sewage Wastewater is water that has undergone an alteration in its physical, chemical or biological properties as a result of human activities. This water is not suitable for human consumption and contains a high load of contaminants that must be treated before being discharged back into the environment or reused. This article addresses the fundamental characteristics of wastewater, the sources that generate it and the treatments necessary for its effective purification.
What is wastewater
Wastewater is produced when we use water for domestic, industrial or agricultural purposes and alter its original characteristics. These alterations can be chemical, biological or physical, which makes it a danger to the ecosystem and human health. Therefore, adequate treatment is necessary to return it to the natural cycle or prepare it for other uses, such as agricultural irrigation or industry.
Water that is discarded during daily activities such as washing, cleaning or even rain can carry contaminants that need to be treated. Although its contaminating potential varies depending on its origin, all wastewater requires thorough treatment to reduce its environmental impact.
Origin and types of wastewater
Wastewater classification is directly linked to its origin. Knowing where it comes from is essential to determine its treatment, as pollutants can vary significantly. The most important types of wastewater are:
- Domestic wastewater: Resulting from use in homes, offices and residential areas, these waters contain high levels of organic compounds, detergents and pathogenic microorganisms that must be eliminated.
- Industrial wastewater: Generated in industrial processes. These waters can be highly polluting, with the presence of heavy metals and toxic chemicals that require highly specialized treatments.
- Agricultural waters: They come from agricultural activities in which fertilizers, pesticides and other chemical products are used. These residues are transported by irrigation water or by rain runoff.
- Stormwater: Rainwater can carry atmospheric pollutants, such as heavy metals, and urban waste such as oils or plastics, which must be treated before reaching rivers and seas.
Physicochemical characteristics of wastewater
The characteristics of wastewater vary depending on its origin, but we can highlight several elements that are always present and that must be controlled during the treatment process:
- Organic material: Mainly composed of plant and animal remains, as well as human waste. It is essential to control the level of decomposition of this material to avoid the reduction of oxygen in the water.
- Suspended solids: These include visible and microscopic debris that affects the clarity of the water and its overall quality.
- Chemical Oxygen Demand (COD): It indicates the amount of oxygen needed to oxidize organic matter and some inorganic contaminants present in the water.
- Biochemical Oxygen Demand (BOD): It measures the amount of oxygen that microorganisms need to decompose organic matter in a given period of time. A high BOD value is an indicator of high pollution.
- Nutrients: Elements such as nitrogen and phosphorus generate adverse effects such as eutrophication, which causes an uncontrolled increase in aquatic flora, affecting biodiversity.
- Heavy metals: Metals such as mercury, lead and cadmium, present in many industrial processes, are toxic at very low concentrations and are difficult to eliminate.
Stages of wastewater treatment
Wastewater treatment is carried out in several stages that include physical, chemical and biological processes. The aim is to eliminate or reduce the concentration of contaminants so that the water can be safely returned to nature or reused for specific purposes.
Pretreatment
This is the first phase of treatment and aims to remove the larger, easily separable solids. Screens, sieves and grit traps are used in this stage to remove the larger elements and sand particles. Pre-aeration is also carried out, which helps to reduce bad odours and organic load in this initial phase.
Primary treatment
In this phase, the finest solids and some suspended matter are removed by sedimentation. The heavier particles not removed in the pretreatment settle to the bottom of the sedimentation tanks, while fats and oils are skimmed off the surface. Chemicals (coagulants and flocculants) are sometimes added to increase the efficiency of this stage.
secondary treatment
Secondary treatment is responsible for removing dissolved organic matter and nutrients through biological processes. Activated sludge and trickling filters are two of the most common techniques at this stage. In aerobic processes, oxygen is used to facilitate the decomposition of organic matter, while anaerobic processes break down compounds without the need for oxygen.
Tertiary treatment
In this final phase, residual contaminants remaining after secondary treatment are removed. Processes used in this stage include filtration through sand beds, the use of activated carbon to remove toxins, and disinfection using chlorine, ozone, or ultraviolet (UV) light. This is a crucial stage if the water is to be reused for agricultural or industrial purposes or for discharge into protected environments.
Impact of poor wastewater treatment
Discharging wastewater without proper treatment can have disastrous consequences for the environment and human health. Water bodies that receive contaminated wastewater tend to suffer from eutrophication processes, which cause the death of aquatic species and a decrease in water quality. These effects can result in the loss of biodiversity, the contamination of drinking water sources and increased costs for future water treatment.
In addition, poorly treated wastewater is also a major health risk, as it can contain pathogens that cause diseases such as cholera, dysentery or hepatitis. Its socio-economic impact is also very significant, as sectors such as tourism, fishing and agriculture are affected by the pollution of aquatic ecosystems.
For these reasons, proper wastewater treatment is essential to ensure long-term environmental sustainability and public health.