Defending Septic Waste as a Soil Amendment

Land application of septage sometimes gets a bad rap from neighboring property owners, but soil science proves it’s a beneficial disposal method.

Jim Anderson
Jim Anderson

I continually get questions from homeowners about what happens to the contents of their tank after it has been pumped. As we in the pumping industry are aware, there are several ways to handle septage depending on where the residence is located. If it is located near or in a large metropolitan area, chances are septage is transferred to a publicly operated wastewater treatment plant, a private septage management facility, landfill after dewatering or land application.

Where I reside, the most common methods of septage handling are land application and discharge to a metropolitan wastewater treatment plant. When discharged to a treatment plant, septage is treated and managed as biosolids along with the municipal wastewater and sludge treatment.

Land application occurs in rural areas far away from a municipal treatment plant or areas where the small community wastewater treatment plant manager is concerned that introducing septage will overload or upset the system. The reason for this concern is well placed because septage has a very high BOD (or biological oxygen demand), on average 6,400 milligrams per liter. If it is simply dumped into the waste stream, this can overwhelm the biology in the treatment process, and it takes a long time to fix. There are ways this can be mitigated by controlling when and where septage is introduced to the plant’s waste stream.

If you have an underutilized nearby treatment plant, it may be possible to strike a deal for you and the operator through dumping fees to the plant. Where there is no nearby treatment plant or when the local plant is at or above capacity, land application is a great option.


Land application usually is connected to crop production, either through an agreement with farmers to accept the product or often applied to land the pumper owns and manages for crop production. When septage and other biosolids are applied correctly to land, they are very good for crop yields and soil health.

Land application is subject to the federal 40 CFR Part 503 requirements and applicable state requirements. If you are land-applying, it is your responsibility to know and follow these rules. From the federal rules perspective, there are recordkeeping requirements in terms of the location and amount of septage applied, how pathogens were reduced, and how vectors (insects, birds, etc.) and odors are managed. In terms of total amounts applied to a specific property for crop production, it should be done in the context of meeting the crops’ nutrient (specifically nitrogen) requirements as part of an overall nutrient management plan. Septage typically also has low levels of trace nutrients, such as cadmium, that can pose problems.

Since I am a soil scientist by training, I pay attention to research done on land application of biosolids for crop production. Recently I reviewed a long-term study (eight years) of applying biosolids to wheat land in the state of Washington. The study compared land application of biosolids to commercial fertilizer, with positive results. Here are a few points from the study and about biosolids and septage in general.

As I mentioned above, septage should be applied at agronomic best practice rates of nitrogen for the crop being grown. Since the primary nutrients such as nitrogen in septage are in their organic forms, the nitrogen is not immediately available for plant uptake. It must be biologically converted (mineralized) first. A certain amount is released during the first year, with decreasing amounts in succeeding years. This is where having a nutrient management plan is important because it accounts for residual nitrogen in succeeding years and adjusts application rates to keep the amount of nutrients supplied within the best agronomic practice.

Residual and slow release means biosolids and septage can provide nutrients over a longer time period, which allows the crop to use them more efficiently, resulting in less nutrient loss. This is good for the producer from a cost of fertilizer input perspective and good for the environment because the nutrients are not lost to groundwater or surface waters, where they can cause environmental and human health problems.


Another major plus for land application of biosolids is that they provide stable soil organic matter. Organic matter is one of the primary soil constituents that hold soil particles together, helping to maintain soil infiltration and permeability capacity. And holding particles together makes the soil less susceptible to water and wind erosion. Keeping soil in place ensures a good media for plant growth and sustained yields into the future. Organic matter additions improve the overall soil quality, increasing water-holding capacity, and the exchange capacity to supply essential micronutrients.

The study looked at conservation tillage in addition to the fertilizer comparison. Conservation tillage was used to help reduce erosion potential and maintain organic matter levels. Over the eight years of the study, there were no differences in yield between the commercial fertilizer and biosolids and no difference between conservation tillage and more conventional methods. The biosolids areas did produce more wheat straw, which can help protect the soil surface under conservation tillage, and some eventually is incorporated as organic matter into the soil.

The conclusion of the study was: “Use of biosolids combined with low-disturbance conservation tillage is an environmentally sound practice for dryland wheat production.” The takeaway message for us is that land application is a legitimate crop-production practice that, when done right, can have large benefits to the public, the farmer and the environment.

These types of studies are important to us because they provide background information that can be used in discussions with the public, government officials and others who question why we use land application. In my view, this is another in a long list of studies that demonstrate what we do is very important to the soil, environment and public.


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