Septic tanks and wastewater treatment in Bali

Sizing starts with people, not bedrooms. We work from roughly 150 to 200 litres per person per day, add staff, count the guests a rental actually sleeps rather than the number on the listing, then allow retention time for the treatment stage to do its work. A three-cubic-metre bio unit serves a household of five to eight comfortably; a four-bedroom rental running at capacity with a kitchen and a pool is a different calculation entirely.

What happens after the tank matters as much as the tank. In the clay around Canggu and Kerobokan a soak pit that works in August is a pond by January, so effluent needs a drainage field or a treatment stage producing water clean enough to irrigate with. On the limestone of the Bukit the opposite is true: everything disappears instantly, straight towards the water table — which is precisely why we do not build bottomless pits there.

Bio septic or a conventional tank

A conventional tank is two or three sealed concrete chambers: solids settle, liquid moves on, bacteria work anaerobically and slowly. Built properly it is durable, cheap to run and needs desludging every two to three years. Built the way it too often is here — a single chamber with an open bottom, sold as a septic tank — it is simply a hole discharging raw sewage into the aquifer.

A bio septic unit is a moulded tank with baffled anaerobic and aerobic stages, filter media and usually a disinfection chamber. It takes less ground, produces effluent clean enough for a drainage field or planted trench, and costs more. On tight plots, in clay, or anywhere near a well, it is usually the right answer. What it is not is maintenance-free: media needs checking and sludge still has to come out.

Groundwater, flotation and the calls we get

Three failures account for most of the emergencies. Flotation: an empty plastic tank in a high water table lifts like a boat and cracks its inlet, so ours sit on an anti-flotation slab with straps, backfilled in stages with the tank part-filled. Overloading: a system sized for a family and then run as a six-bedroom rental, which turns a two-year desludging cycle into six months and pushes solids into the drainage field.

And proximity. A septic system upgradient of a well, or within about ten metres of one, will contaminate it — the coliform counts in some south Bali wells make that point loudly enough. We site tanks downgradient wherever the plot allows, and where it does not, we specify treatment rather than distance. Getting this right at design stage costs nothing. Getting it wrong costs a well.