Sludge

Sludge describes a mixture of solid and liquid substances, obtained in wastewater treatment by sedimentation. When the solids are suspended solids that settle in the treatment plant from the water and sink to the bottom ( sediment ). In the liquid material content in addition to water may be a variety of chemical compounds in the dissolved state.

• 4.1 Solar drying of sewage sludge
• 4.2 belt dryer
• 4.3 fluidized bed steam dryer ( WVT )
• 4.4 washer dryer
• 4.5 frying of sewage sludge

• 5.1 Agricultural recycling
• 5.2 Disposal
• 5.3 Thermal Treatment
• 5.4 Sewage sludge

• 6.1 Precautions in handling

Species

A distinction raw sludge and treated sewage sludge. Raw sludge falls on sewage treatment plants as primary sludge in the mechanical cleaning stage or as excess sludge in the biological stage. Excess sludge consists mainly of microorganisms such as protists and bacteria. By aerobic or anaerobic stabilization of the raw sludge obtained less odorous sewage sludge ( treated sewage sludge or stabilized sewage sludge). The anaerobic treatment takes place in larger sewage treatment plants in anaerobic digesters ( sludge ). Sewage sludge is colored fluid and dark in the initial state. By gravity thickening solids contents of about two to five percent can be achieved ( for example, by centrifugal systems ). However, there are even more possibilities such as Siebbandeindickung.

The sewage sludge is rich in nutrients as the bacteria use the wastewater constituents to build the biomass in the biological stage. Are of particular importance especially for the agriculture nitrate, phosphate and other nutrient salts.

Below is a table with the normal levels of nutrients of the sludge, which can vary greatly from time to time and also from plant to plant.

Sewage sludge contains substances that may be problematic for the environment and humans. In particular, the heavy metals, the occurrence of back solutions pipes, among other things. Limits are set by the Sewage Sludge Ordinance.

Organic ( harmful ) substances are not a negligible problem. In sewage sludge, a variety of organic compounds with different properties and effects that come from anthropogenic processes in the waste water to be present. These substances can, for example, carcinogenic, mutagenic, etc. be toxic. The Sewage Sludge Ordinance has limits for the sum parameter AOX, PCB and PCDD. If such substances present in the waste water, occurs a particular threat because they can enter the food chain even in low concentrations after agricultural application accumulate ( " bioaccumulation " ) and.

Below is a table showing these organic substance groups and chem. Compounds and their concentration were detectable in sewage sludge 1980s. Due to the adopted in the meantime statutory prohibitions on using some of the listed weeds and Schädlungsbekämpfungsmittel are several of the indicated compounds in the sludge currently no longer detectable. Toxic organic compounds are also currently still present in the sludge.

The introduction of hormones and pharmaceutical residues, for example, hospitals, nursing homes, etc., is also a problem because they accumulate in the soil and can then enter the food chain or even remain in the treated water and thus enter into the natural water cycle. The variety of materials detection and removal is difficult.

Sludge Treatment

Sewage sludge is treated for the further disposal and uses the following process steps can be used: thickening, conditioning, sanitizing, dewatering and drying. Which methods may be used here and the sequence in which this takes place may be different.

Drainage

Mechanical drainage

Frequently dehydrate mechanical dewatering equipment (centrifuges, filter presses, belt filter presses, screw presses ) the most frequently treated ( stabilized ) sludge prior to treatment, recycling or disposal. To be able to squeeze out water, it is necessary to condition the slurry. This can be done by adding the liquid slurry polymeric flocculant, iron, or lime milk. A lime addition is about 20 to 35% CaO in the solid fraction, and makes a substantial part of the benefit in the recovery as fertilizer. However, while the initial dry mass of the waste quantity is increased, resulting in additional costs. By mechanical dewatering of the solids content of, for example 3.0%, rising to 28% and allows a reduction of the volume or the mass of sludge to be disposed to one tenth of the original amount of wet sludge. The proportion of organic materials (depending on the sludge treatment from about 50 to 60% dry matter) can be reduced with biological drainage.

Biological drainage

Under the biological drainage refers to the application of sewage sludge on Vererdungsbeete. In these predominantly planted with reed beds it first comes to a rapid dewatering of the sludge to about 10% solids content. The seeping through the soil filter water of the sewage sludge is collected with drainage systems and returned to the treatment plant. In the Vererdungsbeeten the Klärschlammvererdungsanlage the organic and mineral solids remain. Through biological conversion processes over a longer period, the organic components are degraded and mineralized, whereby the mass is substantially reduced. This creates high-quality sludge earth, reached after evacuation and post-storage dry matter content of up to 60 % from the rest pulp sludge. Due to the reduction of 50 to 60 % of the organic substances contained in sewage sludge ( mass reduction) Sewage sludge can effectively achieve lower residual amounts.

