There are many different biological water treatments available, all similar in a number of ways but sharing key differences in the specific ways water is filtered. Below is a breakdown of several types of treatment systems in the industry and how they work.
Slow Sand Filtration
This system supports the growth and maintenance of microorganisms that eventually become its primary filtering mechanism. Its layers work to get rid of any potential contaminants that are within the water. Slow sand filtration has been among the most effective filtration systems for preventing the spread of diseases for the last century now. It has been very common in western Europe and the United Kingdom until recently, due to rising labor costs that made the system too expensive. Despite that, it continues to be a very sustainable method of water filtration.
Rapid Sand Filtration
Rapid sand filtration is a similar biological water filtration system. The main difference with this method is that it uses a much larger surface loading rate. This means that microorganisms can reach deeper in the filter media in comparison to the slower sand filtration system. For this system, it’s highly recommended to add disinfectants downstream of the filter media in order to maintain biology activity.
Activated Carbon Filtration
This method of water filtration is mainly used for the removal of specific organic contaminants as well as natural organic matter. It’s based upon the absorption and assimilation of containment. Relying on this method can allow for higher microbial concentrations compared to other systems.
Ozone-Enhanced Biological Filtration
This biological filtration system is used specifically to treat drinking water. It aims to remove several possible contaminants while also improving the appearance of the water itself. Ozone-enhanced treatment is widely used in many drinking water production facilities. One thing to note about this system is that it can cause a noticeable increase in BDOC/AOC levels, which means a biofiltration (sand/GAC) step must be added.
Operational and Design Considerations
Biological water filtration systems are all based on the same objective, which is the maintenance of microorganisms and assurance of biological quality being held to standards and regulations, accomplished through the removal of inorganic and organic matter. There are many factors that contribute to the overall biofilter performance, including maturation time, temperature, oxygen levels, redox control, contact time and adequate backwash conditions.
The time it takes the biofilter to reach an adequate level of biological activity is a very important design consideration. In sand-based filters, this time can be around two months. There are still looming questions over the safety of engineered biofiltration when it is used for drinking water production. Studies often fail to look at every characteristic of the water in-depth, preferring to only cover the removal of organic matter. There’s a risk of absorbed contaminants from the treatment process ending up in the water as well, but more research is required to fully understand this.