Because the sewage smell was getting out of hand, the city of Vechta had to act. After an extensive testing phase, they found a smart solution that is also particularly environmentally friendly.
Oldenburger Münsterland and Vechta right in the middle: A beautiful area in northwestern Germany characterized by abundant nature and agriculture. Out in Feldmark, the district seat with 34,000 inhabitants operates its sewage treatment plant. Complaints came again and again from a nearby residential area. “The westerly wind carries the sewage smell one to two kilometers away,” reports employee Thomas Ecke. He can understand the burden: “That also bothered employees here.”
Numerous importers, intensive production and digested wastewater from the pressure pipeline: Vechta's wastewater workers were faced with a challenge in terms of odour emissions.
The sewage treatment plant on Bokerner Damm, built in 1954 and expanded several times, is designed for an equivalent population of 60,000 inhabitants. Your waste water comes from a sewer network of 330 km in length, which is fed by 135 pumping stations. The daily volume of wastewater is around 6000 m³.
On average, Vechtär wastewater only stays in the sewer system for half a day, but the discontinuous inflow and its composition finally called into question the open design of the computing building in 2015. “We used to have two fans that ran from morning to evening,” says Ecke. These removed the smell from the precision system to the outside. When emissions in the surrounding area were no longer justifiable, the specialist service manager for urban drainage oriented himself on the market.
The approach taken by Bavarian specialist Fritzmeier Umwelttechnik appeared promising. He had developed a filter system that differed in many ways from the usual solutions. The city and manufacturer agreed on a temporary trial of the Volumenmax system, which was set up in summer 2016. It was worth the try, the conditions were right.
The people of Vechta had immediately put the plant into operation provisionally. The original air outlet from the data room was switched off and the detour via DN300 pipes to the self-suction system was placed outside. And very pragmatically: “Through a hole in the door, created without further ado with a jigsaw,” Ecke remembers. Next door is a 400V control box, power connection, done.
The system is a compact, inconspicuous cuboid: its “footprint” is only 131 x 131 cm, with a total height of around 3.8 meters. At the bottom of the side, the raw gas goes in, at the top, it emerges neutralized from the mushroom head and into the open air. Measurements were carried out before commissioning: 40 to 50 ppm of hydrogen sulphide was measured in the raw gas on average, says Ecke. With peaks of up to 200 ppm. Hydrogen sulphide (H2S) is an essential component in wastewater odor.
A sophisticated filter system works inside. The raw gas flows through a filter cascade of physical, biological and chemical components — developed in-house by the manufacturer, partly protected by patent law. Fritzmeier is the only provider of such triple hybrid systems. They are based on COALSI filter technology, which has already been used several thousands of times for street ducts and exhaust air lines using the same operating principle.
“Volumenmax is our development for large companies,” emphasizes COALSI Sales Director Ulrich Bethge. The system is designed for an hourly output of up to 2800 m³. For even larger volumes, parallel operation of several systems is possible. A central element is the powerful activated carbon adsorber with an inflow area of around 8 m². “The leaking clean gas is odourless.”
Ecke confirms the statements: “Now only 3-5 ppm are coming out.” Room and exhaust air are continuously monitored. Even in the immediate vicinity, you could not smell anything. The processing room is evacuated by the extraction system and the entire atmosphere is channeled through the filter. “Since then, there hasn't been a single complaint from local residents,” Ecke underlines.
Vechta decided to buy the plant. The temporary outlet through the door gave way to a core hole in the walls — not much more was necessary. After a total of three years, a balance was also drawn from regular operation: The plant has been running reliably to date, there have been no failures.
There is one point to consider. There are filter mats in the system, the effect of which decreases over time. Microorganisms that metabolize the raw gas sit on them. The lifespan of the mats therefore depends on the gas flow rate. “As a precaution, we swap once a year,” says Ecke. “Always in spring, before the smell increases with warm weather.” Thanks to the fixed cycle, you play it safe and don't have to worry anymore. “You can also test olfactorily and only then change them,” says Bethge: If you smell something, the filters have been used up.
Vechta has the mats replaced regularly by the manufacturer. They are fixed with clips, which minimizes the effort. Bethge emphasizes that this can also be done by the customer on their own. Ecke and his team have not yet made a final decision on this.
In Vechta, the fan installed in the system is not linked to the volume flow, but simply to the pump output. As a base load, it runs on a small scale around the clock. This is not absolutely necessary, but it keeps the microorganisms busy. According to Bethge, the system can also be completely switched off when not required. However, full cleaning performance is only achieved one to two days after restarting: The bacteria need this time to activate. Even with full fan output, energy consumption is moderate at 0.71 kW/h. The electrical operating costs depend on the heating requirement to control the moisture level of the raw gas. In practice, they are usually between four and six euros per day.
A successful concept in the fight against bad odors? Absolutely for Vechta, especially since the usual alternatives were not very convincing: Considerably expanding the minimal use of chemicals to combat odors was considered out of date. Biological fixed-bed reactors were also out of the question, as they take up a comparatively large amount of space, require large amounts of organic carrier mass and can always be kept at an optimal functional level (dehydration, wetting).
The selected floor filter, on the other hand, weighs just 680 kg and requires minimal space, requires no building permit, is as quiet as a refrigerator with an operating noise of less than 50 dB (A) and can be switched on at the push of a button. The filter mats are easy to change and can be disposed of in an environmentally friendly manner as minimal residual waste — a concept supported by everyone. “It's the best choice for us when it comes to environmental protection and use of resources,” they say in unison in Vechta. “But above all, a carefree solution from an operational point of view.”
Other operators also confirm the information. However, mixed forms are sometimes used: “A customer uses the filter in parallel and was able to cut their five-digit chemical costs by about half,” says Bethge. Every treatment situation is different; anything from 30 to 1000 ppm of H2S would occur. Although Vechta's requirements were particularly high due to the importers, they were ultimately typical. “Filtering with 99 percent efficiency is realistic for many applications.”
In sewage treatment plants and in numerous production processes, odors are usually caused by biogenic decomposition, which causes bacteria to multiply as a function of temperature, which in particular form sulfur compounds and ammonia in an anaerobic environment.
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