Dehydration

By drying, the weight and the volume of the sludge is further reduced. The bound water in the sludge is reduced by means of evaporation or vaporization. Mainly due to the different drying methods of contact, convection or radiation procedures, in order to solve the bound water. The calorific value of dried sludge is dependent on the drying method, the residual moisture content and the carbon content in the starting material used. The calorific value of dried raw sludge corresponds to the drying of lignite (up to 19 MJ / kg), with dried sewage sludge about 11 MJ / kg was achieved.

Solar sewage sludge drying

For this, the accumulating and dewatered sewage sludge in wastewater treatment is dried using the power of the sun. The sludge is a large area put into the drying hall. This hall is like a greenhouse and has a transparent building covering consisting of film, polycarbonate or glass. The heating of the drying air in the room is carried out by the direct and diffuse sunlight; Thereby, the air and the sewage sludge heated in stock. In addition, the biological processes produce sludge in the temperature, but which leads to the decrease in the organic carbon content. This heating will increase the vapor pressure in the sludge in relation to the above it air and evaporates the water from the sludge. A built-in ventilation system in the hall (realized eg by ventilation dampers, fans ) ensures controlled air exchange. This moist air is removed or replaced from the drying hall.

The possible degree of drying of the sludge depends on the time, outside temperature and sunlight. It indicates how much the residual moisture in the sludge after drying is present. With a sufficiently long period of residence in the hall to get a summer drying rate of around 90%. In winter, the specific water withdrawal per m² floor area reduced; characterized the evaporation ( and therefore also the degree of drying of the sludge ) is slightly less than in summer.

By means of drying produces a granulate which is a renewable alternative fuel with a calorific value of 8-11 MJ / kg DM (equivalent to approximately 2-3 kWh / kg TS; Conversion: 1 MJ = 0.2778 kWh) used in coal-fired power stations and cement works is or as fertilizer (see section on waste disposal ) can be used. The largest solar sludge drying plant with 7200 sqm drying area is currently being operated in Nicaragua after the WendeWolf process.

Belt dryer

The drying of sewage sludge with a Badtrockner is usually carried out in an air atmosphere. The air heats the sludge strings generated by the piercing die on the belt. The exiting air is purified in a biofilter. The moisture must be condensed under certain circumstances. The energy for heating the air is usually supplied as heat from an industrial process. Therefore, the drying band is often used as an example of the low-temperature drying.

Fluidized bed steam dryer ( WVT )

For the fluidized bed steam dryer usually sludge particles are used, which are fluidized in a fluidized with superheated steam (atmospheric or overpressure). The particles are produced by extruder or meat grinder. The evaporated water is condensed from the product of a heat exchanger and can be thermally re-used ( saturated steam at 4 bar with 140 ° C). The vapor (condensate) is then fed to the treatment plant as a chargeback. As the heating medium normally come saturated steam or thermal oil used.

Washer dryer

When washer dryer the sludge is heated to similar heated slices a mixer. The escaping steam is removed and condensed. The resulting water-insoluble gases are purified. The condensate can (usually atmospheric with 70-90 ° C) be used thermally by heat exchanger and is then fed to the treatment plant as a chargeback. As the heating medium normally come saturated steam or thermal oil with high temperatures used. Therefore, this type of drying has often been used as an example of the high-temperature drying.

Frying of sewage sludge

At the first frying the dehydrated to about 30 % dry solids sludge is pelletized in a type mincer, before the aqueous intermediate product to a moisture Durchlauffritteuse is supplied. After about 8 minutes, the dried pellets are having a residual water content of 4 to 5%. The calorific value is, by the amount of oil containing about 22 MJ / kg DM.

The energy-rich pellets can be utilized to generate electricity. Depending on the energy level of a Nachprozesses both the waste gases of combustion, and the heat in the vapor can be used thermally.

Recycling and disposal

Agricultural recycling

The recycling of sludge from municipal wastewater treatment plants as a fertilizer in agriculture in Germany is subject to the Sewage Sludge Ordinance ( AbfKlärV ).

The use as fertilizer is allowed only on farmland - not on permanent pasture or fruit and vegetable crops. Sludges that meet the requirements of the Sewage Sludge Ordinance regarding pollutant levels and in terms of nutrient contents comply with the requirements of the Fertilizer Ordinance, applicable in Germany as an approved fertilizer. Legally declared sewage sludge fertilizer exactly top quality, which have agronomic advantages of the direct nutrient effect, the humus supply and the Kalkeffekt are called Klärdünger. According to the German Association for Water, Wastewater and Waste (DWA ) were used in Germany in 2003, 56% of treated sludge from municipal wastewater treatment plants as a fertilizer in agriculture and landscaping. Regionally, the recovery rates are very different, the state with the highest recycling rate is Lower Saxony (2005 > 75 %).

