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

Solids are removed from the tank periodically, usually in response to poor performanceor when the volume of solids in the tank reaches a certain limit (see Table 2-3). At the time ofcleaning, it is common to have the septic tank emptied fully without leaving sludge in the tankfor seeding purposes (U.S. EPA, 2002). The material that is removed from the tank is known asseptage. It should be noted that the solids contained in the septage range from old materialaccumulated since the previous tank cleaning event to material deposited immediately prior tocleaning. Thus, the SRT is approximately equal to one half of the time between tank cleaningevents. A comparison of the operational parameters for a typical anaerobic digestion process anda septic tank is shown in Table 2-6.2.3.3 Toxic Compounds in Anaerobic ReactorsThere are many inorganic and organic compounds that at certain concentrations becomeinhibitory or toxic in anaerobic reactors. Toxic compounds can range from inorganic ions such aspotassium, magnesium, or calcium, to metals such as copper, zinc, or lead (McCarty, 1964). Ingeneral, a toxic effect occurs at high concentrations. Conversely, at low concentrations, the effectof these compounds might be stimulatory and favorable to the anaerobic process (McCarty,1964). Moreover, anaerobic conditions must be maintained in the reactor as even small amountsof oxygen inhibit methane forming microorganisms.Earth metal salts commonly found in industrial wastes, such as sodium, potassium,calcium, or magnesium, are highly toxic, causing failure or low treatment efficiency (McCarty,1964; Speece, 1996). It has also been reported that toxicity is related directly to the cation of thesaltTable 2-6. Typical Operating Parameters for Single-Stage Complete Mix Anaerobic Digester and Septic Tank.ParameterUnitsSingle-stage completemix digesterSeptic tankCOD kg/m 3·d 3.2 - 32 0.07 – 0.106 aSRT d 20 574 – 730 b,cHRT d 20 1 – 2Temperature °C 30 - 38 7 – 30Nitrogen mg/g COD 5 – 15 d 11 ePhosphorus mg/g COD 0.8 – 2.5 d 6.8 eAlkalinity mg/L as CaCO 3 2,500 to 5,000 4,500Mixing - Complete Intermittent bybubble activitya Calculated values, see Appendix A.bApproximately equal to one half of the time between tank cleaning events.cCleaning event assumed every three to five years (D’Amato et al., 2008).d Rittman and McCarty (2001).eBased on typical raw residential wastewater values (Crites and Tchobanoglous, 1998; Höglund, 2001).(i.e., Na + ) instead of the anion (McCarty, 1964). For example, in a septic tank study, Weibel etal. (1955) reported that a 1.2 % (12,000 mg/L) mixed salt brine representative of a water softenerbackwash cycle inhibited a non-acclimated anaerobic digestion process for 9.5 d. Weibel et al.,(1955) also reported that digestion activity in acclimated sludge was not inhibited atEvaluation of Greenhouse Gas Emissions from Septic Systems 2-15