Should Onsite Wastewater Follow Agriculture’s Lead?

By Robert K. Rebori
August 2010

The Author is a Staff Writer at Bio-Microbics, Inc., in Shawnee, Kansas, USA. The Paper was reviewed by Allison Blodig, REHS, Manager, Regulatory Affairs & Special Projects.

Wood chip filters have made a small appearance in the onsite wastewater industry as proprietary devices, but they are very expensive and demand a large footprint. There are pros and cons to these filters and more research will likely be required by regulatory agencies on the use of these non-proprietary filters for denitrifying wastewater, but when looking at the results from agriculture and aquaculture this outlook is very promising.

Using Hardwood Chips as a Denitrification Filter

Prior to the new millennium, farmers and conservationists had managed all aspects of agriculture, except one: field drainage. Within the past decade, drainage has gotten increased attention due to the presence of high levels of nitrates in it. Although artificial drainage quickly removes excess water from large areas of farmland, it also unavoidably takes the nitrates from the fertilizers with it. Once this runoff water infiltrates the groundwater, it can enter a community or an individual’s drinking water.

However, if the nitrates find their way to a stream or river, they transport the nitrates into the ocean where they can cause a chain reaction that kills aquatic life. The largest example of this reaction is in the Gulf of Mexico, in the deeper waters of the Mississippi River delta, which contains an anoxic area of around 20,000 square kilometers. With more nitrogen reduction regulations imposed on farmers, they found simple, low-cost alternatives to treating large acres of land by digging trenches and filling them with hardwood chips to capture and treat runoff. Having similar nitrogen regulations in place, could the onsite wastewater industry learn from the experiences of the farming industry?

Experiments with Wood Chips

Since 1998, farmers have experimented with wood chip filters to solve this problem (Hequet, 2008). These filters usually consist of around six-foot-deep and three-foot-wide trenches filled with wood chips to intercept the nitrate-laden water. The length of the trench varies, depending on how much water needs to be treated. After the chips are buried beneath a couple of feet of topsoil, facultative microbes in the soil consume the carbon in the wood chips, and convert the nitrates in the drainage water to nitrogen gas that rises out of the soil and escapes into the atmosphere, where it does no harm.

Generally, wood chip filters remove between sixty and seventy percent of nitrates from tile drainage (Morrison, 2008). Scientists experimenting with filters in Ames, Iowa, USA have found that the amount of nitrates removed decreased as flow rates increased, but that the removal rates per gram of wood increased as flow rates increased. When 50 mg/L of nitrates were added to dosing water flowing at three centimeters per day, 100% of the nitrates were removed. However, when the flow rate increased to almost fourteen centimeters per day, only 30% was removed (Greenan, Moorman, Parkin, Kaspar and Jaynes, 2009). Thus, the flow rate and expected concentration of nitrates in a wastewater treatment system would dictate the size of a wood chip filter used for denitrification.

Dan Jaynes, one of the scientists in Iowa, reported that an existing filter had functioned consistently for eight years, and predicted that the filter would continue for another twenty years (Morrison, 2008). However, he did not specify any maintenance he performed or would need to perform in the future. This information is critical, since it has been found that the nitrates actually consume the wood chips in the denitrification process in aquaculture wastewater (Saliling, Westerman and Losordo, 2007). Replacing contaminated woodchips from a wastewater treatment system would be a costly and undesirable task.

Cost and Convenience

The benefits of using a non-proprietary wood chip filter largely center around cost and convenience. Wood chips are a common commodity, easily obtained, and for probably considerably less money than wood chips that are part of a proprietary denitrification device. Farmers using wood chip filters have found that the chips do not impede drainage, so filters are not likely to impede pretreated wastewater flow either (Morrison, 2008). Installing a hundred-foot-long filter in a field takes a few hours with a backhoe, so installing a filter for residential or commercial use should be done well within a day.

The most pressing reason for further research is the harmful side-effects wood chip filters might have on the environment. Wood chip filters can destroy E. coli, but may also create them in their effluent (Hequet, 2008). Scientists have also been concerned that not all of the nitrogen in the water leaves as stable nitrogen gas molecules. Nitrous oxide is a greenhouse gas that can trap 310 times more heat than carbon dioxide, and already enters the atmosphere from agricultural soil in substantial amounts (Hequet, 2008). Fortunately, the scientists in Iowa found that only .003 to .028% percent of the nitrogen removed was changed to nitrous oxide (Greenan, Moorman, Parkin, Kaspar and Jaynes, 2009).

Full Onsite Treatment

If it were merely about denitrification, the story could end here. Agricultural runoff contains high nitrates but nitrogen in raw wastewater is predominately in the ammonia-nitrogen form. Whether using a proprietary woodchip system or creating a low-cost, alternative denitrification system, an aeration step will be necessary to nitrify the wastewater before it enters the denitrification filter. In some cases, a drain field will be needed to complete the onsite system. It is also important to note that hardwood chip filters are not likely to remove more than seventy percent of the nitrates unless they occupy a considerably large footprint. In areas which require higher levels of removal (<10 mg/L Total Nitrogen) and where the area for wastewater treatment is limited, any hardwood chip filter should be viewed as a secondary denitrification device installed after an aerobic treatment unit that is well-established and reliable at performing nitrification and denitrification.


When extra denitrification is needed or for markets where the Total Nitrogen needs to be less than 10 mg/L, there are alternatives to modifying your onsite system with low cost, do-it-yourself alternatives. Some suggest that simply hanging a permeable bag of wood chips into a septic tank to use as a supplemental carbon source could help with the denitrification process. This could be an answer and it would prove much less costly to get added treatment on the front end. Moreover, it would be easier to maintain. More research will likely be required, but with farmers finding much success with hardwood chips; the onsite industry could also benefit by looking into non-proprietary alternative denitrification devices to meet strict nitrogen reduction regulations.

Works Cited


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