Germany 2021 Wastewater Treatment Survey Fact Sheet

The professional publication of the German Water Association DWA "Korrespondenz Abwasser· The 11th issue of Abfall (Sewage and Waste) 2022 contains the article "Leistungsvergleich kommunaler Kläranlagen" (34th sewage treatment survey)
written by the "Sewage Treatment Plant BIZ-1.
1 – Sewage Treatment Plant Coordination Group" of the German Water Association DWA.

This time, the relationship between the purification effect, energy consumption, influent water quality and the scale and level of sewage treatment plant was investigated
in depth.
Some of the data are excerpted for study, discussion and reference
.

1 Overview of sewage treatment plants

In 2021, there were 8,891 sewage treatment plants in Germany, with a total population of 152.
1 million equivalent, nearly twice the actual population of
Germany.
This is a decrease of 214 from 9,105 in 2020 (33rd survey report), but the total size is an increase of 300,000 compared with the 151.
8 million equivalent population surveyed in 2020
.

In 2021, 5,273 of them were surveyed, with a total survey size of 130.
8 million equivalent population, accounting for 85.
9%
of the total scale.
The size distribution of the surveyed wastewater treatment plants is detailed in Table 1
.

According to the 2021 survey, German sewage treatment plants are still dominated by small and medium-sized sewage treatment plants, with the number of plants below the size of 10,000 equivalent population accounting for 66.
15%, the same
as the 2020 survey.
However, the population of the scale accounts for only 7.
53% of the total population; Compared with the number of sewage treatment plants with a population of more than 100,000 equivalent, the total number is only 4.
23% of the total number, but the total population served accounts for 54.
21%
of the total population of the survey plant.
The layout of this sewage treatment plant in Germany is closely related
to the high level of urbanization, many small towns and urban layout characteristics in Germany.

2 Sewage purification

Table 2 shows the wastewater treatment situation of the survey plants in Germany in 2021 (annual average).

The data in the table reflects:

1) None of the German sewage treatment plants are operating
at full capacity.
The operating load ratio of the surveyed sewage treatment plants (in terms of the ratio of population actually served to population size) averaged 76.
5%, compared with 69.
5%
in Austria.
The highest is in the Hesse/Leigh Palatinate/Saarland region in the central and western part of Germany at 88.
2 percent and the lowest in Bavaria at 67.
1 percent
.

2) Germany's per capita sewage treatment is closely related
to the influent water quality and drainage system.
According to the equivalent population, the per capita sewage treatment capacity in Germany is 112.
3 L/person·d~301.
4 L/person·d, and the corresponding influent CODCr concentration is 400 mg/L~1070 mg/L
.
The comparison countries in Austria were 194.
5 L/person·d and 618 mg/L
, respectively.
Reflecting: First, the per capita treated water volume is inversely proportional to the influent concentration, and is closely related to the proportion of the combined system, such as Baden, its annual average influent CODCr is 400 mg/L, and its combined flow system accounts for 88% (see Figure 1 German combined flow system proportion chart for details), and its per capita sewage treatment capacity is 301.
4 L/person·d is the highest, which includes more rainwater intercepted on rainy days; In the northeast region, the annual average influent CODCr was 1070 mg/L, and its combined flow system accounted for only 15.
1%, and its per capita sewage treatment capacity was 112.
3 L/person·d, which was the lowest
.
Taking the northeastern region with the highest proportion of diversion systems (Mecklenburg-Vorpomerie and Brandenburg Liangzhou) as an example, its resident population is about 4.
3 million people, and its actual service equivalent population is 13.
2 million people, which is about twice the actual resident population in the region, that is, about two-thirds of the equivalent population is industrial
.
The equivalent population treated sewage is 112.
3 L/person·d, which translates to about 340 L/person·d for the residential population, including rainwater
in a small number of combined areas.
This also shows that Germany's water conservation work is very effective, and it should be praised
for Germany's water saving work.
Reflect on whether the per capita comprehensive sewage volume standard stipulated by China's relevant standards is too high? Water conservation is the most effective anti-pollution measure
.

3) The influent water quality of German sewage treatment plants is closely related
to the quality of pipelines.
In addition to sewage, the influent water of the sewage treatment plant also includes external water and rainwater
.
The concentration of pollutants influent from sewage treatment plants in the north and northeast of Germany, which is dominated by the diversion system, is almost 1 times higher than that in the combined system, especially the CODCr concentration of the northeast influent water exceeds 1000 mg/L, which is consistent with the per capita treated water value in the diversion system area is almost half of that in the combined
system-dominated area.
Only in areas with low groundwater tables and cities with better drainage pipe networks in China are basically close to those in German combined water
concentrations.
This also shows that the quality of German sewage pipes and combined pipes is amazing
.
In addition to the high influent water concentration is directly related to the effectiveness of water conservation, it is also closely related
to the effectiveness of Germany's effective elimination of groundwater and other external water infiltration.
"Cleaning up pollution" is an important factor affecting the low concentration of water influent in China's urban sewage treatment plants, and it is of great significance to improve the influent concentration of sewage treatment plants and improve the efficiency of pollutant reduction in sewage treatment plants.

4) The science
of measuring the size of wastewater treatment plants in Germany.
The scale of German sewage treatment plants is measured according to the equivalent population (industrial wastewater is converted according to the CODCr load of 120 g/person·d), which effectively solves the problem of
different drainage systems, different treated water volumes, but the same pollution load of the (equivalent) population.
Its sewage collection rate by equivalent population (currently 99%) also has much fewer influencing factors (e.
g.
, avoids the question of the authenticity, accuracy and reasonableness of the survey of influent water and sewage generation); At the same time, the comparison of electricity consumption, sludge production, etc.
is carried out
on the same platform.

