Wastewater Treatment and Quality: A Comprehensive Guide

Posted on Sep 12, 2024 in Chemistry

What is Wastewater and Its Purpose?

Wastewater is defined as water that has been used for various purposes, incorporating substances that impair its original quality (pollution) and reducing its potential for further use. Its treatment serves two main purposes: protecting public health and preserving water resources.

Types of Wastewater

Domestic Sewage

Domestic sewage originates primarily from residential sources (human waste, bathrooms, kitchens) and similar uses. It is typically collected by sewage systems.

Industrial Wastewater

Industrial wastewater comes from industrial processes. Its amount and composition vary greatly depending on the specific industry, production activities, technology used, raw materials, and other factors.

Measuring Organic Load in Wastewater

Chemical Oxygen Demand (COD) vs. Biochemical Oxygen Demand (BOD)

COD is the amount of oxygen required to chemically oxidize the organic materials in a water sample. BOD measures the amount of oxygen required by microorganisms to degrade the organic matter biologically.

Total Organic Carbon (TOC)

TOC is based on measuring the CO₂ released when organic matter is oxidized with a strong oxidant, compared to a known standard value.

Other Wastewater Constituents

Chlorides and Sulfates
Carbonates and Bicarbonates
Nitrogen and Phosphorus

Coliforms as Indicators of Pollution

Total Coliforms

Total coliforms are a group of bacterial species with common biochemical characteristics. They are important indicators of water and food pollution.

Fecal Coliforms

Fecal coliforms in a water supply indicate potential contamination with sewage or other decomposing waste. They are often found in the surface layer of water or bottom sediments.

Pollutants in Domestic Wastewater

Five common pollutants found in domestic wastewater include:

Free Ammonia
Total Nitrogen
Total Organic Carbon
COD
BOD₅ (20°)

Sewage Collection

Ancient civilizations, such as the Greeks and Romans, built drainage systems to remove wastewater and rainwater. Modern sewage systems, used to collect and transport human waste and other wastewater along with rainwater, emerged in the mid-nineteenth century. These are called mixed or combined systems.

Contamination of Receiving Bodies

Contamination occurs when wastewater discharge deteriorates the quality of the receiving water body. Pollutants causing problems include pathogens, organic matter, suspended solids, nutrients, toxic substances, color, foam, oil and grease, temperature, and radioactive materials.

Factors Affecting Water Quality

Water quality can also be altered by construction projects on watercourses, such as reservoirs and changes in runoff conditions.

Impact of Low Dissolved Oxygen (DO)

If DO levels drop below 4 or 5 mg/L, aquatic life may struggle to survive or migrate away. In extreme cases, DO can disappear (anaerobic conditions), leading to the elimination of most life forms and the production of foul-smelling gases.

Water Quality Modeling

Water quality models are used to represent and predict water quality changes in response to various factors. These models are essential for planning and resource preservation.

Physical-Chemical Wastewater Treatment

Physical-chemical treatment removes organic substances from wastewater. The removed solids require further treatment depending on their composition.

Tertiary Treatment

Tertiary treatment, a final stage of wastewater treatment, may include:

Raising pH
Correcting pH through re-carbonation
Filtering water to remove suspended solids
Re-filtering to remove compounds causing taste and odor

Biological Wastewater Treatment

Biological treatment, mainly used for domestic wastewater, reduces organic matter using natural aerobic microorganisms. These microorganisms oxidize the organic matter into simpler compounds.

Factors Affecting Dissolved Oxygen in Rivers

Organic waste (organic matter) consumes DO.
Plants produce DO during the day (photosynthesis) and consume it at night (respiration).

Objectives of Wastewater Treatment

The main objective of wastewater treatment is to remove or reduce contaminant levels to comply with current regulations.

Principles of Biological Treatment

Biological treatment utilizes natural aerobic microorganisms that reduce organic matter by synthesizing it into new cellular matter (microorganism growth) and oxidizing it into simpler compounds (used as food by microorganisms).