Is the Time Coming to Process Your Own Septage?

Rules governing septage stabilization and acceptable soil and site characteristics for land application may eventually derail your disposal practices.
Is the Time Coming to Process Your Own Septage?
Jim Anderson, Ph.D., is an emeritus professor at the University of Minnesota Department of Soil, Water and Climate and recipient of the pumping industry’s Ralph Macchio Lifetime Achievement Award. Email Jim questions about septic system maintenance and operation at

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Pumpers and haulers who land-apply septage have long been aware of the need to stabilize loads with lime. Alkaline stabilization is the addition of lime to septage solids either before or after dewatering. This is to raise the pH of the septage to 12 for a period of 30 minutes as a pathogen- and vector-reduction strategy. Stabilizing septage before land application gives the pumper more options in terms of the length of time or need to incorporate the septage into the soil, as well as more options on the use of crops from the application site.

Now a recent study funded by the Water Environment Federation may offer an added benefit to septage pumpers. The study itself focuses on sludges derived from wastewater treatment plants in North Carolina, with the aim to establish some baseline values for phosphorus (P) levels that could then be used to evaluate the risk associated with land-applying those materials and phosphorus runoff and leaching.

Up until now, meeting the U.S. Environmental Protection Agency Title 40 Code of Federal Regulation Part 503 Rule required biosolids (including septage) to be applied on a crop nitrogen (N) need basis. Maximum septage application rates are determined based on crop nutrient use. The problem is that N levels in septage — due to handling and processing — are relatively low. This means that the ratio of N to P in septage is virtually 1-to-1, which means on a given site additional P is being added, which accumulates in the soil over time. Excessive algae growth and surface water eutrophication may be a result if the P moves out of the soil or offsite due to erosion and ends up in surface waters.


This has led some states to look at watersheds to determine whether they are sensitive to P additions, and then institute additional requirements in terms of animal manure applications as well as other biosolids including septage. If application rates of septage are controlled by high soil test values and total P values in soil and septage, application rates of septage to cropland would be reduced and require more sites and more acres to land-apply septage.

Research primarily directed at manure applications has identified that it is not so much total P that controls whether the P in land-applied materials is a problem, but that portion of total P that is soluble in water, called water-extractable P (WEP). This makes sense because this would be the fraction that could be incorporated into surface runoff as well as being subject to leaching through the soil profile, which can ultimately find its way into nearby surface waters.

The other major pathway would be loss of soil particles and P due to soil erosion, which can wash into surface waters, resulting in the soluble fraction being available for algae growth. This is one of the reasons that erosion control measures are generally required for any land application site.

There’s one additional reason to be aware of these developments if you are using land-applied septage as a part of a farming operation and are requesting federal assistance of any type through the Natural Resources Conservation Service. A nutrient management plan is required for the site and in those sensitive watershed areas some type of P assessment is required. The type of tool used to make this assessment varies from state to state but will be based on some type of P index system tied to soil test values and P additions along with crop requirements.

Through alkaline stabilization, the amount of WEP is reduced. This is probably due to the formation of calcium-phosphate precipitation and absorption with the added lime material as well as within the soil. Just adding the lime as a slurry also adds a dilution factor that decreases the amount of total P along with the soluble P. When P is bound within the calcium phosphates, it is not available for leaching and is relatively insolvable.


There is one large caution: Even though it looks like using lime to stabilize the septage also helps reduce the availability of phosphorus and helps to tie the P up long term, we are not totally off the hook. In particular, under acidic soil conditions, calcium-associated P in alkaline-stabilized materials will slowly solubilize, presenting an increased runoff risk prior to being transformed into more stable iron and aluminum phosphates within the soil profile.

Evaluation of soil and site characteristics for land application will become much more important and come under more scrutiny in sensitive areas. As I mentioned earlier, land application of septage based on both P and N may decrease overall application rates, reduce the number of fields eligible to receive septage, and shorten the time it can be applied. This may make land application more costly and difficult.

Depending on your region, this may be another reason to consider developing a stand-alone sewage treatment facility, either through your own business or in partnership with others. As the cost of land application rises and access to municipal sewage treatment plants is further curtailed, establishing your own facility may become more economically viable. In addition, with additional processing the problems due to P can be mitigated, while still supplying a beneficial soil amendment.


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