- Pathogenic bacteria and other organisms that could cause diseases. This is very important if the resulting water is going to be reused as drinking water or if there is a big chance of contact with people.
- Biodegradable organics or also known as BOD. These are contents that could give nutrients to microorganisms residing in the receiving body of water. Microorganisms that have enough biodegradable organics mixed with oxygen could stay healthy and thrive to grow in numbers. The same oxygen is needed by other aquatic beings such as fishes. Heavy organic content could cause pollution and may result to dead zone, areas where fish could not thrive. If these heavy organic contents are released into the water, this could result to fish kills.
- Suspended solids. These are the physical particles that could cause clogs in channels and rivers if ever they sink under due to gravity.
- Nutrients. Examples of these are phosphates and nitrates which could aid in the growth of unwanted algae. High concentrations of these algae could be a source of biodegradable organics.
The above mentioned toxic waste and chemicals could be eliminated or neutralized through treatment processes. Though there are many sewage collection and treatment methods, these treatments should be done inside the industrial plane before discharging the liquid waste into the water courses or municipal sewers. Wastewater treatment has three levels which are more commonly called as: primary, secondary, and tertiary or the advance level.
- Primary treatment. This level is also called as the mechanical treatment. This treatment is done in order to remove solids that are suspended and floating in the raw sewage. During this level, screening is done to trap the solid objects while sedimentation through gravity is employed to eliminate suspended solids. During sedimentation, chemicals are used for faster results. During the primary treatment, the biodegradable organics are reduced by 20 to 30 per cent while the suspended solids are reduced by 50 to 60 per cent. Wastewater treatment is usually started using this as a first stage. It was the first process used in many industrialized countries. Other treatments were added after innovation and development of new technologies and methods. The increase in the wastewater and the available resources have also triggered for more treatment options.
- Secondary treatment. This level is also called as biological treatment. Any organic matter that was dissolved and was not totally cleared in the primary treatment is dissolved at this process. This is done by using microbes to consume the organic contents which are then converting into water, carbon dioxide, and energy so they could grow and multiply. During this level, biodegradable organics and suspended solids are reduced by 85 per cent.
- Tertiary treatment. This additional treatment is designed to remove more than 99 per cent of the impurities. After this process, the liquid waste’s quality is almost the same as drinking water. This technology is more expensive and needs to be done by professionals who know the technical aspects. It also needs uninterrupted energy supply and other equipment that may not be always available.