As the concept and use of decentralized wastewater treatment has advanced, systems are increasingly installed in areas where additional pre-treatment beyond the traditional septic tank is required. This can be due to replacement of previous systems that failed, placing new systems on substandard lots, or building in nutrient management areas requiring water-quality protection. Whatever the reason, today’s service provider will see more of these advanced systems. So it becomes more and more important to know how systems are supposed to work and to be able to evaluate whether the system is functioning correctly.

Many manufacturers now require service providers to take system-specific training. This ensures contractors understand the technology and are able to obtain the right replacement parts. But it also means they are almost certain to come across unfamiliar systems and manufacturers in the field. If advanced systems are going to be part of a pumper’s business, having a basic understanding of operation and maintenance will be important. These systems require more work than pumping the septic tank every couple of years.

Getting to the basics

Here are some general comments about how ATUs operate:

Aerobic treatment units are small packaged plants used to treat the wastewater. The primary objective is to remove organic matter and suspended solids from wastewater. This provides cleaner wastewater from an organic-loading standpoint, which reduces the organic load at the infiltrative surface in the final soil dispersal area. The intent is to make these systems function more efficiently and for a longer period of time.

It’s important to remember that this reduction is only maintained if the system is operating properly and, unlike a septic tank, as a pre-treatment device. There are – depending on the type of system – mechanical parts, pumps and blowers requiring consistent attention to maintain the reduction in BOD (biochemical oxygen demand) and solids.

In addition, there will be some reduction of the pathogenic organisms. This is limited to only a reduction. You’ll not get as high a quality of reduction in pathogenic organisms as in media filters. Additional treatment will be required before the wastewater can be discharged to the environment.

Sensitive environments

That means the effluent will need to be passed through soil or a disinfection component will be needed. This adds another component to the system that requires constant attention. Depending on manufacturer or regulatory requirements, ATUs may need to be checked as often as every month.

One use of ATUs coming to the forefront is designing systems to provide additional treatment of nitrogen. In many areas of the country – particularly in densely populated areas with coarse textured sandy or loamy soils – groundwater has become contaminated with excess nitrogen in the form of nitrate-nitrogen. The human health standard for nitrates in drinking water is 10 mg/l or 10 parts per million.

Nitrogen can also negatively impact aquatic organisms in coastal estuaries. In these locations, management areas are being set up to reduce nitrogen loading. Currently there is a big effort to reduce nitrogen levels in the Chesapeake Bay watershed. ATUs are one system being evaluated as a “best management practice” to reduce nitrogen loads.

Converting nitrates

Nitrogen reduction requires ATUs to convert nitrates back to nitrogen gas in a process called denitrification. This process only happens in low-oxygen or anaerobic conditions in the presence of bacteria with a food source. Since ATUs aerate effluent, converting the ammonia in the effluent to the nitrate form of nitrogen, another step is required to run it back through an anaerobic environment. This process is conducted differently, based on the type of ATU installed.

No matter how the ATU is being evaluated, the aerobic treatment process can be generally broken down into five components. There is a trash tank or compartment at the front end of the process to remove the large solids and inert materials and for anaerobic treatment. This tank should be smaller than a normal septic tank because removing too much of the BOD reduces the food source for bacteria in the ATU, resulting in less treatment.

Aerobic treatment takes place where food and organisms are mixed together. There needs to be an air supply system, a clarification process where the material is removed from the water, and lastly, the sludge return (usually called separation) process where the clarified material is put back into the aeration chamber or the pretreatment device. No matter the manufacturer’s technology, the service provider should be able to find or identify these five components in the units.

One last comment on treatment capability: One common way of achieving denitrification is to run some of the nitrified effluent back through the trash or septic tank to obtain the reductions.

Knowing the system

As already mentioned, there are many different makes of ATUs, but they can all be placed in several broad categories based on how they aerate the effluent and provide a place for the bacteria to treat the waste. These unit types are suspended growth, attached or submerged growth/fixed media process, the sequencing batch reactor, and rotating biological contactor.

Suspended growth means the bacteria in the process are suspended in the liquid being aerated in the unit. Attached growth means there is some type of media that the bacteria grow on and the aerated liquid comes in contact with the media. Sequencing batch reactor indicates there is a set process sequence that is followed where the air is turned on and off during the process. The rotating contactor is a rotating drum with the bacteria growing as the drum or disk moves through the wastewater.

Next month I will walk through some of the steps a service provider should take when evaluating how these systems are working from an operation and maintenance perspective.