Septic system design: a tank with an inflow pipe and an outflow pipe used in the place of central sewage system
The typical tank has two chambers, one containing aerobic bacteria oxygenated by airflow in pipes caused by flushing, and the second chamber containing anaerobic bacteria. The bacteria digest waste in the wastewater as it flows into the tank via inflow pipe. Indigestible materials sink to bottom of tank as sludge. The broken down materials are in aqueous solution that slowly move through the system, with the materials being broken down in a floating layer called the scum layer. The aqueous solution flows through the outflow pipe into the surrounding earth in a leech field. The surrounding soil filters the water as it percolates down as groundwater recharge. The type of soil the leech field has is very instrumental in the efficiency and effectiveness of the septic tank. Soil composed of high percentages of clay will not work well with a septic tank because it does not allow water to flow through it, thus causing back-ups in the septic tank. Soil with large particles will not filter the wastewater thus possibly contaminating the groundwater supply. Because approximately half of the houses in Puerto Ayora have septic tanks, this could in part contribute to the cholera outbreaks that have occurred there, paired with the leaky water pipe system.
In the interview with Mr. Erickson, he advised against a traditional septic tank system for every household in the city, as the city structure is compact. For a four person family, the general size of a septic tank is 20,000 gallons, which needs pumped of sludge every 2 to 3 years depending on usage. The ground surrounding the septic tank and the leech field must be able to support the volume of water flowing through the system, otherwise it becomes too saturated with waste water that, although very fertilizing, isn’t going to drain sufficiently and may contaminate groundwater supply by over-saturation. Thus, depending on the soil, only a few septic tanks can be functioning in an area at a time.
A second option would be for non-draining septic systems, that must be pumped quite frequently. The response in the interview advised against this as well, due to the fact that the amount of digging to put in very large tanks for septic systems would like be more extensive than a sewage system and the overall time vs. power efficiency of frequent pumping would negate the environmental and civil good of leaving the streets intact by foregoing a traditional sewage system.
As far as the sewage system is concerned, it is advised that a traditional small-town gravity flow system be established to minimize energy consumption by tanks. As Puerto Ayora is conveniently established on an incline towards the coast, a water treatment plant could be built near the coast outside of town. Depending on flow rate and usage in specific areas, the typical house would need a 4-inch diameter pipe leading from house to street, where the main vane is put under the stone streets, which will be easier to replace and repair than pavement streets. The main vane would typically be a 10 to 12-inch diameter pipe leading to the water treatment plant.
Traditional wastewater treatment plants are very complex and require a great deal of space for their construction. In addition, they are not very attractive. A system could occupy up to half a square mile depending on its waste-load for the treatment area.
Sources:
Erickson, Alan. Interview. 21 November, 2004.
Municipal Wastewater Treatment. Becher-Hoppe Associates Inc. 2004.
Brain, Marshall. Urban Wastewater Systems. HowStuffWorks Inc. 2004.