But with the huge growth in world population and the concentration of that growth in urban centers, human waste has been disconnected from the cycle. Today our wastes seem to miraculously vanish simply by flushing the toilet. But that's where the problems begin. Little more than half the solid waste sent through sewage pipes to a primary treatment facility can be removed before discharge. Secondary treatment plants remove more of the sludge, but the effluent discharged is still contaminated by metals, organic chemicals, and viruses.
Septic systems are no better.
Today in the US, over one-third of all homes use septic tanks for wastewater disposal. Soil and ground water contamination related to the tanks is common. The denser the population, the greater the likelihood of contamination. Fortunately, several visionaries are setting their sights on answering that question. The key to accomplishing these tasks is through ecological engineering.