What breaks down sewage in a HSTP?

What is a HSTP?

A home sewage treatment plant also called a HSTP, is a device which treats both greywater and blackwater in one place. In principle, a HSTP is loosely based around a community sewage treatment system, however, is suited to one home, as opposed to an entire street or area.

There are various kinds of HSTPs, however, the usual parts include :

• Primary treatment chamber: Also called the ‘septic tank chamber’ or the ‘trash trap’, where wastewater starts the treatment process.
• Secondary treatment chamber: Also called the ‘aeration chamber’, where wastewater is aerated to allow bacteria to do its cleaning job.
• Settling, disinfection and pump out chamber: The water is cleaner and lighter than before, allowing pollutants to settle on the bottom of this chamber.
• Irrigation Pump: Transfers the treated water into the environment.

How does a HSTP break down sewage?

HSTPs use natural bacteria to break down the sewage in a HSTP.

Solids and liquids are separated by transferring both the blackwater and greywater between varying chambers, which also allows anaerobic bacteria - bacteria that don’t need oxygen to survive - to be added, as well as the aeration of the liquids. After this process is complete, the resulting water can be used for irrigation of your garden safely and securely.

… the (HSTP) system is designed to hold the polluted or degraded water in specially designed compartments where various types of bacteria can flourish and in doing so, “treat” the water, naturally, without dangerous chemicals or additives. - Taylex

HSTPs and the sewage process

1. Black water - from toilets - and grey water - from showers, kitchen sinks, dishwashers, basins, washing machines and baths - makes its way into the primary tank.
2. Solids that are untreatable, or ones that are slow to be treated, stay in the primary chamber and drop to the bottom. Eg. oils, fats, grit. Lighter matter floats to the top.
3. As the chamber has little to no dissolved oxygen, it is basically a primary filter.
4. Effluent moves from the mid-water level into the secondary pre-treatment chamber, where anaerobic bacteria - bacteria that do not need oxygen to live - use the pollutants in the water as food and start to break them down.
5. The effluent is then moved to the aeration chamber, where the air is disbursed into the liquid through a fine bubble aerator.
6. As the tiny bubbles rise up, gaseous oxygen transfers into the water, which then becomes dissolved oxygen. This is a form that aerobic bacteria - bacteria requiring oxygen to live - can get to and they are much faster working than their anaerobic counterparts. They can double their numbers very fast when given a food source, contained in polluted water, abundant oxygen and time.
7. Lots of different types of bacteria multiply rapidly, some eating the original food and some eating other bacteria. This is just like what happens in nature and results in the biomass of billions of naturally occurring creatures cleaning the water.
8. The effluent is then sent through to the clarification or settling stage. The cleaned water is lighter than polluted water, so the biomass will settle to the bottom if the clarifier is not disturbed.
9. The top-level water in the clarifier flows out by displacement, caused by water entering the primary chamber, and flows through an ultraviolet disinfection tube or through a chlorine tablet dispenser into the pump-out/chlorine contact chamber.
10. The irrigation pump will transfer the now clear water out into the environment where it can be safely used for irrigation in your garden.

Resources: https://taylex.com.au/wastewater/secondary-wastewater-treatment-systems/