What you should know: Everything about wastewater treatment and small sewage treatment plants (1)

What is wastewater anyway?
#Wastewater is a generic term for #water coming from different sources, which is carried away via structures:

#Rainwater: Rainwater running off paved surfaces

#Dirty water: water contaminated by use, which is further differentiated:

#Grey water: according to EN 12056-1, faecal-free, slightly polluted waste water, such as that produced when showering, bathing or washing hands, but also comes from the washing machine and can be processed into service or process water. Rainwater running off the roof or balcony is also included. #Black water: according to ISO 6107-7:1997 domestic waste water with urine and/or faecal solids; Blackwater can be further subdivided into:

#Yellow water: Urine with flushing water#Brownwater: faeces, dishwater and toilet paper without urine

#Internal water: Water entering the sewage system due to structural damage

What is domestic wastewater made of?
Wastewater from households consists of a mixture of water and various contaminants, usually from bathrooms, toilets, kitchens and washing machines.

Why does wastewater have to be treated?
Groundwater is used, among other things, to obtain drinking water and must therefore be specially protected. Most small sewage treatment plants discharge treated wastewater into the ground, where it mixes with the groundwater.

What is the difference between municipal and private small sewage treatment plants?
Municipal sewage treatment plants
Wastewater collected from sewers is treated in municipal #sewage treatment plants. Municipal sewage treatment plants are monitored by the authorities and regularly adapted to the latest state of the art. The costs incurred for the #wastewater treatment and maintenance of the sewer systems are passed on to the citizens.

Municipal sewage treatment plants are mainly built as activated sludge plants, which we will come back to later. Activated sludge treatment plants have relatively small dimensions and are highly efficient. Due to the high technical standard, there are hardly any odor nuisances. With the appropriate effort, even bathing water quality can be achieved. Municipal sewage treatment plants that treat rainwater and waste water together usually discharge part of their wastewater untreated during heavy rainfall. The treated wastewater is mainly discharged into larger watercourses.

Citizens who are not yet connected to a municipal sewage treatment plant via a sewage system are faced with the question of whether this will happen in the foreseeable future or whether a connection is not planned. The respective city administration/municipality must have drawn up a #wastewater disposal concept that shows whether and, if so, when a #sewer connection is planned.

Private sewage treatment plants
Private sewage treatment plants for domestic waste water that is produced on one's own property are called small sewage treatment plants. The treated wastewater is usually discharged underground on your own property. If a watercourse borders the property, it should preferably be discharged here.

The property owner is responsible for proper waste water treatment and discharge. Since small wastewater treatment plants are no longer seen as a temporary solution until the sewer connection is made, but as a permanent solution, technical development has led to a great variety of systems. The property owner must choose the system and manufacturer that is right for him. Important: Serially produced small wastewater treatment plants must have a building inspectorate approval for retrofitting and, as a general rule, have a CE mark. The systems not only differ enormously in their cleaning performance and the energy costs incurred, ease of maintenance and robustness must also be taken into account when making the choice.

Authorities demand a #minimum effluent quality at the discharge point of the treated wastewater. In Germany, the requirements for the effluent of small sewage treatment plants are divided into effluent classes:

C = Carbon removal - this minimum requirement is sufficient in most cases

N = Nitrification/decomposition of ammoniacal nitrogen - reduction of oxygen consumption in surface waters

D = Denitrification/Removal of Total Nitrogen - Reduction of nitrate to protect ground and surface water

P = Phosphorus elimination - minimizes the phosphorus input in surface waters to avoid over-fertilization of the water bodies

H = Hygienization - germ reduction when discharged into standing water and bathing lakes.

Phosphorus elimination and sanitization* can also be retrofitted into existing systems. The legal requirements for the water quality to be achieved form the basis for assessing the required discharge class.

When is a sewage treatment plant referred to as a small sewage treatment plant?
A small sewage treatment plant is a plant that is intended for the water volume of a maximum of 50 connected inhabitants (PE). The legal requirements for monitoring are less strict than for larger sewage treatment plants.

Which waste water is allowed in a small sewage treatment plant?
Only wastewater from households may be discharged into small sewage treatment plants. Rainwater or drainage water is referred to as extraneous water that must not be discharged into the sewage treatment plant. If the groundwater level is high, care must be taken to ensure that no water can penetrate the ground pipes and shafts from the outside. In order to drain off the treated waste water, the lower edges of seepage pipes or seepage shafts must be installed at least 1m above the highest groundwater level.

What are harmful substances for the small sewage treatment plant?
Harmful substances are dissolved substances such as cleaning agents and disinfectants, but also solids that normally belong in the garbage can. If the use of cleaning agents is within the usual household range, this is not a problem. There are now many suitable cleaning agents** on the market, which can be used for cleaning in the household without any problems.

So, enough material for today! In the next post in this blog series, we'll then ask how wastewater is treated with small wastewater treatment plants, talk about the various treatment processes and explain legal issues, before finally getting into the more practical side of things with the different systems. So stay tuned!