Urban development and climate: Green energy from black water

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Every time Beatrixe Freier goes to the bathroom, it sounds like she's just taking a plane trip. Pressing the button on her toilet bowl unleashes in her bathroom the typical sucking sound of toilets in large passenger aircraft.

"It was funny in the beginning, but I got used to the noise," says Freier. The 70-year-old is standing next to the bowl and shows off her vacuum toilet. Each of her neighbors has a toilet like her. Freier is one of the first 120 residents of a development area in the east of Hamburg. Construction cranes are still standing at some of the blocks in the district Jenfelder Au. In a few years, all of the more than 830 apartments will be occupied.

The built-in high-tech toilets consume just under a liter of water per wash cycle. For standard toilets, there are up to eight. Although lower-pressure toilets cost more, but the water charges are lower. But that's not the only plus of the special latrine. They are part of a concept with which Hamburg wants to break new ground in the treatment of wastewater.

For this, Ms. Freier has to follow meticulous rules on her vacuum toilet - the system is more sensitive. Leftovers or wet wipes are not in the toilet, otherwise the pipes clog quickly. This also applies to conventional toilets. But vacuum toilets are much more sensitive. Once even some paper towels Freiers toilet lame.

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If humankind wants a chance to fight global warming and protect the environment, it must save energy and use more sustainable power generation techniques. Sewage treatment plants are often the largest municipal energy consumers. Around the clock, pumps blow oxygen for faeces-eating microorganisms into basins, huge motors drive agitators.

In order to achieve the Paris climate protection goals, there will hardly be a way to use more energy-efficient wastewater treatment methods. In Jenfeld, therefore, an idea is tested in which the wastewater from households serves as an energy source. The system of the Hamburg Water Cycle works like this:

Used water from the apartments of the Jenfelder Au is split directly with the consumer and flows into two different pipes. In Germany, about 70 liters of water fall on average per inhabitant per day when bathing, showering, washing dishes or from the washing machine. This so-called gray water lands in Jenfeld in its own pipe system. The other is there for the black water and starts in the toilet. If Mrs. Freier pulls the trigger, some water is flushed into the bowl, then a valve opens to the pipe. Here there is a pressure of minus 0.65 bar compared to that of the atmosphere. Due to the negative pressure, the feces are sucked into the line.

In the classic sewage system, they would eventually arrive at the central sewage treatment plant in the Port of Hamburg. On the way, they flow through mighty pipelines, in which a slope is installed. But the so-called alluvial canalization needs a lot of water. And energy: sometimes pumping stations also make sure faeces move forward. With the pressure lines in Jenfeld the material transport works easier. Relatively small pumps for generating negative pressure are sufficient to move the feces-water mixture. Another advantage: The pipes are much slimmer than in the conventional system and do not have to be laid so deeply. This saves material, construction costs and energy.

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And that is exactly what is being produced from the black water. For this purpose, the rich organic material wastewater is enriched with fat residues from the catering and collected in a container. They drive microorganisms that initiate fermentation processes at temperatures above 35 degrees Celsius. Methane-containing biogas is produced. The generator of a combined heat and power plant converts the gas into electricity, which is fed into the grid.

At the same time, heat is generated, which helps to heat the apartments of the neighborhood. So the entire system is not only energy self-sufficient. Thanks to Jenfelder Fäkalien it even produces surplus: According to the operator Hamburg water generated so about 450,000 kilowatt hours of electricity and 690,000 kilowatt hours of heat per year - at least if at some point all neighbors of Mrs. Freier are drafted. Based on the Hamburg average of 1.8 persons per household, this covers the electricity requirement of 225 households and the heat requirement of 70.

Biogas is also obtained at sewage treatment plants and used for energy production. But the effort is greater because the wastewater previously passes through three different treatment stages, in which much energy must be expended. In Jenfeld this is not necessary, the black water can be recycled uncleaned.

The three-step concept is decades old. Apart from discussions about a further purification stage for the containment of harmful substances (read more here) from medicines, the cleaning technology has since then not fundamentally improved. Wastewater was seen as a disposable product of our consumer society for decades.

This is how a wastewater treatment plant works (click on the numbers):

1

The waste water first passes through a rake, which holds back coarse debris (paper, bottles, branches, cans, etc.) and removed with an automatic scraper .

2

In the sand trap , the drainage channel widens , which reduces the speed of the still flowing wastewater and deposits coarse materials such as gravel and sand, which are heavier than water, at the bottom.

3

In the primary clarifier, the water is held back for about two hours. In this large, rectangular or round basin, the fine suspended matter can settle on the ground as mud. This raw sludge is sucked off, thickened (10.) and transported into a digester (11.) . So-called lightweight materials, which are distended to the water surface (fats, mineral oils, etc.) are drained into a special container.

