The Language of Biological Wastewater Treatment
- What wastewater really contains.
- The meaning of BOD, COD and TSS.
- What Hydraulic Retention Time (HRT) means.
- Why these measurements are important.
- How biology influences wastewater performance.
4.1 What Is Wastewater?
Wastewater is simply water that has been used and contains dissolved or suspended substances that must be removed before the water can safely return to the environment.
It originates from many different sources, including homes, farms, lodges, restaurants, shopping centres, food processing facilities, industries and municipal sewer systems.
Although every wastewater stream is different, they all contain one thing in common: organic matter.
That organic matter becomes food for billions of naturally occurring microorganisms, making biology the heart of every successful wastewater treatment process.
4.2 What Does Wastewater Contain?
A typical wastewater stream may contain:
- Organic solids.
- Human waste.
- Food waste.
- Fats, oils and grease (FOG).
- Proteins.
- Carbohydrates.
- Detergents.
- Nutrients such as nitrogen and phosphorus.
- Suspended solids.
- Microorganisms.
The exact composition depends on where the wastewater originates. For example, domestic wastewater contains different materials to wastewater from a restaurant, dairy, abattoir or food processing plant.
Understanding what is present helps determine the most appropriate biological treatment programme.
Every type of organic waste eventually becomes food for specialised microorganisms. The healthier the biological community, the more efficiently that waste is recycled.
4.3 Biological Oxygen Demand (BOD)
One of the most important measurements in wastewater treatment is Biological Oxygen Demand (BOD).
BOD measures the amount of oxygen that microorganisms require to break down biodegradable organic matter. Think of BOD as the food demand placed upon the bacterial population.
The higher the BOD, the more organic material is available for bacteria to digest. A healthy biological system can comfortably process normal BOD levels.
However, if excessive organic waste enters the system, the bacterial population may become overloaded, reducing treatment efficiency and increasing the risk of odours, sludge accumulation and poor water quality.
4.4 Chemical Oxygen Demand (COD)
Chemical Oxygen Demand (COD) measures the total amount of oxidisable material present in wastewater.
Unlike BOD, which measures only biologically degradable material, COD also includes substances that are difficult or impossible for bacteria to digest. COD is therefore usually higher than BOD.
Comparing BOD and COD provides valuable information about how much of the wastewater can realistically be treated using biological processes. The more biodegradable the wastewater, the more effective biological treatment is likely to be.
4.5 Total Suspended Solids (TSS)
Not everything in wastewater is dissolved. Many particles remain suspended within the water. These are known as Total Suspended Solids (TSS).
Examples include:
- Food particles.
- Paper fibres.
- Organic debris.
- Soil.
- Biological floc.
- Other suspended materials.
Excessive suspended solids can reduce treatment efficiency, increase sludge production and place additional stress on pumps, pipes and treatment equipment. Healthy biological activity assists in reducing and stabilising many organic suspended solids over time.
4.6 Hydraulic Retention Time (HRT)
Hydraulic Retention Time, commonly called HRT, is one of the most important design concepts in wastewater treatment.
HRT simply refers to how long wastewater remains inside a treatment system before flowing out.
Biology requires time. Bacteria cannot digest large quantities of organic material instantly. The longer wastewater remains in contact with healthy bacterial populations, the more complete the biological treatment process becomes.
Although the ideal HRT varies according to system design and treatment objectives, biological wastewater systems are generally designed to provide sufficient retention time for bacteria to perform their work effectively.
An overloaded system often reduces HRT, allowing wastewater to leave before treatment has been completed.
In most biological wastewater treatment systems, hydraulic retention times typically range from several hours in high-rate activated sludge plants to one or more days in septic tanks, lagoons and other passive treatment systems. Providing adequate retention time is essential for stable biological performance.
4.7 When Biology and Engineering Work Together
Successful wastewater treatment depends on two equally important components.
The first is good engineering. Well-designed tanks, pumps, aeration systems and pipework create suitable operating conditions.
The second is healthy biology. Living bacteria recycle organic matter, reduce pollutants and stabilise wastewater.
Neither can perform effectively without the other. Engineering creates the environment. Biology performs the treatment.
4.8 The BactiDigest® Approach
Every wastewater system, regardless of its size, depends on maintaining a healthy biological population capable of processing the incoming organic load.
The BactiDigest® biological programme has been developed to strengthen these natural microbial communities across a wide range of wastewater applications.
Whether supporting a residential septic tank using SeptiDigest®, reducing FOG with FatDigest®, maintaining sewer infrastructure through SewerDigest®, or optimising biological performance in larger treatment facilities with BioDigest®, the objective remains consistent:
Support healthy biology so that wastewater can be treated naturally, efficiently and sustainably.
Wastewater treatment is more than pumps and pipes. It is a balance between engineering and biology. Measurements such as BOD, COD, TSS and HRT help us understand how effectively that balance is being maintained.