Caring for Media Filters

Proper maintenance of all components is necessary to make sure single-pass or recirculating filters perform well and last long
Caring for Media Filters
Above-ground bottomless sand filter.

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For those riding the Treatment Train for the first time, welcome aboard the systematic process of evaluating the components of an onsite wastewater treatment system. In this edition, for those who may have siting or soil limitations, we will discuss one option for advanced treatment: media filters.

One of the first questions we hear from system owners is: Why do I need this fancy system? While we hope this question has already been addressed by the system designer, we often find ourselves patiently explaining the site and soil limitations that can dictate advanced treatment.

I never try to determine what the deciding factors on a particular site were because I am not a designer and was not involved in that process. Instead, I explain how certain conditions such as high groundwater, soils that allow fast movement of water, soils with a limiting layer that restricts flow of water, or steep slopes may be cause for advanced treatment.

There are two basic categories of advanced treatment technologies: media filters and aerobic treatment units (ATUs). We will address ATUs in the next edition of “O&M Matters”. Media filters come in many configurations and sizes. Some are engineered and custom built while others are packaged, off-the-shelf proprietary units. Either way, the basic operation and maintenance requirements are the same.

 

What is a media filter?

There are many types of media used in filter design and construction, including sand, gravel, peat, textile fabric, and other synthetic materials. We typically see two basic configurations: a single-pass or recirculating flow path.

In most cases a dosing pump tank is used to apply a specific volume or dose of pretreated wastewater or septic tank effluent to the media surface. Proper dosing is important, as the designer has sized the media filter to ensure that the effluent that has passed through the filter (filtrate) is adequately treated before dispersal into the soil treatment unit, which most likely has a limiting condition.

Single-pass media filters can have an internal filtrate collection system, known as an under-drain, that uses a discharge pump tank. They can also be bottomless, allowing the treated effluent to be dispersed back into the natural environment below.

The latter are typically referred to as above-ground systems and include mounds, bottomless sand or peat filters, or at-grade systems. These aboveground systems serve both as the advanced treatment unit and the soil treatment unit and are often used for sites where high groundwater is an issue.

Recirculating media filters use a bit more energy but have the benefits of a smaller footprint and the ability to provide a higher level of nitrogen reduction. The bottomless single-pass is less maintenance-intensive.

 

Why perform O&M?

To understand why media filters need maintenance, consider two words: biological and filter. As with any biological process, bacteria reproduce and die off, creating biosolids. The filter aspect speaks for itself: It is a physical process of removing small particles.

If biosolids and other particulates are not removed from the distribution system that applies wastewater to the filter, the solids will be pushed through the distribution orifices into the media, where it will accumulate and eventually cause fouling.

In addition, there are numerous system control sensors, pumps and in some cases, valves that if not serviced may fail, causing much greater problems, such as flooding of the media, resulting in undesirable saturated conditions.

If the media becomes saturated, the aerobic bacteria die off, and anaerobic conditions occur. It is important to note that aerobic bacteria metabolize at about 10 times the rate of anaerobic bacteria. That’s why unsaturated conditions are important for achieving the expected performance of a media filter.

So let’s start with the basics of performing O&M on the simpler single-pass systems. As with all onsite system service, it is important to inspect the entire system area first to ensure there are no unacceptable conditions before we begin adding service-related water to the system.

Most engineered media filters are designed with inspection ports to verify any subsurface ponding conditions within and around the filter. Record the liquid levels in all inspection ports before service. In some cases, inspection ports or the design of proprietary units allow inspection of the media itself. Routine inspection of the media can identify issues before they become problems.

Also take note of the condition of the media containment structure and of the toe of the mound and at-grade systems. The toe is the external area where the constructed portion of the unit meets the natural soil and is the place most likely for seepage if the native soil was not adequately prepared or was damaged during construction.

We want to note that the vegetation around and atop the unit is well maintained and not a type that will produce deep roots that might affect the media or distribution laterals. In addition, record all control panel data, such as the cycle counter and elapsed time meters, as this data is most valuable in calculating system use and flow information.

 

Effluent distribution

All media filters have some type of effluent distribution system designed to evenly distribute pretreated wastewater over the media surface. In some peat filter applications, it could be gravity, and in other cases it might be a spray system, but in most cases it is pressure-dosed distribution laterals with orifices and shields.

