[USCC] Compost Digest, Vol 29, Issue 6. Methane emissions

[Edo McGowan]

The following is based on data generated by a study, commissioned by the California Integrated Waste management Board (CIWMB) and prepared by the University of California, Riverside, College of Engineering-Center for Environmental Research and Technology. The entire article can be read by obtaining a web copy from Forester Pubs, the latest issue (http://www.gradingandexcavating.com/msw.html). The paper is entitled Evaluation of Environmental Impacts of Theromchemical Conversion Technologies Using Municipal Solid Waste Feedstock. MSW Management, Elements 2007, Vol 16, No 4. 

Using this study as a model, I have merely substituted data but essentially used their methodology. In the case presented, we are discussing the movement of about one half million tons of sewer sludge from the Greater LA Basin into the San Joaquin Valley for land application. Thus, I will look at the added air quality impacts in this case from trucking and then the evolution of CH4 from that sludge. Please remember this is in essence a transfer of smog out of LA and into the already oversubscribed air basin of Bakersfield. 

It should be of some interest to also note that the wine industry of the San Joaquin Valley has been hit with an air quality fee. Among the questions I raise here as an aside, is the subsidizing of LA sewage impacts by the Valley wine industry, an obvious externality to that agricultural industry. This says nothing of reduced agricultural production caused by increased air quality impacts. These costs are also external to production. Further, the issue of human health and thus diminished desirability of the area will at some point begin to see economic consequences.  

TRUCKING IMPACTS 

Assume the following. 500,000 tons of sewer sludge is transferred from the LA Basin to the Bakersfield area. Over the grapevine route assumes an elevation shift of about 4,000 feet. The trip from Hyperion to Bakersfield is about 100 miles which will be our assumed one-way distance. The average truck is assumed to carry 25 tons and has an average fuel consumption rate of 5-miles/gallon.

Thus far we can compute some data.

At 25 tons/truck, we will be making 20,000 trips to Bakersfield and back. That is 55 trips/day. At a round trip of 200 miles this consumes 40 gallons of diesel. At 55 trips per day, this is 2,200 gpd or about 803,000 gallons of diesel per year. 

HEAVY DUTY DIESEL NOX EMISSIONS

Cocker et. al. measured NOx emissions with a mobile laboratory that serves as the semi-trailer for a heavy duty diesel truck-tractor (1). With a gross vehicle weight (GVW) of 60,000 lbs, NOx emissions averaged 20 g/mile for the 106-mile round trip between Riverside and Victorville, CA (with an altitude change of 3,000 feet between the two cities).

Using the Cocker data and substituting the Hyperion to Bakersfield run, we get the following (note the elevation difference in our example is 1,000 feet more thus using the Cocker example directly will be an underestimate). 

CARB EMFAC gives NOx emissions from 13.4 to 23 g/mile for heavy duty vehicles manufactured between 1984 and 2002 (http://www.arb.ca.gov/msei/on-road/downloads/tsd/HDT_Emissions_New.pdf). 
The hypothetical distance used by Cocker was 53 miles, (our one-way trip is approximately double that). From Cocker, NOx emissions for a heavy-duty diesel truck (class 8) making this roundtrip will be approximately 2 kg (using an emission factor of 20 g NOx/mile) (2). Assume a payload of approximately 25 tons (50,000 lbs. Maximum gross vehicle weight is 80,000 lbs). Thus based on this assumption, the 55 trips from Hypperion to Bakersfield and back will generate 110 pounds of NOx

PM AND CO2 EMISSIONS

>From CARB EMFAC, approximate emission factors for PM and CO2 are 0.4 g/mile and 2000 g/mile respectively. Therefore, the avoided emissions of diesel engine PM and CO2 due to MSW transport from the transfer station to landfill are about 0.2.4 and 1200 tons respectively.

EVOLUTION OF METHANE BY TRANSFERRED SEWER SLUDGE

Assuming Loren Faundahl and her staff are correct that each ton of sewer sludge will produce on the average about 3,000 cuft of CH4. The roughly one-half million tons of LA Basin sewer sludge will produce 1.5 billion cubic feet of CH4 or 21 times that amount of CO2 will eventually evolve. 

SUMMARY

>From the above, assuming assumptions, it should be clear that the transfer of sewer sludge from one area to another is not without consequence. In preparing a proper economic analysis of benefits from the land application of sewer sludge or its conversion via composting, the above externalities (this is just a short list) need to be backed out to obtain a more accurate cost-to-benefit analysis. Until such an economic analysis has been presented, it will be difficult to continue any reasonable discussion of the merits of this activity. 

Notes _______________________________

(Cocker, D.R.; Shah, S.D.; Johnson, K.; Miller, J.W.; and Norbeck, J.M. 2004. “Development and Application of a Mobile Laboratory for Measuring Emissions From Diesel Engines. 1. Regulated Gaseous Emissions.” Environmental Science and Technology, 38[7Apr 1], 2182-2189).
(2) Cocker estimated NOx emissions for transporting solid waste from the transfer station to the landfill are, therefore, 0.1 lb/ton (0.046 kg/ton). The 129,780 tons of MSW no longer trucked to a landfill result in a reduction of approximately 6.1 tons of NOx emissions emitted to the atmosphere annually. 
+++++++++++++++++++++++++
Cheers------------------Edo