretical Oxygen Demand and Practical Applications in Treatment Design
Theoretical oxygen demand, or TheoD, is a calculated value derived from the stoichiometric oxidation of organic matter whose chemical composition is known. Organic matter of animal or vegetable origin in wastewater generally consists of carbon, hydrogen, oxygen, nitrogen, and other elements arranged in complex molecular structures. If the exact chemical formula of an organic compound is established, the amount of oxygen required to fully oxidise it to carbon dioxide and water can be determined using fundamental stoichiometry.
The relationship between the three oxygen demand measures is instructive. If oxidation is carried out by bacteria, the measured demand is BOD. If oxidation proceeds through a chemical process, the measured demand is COD. When a combination of both biological and chemical pathways is involved, or when the demand is calculated from first principles, the result is termed theoretical oxygen demand. In practice, TheoD serves as a quality control benchmark. Large discrepancies between theoretical and measured values indicate the presence of inhibitory compounds, analytical errors, or uncharacterised organic fractions that require further investigation.
Modern wastewater treatment plants integrate multiple process stages to address the full spectrum of oxygen demand. Preliminary treatment removes grit and large solids, primary sedimentation settles suspended organic matter, and secondary biological treatment addresses the dissolved BOD fraction. For facilities handling complex industrial effluents or high-strength domestic sewage, advanced wastewater treatment system configurations may be required to meet stringent discharge limits. These systems often incorporate tertiary filtration, nutrient removal, and disinfection stages to produce effluent suitable for reuse or sensitive environmental discharge.
Nutrients, Alkalinity and the Significance of Chemical Parameters
Beyond oxygen demand, the chemical characterisation of sewage includes several additional parameters that influence both treatment performance and environmental fate. Nitrogen and phosphorus are the primary nutrients of concern. Nitrogen appears in sewage as organic nitrogen, ammonia, nitrite, and nitrate, while phosphorus exists as orthophosphate, polyphosphate, and organic phosphate. These nutrients, if discharged untreated, stimulate algal blooms and accelerate eutrophication in receiving water bodies.
Alkalinity in wastewater arises from bicarbonates, carbonates, and hydroxides and provides essential buffering capacity. Adequate alkalinity is necessary to maintain stable pH conditions during biological treatment, particularly in nitrifying systems where the oxidation of ammonia consumes alkalinity and can depress pH below optimal levels. A well-designed treatment strategy must consider both the organic load measured by BOD and COD and the nutrient and alkalinity balance required to sustain healthy biological processes. Selecting among the available technology options requires a clear understanding of the differences between them, and reviewing important types of wastewater treatment systems helps identify the most appropriate configuration for a given application, whether it involves trickling filters, activated sludge, sequencing batch reactors, or membrane bioreactors.
The comprehensive chemical characterisation of sewage using BOD, COD, TheoD, nutrient analysis, and alkalinity measurements provides engineers with the data needed to design effective, efficient, and environmentally responsible wastewater treatment systems. Each parameter contributes a distinct piece of information that, when assembled, creates a complete picture of the wastewater’s composition and the treatment strategies required to protect public health and the natural environment.