The first three stations together form the first cleaning stage. Since the treatment of the waste water takes place here only in a mechanical way, it is called mechanical cleaning. Here it is possible to extract about 30 percent of the total amount of pollutants fed into the system.

4

In the second purification stage , also called biological purification, one makes use of a thoroughly natural process by creating favorable living conditions for microorganisms in an activated sludge basin by supplying oxygen, which are able to absorb dissolved organic wastewater in connection with atmospheric oxygen as food and in to build one's own organism. It also pollutants, such as heavy metals, are included.

5

Many microorganisms form colonies, which sink as visible sludge flocs in the following sedimentation tank to the soil and are either pumped back again into the aeration tank or transported to the primary clarifier (3) for the purpose of sludge disposal.

With the disposal of the sewage sludge from the wastewater so the biodegradable pollutants are removed. In smaller sewage treatment plants you will often find trickling filters, round concrete boilers, loosely filled with porous rocks. Here, the large surface is important on which bacteria settle and form a "biological lawn" (corresponding to the sludge flakes in the activated sludge process), when raining over wastewater. After passing the mechanical and biological purification stages, the wastewater is now about 90 percent cleaned.

6

From a precipitating metering station , a chemical solution is added to the wastewater at the same time with thorough mixing.

7

As an example in the display panel, phosphate-rich water from the secondary clarifier number 5 first reaches a flocculation basin

8th

This precipitant chemically reacts with the phosphates to form a water-insoluble compound. The residual dirt "flocculates" and can settle in a secondary clarifier as sludge, which - thickened - is fed to the digester.

9

The purified water can now be discharged into a natural body of water.

10

With the help of thickeners , this content is on

96 percent

11

Now, the sludge is transported to a digester, where under exclusion of air rot bacteria in a fermentation process at 35 degrees Celsius produce a biogas, consisting of 2/3 of methane and 1/3 of carbon dioxide.

12

This gas is stored in a container and used for heating purposes.

13

After about 4 weeks, the sludge is rotted, odorless and can be dehydrated in dry beds.

Source: BMU

"In the past, we knew too little about the resources of the sewage, today we are smarter," says Wolfgang Kuck of Hamburg Wasser about the old times. He stands on the depot in Jenfeld and points to the digester, in which the black water in front of him. "There is more in sewage than you think," says the project manager.

In recent years, the urban water management has changed, the entire cycle is in Jenfeld on the test. Wastewater, the foul-smelling mixture of urine, feces and many other substances, is now traded as a valuable resource. Not only does it serve as an energy source, it is also rich in nutrients such as carbon or nitrogen. Phosphorus helps plants to grow. The fabric is imported to Germany and getting scarcer. In the future, sewage plant operators hope, it can be won from sewage sludge. Hamburg is therefore currently building the world's first phosphorus recycling plant next to the wastewater treatment plant in the port.

Phosphorus is also contained in the residues from black water fermentation in Jenfeld. Therefore hopes Wolfgang Kuck that their waste product will eventually be used as a fertilizer. So far, the residues are treated as sewage sludge and migrate into the combustion.

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It is still questionable how the gray water should be cleaned. It contains, for example, microparticles and nanoplastic particles made from textiles. The classic cleaning technique is not designed for such problematic substances. The gray water is still draining into the sewage system. But in the coming year researchers are testing new procedures on site. The water is pressed through tiny filter membranes, which can also capture microparticles. However, this technique is still relatively expensive and energy-intensive.

Similar projects like the one in Jenfeld already existed in Lübeck and other European cities. However, the complex double pipe systems are reaching their limits and are far from profitable - despite the energy surplus. The network can not be expanded arbitrarily large. So sewage treatment plants will continue to exist.

But Jenfeld is a start. And with regard to climate change, people have been thinking about another part of the sewage, the rain. It is increasingly challenging network and wastewater treatment plant operators. Because of climate change, researchers have long predicted an increase in heavy rainfall. At the same time, Germany's cities are growing, and as a result, the proportion of concreted and asphalted land is increasing.

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With heavy reefs this leads to problems because rain water due to the sealed soil can flow off worse. In addition, the water flushes substances such as tire abrasion or chemicals from the streets. The polluted rain ends up in the sewage system, but the nets are not designed for the ever larger volumes of water. In Hamburg, this leads to overflowing the Gullis and flooding. In this way, contaminated rainwater gets into streams, rivers and lakes.

You want to prevent flooding in Jenfeld. Retention basins should be able to catch even rainfall, as statistically only every few years occur. The water can seep in peace, at the same time it waters green areas and evaporation provides for cooling on hot days. And by the way, Ms. Freier is happy that she is looking directly at a pond when she opens her front door.

In summary: A urban development project in Hamburg-Jenfeld is testing new methods of wastewater treatment and separating black, gray, and rainwater. For the black water, the faeces of the inhabitants, water-saving vacuum toilets are used. Subsequently, a biogas plant produces electricity and heat from the feces.