So you flush the toilet and that’s that, right? Not quite. For those who are connected to a public wastewater system, the following article will describe the process that wastewater undergoes.
Up first is the collection system itself. To carry away the wastewater, a conduit is needed. The wastewater is conveyed to the treatment plant by a gravity main and/or a pumping station. A typical pumping station basically has a dry well (for the pumps) as well as a wet well to collect the wastewater. As the wet well fills, floats and switches activate the pumps and conveys the wastewater to the treatment works.
At the entrance (inlet) of a typical treatment plant, a bar screen is employed to remove large debris (rags, wood etc.) and a grit chamber slows the flow rate down to approximately 2 feet per second so particles like egg shells can be removed. You don’t want that kind of stuff in the pumps. Other grit removal machines employ centrifugal force.
At the primary treatment stage, large settling tanks are constructed and sized so as to allow the solids to settle out as a sludge at the bottom of the tanks where it is removed for either further treatment or complete removal and disposal. The average settling time in these tanks is two hours. Approximately 20% of the suspended solids and biochemical oxygen demand (BOD) is removed in this phase of treatment. The definition of “BOD” is as follows: The amount of oxygen required to stabilize organic matter at 68 degrees fahrenheit in 5 days. It is measured in milligrams per liter (mg/l) or parts per million (ppm).
In the next stage, two of the most popular secondary treatment methods used are trickling filters and activated sludge systems. In the trickling filter method, the effluent from the primary settling tanks is distributed over a bed of certain medium by moving pipe laterals in a circular motion continuosly from a central hub. Microbial action treats the wastewater as it contacts the medium downwards. In the activated sludge system, the primary tank effluent flows into large aeration basins or tanks and mixes with the bio-mass (called mixed liquor) that is already present. A simple way to describe this process is as follows: Under excess aeration, the solubilized organics are metabolized into carbon dioxide, water and new bacterial cells. Elements such as nitrogen and phosphorus are altered during this treatment phase. Nitrification takes place in the warmer temperatures. A certain amount of this bio-mass is returned back into the basins to keep the microbial population healthy. Some of the bio-mass is removed (wasted) from the basins periodically to maintain the same aforementioned balance. Approximately 80% of the suspended solids and “BOD” are removed in this phase of treatment.
If there is no tertiary treatment required by the regulatory agency, the effluent from the secondary system enters the disinfection phase before it is discharged into the receiving stream or deep-well injection. Some plants employ chlorination; some use ultraviolet bulb systems. Nowadays, dechlorination is employed after chlorination because chlorine can harm fish. Some very small treatment systems use sand filters for their tertiary treatment, thereby producing a very high quality effluent, especially for a smaller sized receiving stream.
The subjects of odor control, proficient operations and cost-effectiveness cannot be discussed here in this limited article. As far as this writer is aware, pharmaceuticals and some chemicals may not be removed from the treatment plant. It is very important to remember that no one should flush any pharmaceuticals down the toilet. As far as the commercial users of wastewater systems, it is of paramount concern and consequence that no illegal dumping of chemicals ever takes place, ever.
Author’s note: This writer is a former licensed wastewater treatment operator having commenced with an apprenticeship in the United States Air Force.