As more water is required to support human activity worldwide, sources once considered too contaminated or expensive to treat are quickly becoming necessary options. In this area, biological treatment of groundwater is finding its niche as an option for dealing with the challenges of utilizing formerly questionable water sources.
In the last decade, biological treatment for potable water has been permitted in many states by regulators and has gone from unconventional water treatment oddity to international success story. The idea of leveraging naturally occurring bacteria and their simple ability to change organic and inorganic contaminant properties fits in nicely with today’s high demand for water, environmental consciousness, and sustainability.
As water treatment solution providers, our role in society is not only to provide a solution, but also to educate the public, design engineers, private or public utilities, and governmental regulatory agencies on assurances that public safety will be preserved. Supporting statistics and assurances come from piloting new technologies, collecting data, discussing that data with regulators relative to their requirements, and finally combining all these into solutions for treating contaminants deemed unsafe for public consumption. To aid in that effort, the goal of this article is to address some common questions, myths, and concerns about biological groundwater treatment.
How Biological Treatment Of Groundwater Works
Biological treatment refers to the way that naturally occurring bacteria, in the right environment, modify contaminants. In the case of groundwater, contaminants such as nitrate, nitrite ammonia, volatile organic carbons, and other man-made contaminants polluting an aquifer are consumed or converted into non-harmful forms by bacteria.
How To Speed Up Nature
When speeding up a biological process, the bacterial biomass needs a food source in order to reproduce and to speed up contaminant removal. If the untreated water does not contain enough nutrients to meet the engineered speed of contaminant removal, a supplementary phosphorus source is required. This is often in the form of a liquid-phase orthophosphate or phosphoric acid. In some cases, a supplemental carbon source might be needed to help the biologicals repopulate. An organic acid such as acetic acid is often used for potable groundwater sources.
Creating The Right Environment
Given the right conditions, biological treatment engineered to catalyze the natural process can remove a wide range of contaminants. Under the right natural conditions, specific contaminant removal can be targeted, the natural process accelerated, and the generated waste recycled just as nature intended. The two conditions that are most common for contaminant removal are oxidizing environments and reducing environments, explained more in detail below.
This type of environment is generated using oxygen — often atmospheric air — to increase air-to-water contact and elevate the dissolved oxygen (DO) above that of the raw well water or all the way to its saturation point. Saturated DO capability varies with several factors, such as water temperature, water pressure, contaminants in the water consuming DO, organic matter, and several other factors. With elevated DO concentrations, an aerated and natural oxidizing environment is created. This environment differs from chemical oxidizing environments, such as with sodium hypochlorite for disinfection, in that chemical oxidizing environments could very easily eliminate or reduce biological growth. That runs contrary to the goal of a biological treatment process. Aerobic environments are ideal for changing the contaminants in the water to a more oxidized state, such as converting nitrites (NO₂) to nitrates (NO3). Volatile substances in the water are reduced by exposing the contaminants to excess air, allowing them to transfer into the air as trace contaminants.
Contaminant removal in high-aerobic environments commonly includes the following:
Anoxic & Anaerobic Environments
These types of environments are generated using a biological process to eliminate first the free — i.e., dissolved — oxygen (anoxic), and then the total oxygen bound in NO₂, NO3, etc. (anaerobic) from the water. Doing so biochemically reduces the contaminants by removing the bound oxygen in nitrate and nitrite contaminants, reducing them to nitrogen gas, which makes up 79 percent of the natural atmosphere we breathe. Through these processes, contaminants are changed to non-hazardous components of the natural environment.
Contaminant removal for anoxic and anaerobic environments commonly includes the following:
The contaminants treated in this environment can often be identified as having one or more oxygen molecules in their chemical structures.
Myth: “Biological Treatments Leave Bacteria In My Drinking Water”
Biological treatment of drinking water was once looked upon cynically — and still is by some — because it uses ‘bacteria’ in water that the public would consume. With the significant need for more water sources, this myth has been diminished by a higher level of understanding on how to protect the public with this approach.
Although biological treatment does use natural bacterial colonies as the workhorse of contaminant removal, only a small percentage of those colonies is harmful. Yes, some of the ‘bad’ biology is present in the untreated water. In the case of biological denitrification, however, the denitrification biology will dominate while naturally keeping unwanted biological colonies to a minimum. Yet again, Mother Nature guides the way.
As a secondary safety component, state regulators require biological treatments to include filtration after the biological main process to reduce turbidity in the form of excess biomass or oxidized solid contaminants. In the case of ammonia removal, the contaminant is oxidized through aeration, along with any iron, manganese, and hydrogen sulfide. Some of the contaminants are converted into solid form and filtered out of the water in this required step. The lower the turbidity of the water, the more effective the next step of disinfection can be.
Disinfection is the final assurance that the public is safe when consuming water from a biological treatment process. This step is heavily monitored by regulators and well understood by water plant operators. Disinfection is done in all municipal drinking water systems, often with sodium hypochlorite (NaOCl) used as a disinfectant. This chemical is long proven to be both physically effective and cost effective. NaOCl is known to deactivate unwanted biological species — such as viruses and pathogens — very effectively and reliably. Engineers are required to design the disinfection system with very tight parameters. The water exiting a biological treatment system is likely to be of better quality than many privately owned well-water systems, which are largely accepted among the general public.
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