Do frequent septic system pumpouts reduce watershed nitrogen loading?

by Dr. Joe Costa

As more municipalities and residents come to appreciate the fact that groundwater discharges of nitrogen from septic systems are degrading water quality and living resources in their estuary, we receive this question from time to time.

The Massachusetts Department of Environmental Protection recommends (and we agree) that septic systems should be pumped out every three years to prevent sludge and grease from accumulating in the septic tank to such a degree that it overflows into, and clogs, the leaching field. Replacing a clogged leaching field can cost a homeowner many thousands of dollars.

We have long recognized that pumping out the contents of a septic tank also removes some nitrogen that might otherwise discharge to groundwater and eventually reach our estuaries. If this is true, some have asked, would pumping out a septic system annually further help reduce nitrogen to coastal waters?

Intuitively it would seem true that there would be at least a small benefit from frequent pumpout because you are diverting some volume of wastewater from reaching groundwater. If this assumption is true, perhaps the more important questions are “How large is the benefit?” and “Is this a cost effective strategy?” There are a number of assumptions that need to be made to more precisely answer these questions.

First, while it is true that the total nitrogen content of septage (what is pumped out of septic systems) can be quite high, the reality is that septic system sludge (which contains a lot of nitrogen) keeps accumulating in thickness on the bottom of the tank until it is pumped out. However, this nitrogen in the sludge component of septage may be quite refractory; that is, it does not degrade easily. Therefore, from a conservative management point of view, we would assume that when a septic tank is pumped out, we are losing only the nitrogen that would have flowed out during the time an emptied 1500 gallon tank is refilling with household sewage.

To contradict this assumption, we would have to find a study that shows nitrogen discharge was lower for weeks and months after the system was pumped out because of increased performance. However, the opposite may be true. For example, the point is sometimes made, as in this technical study by Bounds, 1997 , that the sludge layer provides an important environment for degradation and mineralization of waste, and too frequent pumpouts may adversely affect septic system performance. However, for the purpose of the analysis presented here, we will assume that little denitrification occurs in septic tanks, irrespective of the depth of the sludge layer.

Therefore, using the assumption that septic system nitrogen mineralization and denitrification rate is not affected by pumpout frequency, then, with 50-gpd per person flows and 2-person occupancy (approximate averages for our area), pumping out a septic tank could suspend discharge from the leach field for 15 days. This is 4.1% (15/365) reduction of annual flow and nitrogen discharge.

The Commonwealth of Massachusetts recommends pumpouts every 3 years to protect the leaching field from clogging. Using the above assumptions, this is the equivalent of a 1.4% per capita annual nitrogen reduction. If the homeowner pumped every year instead, he would reduce his nitrogen another 2.7% over normal recommended maintenance pumpout frequency.

In a watershed where 50% of the load is from septic systems, that would equate to a 1.4% annual reduction in total loading. This helps a little in nitrogen management, but it would not be discernible in the water quality data from the estuary because such a decline would be masked by the normal annual variation in water quality that results from changes in summertime precipitation and other variability in weather.

Are frequent septic pumpouts a cost effective strategy compared to sewering, for example? Consider a watershed with 1000 septic systems. If everyone followed the state’s guidelines, over a 20-year period, each home would have 7 pumpouts. If pumpouts cost an average of $250, over the twenty-year period, the homeowners would spend $1.75 million. If they pumped every year instead, these same homeowners would pay $5.00 million (this ignores rising costs due to inflation).

What would the extra $3.25 million over 20 years get you? With a 1000 homes with a 2 person occupancy and 5.9 lb per capita assumed watershed loading, the 2.7% annual load reduction would be 319 pounds of nitrogen in the watershed each year.

In the above example, we chose a 20-year period to make possible a comparison to sewer betterments. Some recent 20-year sewer betterments, together with other homeowner costs have been as high as $30,000 per home. With $500 per year sewer fees, over twenty years a homeowner will pay an additional $10,000 (this is a minimum based on the current rates; sewer fees tend to rise with inflation). Therefore, at a $40,000 per homeowner cost, the $3.5 million spent for extra pumpouts would completely pay for sewering 88 homes (including their annual sewer fees). Eighty-eight homes (with two-person occupancy) contribute 1,038 pounds of nitrogen per year. Thus, from a nitrogen management point of view, it would be three times more cost effective to sewer a small number of homes than to pump-out more frequently a large number of septic systems.

Of course, there are all sorts of limitations to this analysis, and many watershed-specific factor come into play. Septic systems need to be maintained in perpetuity, sewer betterments go away after 20 years, and you are left only with maintenance costs. (Of course, after 20 or 30 years a homeowner may have to replace or upgrade a septic system and municipalities may have to replace or upgrade their wastewater facility starting a new round of costs, and towns may have to replace damaged or failed sewer systems in the distant future.) Another issue is the location of the wastewater facility. If the septic system septage goes to a sewage treatment plant in the same watershed, the benefits may not be as great. Similarly, sewering homes and connecting to a treatment plant in the same watershed would diminish the effectiveness of that strategy. In those situations the treatment plant’s performance in removing nitrogen would be an important consideration in the analysis. In addition, the effectiveness of nitrogen removal by sewering (or pumpout frequency for that matter) also depends on whether the septic systems considered in the analysis are in the upper watershed where there is some natural attenuation (loss) of nitrogen, or in the lower watershed where losses are less likely to occur.

Sewering is not the only possible way funds for more frequent pumpouts could be utilized. Another potential use of $3.5 million dollars of private funds spent on extra pumpouts could be used to pay for infrastructure upgrades to a wastewater plant to improve its capacity to remove nitrogen. Again this is site specific solution, but such upgrades could potentially eliminate the equivalent of many hundreds of homes if the plant discharges 10 ppm nitrogen or higher.

Based on the above analysis (and given the stated assumptions), the BBNEP would not recommend annual septic system pumpouts as a cost effective general strategy to manage nitrogen.

Comments about this page should be directed to Joe Costa.