5) German influent pollutant equivalent regulations are accurate
.
Surprisingly, according to the actual amount of treated water and the concentration of influent pollutants, the equivalent value of pollutants produced by the average population of the remaining pollutants in each region is very close to the equivalent population production equivalent specified in the German standard, which are: CODCr: 120 g/person·d, total nitrogen: 11 g/person·d
.
The actual total phosphorus equivalent value is lower than the standard value of 2.
5 g/person·d due to the continuous introduction of phosphorus-free detergents in Germany
.
In recent years, there is no longer BOD5 data in German surveys, which is closely related to
Germany's encouragement of the use of CODCr for design calculations and assessments since 2000.

6) Pollutant reduction in German sewage treatment plants remains at a high level
.
In 2021, the reduction rate of CODCr of German surveyed sewage treatment plants was 94.
7%~96.
3%, the total nitrogen reduction rate was 76.
6%~91.
8%, and the total phosphorus reduction rate was 90.
4%~96.
2%, see Figure 2
for details.
The effluent quality of various pollutants is better than, or even significantly better than, the average of
EU standards.

Figure 2 Reduction of pollutants in wastewater treatment plants across Germany

3 Analysis of pollutants in influent water in Germany

The 2021 survey specifically investigated the change of water volume and influent pollutants in sewage treatment plants of various scales, and the results were very interesting, but the reasons
were not explained.
It can be summarized as:

1) The amount of water treated by the equivalent population is basically inversely proportional
to the scale and grade of the sewage treatment plant.
The higher the proportion of diversion systems within the service area of the sewage treatment plant, the lower the amount of rainwater in the influent (ideally zero).

The wastewater treatment plants in northern and northeastern Germany have significantly less water treated by the equivalent population than other regions, so the concentration of pollutants in the influent water in these two regions is significantly higher than in other regions
.
At the same time, the survey also shows that the amount of water treated by the equivalent population decreases as the scale of the sewage treatment plant increases, and the higher the scale level, the smaller the amount of water treated by the population, as shown in Figure 3
.

2) The concentration of CODCr in the influent water is basically proportional
to the scale and grade of the sewage treatment plant.
The survey shows that unlike other influent pollutants, the concentration of influent CODCr is basically proportional to the scale grade of the sewage treatment plant, that is, the larger the scale, the higher
the concentration of influent CODCr.
See Figure 4
for details.

3) The total nitrogen generation equivalent of the population is inversely proportional
to the scale and grade of the sewage treatment plant.
The survey showed that the total nitrogen concentration in the influent water in the northern and northeastern regions was inversely proportional
to the scale of the sewage treatment plant and the CODCr concentration of the influent water.
That is to say, the larger the sewage treatment plant, the lower
the total nitrogen concentration of the influent water.
There is no pattern in other areas, as shown in Figure 5
.
However, the total nitrogen equivalent generated by the influent population is clearly inversely proportional to the size of the sewage treatment plant, as shown in Figures 6 and 7
.
This means that the smaller the wastewater treatment plant, the higher the requirements for total nitrogen in the
influent water.

4) The total phosphorus production equivalent of the population is inversely proportional
to the scale and grade of the sewage treatment plant.
The survey showed that the total phosphorus concentration in the influent water was not regularly displayed with the scale level of the sewage treatment plant, as shown in Figure 8
for details.
However, the total phosphorus production equivalent of the influent population is also inversely proportional to the scale of the sewage treatment plant, as shown in Figures 9 and 10
.
It also means that the smaller the wastewater treatment plant, the higher the total phosphorus requirements of the
influent water.

4 Effluent pollution load investigation

The survey also investigated the effluent pollution load of sewage treatment plants of various scales, and the survey showed that the population size of sewage treatment plants below 10,000 equivalent population (level 1~3) accounted for only 7.
53% of the total scale of the survey (see Table 1), but the total phosphorus discharge load accounted for only 24%
of the total phosphorus discharge load of the survey plant.
Therefore, further reducing the phosphorus concentration in the effluent of small sewage treatment plants is an important task
for German sewage treatment plants.
See Figure 11
for details.

5 Energy consumption of wastewater treatment plants in Germany

The energy consumption survey of 5,273 surveyed sewage treatment plants is detailed in Table 3
.
There is not much
difference in electricity consumption between states.
The lowest electricity consumption per capita is in Austria and the northeastern states, and the highest in Baden.

Table 3 shows that the electricity consumption of treated water in Germany is 0.
31~0.
69 kWh/m3, with an average of 0.
38 kWh/m3
.
This power consumption includes sludge treatment
.
While actively saving energy, Germany also vigorously promotes energy self-sufficiency, and the energy self-sufficiency rate of large-scale sewage treatment plants can achieve more than 50%, which is directly related to
their emphasis on the use of biogas for anaerobic digestion of sludge.
On this basis, Germany is also vigorously promoting CO collection, biogas power generation, photovoltaic power generation, wind power generation, and improving hydraulic conditions to achieve energy self-sufficiency in sewage treatment plants
.
The 2017 survey also looked at biogas production, biogas generation and efficiency
.
Based on an average population size of 23 L/person·d, a population size of 151.
8 million equivalent and a load ratio of 77.
5%, the biogas production of urban wastewater treatment plants in Germany is 2.
7 million m3
per day.
It is worth thinking about how much biomass energy is lost in our many wastewater treatment plants?