Table 3.1 Estimated results of monthly effluent flow and septic tank volume
Month
Design Wastewater
Flow (m3/d)
Average Daily Precipitation
(m/d)
Area (m2)
Runoff (m3/d)
Inflow (m3/d)
Septic Tank Volume (m3)
Jan 0.275 0.000752 112.5 0.0677 0.34 0.68
Feb 0.275 0.00115 112.5 0.1035 0.38 0.76
Mar 0.275 0.002132 112.5 0.1919 0.47 0.94
Apr 0.275 0.00493 112.5 0.4437 0.72 1.44
May 0.275 0.008226 112.5 0.7403 1.02 2.04
Jun 0.275 0.008633 112.5 0.7770 1.05 2.10
Jul 0.275 0.008245 112.5 0.7421 1.02 2.04
Aug 0.275 0.008458 112.5 0.7612 1.04 2.08
Sep 0.275 0.01113 112.5 1.0017 1.28 2.56
Oct 0.275 0.00759 112.5 0.6831 0.96 1.92
Nov 0.275 0.001547 112.5 0.1392 0.41 0.82
Dec 0.275 0.000219 112.5 0.0197 0.29 0.58
Mean 0.275 0.005251 112.5 0.4725 0.75 1.50
The maximum inflow into the septic tank was estimated at 1.28m3/d in the month of September.
Thus, the septic tank minimum volume was 2.56m3. Septic tank geometry is an important consideration that affects residence time of solids in the tank. For this study, commercially available cylindrical plastic tanks were considered for the septic system, not only to reduce the construction costs, as they were cheaper than reinforced concrete tanks, but also due to the temporal nature of the study. For enhanced suspended solids and organics removal, a two compartment septic tank system was adopted with a volume ratio V1:V2 = 1:1.
PKS based Field-Scale HSSF CW Design
The presumptive design method, which assumes that a certain amount of pollutant was removed by the primary treatment and uses the remaining contaminant concentration to size the system,
was used in this study due to paucity of data on the treatment performance of septic tank systems as primary treatment units for slaughterhouse wastewater in Nigeria. Most HSSF CWs are designed for BOD removal. However, there are recommendations for wetland sizing to be based on the parameter that requires the largest footprint for treatment, which is considered the limiting design parameter (LDP). This parameter controls the dimensions of the wetland to ensure effluent target for all the parameters of interest are met (Reed et. al., 1995a). For this study, the wetland was sized for BOD, TSS, NH4-N, NO3-N and PO43- removal.
Maximum raw wastewater parameter values obtained from the Agulu slaughterhouse during the characterization study were: BOD = 736.2 mg/l; TSS = 586.2 mg/l; NH4-N = 92 mg/l; NO3-N = 61 mg/l and PO43- = 14 mg/l. These values were taken as the maximum strength of wastewater that will be discharging into the septic tank from the slaughterhouse. The removal efficiency of septic tanks treating different types of wastewater in the literature range from 50 - 70% for organics, 40 - 80% for solids and 30 - 65%, for nutrients (USEPA, 2000; Rahman et al., 1999).
For this study, conservative estimates of 50% organics, 40% solids and 30% nutrients removal were used. Therefore, the influent wastewater concentrations for the HSSF CW design were:
BOD = 368.1 mg/l; TSS = 351.72 mg/l; NH4-N = 64.4 mg/l; NO3-N = 42.7 mg/l; PO4
= 9.8 mg/l. The wetland was designed to meet values prescribed by FEPA (1991)/NESREA for effluent discharge in Nigeria. The target effluent concentrations for the design were: BOD = 50 mg/l; TSS = 30 mg/l; NH4-N = 10 mg/l; NO3-N = 20 mg/l; PO4
= 5 mg/l.
The system was sized using the modified first order kinetic model (Equation 2.7, Chapter 2).
The rate constants and temperature coefficients estimated for contaminants removal in PKS column experiment, as well as the lowest values of the irreducible background concentrations obtained during the preliminary experimental run were used in the design, except for NH4-N which was found to be significantly higher than the theoretical background of zero. These parameter values are shown in Table 3.2.
Table 3.2 k-C* Model Parameters for Sizing the PKS based HSSF CW
Parameter K20(d-1) .
𝜽
C* (mg/L)BOD 0.6042 0.9948 23.0
TSS 0.6236 1.0928 25.6
NH4-N 0.2783 1.0505 0
NO3-N 0.3235 1.016 0.36
PO4
3-0.3065 0.9537 0.42
Because the value of temperature coefficient for BOD was counterintuitive and contradicted values reported in the literature, system design for BOD removal was not adjusted for temperature as recommended by Dotro et al., (2017).
Water depth is a very important system design, operation, and maintenance parameter. There is no consensus as regards the appropriate depth of substrate for HSSF CWs in the literature.
Depths of between 0.4 m and 0.6 m are considered very effective (USEPA, 1999). From the pilot study on the growth and performance of macrophytes, it was established that both the Thalia Geniculata and Colocasia Esculenta roots and rhizomes penetrated more than 0.3 m into the substrate. Therefore, a bed depth of 0.5 m was selected for the system.
The calculation results are presented in Appendix B. Based on the average inflow of 0.75 m3/d, NH4-N required the largest area of 17.21 m2 for treatment, which was set as the area of the HSSF CW. The average inflow was used instead of the maximum inflow to minimize the need for supplemental water in the dry season when the inflow was at its lowest, and also because of the fact that the actual slaughterhouse wastewater design inflow of 0.275 m3/d was well below the average wetland inflow and the additional inflow from rainwater did not contribute significantly to the pollutant load.
Theoretically, longer channels are considered more likely to generate flows that are closer to plug flow, than wider channels. Good general ratios are considered to be 3:1 or 4:1 (1 m width for 3 or 4 meter length) (Mitsch and Gosselink, 2007; Kadlec and Knight, 1996; USEPA, 1999).
An aspect ratio of 3:1 was set for this study. Thus, for the set surface area of 17.21 m2, the width of the system was 2.4 m and the length was 7.2 m. The flow that can pass through the bed in subsurface conditions was determined using Darcy's equation. If the Darcy's Q was less that the design flow, then surface flow was possible. Thus the dimensions must be adjusted until Darcy's Q was equal to or greater than the design flow. The hydraulic conductivity of the PKS was assumed to be equal to that of gravel of the same size fractions, which was estimated as 480m3/m2/d (USEPA, 1999). The design cross-sectional area of the bed was 1.2 m2. A slope of 1% was chosen for ease of construction (s = 0.01). The calculated Q was 5.76 m3/d which was higher than the peak wet season flow of 1.28m3/d. Therefore surface flow was ruled out. The design drawings are shown in Appendix C.