Key Considerations For Biological Drinking Water Treatment

Interested in converting to biological drinking water treatment? Here’s what you need to know.

By Thomas Bell-Games, PE, project manager, Burgess and Niple

Biological treatment techniques have been a mainstay in wastewater treatment for generations. Today, the use of biological processes in the treatment of drinking water is a relatively new concept. During this process, solids are separated out while dissolved organics and other compounds are consumed by the biology.

Conventional water treatment technologies, such as coagulation/sedimentation and lime softening, involve chemical addition and generation of a waste sludge. In addition to physical separation, biological filtration uses the filter media as a substrate to support the biology. This results in the removal of target contaminants as they are consumed by the biomass. In general, the waste backwash water is very similar in character to the waste backwash water of conventional filtration.

Advantages Of Biological Drinking Water Treatment
One advantage of biological filtration is its ability to treat a broad range of contaminants. Target contaminants include, among others:

• Natural organic matter (NOM)/dissolved organic carbon (DOC) — disinfection byproduct (DBP) precursors
• Color
• Perchlorate
• Chloroform
• Algae and algal toxins
• Iron/Manganese
• Nitrate/Nitrite
• Chromate.

Another advantage is the increase in water stability within the distribution system due to the reduction in assimilable organic carbon (AOC) as well as biodegradable dissolved organic carbon (BDOC). With the reduction of these forms of organic carbon, formation potential of DBPs, such as trihalomethanes (THM) and haloacetic acids (HAA), is also reduced.

A third advantage is the general resilience and flexibility of biological systems. Over time, as the nature of the constituents within untreated water changes, the biology adjusts to consume these varying constituents. While the biology is not something that can change quickly, it can adjust to seasonal variations, such as taste and odor-causing compounds.

Factors affecting the efficacy of biological treatment are those that also impact biological treatment of wastewater. These include:

• Time since start-up
• NOM loading rate
• Temperature
• Filtration rate/Empty bed contact time (EBCT)
• Backwashing method.

Natural Organic Matter
Typically, filters are not seeded; instead, naturally occurring microorganisms in the filter influent serve to establish the biological matrix once disinfection prior to filtration is discontinued. NOM can serve as a food source for the biology. Preconditioning through such techniques as ozonation can convert much of the NOM into more readily assimilable, less complex forms that are more easily consumed by the biology. Ozone in the water breaks down into oxygen, which results in a filter influent with high-dissolved oxygen, which also aids the biology.

Ozonation + Biologically Active Filtration
The practice of combining ozonation with biologically active filtration has become increasingly common over the past 20 years. The warmer the water, the higher the metabolism of the biology and the more effective the system is in treating contaminants of concern. The longer the EBCT (i.e., the lower the filtration rate), the more time the contaminants in the water have contact with the biology, thereby increasing removal rates.

Biological filtration has the potential to generate increased headloss; therefore, systems typically incorporate an aggressive backwash regime, such as the addition of an air scour step. This often is implemented as air alone, followed by simultaneous air/water, followed by hydraulic backwash alone. Each step serves a separate function in the overall backwash process.

Filter Influent
Another factor for consideration is the balance of nutrients in the filter influent. Generally, a C:N:P ratio of 100:10:1 is recommended. If phosphorus is particularly low, some systems supplement through a readily available form of phosphorus such as phosphoric acid. Research has shown that when nutrients are out of balance, the stressed biological matrix will respond by increased generation of extracellular polymeric substances (EPS). A certain amount of EPS is beneficial in that it helps form the structural integrity of the biofilm. Excess EPS, however, can result in increased headloss across the filter and is a potential source for undesirable fouling of filters.

Monitoring
As a biological process, some additional monitoring techniques are typically practiced beyond conventional parameters. Examples include turbidity, run-times, differential head, and particle counting. Systems frequently find monitoring of total organic carbon, DOC, AOC, and BDOC across the filters to be helpful. Adenosine triphosphate (ATP) analysis before and after filter backwashing can assist in monitoring biological activity. Molecular microbial community analysis also can be performed as part of trouble shooting, if required, to understand the characterization of the biological matrix. All of the testing and monitoring parameters should be logged and trended over time to establish baseline values. This will take at least a year to account for settling in and seasonal variation. The Water Research Foundation Monitoring and Control Toolbox Guidance Manual has a useful table for recommended frequency of monitoring with respect to normal operations as well as start-up/ optimization and troubleshooting.

Considering Conversion?
Utilities considering conversion to biological filtration should be aware of some unique characteristics of this type of filter system. Often, there is an oily/ dirty appearance to the water surface. This is natural, but quite different from the clean appearance of water produced by conventional rapid sand filters.

There is potential for shortened filter run-times due to excess build-up of biomass within the filter. While backwashing with chlorinated water is not typically performed, it has been found to have a minimal impact on the performance of the biology, with biological processes re-establishing themselves quite quickly. This is another example of the resiliency of these systems.

Additional Resources
A great deal of research is currently ongoing with respect to many aspects of biological drinking water treatment. Other sources of additional information include the AWWA Research Foundation’s publication, “Optimizing Filtration in Biological Filters,” and the Water Research Foundation’s “Engineered Biofiltration for Enhanced Hydraulic and Water Treatment Performance.”

Example Of The Process Of Conversion

Process Flow: Existing

Process Flow: Revised

About The Author
Thomas Bell-Games is a project manager with Burgess and Niple and a licensed Professional Engineer in Ohio and Virginia with over 20 years of practical experience in water/wastewater treatment design. He sits on state and association level committees with AWWA, and is also a member of WEF and the International Water Association. He holds a Bachelor of Science degree in Civil Engineering from The Ohio State University and is pursuing a Masters in Engineering Management from Ohio University.