Waste Treatment Lagoon

A waste treatment lagoon is a sealed outdoor basin that takes all of a household's wastewater, toilet waste included, and holds it still while bacteria break it down. It is not a better septic system but the answer where one cannot work, because the ground drains too slowly to take the buried pipe field a septic tank needs; extension guidance limits the method to lots of 1.2 hectares (3 acres) or more. The stabilised liquid is spread on land under separate rules rather than reused around the house. Building one takes a permit and an engineered design, and it is built to last about fifteen years.

Depth is what makes it work. Residential guidance gives 0.6 to 1.5 m (2 to 5 ft) of wastewater, plus 30 cm (1 ft) of empty bank so a storm cannot push it over the edge. Shallower and the floor dries, the smell rises and rooted plants take hold; deeper and treatment stops working as designed. It is sealed with clay or a liner tight enough to pass no more than about 3 mm (an eighth of an inch) a day: a leak contaminates groundwater silently and the seal is all that prevents it.

Bacteria slow in the cold, so the smell arrives in spring when the water warms and a winter's load is worked through together. Two habits follow: start a new basin in late spring or early summer so the bacteria establish before the first cold weather, and load it steadily, at least weekly and preferably daily, since a lagoon fed after a long gap performs worst. It is fenced because open water is a drowning risk to children and stock. Grass inside is mown short and woody growth kept off the bank, so air moves over the surface and roots stay out of the seal. Floating plants are raked out rather than sprayed: a herbicide returns the dead plant as fresh load. Solids settle and stay, so the sludge is measured every few years and pumped out before it rises within 45 cm (18 in) of the surface.