My system failed! Should I put in an aerobic treatment unit? This is another homeowner question that usually follows or precedes a question about how and when oxygen is important in a system. In a previous column we discussed formation of the biomat and the relationship between the organic loading through measures of BOD and FOG, and development of a more resistant and thicker biomat.

Adding more pretreatment components – along with altering household practices – is a way to address the organic loading issue and keep biomat development at levels where the soil is able to hydraulically accept the water generated by system users. Pretreatment components involve oxygen as a part of the process of breaking down and removing the organic waste constituents before they reach the soil treatment unit.

PROBLEM SOLVING

Several types of pretreatment components can be used. These include a variety of media filters (sand, peat, textile) in addition to aerobic treatment units (ATUs). Whether adding any of these components will help solve the situation where a drainfield has already failed depends on the factors discussed previously: that the condition is the result of hydraulic and organic overloading and lack of proper septic tank maintenance, or it is due to soil compaction or smearing. In most cases where there is an existing problem, additional pretreatment efforts will need to be accompanied by a resting period, which also may mean some modification to the soil treatment area.

Determining the appropriate pretreatment component for a specific situation requires knowledge about how each component works. In an ATU, wastewater is pretreated by adding air to break down organic matter, reduce pathogens and transform nutrients. There is usually a septic tank in front of the ATU that acts as a trash tank to remove the larger solids and foreign material such as plastic products or baby wipes.

A highly oxygenated environment is created by bubbling compressed air through the liquid effluent to promote aerobic bacteria in the unit. These bacteria are more efficient at breaking down the organic matter than the aerobic bacteria that survive in a septic tank. Effluent delivered to the soil treatment area from a properly operating unit is much cleaner. There are still relatively high levels of nutrients and pathogens present but much lower organic loading. There are also now aerobic units set up and operated to address nutrients as well.

In other than specific types of peat filters, septic tank effluent is delivered periodically through a set of pressure distribution laterals. In all cases effluent is uniformly distributed to the media. Treatment mechanisms in media filters involve physical filtering of solids, ion exchange (alteration of compounds by binding the media), decomposition of organic waste, trapping of pathogens and transformation of nutrients.

By using equal distribution and periodic loading over time, septic effluent comes in contact with oxygen in the unit and allows aerobic bacteria time to break down the waste. In areas where nitrogen is a public health or environmental concern, recirculating media filters can be installed to address this specific problem.

KNOW THE LOADS

As wastewater moves through a recirculating media filter, the filter becomes oxygenated and the nitrogen present as ammonia in the effluent is transformed to the nitrate form of nitrogen. The effluent is then circulated back to the septic tank, which is low in oxygen and high in organic matter; there the nitrate is converted to nitrogen gas and released through a process called denitrification.

The pretreatment component selected will depend on the organic loading, as well as the level of desired treatment of not only the organic loads but also nutrients and pathogens and the consistency of the waste generated. For instance, an ATU would not be selected for a situation where the system only periodically receives wastewater. So they would not be an ideal choice for seasonal or periodic-use residences because it is important that the aerobic bacteria are consistently fed. If they go through time periods of low loading, the bacteria do not survive and thrive well. So when they are hit with a higher load, they are not able to break down the waste as efficiently.

All of the pretreatment components have the potential to improve the function of a failed system and, by “cleaning up” the effluent, allow the system to recover over time. Other potential applications include use in areas where vertical separation distance to a limiting layer – such as bedrock, high water tables and wellhead protection areas – ensures adequate treatment before discharging into the environment.

Homeowners should be made aware that all of these pretreatment components will require more maintenance and management than they have been accustomed to with their septic tank. Since one of the causes of the failure may be lack of septic maintenance, having this discussion with the end user is important. It will take some education to get them to understand that continuous maintenance is required to keep these systems operating efficiently. That is why many states and local authorities require maintenance programs if these components are used.