What you should know: Everything about wastewater treatment and small sewage treatment plants (2)

How is wastewater from small wastewater treatment plants treated?
All biological and chemical processes in #wastewater treatment also take place in nature and without human intervention: A wide variety of microorganisms, which are part of the food chain, feed on the waste, so that in the end only mineral material remains. About 130 years ago, this fact was the inspiration for the development of #sewage treatment plants: to clean the #wastewater in the smallest space and in the shortest possible time so that it can be returned to nature without hesitation.

The main constituents of domestic wastewater are carbon, nitrogen and phosphate compounds in a certain ratio. The #carbon compounds thereby form the organic substance for feeding the #microorganisms in the wastewater. For #wastewater treatment, these microorganisms are supposed to multiply en masse to initiate wastewater treatment and eliminate nitrogen and phosphate compounds in the course of #bacterial activity.

Since the composition of the #wastewater contents differs due to the different origins, logically different microorganisms also form in the wastewater. It goes without saying that this living #biomass must not be impaired by the introduction of harmful substances, especially in larger quantities. When purchasing cleaning agents, it is therefore always important to ensure that they are biodegradable. These items, used in normal household quantities, are harmless.

#Small wastewater treatment plants consist of a #primary treatment, a biological part and a #secondary treatment: In the primary treatment, the coarse substances are retained, in the biological part the actual #wastewater treatment (see above) takes place and in the secondary treatment the #biomass sludge is retained.

Technical development has not stopped at #sewage treatment plants either, so that there are mature and safe systems on the market today.

What are the cleaning methods?
The near-natural processes use the soil body for biological #wastewater treatment. These include vegetated #soil filters (#plant treatment plants*), #subsoil trickling, the filter ditch and #clarification ponds. The area required for these systems is relatively large.

In the case of technical plants, the waste water treatment is carried out using technical devices that are housed in one or more containers. The automatic operation is ensured by a system control. When it comes to wastewater treatment in technical plants, a distinction is made between biofilm and activated sludge plants: In biofilm plants, the biomass settles on a filling material that is either constantly immersed in water or just trickled through by the water. This #biofilm is also known as a biological lawn. The #biomass in activated sludge systems consists of #sludge flakes that float freely in the water. Most municipal sewage treatment plants work with this principle. In both cases, maintaining the biomass requires oxygen to breathe and nutrients from the wastewater.

When do systems have to be renewed or converted/converted?
Since #DIN4261 Part 1 was published in 2002, underground drainage systems, filter ditches and settling ponds for wastewater treatment are no longer permitted in most federal states. As a result, such systems may no longer be operated after the #discharge permit has expired at the latest. In this case, the systems must be converted or renewed. At the latest after the discharge permit has expired, the authorities check whether the small sewage treatment plant still meets the applicable requirements. If this is not the case, the authority can request a conversion of the existing system or the construction of a new one.

What do you have to watch out for in the case of non-approved systems?
Non-approved systems must be constructed and operated in accordance with the specifications of Worksheet #DWA-A 262, March 2006 edition.

What are these requirements?
The form of overgrown soil filters that is common today consists of a #film tank with a surface area of ​​at least 4m2 per inhabitant and a minimum depth of 90 cm. Appropriate types of reed are recommended for planting. The waste water to be cleaned first enters a pre-clarification tank, which is usually designed as a three-chamber pit. The #pit content depends on the size of the system and is between 500 and 1,500 l per inhabitant. The required pit content is larger than for technical systems. The waste water to be cleaned is usually applied to the surface in surges via perforated pipes using pumps. Some manufacturers also lay these pipes under the surface of the bed. The waste water, which should be applied to the #filter surface as evenly as possible, seeps through a 50 cm thick layer of washed gravel with a prescribed grain composition. The drainage pipes laid in the lower area of ​​the filter collect the now cleaned water for drainage. The derivation in sloping terrain can be done with free gradient; otherwise the water usually has to be drained off via a pump in a separate shaft.

What should be particularly considered?
During construction work, particular attention should be paid to the fact that the foil thickness must be at least 1 mm. Observing the proposed grain composition of the filter material is crucial for the function and service life of the system. The perforated pipes in the upper area must be completely drained after each #loading so that there is no frost damage in winter. In order to recognize the risk of backwater in the inlet channel, an acoustic alarm is required if the pump fails. Of particular importance is the retention of coarse materials from the preliminary clarification, so that there are no malfunctions in the #feed pump and the filter material does not clog up. If the #loading pipes are laid openly on the surface, #odor nuisance can occur. If the system is no longer required or the filter material has to be replaced, an examination for harmful substances is required for proper disposal.

How often do checks have to be carried out?
The system must be serviced at least once a year by a specialist company. Planting can die off in the summer months if the soil filter is not used for a long period of time. The reeds should be cut off in late autumn so that the vegetation can develop well again next spring.

Can cleaned wastewater be reused?
In principle, there is nothing to be said against using the cleaned wastewater for #garden irrigation. However, it should be noted that this water is not 100% sterile. This water must therefore not be sprayed for irrigation, but only trickled directly onto the surface or into the ground. The cleaned waste water can also be sterilized using special processes, which then also makes other uses possible.

So much for the basic questions about wastewater treatment and small sewage treatment plants. In the third and next part of our series, we will get into the practical side of things with detailed knowledge about the different treatment systems. So stay tuned!