The current ( 20xx ) still permissible contaminant levels for agricultural discharged sewage sludge are in discussion for some time. An amendment of the German Sewage Sludge Ordinance is provided. A tightening of emissions limits and the inclusion of additional criteria can be expected.

In Switzerland, the spreading of sewage sludge is completely banned since 1 October 2006, on food and vegetable surfaces, the application is already prohibited since 1 January 2003. The dried sludge is incinerated in waste and sludge incineration plants and cement plants. Due to a shortage of capacity, small amounts were deposited and exported, mainly in lignite-fired power plants in Germany. Reasons for the ban in Switzerland, the mercury content of the sludge and the content of endocrine disrupting substances.

In the Austrian provinces of Tyrol and Salzburg land application of sewage sludge is prohibited, while in the rest of Austria restricted (as 20xx ) possible.

Disposal

Due to the high content of organic matter ( about 50 %), which positively affected in the utilization as fertilizer the humus balance of the land, the removal of the sludge by landfilling in Germany since June 1, 2005 is no longer possible. According to German Waste Disposal Ordinance ( AbfAblV ) since that date may only waste with more than 5 % organic solids can be landfilled.

Thermal treatment

Not as fertilizer recycled sewage sludge (incineration or gasification ) are used in thermal processes. Whether combustion of sludge is considered as a recovery operation depends upon the type of procedure. For the incineration of calorific value is important. A sufficiently high heating value can be achieved by pre-drying, however, has a high energy requirement. The following thermal processes serve to sludge disposal:

• In co -fired with solids power plants ( coal-fired power plant, lignite power plant )
• Energetic and material use of sewage sludge in cement plant
• Co-incineration in waste incineration plant
• So-called mono-incineration ( exclusive use sewage sludge)
• Gasification process in mono or with biomass (production of fuel for a CHP )
• In Switzerland, a pilot plant for the production of Lefinoel is operated.

In the thermal co-treatment, for example in power plants or cement plants, the plant nutrients contained in sewage sludge lost to the natural cycle, because the nutrients contained in the sludge are strongly diluted by the main ash. These ashes can not be used sense for a nutrient recovery today usually. For mono processes in which only sewage sludge is used, the phosphorus content in the ash is so high, so that a recovery of phosphorus in the foreseeable future due to the shortage of resources could be economically viable. In process of co-combustion in phosphorus recovery is problematic because of the mixing with ashes of the other fuels.

Sewage sludge

Another recycling method for sewage sludge can be achieved with the Sewage sludge. After dewatering in reed beds, the contents of organic dry mass are largely reduced in sewage sludge by microbial degradation, which also changes in material properties, pore volume, and further consequences are connected. The result is sanitized and humus -like sludge soil, suitable for the manufacture of technical floors for use as a plant substrate in horticulture and landscaping and water management layers in landfill reclamation, where recyclables recycled back into the material cycle, and can not be destroyed as with incineration. Aspects of soil and groundwater protection would be observed in these types of use. Sewage sludge soil can be dewatered sludges as others also burned.

Hazard for humans

According to § 3 BioStoffV ( Biological Agents ) Sewage sludge falls into the less serious ' risk group 2' (of 4 groups). Sewage sludge can contain microorganisms that cause human infectious diseases. A spread of such infectious diseases in the population by sewage sludge is unlikely. The inclusion of sewage sludge (eg, by ingestion, through the damaged skin ( eg open wounds, eczema ) or by inhalation ( aerosols) ) should be avoided.

In Germany since 1992 sludge may not be used on permanent grassland as well as fruit and vegetable crops to prevent the direct recording of any adhering to plant sludge (§ 4 AbfKlärV ). In Switzerland in early 2003 was generally prohibited a spreading of sewage sludge on pastures and vegetable crops.

Precautions when handling

• To protect against ingestion basic hygiene rules must be observed, such as washing hands before eating, drinking and smoking or using the toilet.
• If direct contact can not be avoided, is to wear appropriate personal protective equipment. Protective gloves must be impervious to microorganisms and are marked (symbol " Impermeable to micro-organisms "). This mark shall be printed on the protective glove.
• At very hazardous pollution work of the body can be protected with a spritzwasserundurchlässigen disposable coverall.
• At work, in which the eventual emergence of aerosols through technical means is unavoidable, shall be borne P3 to avoid inhalation of dust masks of the highest filter stage. This is available as a one-off masks for mouth and nose or as particle filtering half mask with filter change possibility.