Evaluation of Septic Tank and Subsurface Wetland for

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loading of 3.85 USgal/ft2-d (147 L/m 2 -d) and 5.34 USgal/ft 2 -d (204 L/m 2 -d). Insufficient hydraulic conductivity (i. e. high resistance to horizontal flow through the media) was the cause of surface flooding. The 6.5:1 length to width ratio of this wetland bed reduces the possibility of short circuiting so that treatment is improved but decreases hydraulic conductivity which will result in surface flooding when maximum flow capacity is reached. The maximum flow capacity was not determined. The outlet water level could have been lowered to reduce or eliminate the flooding, but no changes were made in order to keep conditions constant through the evaluation. The high water level in the manhole may have been caused by partial plugging of the slits in the inlet distribution pipe or solids accumulation in the media at the inlet zone. During installation of the midwetland sample pipe the plant roots were observed to be a thick mat in the top 4 inches of the media and did not significantly reach beyond this depth. This root mat could have created resistance to surface flooding thus forcing the water in the manhole to back up until sufficient head pressure was available to overcome horizontal flow resistance. The surface flooding events provide some useful data for estimating the hydraulic conductivity of the front end of the wetland. Darcy’s law is meant to represent flow through a porous media and is used in wetland design to determine head loss through the media based on an assumed hydraulic conductivity. Darcy’s law is written as follows: Q= K Ac S where Q = flow rate (m 3 /d); K = hydraulic conductivity (m 3 /m 2 -d); Ac = cross sectional area (m 2 ); and S = hydraulic slope gradient (m/m). The cross sectional area Ac = W Dw where W = width of wetland and Dw = average depth of water. The hydraulic slope S = dh/dL where dh = change in water height (i. e. head loss) and dL = length of travel through wetland corresponding to dh. This results in the following formula: Q = K W Dw dh/dL or K = Q dL / dh W Dw. The lowest flow rate that seemed to create surface flooding was 4.73 m 3 /d which when combined with precipitation was 5.055 m 3 /d. The length of wetland travel was half the full length of 47.5 ft (14.48 m) or 7.24 m. The manhole elevation is approximately 2 inches higher than the 24-inch high wetland walls that contain 20 inches of rock media. 35
The water level in the middle of the wetland was 19.5 inches. Assuming there was no overflow at the manhole, the water level in the manhole was 6.5 inches (0.165m) higher than in the middle of the wetland bed. The width of the wetland is 7.3 ft or 2.23 m. Using wetland bottom as reference, the average depth of water between the manhole and mid-wetland point is 26 inches (0.660 m) minus 19.5 in (0.495 m) or 0.576 m. This data may be summarized as Q = 4.73 m 3 /d, dL = 7.24 m, dh = 0.165 m, W = 2.23 m, and Dw = 0.576 m. Applying Darcy’s Law equation K = Q dL / dh W Dw results in the hydraulic conductivity K = 172 m 3 /m 2 -d. This is a reasonable estimate for the average hydraulic conductivity of the first 50% of a wetland bed that has been in service for two years. The field measured porosity of 37.7% corresponds to a hydraulic conductivity (K) in the range of 1,000 to 50,000 m 3 /m 2 -d (Reed, 2001). It is recommended that 1% of the clean conductivity be used for the initial 30% of the bed and the remaining 70% of the bed is assumed to have a K of 10% of the clean K value (USEPA, 2000). If it is assumed that a clean K equals 10,000 m/d, then the first 30% of the bed would have a K of 100 m/d and the remainder of the bed K would have a K of 3,000 m/d. Since wetland had an unmeasured quantity of effluent every week except for two weeks when the water supply was unavailable and the outflow was zero, the ET rate was based on data from these two weeks. During the water outage, buckets were used to supply a minimal amount of water for flushing so precipitation was the major source of water entering the system. ET based upon these two weeks was an estimated 0.24 USgal/ft2-d or10 mm/day. Under normal operating conditions the water level is high in this wetland so the water reaches the plant roots. During the two weeks when the water supply was off, the water input was so low that the water level dropped below the root zone which may have caused a lower than normal ET rate. 36
Page 1 and 2: Evaluation of Septic Tank and Subsu
Page 3 and 4: Abstract Locally designed wastewate
Page 5 and 6: Acknowledgments Major funding was p
Page 7 and 8: Chapter 6 - Costs ……………
Page 9 and 10: Table 5-6 Parameters Influencing Bi
Page 11 and 12: The predominant geological formatio
Page 13 and 14: plants discharging a volume of 20m
Page 15 and 16: lack access to acceptable excreta d
Page 17 and 18: Critical design parameters for sept
Page 19 and 20: Subsurface Flow Constructed Wetland
Page 21 and 22: inging larger rock from a distant q
Page 23 and 24: Chapter 3 - Site Descriptions Figur
Page 25 and 26: Wetland inlet sample point Flush to
Page 27 and 28: Figure 3-4 Pisgah wetland beds befo
Page 29 and 30: times over a two-week period, grew
Page 31 and 32: Figure 3-7 Retrieve youth in front
Page 33 and 34: Chapter 4 - Methodology Field Data
Page 35 and 36: out (see Figure 4-2). Locating the
Page 37 and 38: Figure 4-3 Media porosity measureme
Page 39 and 40: Laboratory Water Quality Analysis A
Page 41 and 42: NEPA laboratory also does not test
Page 43: Since evapotranspiration decreases
Page 47 and 48: BOD was determined to be 53% at Pis
Page 49 and 50: solids than a sample point that pul
Page 51 and 52: Table 5-4 Ammonia (NH3-N) for Pisga
Page 53 and 54: Table 5-5 Nitrate (NO3-N) for Pisga
Page 55 and 56: o Alkalinity Nitrification and deni
Page 57 and 58: system were measured to be 3.3 mg/l
Page 59 and 60: coliform by 88%. The Retrieve septi
Page 61 and 62: Chapter 6 - Cost of Pisgah and Retr
Page 63 and 64: The septic tanks will have to be pu
Page 65 and 66: Jamaica may refer to the hydraulic
Page 67 and 68: Chapter 8 - References APHA (Americ
Page 69 and 70: Pennant, Joseph, (2005), Assistant
Page 71 and 72: Chapter 9 - Appendices Appendix 1 -
Page 73 and 74: 7-Dec- 2004 SET FIVE Sample BOD TSS
Page 75 and 76: Appendix 3 - Determination of 90% C
Page 77 and 78: Appendix 4 - Conductance of Water C
Page 79 and 80: Appendix 6 - Total Dissolved Nitrog
Page 81 and 82: Appendix 8 - Evapotranspiration Det

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