Evaluation of Greenhouse Gas Emissions from Septic ... - Geoflow

5.3 Sources of Variability in Gas EmissionsA number of factors contributed to the wide variability in the gas emission measurementsrecorded for the septic systems included in this study. The influence of factors such as waterhardness, presence of scum layer, pumping intervals, turnover events, presence of invertebrates,temperature, and the septic tank effluent filter are discussed in this section.5.3.1 Water Hardness Influence on Carbon Dioxide EmissionsAs mentioned previously, the water supply for the Auburn Lakes Trails (ALT)development, encompassing Sites 1 through 7, originates in the Sierra and flows into theStrumpy Meadows Reservoir; it has an average hardness content of 9 mg/L as CaCO 3 (GDPUD,2009) which indicates a soft water supply. In contrast, the water that serves the tank located inDavis is pumped out from a groundwater well and has an average hardness of 212 mg/L asCaCO 3 (CDPW, 2009) indicating a hard water supply.A comparison of the carbon dioxide emission rates data between the sites located in ALTand the tank for Site 8 (Figure 5-2) indicates different distributions. It is hypothesized that thehardness content of the water supply in each area may be a reason for this differing behavior. Inaddition to the difference in source water, Site 8 had other distinguishing characteristics thatpotentially impacted emissions. Unique aspects at Site 8 include the diversion of laundry waterto a gray water system, a higher number of occupants per tank volume, and the vegetarian diet ofthe house occupants. It is conceivable that these aspects may also influence the overall anaerobicand facultative processes occurring in the tank and hence the carbon dioxide emission rates.5.3.2 Influence of Scum Layer on Gas EmissionsTwo different groups of sites were compared to determine the influence of the scum layerin the overall emissions from the septic tank. The first group was composed of the tanks fromSites 2, 3, and 7 that share as a common characteristic a thick scum layer varying from 3 to 5inch in depth, with similar appearance, black color, humus like texture and usually covered theliquid surface of the septic tank. The septic tanks at Sites 1, 5, and 6 formed the second group;these tanks were characterized by patchy, light scum (less than 1"), with a light brownish color.As shown on Figure 5-9, the average methane and carbon dioxide emission rates from the sitesthat have a thick scum layer are similar to the sites without scum and the slopes are also similar.The data from the septic tank at Site 8 (light scum, hard water) was plotted separately forcomparison.5.3.3 Emissions from Recently Cleaned TankThe septic tank located at Site 4 was not a typical septic tank. It did not have any scumformation; the sludge was less than 14" in depth, with a light brown coloration and a particularpine odor was emitted when the tank lids were opened. The tank was last pumped out in late Julyof 2009 (measurements took place during September 2009). The methane and carbon dioxide gasemission rates from the tank at Site 4 in g/capita·d, averaged 0.2 and 3.2, respectively. Nitrousoxide emission rates were negligible. From the results of the overall GHG emission rate and thecharacteristics of the tank discussed above, it can be assumed that methanogenesis was notoccurring in this tank, which is consistent with results reported by Weibel et al. (1955) forrecently pumped septic tanks, which had a lag phase in the gas production.Evaluation of Greenhouse Gas Emissions from Septic Systems 5-11