How blackwater treatment and recycling systems work

We are the proud owners of a blackwater recycling system. It’s a step beyond greywater recycling, in that everything that goes down our drains, including toilet water and what it carries, is recycled.

Without this system, given the current drought and water restrictions in Australia, much of our garden would be struggling.

How blackwater recycling works

All the water we use in our house is routed to an initial tank via gravity. The blackwater is given time to settle and a primary colony of bacteria goes to work for 24 hours, chewing through the chunky bits; much like a normal septic anaerobic (without oxygen) system. The settled blackwater is then diverted into a secondary treatment tank that’s divided into 3 separate chambers – Aeration, Sludge settling and Irrigation.

Blackwater Aeration stage

Water and air are injected into the aeration chamber and timed intervals causing churn in the tank contents. Bacteria settle and multiply on the sludge particles, digesting a variety of nutrients and oxygen from the sludge.

Sludge Settling Chamber

The result of the aeration stage is then piped into a sludge settling chamber. Sludge sinks to the bottom and partially treated water is forced upwards through a mechanism that has another bacteria biomass covering it. This colony of bacteria then consumes most of the oxygen in the mix and breaks down any remaining solid particles.

Irrigation Chamber

The remaining effluent passes into the irrigation chamber where it is clarified and chlorinated; a process that is required by our local health authorities. I believe the amount of chlorine used is minimal as the water doesn’t have a chlorine smell; unlike the mains water in our area.

At this stage, treatment is completed and pumped out over our garden irrigation system automatically.

Our plants absolutely love the recycled blackwater as it is still comparitively nutrient rich, just without the dangerous levels and types of pathogens. We never really need to use fertilizer in our garden where the irrigation is used.

The system is arranged in such a way that raw sewage is unable to contaminate the treatment tank. Settled bacteria-rich sludge is also pumped back into the primary tank at regular intervals for the bacteria to continue digesting, thus reducing buildup of sludge and increasing the overall efficiency of the system.

Owning a blackwater treatment system doesn’t pose any serious problems that we’ve noticed so far. It’s serviced every 3 months by the company that installed it. We do need to be careful of what we send down our drains as chemicals and anti-bacterial products can destroy the bacteria colony; but that’s been a good thing as it means we use more environmentally friendly products.

People often ask me if there’s a nasty odor associated with blackwater systems and I’m pleased to say there’s not. In fact, if your blackwater system is working correctly, there’s very little odor. If it smells, it usually means that the bacteria are struggling; likely due to something you’ve put down the drain that you shouldn’t have; such as disinfectant.

A standard domestic blackwater system isn’t suitable for use on vegetable gardens and certainly not for drinking water. There are blackwater setups around that can achieve a purity level suitable for human consumption, but they are very expensive and I don’t think we’re quite ready to take that step just yet :).