First lets take a look into wastewater treatment process. In general multiple stages of water treatment exist. Usual principle is that on every stage we have our waste cleaner compared to previous step. Please take a look into typical home wastewater treatment stages:

Following rules apply:

- In steady state when adding waste water to a tank 1 then same amount of water flows from tank 3. Basically all tanks are full and even one drop of water in added then tank 1 cannot hold any more and same amount of water is transfered to another tank by dedicated pipe. Same story goes for tank 2 and tank 3.

- Out water is water with much better quality than in waste water. This is done by 3 step process where every step happens in different tank. In tank 1 heavy elements are pulled down by gravity forces and if tank is large enough then none of heavy contents of waste water is transfered to second tank. Tank 1 is place where fermentation happens. In second tank oxygen is injected to water. Oxygen allows oxygen bacteria to work. This process happens by having dedicated pump to inject air at the bottom of tank. Air creates foam and every solid element is put into movement and ideally every solid element shall covered by air bubbles. Tank 3 is full of dedicated fittings which allows oxygen bacteria to process remaining parts of waste.

- Once per while Tank 1 must be emptied. As heavy stuff cannot be transfered further to tank 2. Tank 2 is safe from that manner as floating air won't allow heavy elements to stay at bottom. Problem remains at tank 3. After period of time solid elements will stuck at bottom of Tank 3. It is not convenient and nor efficient to drain water from tank 3. Firstly because it complicates maintenance of waste water treatment plant and secondly which is far more important is oxygen bacteria. Draining content of tank 3 would eliminate 'goodies' from treatment plant as bacteria evolves overtime and adjust them self for wastewater profile. It would take a time to recover from such drain.

Controller

This is a place where controller is required. Controller has to

1. Run air pump so that enough oxygen is injected to wastewater in tank 2
2. Control a circular pump that allows to transfer back parts of water from tank 3 down to tank 1

Executive elements

Following is just example of widely available elements that can be used for purpose of building waste water controller. What we would need is air pump and valves that would control air flow. Please note that air pump pressure can be easily used to transfer water between Tank 3 and Tank 1 by using simple pressure pump.

In next article I will describe off the shelf example of controller.