Evaluation of Septic Tank and Subsurface Wetland for

to an elevated tank when the municipal supply was not in service. The US$4,821 for the
solar powered pumping system was not included in Table 6-1. Community labor was
utilized at both sites to different degrees. This prevented direct comparison of labor costs
between the projects and prevented direct transfer of per unit cost for estimating the
expense of future projects. However, the available figures may be valuable for general
knowledge and perspective.
The per capital water usage of 1.3 L/p-d at Pisgah versus the 48.3 L/p-d water usage at
Retrieve illustrates the difficulty in predicting the appropriate design flow rate for new
sanitation systems. Recall that both of these schools had pit latrines before conversion to
the flush toilet systems. Both sanitation systems were sized based on reported school
populations and the same estimate for per capita water use. It should also be noted that
the Retrieve system was designed for approximately twice the number of students than
were attending during the evaluation.
Because of the variation in per capita water use, dollar per liter of wastewater treated is
not a useful cost metric. A more helpful capital cost comparison is dollar per liter of
design capacity. Dividing the Pisgah sanitation system construction cost of US$ 13,396
by the 2,580 liters/day design capacity gives US$ 5.19 per liter per day. Dividing the
Retrieve sanitation system construction cost of US$ 8,699 by the 1,290-liters/day design
capacity gives US$ 6.74 per liter per day. These costs may be valuable for projecting the
capital cost of future designs.
Operating Costs
The operation cost is expected to be low for any sanitation system such as that at Pisgah
which uses rainwater harvesting for water supply and gravity instead of electricity for all
water flow. The Retrieve system solar panels will not require utility payments although
upkeep of an electric pump does increase future costs.
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