In any case, routine cleaning of the distribution system is important for the longevity of the system. When pressure-dosed laterals are used, a distal head test reveals the condition of the distribution system before service. You might find some laterals have a higher distal head than others, indicating either plugged orifices or possible leakage in the distribution piping.

 

Flush, clean, flush

We then will flush, clean, brush or hydrojet the laterals, then flush them again to make sure all loosened solids are removed. The configuration of the distribution lateral distal end can greatly affect serviceability. Good access and the ability to brush or hydrojet the laterals is critical.

You will most likely need to add water to the pump tank to perform the service, so it is important to measure tank solids levels before performing service to determine if pumping and cleaning will be necessary – you don’t want to send unwanted solids that could plug orifices into the distribution system.

Next, as good practice from a safety perspective, lock out and tag out the electrical breakers to the system controls before disassembling, inspecting and cleaning all dosing tank components. Verify that all sensors, pump screens and pumps are operable and that all plumbing and electrical connections are in acceptable condition.

Upon reassembly of the dosing tank components, flush the distribution system again to make sure any debris loosened from other components is cleared from the system. Now, perform a post-service distal head test and balance the distribution system as necessary.

This is also a good time to verify the pump delivery rate through a drawdown test. If the pump delivery rate has changed, consider recalibration of the system. Over time, pumps will wear and will not deliver the same volume of fluid as when they were new. While recalibration is not as much a factor in demand-dosed applications, it is important if the pump relies on a timer, as a weaker pump will not deliver the specified dose to the media in the original set dosing time.

 

Internal pump basin

Now, let’s add an internal pump basin for a single-pass filter that collects and stores filtrate for discharge to a separate soil treatment unit. All the tasks noted above for the dosing tank will need to be repeated for the internal pump basin.

In addition, you now have a filtrate collection underdrain to inspect and service. The under-drain collects filtrate that has moved through the filter media as gravity then diverts it to the internal discharge pump basin or to a recirculation valve if applicable. It may be necessary to periodically clean, brush or hydrojet the underdrain to prevent the buildup or transfer of unwanted biosolids.

 

Recirculation components

If the design incorporates a recirculating filter, you now have a recirculation/process tank to service, as well as some type of recirculation device. Before servicing any system involving a recirculating device, remove the device so as not to send any debris from the service activities to the discharge tank.

Service the tank as noted above, to refresh, lock out/tag out, disassemble, inspect, clean and verify operation of all plumbing and electrical components. Next, disassemble, inspect, clean and reassemble the recirculation device. Be sure to inspect and lubricate any rubberware that may be prone to drying and cracking.

It is also a good practice to verify the recirculation ratio of the filter to ensure that it is within the parameters specified by the designer. The recirculation ratio is the number of times the volume of effluent that is discharged to the soil treatment unit is passed through the media in 24 hours.

In other words, if a system is discharging 300 gallons of treated effluent daily to the soil treatment unit, the filter must have recirculated 1,200 gallons during the 24 hours before discharge to achieve a 4:1 recirculation ratio. Here is an instance where the cycle counters and elapsed time meters provide data that enable you to program the controller to reset the recirculation ratio.

So, let’s recap some of the important aspects of servicing media filters:

Inspect all tanks and their components to ensure proper operation (control sensors, pumps, plumbing, electrical)

Verify that all site conditions surrounding the media filter are acceptable (no surfacing effluent, erosion or inappropriate vegetation)

Monitor and record all liquid levels in available inspection ports

Record all control panel data to calculate average daily flow

Inspect the media if inspection ports or removable covers allow

Service the distribution system, cleaning the laterals, testing the distal head, and balancing of the pressure distribution system, if applicable

If there is an underdrain, inspect and clean as needed

Remove, inspect and service the recirculation device

Calculate the recirculation ratio based on data loggers

Recalibrate the recirculation pump if necessary

In our next “O&M Matters,” we will look at aerobic treatment units. Watch for future articles on disinfection and drip dispersal. The content of this article is based on the Consortium of Institutes for Decentralized Wastewater Treatment O&M Service Provider training program, delivered nationally by the National Association of Wastewater Transporters. Check the NAWT website (www.nawt.org) for a program near you.

About the author

Kit Rosefield is an adjunct instructor at Columbia Community College and a trainer for NAWT and the California Onsite Wastewater Association. His company, Onsite Wastewater Management in Mi Wuk Village, Calif., has a consumer education service at www.septicguy.com. Reach him at 209/ 770-6760 or kit@septicguy.com.



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