Sewage sludge digestion

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Sewage sludge is an aqueous residue that accumulates in a sewage treatment plant and consists of at least 95% water . The rest are organic and mineral solids bound and dissolved in the water . The aim of an economically oriented sewage sludge digestion is the highest possible separation of these solid components from the water, which serves to reduce recycling or disposal costs.

Recovery paths

Sewage sludge can be used in several ways:

  • the application of wet, thickened or drained sludge in agriculture or
  • recycling in recultivation measures and landscaping ( noise barriers , road and rail construction, etc.)
  • the dumping of sewage sludge (no longer possible for non-thermally treated sludge since June 2005)

Drained, dried or earthed sewage sludge can also be incinerated via thermal treatment, e.g. B. in co-incineration in coal-fired power plants or waste incineration plants or in a specially built mono- incineration plant .

Methods of sewage sludge dewatering

  • mechanical sewage sludge dewatering ( pressing , centrifugation ), if necessary with the addition of additives such as lime , polymers and / or iron ,
  • Drying of sewage sludge (as a subsequent treatment step of mechanical sewage sludge dewatering),
  • biological drainage processes (soil with reeds , soil with grass ).

Sewage sludge humification with reeds or grass

Sewage sludge digestion is a long-lasting and sustainable biological dewatering process for sewage sludge treatment.

advantages

  • High dewatering performance ( TS content as a result of up to 40%)
  • Low operating costs, low primary energy requirement
  • Natural material conversion with mineralization of the organic matter = reduction in the amount of recycling
  • Increase in quality of the end product ( similar to humus , but legally still sewage sludge)
  • Without chemicals and additives, CO 2 -neutral

disadvantage

  • Relatively large space requirement, approx. 1 m² per inhabitant value, depending on the sewage sludge properties
  • Concentration of heavy metals (due to the breakdown of organic matter)

Process description

Aerobic (activated sludge processes by oxygen) or anaerobic (by fermentation processes with putrefactive bacteria ) stabilized sewage sludge with about 1 to 5% dry matter content is without mechanical pre-treatment to the soil humification beete passed. The modularly designed soil beds are planted with reeds or grass. Reed ( Phragmites australis ) has to be specially adapted to the high performance requirements in order to achieve good growth and root penetration of the soil. As a water plant, reeds are ideally suited as an oxygen supplier for the soil matrix due to their anatomy . Through the air conduction tissue (aerenchyma) in the reed stalk, atmospheric oxygen is permanently conducted into the area of ​​the sludge close to the roots, which promotes the multiplication and maintenance of the microorganisms necessary for material conversion in the soil. Grass cannot do this.

In the case of gravity-induced seepage of the soil, the sewage sludge is initially quickly drained to an initially 8 to 10% dry matter content. The less polluted filtrate water, which is collected via a drainage system, is returned to the sewage treatment plant . The organic and mineral solids remain in the beds, which are deposited in the rhizosphere , a primarily aerobic but also anaerobic soil matrix, through the symbiosis of reed plants, microorganisms and small organisms, and which are split up over a longer period of time. The organic content is largely mineralized and breathed in, which, in contrast to mechanical dewatering processes, results in a strong reduction in the amount and a significant reduction in recycling and disposal costs.

Over the course of several years, sewage sludge digests into a crumbly earth that resembles humus products in terms of appearance, smell, material properties, pore volume , water retention capacity and other properties.

Clearance and recycling of sewage sludge soil

Before a bed module is cleared, it is no longer filled with liquid sludge for several months; During this time, other bed modules take over the sewage sludge that still accumulates. The dry matter content at the time of evacuation is usually 20 to 40% dry matter, depending on the duration of the dry phase and the weather at the time of evacuation. The sewage sludge cleared with heavy equipment is optionally transported to a pile for post-storage and is regularly moved. As a result, this process achieves dry matter proportions of up to 60% with further degradation of the organic matter. Due to the combined dewatering and mineralization processes, sewage sludge digestion achieves even better results than complex drying processes with around 70% dry matter content, which do not achieve any organic degradation. By adding special minerals , mulch materials, etc., the cleared and downstream sewage sludge soil can be additionally refined in order to optimize it for a desired recycling path in landscaping ( composition of technical floors).

Recommissioning of the earthing modules after clearing

When clearing a grounding module, the grounding material is not completely cleared. In the remaining remains there are root remains from which new reeds sprout. Missing spots can be replanted with little effort. The overall system achieves running times of more than twenty years, including several beeträumungen per earthing module.

Important factors for good drainage and digestion performance

  • Professional planning and dimensioning
  • Competent system control and system operation
  • Sufficient degree of stabilization of the sewage sludge
  • Gentle start-up operation in the stabilization phase of the biological system
  • Plant assessment and maintenance of the entire system over several years
  • Careful clearing of the soil beds without compaction of the soil and securing the start-up phase

Application of sewage sludge digestion

Suitability of the procedure

Sewage sludge digestion is suitable for aerobic and conditionally for anaerobic sewage sludge. If the available space is not a problem, this near-natural process is usually the most economical and effective form of sewage sludge dewatering: the decisive factor here are low operating costs in the digestion process in combination with low recycling costs for sewage sludge soil, which result from the reduction in mass, the positive change in the material properties and the diverse usage options from landscaping to agriculture and incineration. Earlier information on the suitability for sewage treatment plants smaller than 5,000 PE are considered to be outdated because, according to statistical analysis [IB Gödecke, 2002], the main area of ​​application for plants in Germany is between 1,000 and 30,000 PE.

distribution

  • mainly in Central Europe, especially Denmark and Germany with over 100 systems each, but also in Austria, the Netherlands, France and many more
  • worldwide application, e.g. B. in USA, Canada, China

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